1. Pennsylvania Stormwater Best Management Practices
  2. Manual
    1. December 2006
    2. Table of Contents
      1. Foreword
      2. Chapter 1 Introduction and Purpose
      3. Chapter 2 Making The Case For Stormwater Management
      4. Chapter 3 Stormwater Management Principles and Recommended
      5. Control Guidelines
      6. Chapter 4 Integrating Site Design and Stormwater Management
      7. Chapter 5 Non-Structural BMPs
      8. Chapter 6 Structural BMPs
      9. Chapter 7 Special Management Areas
      10. Chapter 8 Stormwater Calculations and Methodology
      11. Chapter 9 Case Studies: Innovative Stormwater Management
      12. Approaches and Practices
      13. Appendix A - Water Quality
      14. Appendix B - Pennsylvania Native Plant List
      15. Appendix C - Protocols for Structural BMPs
      16. Appendix D - Stormwater Calculations and Methodology – Case Study
      17. Glossary
  3. Pennsylvania Stormwater Best Management Practices
  4. Manual
  5. Chapter 1
  6. Introduction and Purpose
    1. Chapter 1 Introduction and Purpose
      1. 1.1 Purpose of this Manual………………………………………………………………1
      2. 1.2 How to Use this Manual…………………………………..………………………….1
      3. 1.3 Overview of Pennsylvania’s Existing Stormwater Management Program….3
  7. Pennsylvania Stormwater Best Management Practices
  8. Manual
  9. Chapter 2
  10. Making the Case for Stormwater Management
      1. Chapter 2 Making the Case for Stormwater Management
      2. 2.1 A Brief Review of Stormwater Problems in Pennsylvania
      3. 2.2 The Hydrologic Cycle and The Effects of Development
      4. 2.2.1 Rainfall, Runoff, and Flooding
      5. Runoff Volume from
      6. Woodland and Impervious Surfaces
      7. 2.2.2 The Impacts of Vegetation Loss and Soil Changes
      8. 2.2.3 Groundwater Recharge, Stream Base Flow, and First-Order Streams
      9. Table 2-2. Common Bulk Density Measurements
      10. 2.2.4 Stream Channel Changes
      11. 2.2.5 Water Quality
      12. Section 2.3 References
  11. Pennsylvania Stormwater Best Management Practices
  12. Manual
  13. Chapter 3
  14. Stormwater Management Principles and Recommended Control Guidelines
      1. Chapter 3 Stormwater Management Principles and Recommended Control Guidelines
  15. Best Management Practices
  16. Manual
  17. Chapter 4
  18. Integrating Site Design and Stormwater
  19. Management
    1. Chapter 4 Integrating Site Design and Stormwater Management
      1. 4.1 A Recommended Site Design Procedure for Comprehensive
      2. Stormwater Management…………………………………………………1
      3. 4.2 The Site Design Checklist for Comprehensive Stormwater
      4. Management…………………………………………………………………3
      5. 4.3 Importance of Site Assessment………………………………………….7
      6. 4.3.1 Background Site Factors…………………………………………..7
      7. 4.3.2 Site Factors Inventory………………………………………………8
      8. 4.3.3 Site Factors Analysis……………………………………………….8
      9. 4.1 A Recommended Site Design Procedure for Comprehensive
      10. Stormwater Management
      11. 4.2 The Site Design Checklist for Comprehensive Stormwater
      12. Management
      13. SITE ANALYSIS
      14. BACKGROUND SITE CONDITIONS
      15. 4.3 Importance of Site Assessment
      16. 4.3.1 Background Site Factors
      17. 4.3.2 Site Factors Inventory
      18. 4.3.3 Site Factors Analysis
  20. Pennsylvania Stormwater Best Management Practices
  21. Manual
  22. Chapter 5
  23. Non-Structural BMPs
      1. Chapter 5 Non-Structural BMPs
      2. Chapter 5 Comprehensive Stormwater Management: Non-Structural BMPs
  24. 5.4 Protect Sensitive and Special Value Resources
  25. BMP 5.4.1: Protect Sensitive and Special Value Features
  26. BMP 5.4.2: Protect /Conserve/Enhance Riparian Areas
  27. BMP 5.4.3: Protect/Utilize Natural Flow Pathways in Overall Stormwater
  28. Planning and Design
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Construction Issues
      6. Maintenance Issues
      7. Cost Issues
  29. 5.5 Cluster and Concentrate
  30. BMP 5.5.1: Cluster Uses at Each Site; Build on the Smallest Area
  31. Possible
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Issues
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  32. BMP 5.5.2: Concentrate Uses Area wide through Smart Growth
  33. Practices
      1. Variations
      2. Applications
      3. Design Considerations:
      4. Detailed Stormwater Functions
      5. Construction Sequence
      6. Maintenance Issues
      7. Cost Issues
  34. 5.6 Minimize Disturbance and Minimize Maintenance
  35. BMP 5.6.1: Minimize Total Disturbed Area - Grading
      1. Description
      2. Detailed Stormwater Functions
      3. Design Considerations
      4. Construction Issues
      5. Cost Issues
      6. Stormwater Management Calculations
      7. Specifications
  36. BMP 5.6.2: Minimize Soil Compaction in Disturbed Areas
      1. Description:
      2. Applications
      3. Detailed Stormwater Functions
      4. Construction Issues
      5. Maintenance Issues
      6. Cost Issues
      7. Specifications
      8. References
  37. BMP 5.6.3: Re-Vegetate and Re-Forest Disturbed Areas, Using
  38. Native Species
      1. Description of BMP
      2. Detailed Stormwater Functions
      3. Design Considerations
      4. Maintenance Issues
      5. Construction Issues
      6. Cost Issues
      7. Stormwater Management Calculations
      8. References
  39. 5.7 Reduce Impervious Cover
  40. BMP 5.7.1: Reduce Street Imperviousness
      1. Description
      2. Applications
      3. Cost Issues
  41. BMP 5.7.2: Reduce Parking Imperviousness
      1. Description
      2. Applications
      3. Cost Issues
  42. 5.8 Disconnect/Distribute/Decentralize
  43. BMP 5.8.1: Rooftop Disconnection
      1. Description
      2. Variations
      3. Applications
      4. Cost Issues
      5. References
  44. BMP 5.8.2: Disconnection from Storm Sewers
      1. Description
      2. Variations
      3. Applications
      4. Cost Issues
  45. 5.9 Source Control
  46. BMP 5.9.1: Streetsweeping
      1. Description
      2. Variations
      3. Applications
      4. Cost Issues
      5. Pollutant Removal Performance
      6. References
  47. Pennsylvania Stormwater Best Management Practices
  48. Manual
  49. Chapter 6
  50. Structural BMPs
      1. Chapter 6 Structural BMPs
  51. 6.4 Volume/Peak Rate Reduction by Infiltration BMPs
  52. BMP 6.4.1: Pervious Pavement with Infiltration Bed
      1. Description
      2. Variations
      3. Pervious Bituminous Asphalt
      4. Pervious Concrete
      5. Pervious Paver Blocks
      6. Reinforced Turf and Gravel Filled Grids
      7. Applications
      8. Design Considerations
      9. Detailed Stormwater Functions
      10. Maintenance Issues
      11. Special Maintenance Considerations:
      12. Cost Issues
      13. Specifications
      14. TESTING, INSPECTION, AND ACCEPTANCE
      15. References and Additional Sources
  53. BMP 6.4.2: Infiltration Basin
      1. Description
  54. Variations ?
  55. Applications ?
      1. Detailed Stormwater Functions
      2. Infiltration Area
      3. Construction Sequence
  56. BMP 6.4.3: Subsurface Infiltration Bed
      1. Description
      2. Variations
      3. Applications
      4. Other Applications
      5. Design Considerations
      6. Detailed Stormwater Functions
      7. Construction Sequence
      8. Maintenance Issues
      9. Cost Issues
      10. Specifications
  57. BMP 6.4.4: Infiltration Trench
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance and Inspection Issues
      8. Cost Issues
      9. Specifications
      10. References
  58. BMP 6.4.5: Rain Garden/Bioretention
      1. Description
  59. Rain Gardens / Bioretention function to: ??
  60. Pretreatment (optional) ?
  61. ???????
  62. Ponding area ????
  63. Plant material ???
  64. ?
      1. Variations
      2. Flow Entrance: Curbs and Curb Cuts Flow Entrance: Trench Drain
      3. Positive Overflow: Domed Riser
      4. Positive Overflow: Inlet
      5. Applications
      6. Design Considerations
      7. Detailed Stormwater Functions
      8. Maintenance Issues
      9. Cost Issues
      10. Specifications
  65. BMP 6.4.6: Dry Well / Seepage Pit
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  66. BMP 6.4.7: Constructed Filter
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Maintenance and Inspection
      7. Winter concerns
      8. Cost Issues
      9. Specifications
      10. References
  67. BMP 6.4.8: Vegetated Swale
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
  68. BMP 6.4.9: Vegetated Filter Strip
      1. Description
      2. Filter Strip Example #1: Turf Grass
      3. Filter Strip Example #2: Native Grasses and Planted Woods Grass
      4. Filter Strip Example #3: Indigenous Woods
      5. Variations
      6. Applications
      7. Design Considerations
      8. Required Length as a Function of Slope, Soil Cover
      9. Detailed Stormwater Functions
      10. Construction Sequence
      11. Maintenance Issues
      12. Cost Issues
      13. Specifications
      14. References
  69. BMP 6.4.10: Infiltration Berm & Retentive Grading
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
  70. 6.5 Volume/Peak Rate Reduction BMPs
  71. BMP 6.5.1: Vegetated Roof
      1. Description
      2. Variations
      3. Design Considerations
      4. Detailed Stormwater Functions
      5. Construction Sequence
      6. Maintenance Issues
      7. Cost Issues
      8. Specifications
      9. References
  72. BMP 6.5.2: Runoff Capture & Reuse
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications:
      10. References
  73. 6.6 Runoff Quality/Peak Rate BMPs
  74. BMP 6.6.1: Constructed Wetland
      1. Design Considerations
      2. Detailed Stormwater Functions
      3. Construction Sequence
      4. Maintenance Issues
      5. Cost Issues
      6. Specifications:
      7. References
  75. BMP 6.6.2: Wet Pond/Retention Basin
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications:
      10. References
  76. BMP 6.6.3: Dry Extended Detention Basin
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  77. BMP 6.6.4: Water Quality Filters & Hydrodynamic Devices
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  78. 6.7 Restoration BMPs
  79. BMP 6.7.1: Riparian Buffer Restoration
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Construction Sequence
      6. Maintenance Issues
    1. Specifications
      1. Design Criteria
      2. Maintenance Guidelines
      3. Planning Considerations
      4. References
  80. BMP 6.7.2: Landscape Restoration
      1. Description
  81. BMP 6.7.3: Soil Amendment & Restoration
      1. Problem Description
      2. Construction Sequence
      3. Maintenance Issues
      4. Cost Issues
      5. Specifications
      6. References
  82. BMP 6.7.4: Floodplain Restoration
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  83. 6.8 Other BMPs and Related Measures
  84. BMP 6.8.1: Level Spreader
      1. Description
      2. Variations
      3. Applications
      4. Detailed Stormwater Functions
      5. Construction Sequence
      6. Maintenance Issues
      7. Cost Issues
      8. Specifications
      9. References
  85. BMP 6.8.2: Special Detention Areas – Parking Lot, Rooftop
      1. Description
      2. Variations
      3. Applications
      4. Design Considerations
      5. Detailed Stormwater Functions
      6. Construction Sequence
      7. Maintenance Issues
      8. Cost Issues
      9. Specifications
      10. References
  86. Pennsylvania Stormwater Best Management Practices
  87. Manual
  88. Chapter 7
  89. Special Management Areas
      1. Chapter 7. Special Management Areas
      2. Waters)
      3. Increased storage
      4. Pollution control/water quality
      5. 7.6 Stormwater Management Near Water Supply Wells
      6. Stormwater infiltration BMPs near water supply wells
      7. Non-infiltration BMPs near water supply wells
      8. Appropriate BMPs
      9. What if the stormwater cannot be infiltrated?
  90. Pennsylvania Stormwater Best Management Practices
  91. Manual
  92. Chapter 8
  93. Stormwater Calculations and Methodology
    1. Chapter 8 Stormwater Calculations and Methodology
      1. 8.1 Introduction to Stormwater Methodologies……………………………………….1
      2. 8.2 Existing Methodologies for Runoff Volume Calculations and their
      3. 8.3 Existing Methodologies for Peak Rate/Hydrograph Estimations and their
      4. 8.4 Computer Models……………………………………………………………………....4
      5. 8.5 Precipitation Data for Stormwater Calculations………………………………….6
      6. 8.6 Stormwater Quality Management…………………………………………………...7
      7. 8.7 Guidance for Stormwater Calculations for Volume Control Guideline 1 and
      8. 8.8 Non-structural BMP Credits………………………………………………………..17
      9. 8.9 References and Additional Sources………………………………………………45
      10. 8.1 Introduction to Stormwater Methodologies
      11. 8.2 Existing Methodologies for Runoff Volume Calculations and their
      12. Limitations
      13. 8.2.1 Runoff Curve Number Method
      14. 8.2.2 Small Storm Hydrology Method (SSHM)
      15. 8.2.3 Infiltration Models for Runoff Calculations
      16. 8.3 Existing Methodologies for Peak Rate/Hydrograph Estimations and their
      17. Limitations
      18. 8.3.1 The Rational Method
      19. 8.3.2 SCS (NRCS) Unit Hydrograph Method
      20. 8.4 Computer Models
      21. 8.4.1 HEC Hydrologic Modeling System (HEC-HMS)
      22. 8.4.2 SCS/NRCS Models (WIN TR-20 and WIN TR-55)
      23. 8.4.3 NRCS NEH 650 Engineering Field Handbook, Chapter 2 (EFH2)
      24. 8.4.4 Storm Water Management Model (SWMM)
      25. 8.5 Precipitation Data for Stormwater Calculations
      26. 8.6 Stormwater Quality Management
      27. 8.6.1 Analysis of Water Quality Impacts from Developed Land
      28. 8.6.2 Analysis of Water Quality Benefits from BMPs
      29. 8.6.3 Water Quality Analysis
      30. 8.7 Guidance for Stormwater Calculations for Volume Control Guideline 1 and
      31. Volume Control Guideline 2
      32. 8.7.1 Stormwater Calculation Process
      33. 8.7.2 Water Quality Calculations (Flow Chart D)
      34. 8.8 Non-Structural BMP Credits
      35. Worksheet 1. General Site Information
      36. Worksheet 2. Sensitive Natural Resources
      37. PROJECT: SUB-BASIN:
    2. WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
  94. FLOW CHART CControl Guideline 2 Process
      1. WORKSHEET 7. CALCULATION OF RUNOFF VOLUMES (PRV and EDV) FOR CG-2 ONLY
      2. PROJECT: DRAINAGE AREA:
      3. WORKSHEET 8 . STRUCTURAL BMP VOLUME CREDITS
      4. Flow Chart DWater Quality Process
      5. WORKSHEET 10. WATER QUALITY COMPLIANCE FOR NITRATE
    1. WORKSHEET 11. BMPS FOR POLLUTION PREVENTION
      1. WORKSHEET 12. WATER QUALITY ANALYSIS OF POLLUTANT LOADING FROM ALL
      2. DISTURBED AREAS
      3. 8.9 References and Additional Information Sources
  95. Pennsylvania Stormwater Best Management Practices
  96. Manual
  97. Chapter 9
  98. Case Studies: Innovative Stormwater Management
  99. Approaches and Practices
    1. Chapter 9 Case Studies: Innovative Stormwater Management
    2. Approaches and Practices
      1. 9.1 Introduction………………………………………………………………………...…1
      2. 9.2 Outline of Information Needed for Case Studies…………………………..…..2
      3. 9.3 Case Studies……………………………………………………………………….…3
      4. 1: Penn State University - Centre County Visitor Center, Centre County...3
      5. 2: Dennis Creek Streambank Restoration, Franklin County………….……10
      6. 3: Commerce Plaza III, Lehigh County…………………………………………11
      7. 4: Flying J. Truck Plaza for Welsh Oil of Indiana……………………………..13
      8. 5: Ephrata Performing Arts Center, Lancaster County………………………16
      9. 6: Lebanon Valley Agricultural Center, Lebanon County……………………18
      10. 7: Penn State University Berks County Campus, Berks County……..…….19
      11. 8: Warm Season Meadows at Williams Transco, Chester County………….22
      12. 9: Hills of Sullivan Residential Subdivision, Chester County……………….24
      13. 10: Applebrook Golf Course Community, Chester County…………………..28
      14. 11: Swan Lake Drive Development, Delaware County…………………….…..30
      15. Case Studies: Innovative Stormwater Management Approaches and Practices
      16. 9.1 Introduction
      17. 9.2 Outline of Information Needed for Case Studies
      18. PADEP Stormwater Manual Case Study
      19. 9.3 Case Studies
      20. Engineering Plans
      21. Case Study 2: Dennis Creek Streambank Restoration, Franklin County
      22. Case Study 3: Commerce Plaza III, Lehigh County
      23. Cumberland County
      24. Project Background
      25. Site Description
      26. Design Images and Details
      27. Case Study 5: Ephrata Performing Arts Center, Lancaster County
      28. Case Study 6: Lebanon Valley Agricultural Center, Lebanon County
      29. Case Study 7: Penn State University Berks County Campus, Berks County
      30. Case Study 8: Warm Season Meadows at Williams Transco, East Whiteland Township,
      31. Chester County
      32. Case Study 9: Hills of Sullivan Residential Subdivision, London Grove Township,
      33. Chester County
      34. Case Study 10: Applebrook Golf Course Community, Chester County
      35. References
      36. Case Study 11: Swan Lake Drive Development, Delaware County
  100. Pennsylvania Stormwater Best Management Practices
  101. Manual
  102. Appendix A – Water Quality
  103. Pollutant Event Mean
  104. Concentrations by Land Cover & BMP Pollutant Removal Efficiencies
  105. Pollutant Event Mean Concentrations by Land Cover
      1. TABLE A-1. EVENT MEAN CONCENTRATIONS (EMCs) FOR TOTAL SUSPENDED SOLIDS
  106. BMP Pollutant Removal Efficiencies-
  107. Percent Efficiency
      1. Structural BMP
      2. COMPREHENSIVE BMP LIST
      3. Non-Structural BMP
  108. BMP Pollutant Removal Efficiencies- Inflow vs. Outflow Pollutant concentrations
  109. Pennsylvania Stormwater Best Management Practices
  110. Manual
  111. Appendix B – Pennsylvania Native Plant List
  112. Pennsylvania Stormwater Best Management Practices
  113. Manual
  114. Appendix C – Site Evaluation and Soil Testing
      1. Protocol 1
      2. Site Evaluation and Soil Infiltration Testing
      3. Protocol 2 Infiltration Systems Design and Construction Guidelines
  115. Pennsylvania Stormwater Best Management Practices
  116. Manual
  117. Appendix D – Stormwater Calculations and
  118. Methodology: Case Study
      1. WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
      2. Storm Event
      3. Peak Flow without BMPs
      4. (cfs)
      5. Volume Control BMP Storage
      6. Ave. Residence Time/
      7. Time of Conc.
      8. Increase (min.)
    1. WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
    2. GLOSSARY

PENNSYLVANIA
Stormwater BMP Manual
December 30, 2006

363-0300-002 / December 30, 2006
DEPARTMENT OF ENVIRONMENTAL PROTECTION
Bureau of Watershed Management
DOCUMENT NUMBER:
363-0300-002
TITLE:
Pennsylvania Stormwater Best Management Practices Manual
EFFECTIVE DATE:
December 30, 2006
AUTHORITY:
Pennsylvania Clean Streams Law (35 P.S. §§ 691.1-691.1001);
Pennsylvania Stormwater Act (32 P.S. §§ 680.1-680.17); Federal
Clean Water Act (33 U.S.C.A. § 1342), 40 CFR Part 122 and 25 Pa
Code Chapters 92, 93, 102, 105 and 111.
POLICY:
The Department will ensure that activities and plans approved under
its authority will employ stormwater management plans utilizing best
management practices to control the volume, rate and water quality of
post construction stormwater runoff so as to protect and maintain the
chemical, physical and biological properties of waters of the
Commonwealth. These best management practices must, at a
minimum, protect and maintain water resources, preserve water
supplies, maintain stream base flows, preserve and restore the flood
carrying capacity of waters, preserve to the maximum extent
practicable the natural stormwater runoff regimes and natural course,
current and cross section of waters of the Commonwealth, and protect
and conserve ground waters and ground-water recharge areas.
PURPOSE:
Clean, reliable water resources are critical for sustaining the
environmental health of our natural resources, protecting the public’s
health and safety, and maintaining the economic vitality of the
Commonwealth. The purpose of this guidance manual is to ensure
effective stormwater management to minimize the adverse impacts of
stormwater on ground water and surface water resources to support
and sustain the social, economic and environmental quality of the
Commonwealth.
APPLICABILITY:
This guidance applies to all persons conducting or planning to conduct
activities that require a written post-construction stormwater
management plan.
DISCLAIMER:
The policies and procedures outlined in this guidance are intended to
supplement existing requirements. Nothing in the policies or
procedures shall affect regulatory requirements. The guidelines herein
are not an adjudication or a regulation. The Department reserves the
discretion to vary from this guidance as circumstances warrant.

363-0300-002 / December 30, 2006
PAGE LENGTH:
642
LOCATION:
Volume 34, Tab 20
DEFINITIONS:
See Title 25 Pa. Code, Chapters 92, 93, 102, 105 and 111.

363-0300-002 / December 30, 2006
Pennsylvania Stormwater
Best Management Practices

Back to top


Manual
December 2006

363-0300-002 / December 30, 2006

363-0300-002 / December 30, 2006
Table of Contents
Foreword
Chapter 1 Introduction and Purpose
1.1 Purpose of This Manual................................................................................................................ 1
1.2 How to Use This Manual............................................................................................................... 1
1.3 Overview of Pennsylvania’s Existing Stormwater Management Program .................................... 3
Chapter 2 Making The Case For Stormwater Management
2.1 A Brief Review of Stormwater Problems in Pennsylvania ............................................................ 1
2.2 The Hydrologic Cycle and The Effects of Development ............................................................... 4
2.2.1
Rainfall, Runoff, and Flooding............................................................................... 6
2.2.2
The Impacts of Vegetation Loss and Soil Changes ............................................ 10
2.2.3
Groundwater Recharge, Stream Base Flow, and First Order Streams ............... 10
2.2.4
Stream Channel Changes................................................................................... 13
2.2.5
Water Quality ...................................................................................................... 14
2.3 References ................................................................................................................................. 19
Chapter 3 Stormwater Management Principles and Recommended
Control Guidelines
3.1 Introduction................................................................................................................................... 1
3.2 Recommended Site Control Guidelines ....................................................................................... 1
3.3 Recommended Volume Control Guidelines.................................................................................. 2
3.3.1
Volume Control Criteria......................................................................................... 4
3.3.2
Volume Control Alternatives.................................................................................. 5
3.3.3
Volume Control Guideline 1 .................................................................................. 6
3.3.4
Volume Control Guideline 2………………………………………………………….…7
3.3.5
Retention and Detention Considerations .............................................................. 7
3.4 Recommended Peak Rate Control Guideline............................................................................... 8
3.5 Recommended Water Quality Control Guideline.......................................................................... 8
3.6 Stormwater Standards for Special Areas...................................................................................... 9
Chapter 4 Integrating Site Design and Stormwater Management
4.1 A Recommended Site Design Procedure for Comprehensive Stormwater
Management................................................................................................................................. 1
4.2 The Site Design Checklist for Comprehensive Stormwater Management .................................... 3
4.3 Importance of Site Assessment .................................................................................................... 7
4.3.1.
Background Site Factors....................................................................................... 7
4.3.2.
Site Factors Inventory .......................................................................................... 8
4.3.3.
Site Factors Analysis............................................................................................. 8

363-0300-002 / December 30, 2006

363-0300-002 / December 30, 2006

363-0300-002 / December 30, 2006
Chapter 5 Non-Structural BMPs
5.1 Introduction ................................................................................................................................... 1
5.2 Non-Structural Best Management Practices................................................................................. 1
5.3 Non-Structural BMPs and Stormwater Methodological Issues..................................................... 3
5.4 Protect Sensitive and Special Value Resources........................................................................... 5
BMP 5.4.1 Protect Sensitive and Special Value Features ...................................................... 7
BMP 5.4.2 Protect/Conserve/Enhance Riparian Areas ........................................................ 13
BMP 5.4.3 Protect/Utilize Natural Flow Pathways in Overall Stormwater Planning
and Design .......................................................................................................... 21
5.5 Cluster and Concentrate............................................................................................................ 27
BMP 5.5.1 Cluster Uses at Each Site; Build on the Smallest Area Possible ........................ 29
BMP 5.5.2 Concentrate Uses Areawide through Smart Growth Practices ........................... 37
5.6 Minimize Disturbance and Minimize Maintenance..................................................................... 47
BMP 5.6.1 Minimize Total Disturbed Area – Grading .......................................................... 49
BMP 5.6.2 Minimize Soil Compaction in Disturbed Areas .................................................... 57
BMP 5.6.3 Re-Vegetate and Re-Forest Disturbed Areas, Using Native Species ................. 63
5.7 Reduce Impervious Cover ......................................................................................................... 69
BMP 5.7.1 Reduce Street Imperviousness.......................................................................... 71
BMP 5.7.2 Reduce Parking Imperviousness ....................................................................... 77
5.8 Disconnect / Distribute / Decentralize........................................................................................ 82
BMP 5.8.1 Rooftop Disconnection........................................................................................ 85
BMP 5.8.2 Disconnection from Storm Sewers..................................................................... 89
5.9 Source Control........................................................................................................................... 92
BMP 5.9.1 Streetsweeping ................................................................................................... 95
Chapter 6
Structural BMPs
6.1 Introduction ................................................................................................................................... 1
6.2 Groupings of Structural BMPs ...................................................................................................... 1
6.3 Manufactured Products................................................................................................................. 2
6.4 Volume/Peak Rate Reduction by Infiltration BMPs....................................................................... 5
BMP 6.4.1 Pervious Pavement with Infiltration Bed................................................................ 7
BMP 6.4.2 Infiltration Basin .................................................................................................. 27
BMP 6.4.3 Subsurface Infiltration Bed.................................................................................. 33
BMP 6.4.4 Infiltration Trench ................................................................................................ 41
BMP 6.4.5 Rain Garden/Bioretention ................................................................................... 49
BMP 6.4.6 Dry Well / Seepage Pit........................................................................................ 67
BMP 6.4.7 Constructed Filter................................................................................................ 71
BMP 6.4.8 Vegetated Swale................................................................................................. 83
BMP 6.4.9 Vegetated Filter Strip .......................................................................................... 99
BMP 6.4.10 Infiltration Berm & Retentive Grading.............................................................. 113

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6.5 Volume/Peak Rate Reduction BMPs........................................................................................ 123
BMP 6.5.1 Vegetated Roof ................................................................................................. 125
BMP 6.5.2 Runoff Capture & Reuse................................................................................... 139
6.6 Runoff Quality/Peak Rate BMPs............................................................................................... 149
BMP 6.6.1 Constructed Wetland......................................................................................... 151
BMP 6.6.2 Wet Pond/Retention Basin................................................................................ 163
BMP 6.6.3 Dry Extended Detention Basin.......................................................................... 173
BMP 6.6.4 Water Quality Filters & Hydrodynamic Devices ................................................ 183
6.7 Restoration BMPs..................................................................................................................... 189
BMP 6.7.1 Riparian Buffer Restoration............................................................................... 191
BMP 6.7.2 Landscape Restoration ..................................................................................... 211
BMP 6.7.3 Soil Amendment & Restoration ......................................................................... 221
BMP 6.7.4 Floodplain Restoration ...................................................................................... 231
6.8 Other BMPs and Related Structural Measures......................................................................... 241
BMP 6.8.1 Level Spreader.................................................................................................. 243
BMP 6.8.2 Special Detention Areas - Parking Lot, Rooftop................................................ 253
Chapter 7 Special Management Areas
7.1 Introduction ................................................................................................................................... 1
7.2 Brownfields ................................................................................................................................... 1
7.2.1 Site Remediation (i.e. Cleanup)........................................................................................... 2
7.2.2 Site Redevelopment ............................................................................................................ 2
7.3 Highways and Roads.................................................................................................................... 3
7.3.1 Roadway Runoff Quality Issues........................................................................................... 4
7.3.2 BMP Considerations for Roadways ..................................................................................... 5
7.3.3 Specific BMP Considerations .............................................................................................. 9
7.3.4 Gravel Roads..................................................................................................................... 10
7.4 Karst Areas................................................................................................................................. 11
7.4.1 The Nature of Karst ........................................................................................................... 11
7.4.2 Infiltration vs non-infiltration ............................................................................................... 12
7.4.3 Basic Principles ................................................................................................................. 13
7.4.4 BMP Considerations .......................................................................................................... 15
7.5 Mined Lands ............................................................................................................................... 16
7.6 Stormwater Management Close to Water Supply Wells............................................................. 17
7.7 Surface Water Supplies and Special Protection Waters............................................................. 20
7.8 Urban Areas ............................................................................................................................... 21
7.8.1 Highly Impervious Urban Land .......................................................................................... 21
7.8.2 Urban Water Quality .......................................................................................................... 22
7.8.3 Other Urban Stormwater Management Considerations..................................................... 24
7.9 References ................................................................................................................................. 25

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Chapter 8 Stormwater Calculations and Methodology
8.1 Introduction to Stormwater Methodologies ................................................................................... 1
8.2 Existing Methodologies for Runoff Volume Calculations and their Limitations ............................. 1
8.2.1
Runoff Curve Number Method .............................................................................. 1
8.2.2
Small Storm Hydrology Method ............................................................................ 2
8.2.3
Infiltration Models for Runoff Calculations............................................................. 3
8.3 Existing Methodologies for Peak Rate/Hydrograph Estimations and their Limitations ................. 3
8.3.1
The Rational Method............................................................................................. 3
8.3.2
SCS (NRCS) Unit Hydrograph Method ................................................................. 4
8.4 Computer Models ......................................................................................................................... 4
8.4.1
HEC Hydrologic Modeling System (HEC-HMS) .................................................... 4
8.4.2
SCS/NRCS Models: WIN TR-20 and WIN TR-55 ................................................. 5
8.4.3
EFH2 ..................................................................................................................... 5
8.4.4
Storm Water Management Model (SWMM) .......................................................... 5
8.4.5
Source Loading and Management Model (SLAMM) ............................................ 6
8.5 Precipitation Data for Stormwater Calculations ............................................................................ 6
8.6 Stormwater Quality Management ................................................................................................. 7
8.6.1
Analysis of Water Quality Impacts from Developed Land .................................... 8
8.6.2
Analysis of Water Quality Benefits from BMPs ................................................... 10
8.6.3
Water Quality Analysis ........................................................................................ 12
8.7 Guidance for Stormwater Calculations for CG1 and CG2 .......................................................... 13
8.7.1
Stormwater Calculation Process ......................................................................... 14
8.7.1.1 For Control Guideline 1 (Flow Chart B) ................................................... 14
8.7.1.2 For Control Guideline 2 (Flow Chart C) .................................................. 15
8.7.2
Water Quality Calculations (Flow Chart D).......................................................... 16
8.8 Non-structural BMP Credits ...................................................................................................... 17
8.9 References and Additional Sources ........................................................................................... 45

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Chapter 9 Case Studies: Innovative Stormwater Management
Approaches and Practices
9.1 Introduction ................................................................................................................................... 1
9.2 Outline of Information Needed for Case Studies .......................................................................... 2
9.3 Case Studies ................................................................................................................................ 3
Case Study 1: Penn State University - Centre County Visitor Center, Centre County ................ 3
Case Study 2: Dennis Creek Streambank Restoration, Franklin County .................................. 10
Case Study 3: Commerce Plaza III, Lehigh County .................................................................. 11
Case Study 4: Flying J. Truck Plaza for Welsh Oil of Indiana Truck Refueling
Terminal ............................................................................................................................... 13
Case Study 5: Ephrata Performing Arts Center, Lancaster County .......................................... 16
Case Study 6: Lebanon Valley Agricultural Center, Lebanon County ....................................... 18
Case Study 7: Penn State University Berks County Campus, Berks County ............................ 19
Case Study 8: Warm Season Meadows at Williams Transco, Chester County......................... 22
Case Study 9: Hills of Sullivan Residential Subdivision, Chester County.................................. 24
Case Study 10: Applebrook Golf Course Community, Chester County...................................... 28
Case Study 11: Swan Lake Drive Development, Delaware County ........................................... 30
Appendix A - Water Quality
Appendix B - Pennsylvania Native Plant List
Appendix C - Protocols for Structural BMPs
Protocol 1 – Site Evaluation and Soil Infiltration Testing
Protocol 2 – Soil Evaluation and Investigation for Infiltration BMPs
Appendix D - Stormwater Calculations and Methodology – Case Study
Glossary

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FOREWORD
Stormwater runoff and flooding are natural events that, over the millennia, have helped
shape the world around us. Our activities on the landscape routinely alter these natural
drainage patterns by intensifying and redirecting runoff, potentially leading to stream
pollution, property damage and, in extreme cases, loss of life.
Localized flash flooding, stream bank scour and destabilization, siltation, loss of ground
water recharge, declining dry-weather stream flows and habitat destruction are all the
results of unmanaged or poorly managed stormwater. In addition to its physical impact
on the environment, stormwater may carry a variety of pollutants into our waters
including metals, bacteria, oil and grease, pesticides, nutrients and sediment. The
Department’s stream assessment efforts have documented that urban runoff is the third
leading source of stream impairment in Pennsylvania. Moving forward, these historic
problems can be avoided or minimized through a combination of forethought and
planning, and properly constructed and maintained best management practices (BMPs).
By managing stormwater runoff as a valuable and reusable resource rather than as a
waste that must be quickly moved away, a host of opportunities are opened that promote
environmental protection and enhancement while complementing new growth and
development.
This manual is based on the following set of principles:
1. Managing stormwater as a resource;
2. Preserving and utilizing existing natural features and systems;
3. Managing stormwater as close to the source as possible;
4. Sustaining the hydrologic balance of surface and ground water;
5. Disconnecting, decentralizing and distributing sources and discharges;
6. Slowing runoff down, and not speeding it up;
7. Preventing potential water quality and quantity problems;
8. Minimizing problems that cannot be avoided;
9. Integrating stormwater management into the initial site design process; and
10. Inspecting and maintaining all BMPs.
The manual supplements federal and state regulations, and the Department’s
Comprehensive Stormwater Management Policy, by emphasizing effective site planning
as the preferred method of managing runoff while also providing numerous examples of
BMPs that can be employed in Pennsylvania to further avoid and minimize flooding and
water resource problems. This manual has no independent regulatory authority. The
manual is intended to be a technical reference of planning concepts and design
standards that will satisfy Pennsylvania’s regulatory requirements and stormwater
management policies when properly tailored and applied to local site conditions.
Alternate BMPs not listed in the manual may also be used to satisfy regulatory
requirements if they provide the same or greater level of protection. No predetermined
set of practices will be applicable to every building site. Specific considerations such as
soil type, underlying geology, slope, project size and building density will determine
which practices are applicable and feasible for a given project.

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Acknowledgements
The following individuals and organizations participated in developing this manual.
Participation does not infer concurrence or endorsement of the manual or it’s contents.
Stormwater Manual Oversight Committee
Special thanks are expressed to the members of the Stormwater Manual Oversight
Committee who assisted in providing direction, guidance and expertise in the
preparation of this manual
.
Committee members include:
Kevin Abbey, Center for Dirt and Gravel Road Studies, Pennsylvania State University
John Amend, Malcolm Pirnie, Inc.
Mark A. Bahnick, P.E., Van Cleef Engineering Associates
Theresa M. Bentley, Bucks County Planning Commission
Joan Blaustein, Three Rivers Wet Weather, Inc.
Scott A. Brown, P.E., The Pennsylvania Housing Research Center
Frank X. Browne, PhD, P.E., F.X. Browne, Inc.
Albert T. Brulo, P.E., Herbert Rowland & Grubic, Inc.
Rebecca Burns, Pennsylvania Department of Transportation
Paul A. DeBarry, P.E., P.H., Borton-Lawson Engineering, Inc.
Warren Neal Cohn, ACF Environmental
Timothy J. Edinger, P.E., Base Engineering, Inc.
Lawrence A. Fennessey, PhD, P.E., Sweetland Engineering & Assoc., Inc.
Dan Greig, District Manager, Chester County Conservation District
David Klepadlo, P.E., Malcolm Pirnie, Inc.
Anthony Miller, P.E., Pennsylvania Department of Transportation
Timothy J. Murphy, P.E., Natural Resources Conservation Service
Scott Pidcock, P.E., R.A., The Pidcock Company
James W. Pillsbury, P.E., Westmoreland County Conservation District
Bruce R. Snyder, Atlas America, Inc.
Mike M. Stadulis, Realen Homes
Thaddeus K. Stevens, President, Sylvan Glen, Inc.
Albert H. Todd, U.S. Forest Service
Robert G. Traver, PhD, P.E., Civil and Environmental Engineering Department, Villanova
University
Maya K. van Rossum, the Delaware Riverkeeper, Delaware Riverkeeper Network
Paul White, P.G., Walter B. Satterthwaite Assoc., Inc.
Kerry Wilson, Pennsylvania Department of Community and Economic Development
Paul Zeigler, P.E., Governors Green Government Council
Pennsylvania Department of Environmental Protection
Committee Members: Steven Burgo, Stuart Demanski, Durla Lathia, Margaret Murphy,
Kenneth Murin, Kenneth Reisinger, Edward Ritzer, Dennis Stum, and Raymond Zomok
Special technical assistance and editing provided by Patrick Bowling, Stuart Demanski,
Stuart Gansell, Dave Goerman, Joseph Hebelka, Cedric Karper, William Kochnov,
Sharon Hill, William Manner, Jeffrey Means, Claudia Merwin, Barry Newman, Tahmina
Parvin, Frank Payer, Shelby Reisinger, Domenic Rocco, and Diane Wilson.

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Contractor and Supporting Project Teams
Contractor Team
Michele Adams, P.E., Thomas Cahill, P.E., Wesley Hoerner, and Courtney Marm of
Cahill Associates Inc., West Chester, PA
Supporting Project Team
Amy Green, Amy Green Associates, Flemington, NJ
Charles Miller, P.E., Roofscapes, Philadelphia, PA
Ann Smith, Pennsylvania Environmental Council, Philadelphia, PA
Eric Strecker, P.E., and Steven Roy, P.S., GeoSyntec Consultants, Boxborough, MA
Neil Weinstein, P.E., Low Impact Development Center, Inc., Rockville, MD

363-0300-002 / December 30, 2006

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Chapter 1

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Introduction and Purpose

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Pennsylvania Stormwater Best Management Practices Manual
Chapter 1
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Chapter 1
Introduction and Purpose
1.1
Purpose of this Manual………………………………………………………………1
1.2
How to Use this Manual…………………………………..………………………….1
1.3
Overview of Pennsylvania’s Existing Stormwater Management Program….3

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1.1 Purpose of this Manual
The purpose of the Pennsylvania Stormwater Best Management Practices (BMP) Manual is to provide
guidance, options and tools that can be used to protect water quality, enhance water availability and
reduce flooding potential through effective stormwater management. This manual presents design
standards and planning concepts for use by local authorities, planners, land developers, engineers,
contractors, and others involved with planning, designing, reviewing, approving, and constructing land
development projects.
This manual describes a stormwater management approach to the land development process that
strives to:
First, prevent or minimize stormwater problems through comprehensive planning and
development techniques, and
Second, to mitigate any remaining potential problems by employing structural and non-structural
BMPs.
Manual users are strongly encouraged to follow the progression of prevention first and mitigation
second. Throughout the chapters of this manual the concept of an integrated stormwater management
program, based on a broad understanding of the natural land and water systems, is a key and recurring
theme. Such a thorough understanding of the natural systems demands an integrated approach to
stormwater management, so critical to “doing it better, doing it smarter.”
This manual provides guidance on managing all aspects of stormwater: rate, volume, quality, and
groundwater recharge. Controlling the peak rate of flow during extreme rainfall events is important, but
it is not sufficient to protect the quality and integrity of Pennsylvania streams. Reducing the overall
volume of runoff during large and small rainfall events, improving water quality, and maintaining
groundwater recharge for wells and stream flow are all vital elements of protecting and improving the
quality of Pennsylvania’s streams and waterways.
It is important to note that The Pennsylvania Stormwater Best Management Practice Manual has no
independent regulatory authority. The strategies, practices, recommendations and control guidelines
presented in the manual can become binding requirements only through the following means:
1. Ordinances and rules established by local municipalities, or
2. Permits and other authorizations issued by local, state, and federal agencies.
1.2 How to Use this Manual
The following provides a guide to the various chapters of the Manual.
Chapter 1
– Introduction and Purpose
Chapter 2
– Stormwater and the Impacts of Development and Impervious Surfaces
This section provides an overview of the impacts of development on Pennsylvania’s natural
systems and natural resources, including discussions about the effect of increased runoff
volumes, water quality, stream channel erosion, flooding, and lost groundwater recharge and
stream baseflow.

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Chapter 3
– Stormwater Management Principles and Recommended Control Guidelines
This section discusses stormwater management principles to protect water resources and
provides recommended control guidelines for stormwater management. This chapter also
discusses how the recommended guidelines relate to diverse conditions, such as urban areas
rural settings, brownfield sites and karst topography.
Chapter 4
–Integrating Site Design and Stormwater Management
This section discusses the
process
of comprehensive stormwater management, which begins
with better site design and protection of important natural features first, and the use of structural
Best Management Practices to manage stormwater second. An approach to site design and
stormwater management for Pennsylvania is outlined in flowchart and checklist formats.
Chapter 5
– Non-Structural BMPs
This section describes in detail 13 design and development techniques (non-structural BMPs)
that reduce the impact of stormwater. It includes both specific design practices and
recommendations that may be required or encouraged by municipal officials within the context
of zoning and land development ordinances. Use of these “non-structural” BMPs is considered
to be the primary means of stormwater management.
Chapter 6
– Structural BMPs
This section describes in detail 21 specific engineering measures that reduce and mitigate the
impacts of development. The use of the “structural BMPs” is considered the second step in
stormwater design. Chapter 6 includes recommendations (protocols) for the design of
infiltration systems and for soil investigation for infiltration systems.
Chapter 7
– Special Management Areas
This chapter discusses issues and stormwater management implications unique to some
special management areas such as brownfields, highways and roads, karst areas, mined lands,
water supply well areas, surface water supplies, special protection waters, and highly urbanized
areas.
Chapter 8
– Stormwater Calculations and Methodology
This chapter discusses engineering techniques and methods used to perform stormwater
calculations. Improved sources for rainfall estimates (NOAA Atlas 14, 2004) are suggested.
This chapter also provides guidance on developing stormwater calculations based on the
recommended control guidelines in Chapter 3 of the manual. In addition, this chapter includes
optional flowcharts and worksheets to assist stormwater designers and reviewers organize and
conduct their calculations.
Chapter 9
- Case Studies
This chapter presents case studies of projects that have been implemented throughout
Pennsylvania that incorporate innovative techniques and approaches to stormwater
management. This chapter identifies sites in various regions of the state that users of the
manual may visit to observe innovative stormwater management techniques in a range of
development settings.

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Appendix A
– Water Quality
Appendix B
– Pennsylvania Native Plant List
Appendix C
– Protocols for Structural BMPs
Protocol 1 – Site Evaluation and Soil Infiltration Testing
Protocol 2 – Infiltration Systems Design and Construction Guideline
Appendix D
– Storm water Calculations and Methodology – Case Study
Glossary
1.3 Overview of Pennsylvania’s Existing Stormwater Management Program
The Clean Stream Law of 1937 provides the legal foundation for water quality protection and
restoration, and water resources management in Pennsylvania. The Department of Environmental
Protection is primarily responsible for administering the provisions of the act. The Clean Streams Law
has been affected by passage of a series of federal laws, such as the Clean Water Act (CWA) of 1972,
which has also been amended over time. Local government implements specific regulations for land
development and stormwater management. Pennsylvania has 2566 municipalities and 376 designated
stormwater management watersheds, with diverse natural, social, and cultural features. The
Pennsylvania Municipalities Planning Code (MPC) law enables, but does not require, comprehensive
planning, zoning, and subdivision/land development regulation on the municipal, county, and regional
levels. To achieve regulatory status, the recommendations and guidelines in this manual must be
implemented by ordinances and zoning at the municipal level.
The Pennsylvania Storm Water Management Act of 1978 (Act 167) provides the legislative basis for
statewide stormwater management. The Act 167 stormwater management program is mandated,
administered, and funded at a 75 percent level by the state. However, stormwater management plans
must be developed by the respective counties in a given watershed, and be implemented by the
effected municipalities through the adoption of stormwater ordinances. This is a rather uniquely
structured “sharing” of authority and powers by all levels of Pennsylvania government.
In addition to the requirements under local zoning and ordinances, federal regulations require individual
land development projects to obtain National Pollutant Discharge Elimination System (NPDES) permits.
These permits are required for all land development projects that disturb one acre or more. The
permits authorize discharges from erosion and sediment control facilities and approve post-construction
stormwater management plans. The 1999 update to the federal stormwater regulations also required
923 small municipalities and numerous institutions throughout Pennsylvania to obtain NPDES permits
for their stormwater discharges. Each permit holder must implement and enforce a stormwater
management program that reduces the discharge of pollutants to the maximum extent practicable.
More detailed discussions of individual and municipal NPDES construction and stormwater
management permits can be found on the DEP web site under the keyword “Stormwater Management”.

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Chapter 2

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Making the Case for Stormwater Management

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Pennsylvania Stormwater Best Management Practices Manual
Chapter 2
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Chapter 2 Making the Case for Stormwater Management
2.1
A Brief Review of Stormwater Problems in Pennsylvania……………………………………….1
2.2
The Hydrologic Cycle and the Effects of Development…………………………….…………….4
2.2.1 Rainfall, Runoff, and Flooding……………………………………………………………….6
2.2.2 The Impacts of Vegetation Loss and Soil Changes…………………………………….10
2.2.3 Groundwater Recharge, Stream Base Flow, and First-Order Streams……………...10
2.2.4 Stream Channel Changes……………………………………………………………………13
2.2.5 Water Quality…………………………………………………………………………………..14
2.3
References………………………………………………………………………………………………19

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2.1
A Brief Review of Stormwater Problems in Pennsylvania
Pennsylvania is the most flood prone state in the country. It has experienced several serious and
sometimes devastating floods during the past century, often as a result of tropical storms and
hurricanes, and heavy rainfall on an existing snow pack. To a large extent, the flooding that results
from such extreme storms and hurricanes occurs naturally and will continue to occur. Stormwater
management cannot eliminate flooding during such severe rainfall events (Figure 2-1).
Figure 2-1. Flooding impacts are devastating communities,
even with conventional stormwater management programs (F. Thorton).
In many watersheds throughout the state, flooding problems from rain events, including the smaller
storms, have increased over time due to changes in land use and ineffective stormwater
management. This additional flooding is a result of an increased volume
of stormwater runoff being
discharged throughout the watershed. This increase in stormwater volume is the direct result of more
extensive impervious surface areas (Figure 2-2), combined with substantial tracts of natural
landscape being converted to lawns on highly compacted soil or agricultural activities.
Figure 2-2. Parking lots are common impervious surfaces that
affect stormwater runoff.

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The problems are not limited to flooding. Stormwater runoff carries significant quantities of pollutants
washed from the impervious and altered land surfaces (Figure 2-3). The mix of potential pollutants
ranges from sediment to varying quantities of nutrients, organic chemicals, petroleum hydrocarbons,
and other constituents that cause water quality degradation.
Figure 2-3. Pollutant laden runoff degrades water quality.
Increased stormwater runoff volume can turn small meandering streams into highly eroded and
deeply incised stream channels (Figure 2-4). Stream meander and the resulting erosion and
sedimentation is a natural process, and all channels are in a constant process of alteration.
However, as the volume of runoff from each storm event is increased, natural stream channels
experience more frequent bank full or near bankfull conditions. As a result, streams change their
natural shape and form. Pools and riffles that support aquatic life are disrupted as channels erode to
an unnatural level, and the eroded bank material contributes to sediment in the stream and degrades
it’s health by smothering stream bottom habitat. The majority of this stream channel devastation is
intensified during the frequently occurring small-to-moderate precipitation events, not during major
flooding events.
Figure 2-4.Stormwater influenced stream bank morphology in Valley Creek.

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Rainfall is an important resource to replenish the groundwater and maintain stream flow (Figure 2
-
5). When the stormwater runoff during a storm event is allowed to drain away rather than recharge
the groundwater, it alters the hydrologic balance of the watershed. As a consequence, stream
base flow is deprived of the constant groundwater discharge and may diminish or even cease.
During a drought, reduced stream base flow may also significantly affect the water quality in a
stream.
Figure 2-5. Rainfall replenishes the groundwater, which in turn provides stream base
flow.
The groundwater discharge to a stream is at a relatively constant temperature, whereas
stormwater runoff from impervious surfaces may be very hot in the summer months and extremely
cold in the winter months. These temperature extremes can have a devastating effect on aquatic
organisms, from bacteria and fungi to larger species. Many fish, especially native trout, can be
harmed by acute temperature changes of only a few degrees.
Improperly managed stormwater causes increased flooding, water quality degradation, stream
channel erosion, reduced groundwater recharge, and loss of aquatic species. But these and other
impacts can be effectively avoided or minimized through better site design. This chapter discusses
the potential problems associated with stormwater and explains the need for better stormwater
management. The problems caused by impervious and altered surfaces can be avoided or
minimized, but only through stormwater management techniques that include runoff volume
reduction, pollutant reduction, groundwater recharge and runoff rate control for all storms.

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2.2
The Hydrologic Cycle and The Effects of Development
The movement of water from the atmosphere to the land surface and then back to the atmosphere
is a continuous process, with water constantly in motion. This balanced water cycle of
precipitation, runoff, evapotranspiration, infiltration, groundwater recharge, and stream base flow
sustains Pennsylvania’s water resources. This representation of the hydrologic cycle, while
depicting the general concept, over-simplifies the complex interactions that define the surface and
subsurface flow processes of humid regions in the United States.
Changes to the land surface, along with inappropriate stormwater management, can significantly
alter the natural hydrologic cycle. In a natural Pennsylvania woodland or meadow, very little of the
annual rainfall leaves the site as runoff. More than half of the annual amount of rainfall returns to
the atmosphere through evapotranspiration. Surface vegetation, especially trees, transpires water
to the atmosphere (with seasonal variations). Water is also stored in puddles, ponds and lakes on
the earth’s surface, where some of it will evaporate. Water that percolates through the soil either
moves vertically and eventually reaches the zone of saturation or water table, moves laterally
through the soil and often emerges as springs or seeps down gradient or is stored in the soil.
Soils are influenced and formed by vegetation, climate, parent material, topography and time. All
of these factors have some effect on how water will move through the soil. Restrictive soil horizons
may impede the vertical movement of water and cause it to move laterally. It is important to
understand these factors when designing an appropriate stormwater system at a particular
location. Under natural woodland and meadow conditions, only a small portion of the annual
rainfall becomes stormwater runoff. Although the total amount of rainfall varies in different regions
of the state, the basic average hydrologic cycle shown below holds true (Figure 2-6).
Figure 2-6. Annual hydrologic cycle for an undisturbed acre in the Pennsylvania Piedmont region.

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Changing the land surface causes varying changes to the hydrologic cycle (Figure 2-7). Altering
one component of the water cycle invariably causes changes in other elements of the cycle.
Roads, buildings, parking areas and other impervious surfaces prevent rainfall from soaking into
the soil and significantly increase the amount of runoff. As natural vegetation is removed, the
amount of evapotranspiration decreases.
Figure 2-7. Representative altered hydrologic cycle for a developed acre in the
Piedmont region.
These changes in the hydrologic cycle have a dramatic effect on streams and water resources.
Annual stormwater runoff volumes increase from inches to feet per acre, groundwater recharge
decreases, stream channels erode, and populations of fish and other aquatic species decline.
Past practices focused on detaining the peak flows for larger storms. While detention is helpful in
reducing peak flows for the immediate downstream neighbor, it does not address most of the other
problems discussed earlier.

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Figure 2-8. Average annual precipitation in Pennsylvania.
2.2.1 Rainfall, Runoff, and Flooding
In Pennsylvania, average annual precipitation ranges from 37 inches to more than 45 inches per
year (Figure 2-8), and reflects a humid pattern. Nearly all of the annual rainfall occurs in small
storm events (Figure 2-9
)
. Precipitation of an inch or less is frequent and well distributed
throughout the year. However, large storms, hurricanes, and periods of intense rainfall can occur
at any time.
Figure 2-9. Distribution of precipitation by storm magnitude for Harrisburg, PA (Original Data from
Penn State Climatological Office, 1926-2003)
3" - 4"
1%
4" +
1%
65%
0 .1" - 1"
2" - 3"
6%
1" - 2"
27%

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Stormwater management has historically focused on managing flooding from the larger but less
frequent extreme event storms (Table 2-1). Traditional site design has focused on the
peak rate
of
runoff during such events; that is, how fast the stormwater runoff is leaving the site after
development. Detention facilities are built to
slow down the rate of runoff leaving a site
during large storms so that the rate of runoff
after development is not greater than the
rate before development. Regulatory criteria
is often based on controlling the “release”
rate of runoff from the 2-year through 100-
year storm events. Storm frequency is
based on the statistical probability of a storm
being exceeded in any year. That is, a 2-
year storm has a 50% probability of being
exceeded in any single year, and a 100-year
storm, a 1% probability.
Preventing increased runoff rates from large storm events is extremely important but it does not do
enough to protect streams and water quality. With a change in land surface, not only does the
peak
rate
of runoff increase, the
volume of
runoff also increases. While a stormwater detention
facility may slow the rate of runoff leaving a site, there may still be an increased volume of runoff.
This is shown graphically in Figure 2-10. Detention controls the peak runoff rate by extending the
hydrograph. So while the rate of runoff may not increase, the duration of runoff will be longer than
before development because of the increased volume.
Figure 2-10. The hydrograph is an important tool used for understanding the hydrologic
response of a given rainfall event. The area beneath the hydrograph curve represents the total
volume of runoff being discharged
.
2-year 5-year 10-year 50-year 100-year
Philadelphia
3.3
4.1
4.8
6.7
7.6
Pittsburgh
2.4
2.9
3.3
4.4
4.9
Scranton
2.6
3.2
3.7
5.4
6.4
State College
2.7
3.3
3.8
5.2
5.9
Williamsport
2.8
3.5
4.1
6.0
7.0
Erie
2.6
3.2
3.7
5.1
5.8
Frequency of Occurrence (Years)
Location
Table 2-1. Statistical Storm Frequency Events for locations in PA
(24 hour duration) (Source: NOAA National Weather Service
Precipitation Frequency Data Server, 2004).

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On a watershed basis, detention becomes ineffective downstream as the sole management
strategy for stormwater control due to the extended hydrograph and increased volume. There is
even a possibility that the peak flows may
increase
downstream flooding. The combination of
more runoff volume over a longer time period will result in downstream flow rates that are higher
than before development, as indicated in Figure 2-11.
Figure 2-11. This figure illustrates a small watershed comprised of five hypothetical Subbasin development
sites, 1 through 5, each of which undergoes development and relies on a separate peak rate
control detention basin. As the storm occurs, five different hydrographs result for each sub-
area and combine to create a resultant pre-development hydrograph for the overall
watershed. The net result of the combined hydrographs is that the watershed peak rate
increases considerably, because of the way in which these increased volumes are routed
through the watershed system and combine downstream. Flooding increases considerably
in peak and duration, even though these detention facilities have been installed at each
individual development.
The second reason that detention alone is not sufficient for stormwater management is that it does
not address the frequent small storm events in Pennsylvania. Most of the rainfall in Pennsylvania
occurs in relatively small storm events, as indicated for the Harrisburg area (Figure 2-9). In
Harrisburg, over half of the average annual rainfall occurs in storms of less than 1 inch (in 24
hours). Over 90 percent of the average annual rainfall occurs in storms of 2 inches or less, and
over 95 percent of average annual rainfall occurs in storms of 3 inches or less. This pattern is
typical of the entire state.
Detention facilities that are designed to control the peak flow rate for large storm events often allow
frequent small storm events to “pass through” the detention facility. These small frequent rainfall
events discharge from the site at a higher rate and a greater volume of runoff than before
development. There is also an increase in the
frequency
of runoff events because of the change

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in land surface. For example, little runoff will occur from most wooded sites until over an inch of
rainfall has fallen. In contrast, a paved site will generate runoff almost immediately (Figure 2-12).
After development, runoff will occur with greater frequency than before development, and runoff
may be observed with every rainfall. The design of stormwater systems that collect, convey and
concentrate runoff may further degrade conditions.
Runoff Volume from
Woodland and Impervious Surfaces
0.02
0.36
1.03
1.59
2.11
3.6
4.37
0.97
1.49
3.04
3.85
4.54
6.37
7.26
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1 inch Rainfall
1.5 inch
Rainfall
2-yr Storm
(3.27")
5-yr Storm
(4.09")
10-yr Storm
(4.78")
50-yr Storm
(6.61")
100-yr Storm
(7.5")
Runoff (inches)
Woodland
Impervious
Runoff Values for the 1" and 1.5"
storms generated using the Small
Storm Hydrology Methodology (Pitt,
1994), and Runoff values for the
remaining storms generated using SCS
Runoff Curve Number Method (CN=98
for impervious and CN=73 for woods, C
soils, Fair Condition)
Figure 2-12. This graph generally compares the volume of runoff generated from a woodland site
with the volume of runoff generated by impervious area for different rainfall amounts.
Note that the volume increase for small storms is significant.
The combination of more runoff, more often and at higher rates will create localized flooding and
damage even in small storm events. Throughout the state, over 95 percent of the annual rainfall
volume occurs in storm events that are less than the 2-year storm event. The net effect is that
during most rainfall events, stormwater discharges are not managed or controlled, even with
numerous detention basins in place.

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2.2.2 The Impacts of Vegetation Loss and Soil Changes
On woodland and meadow areas, over half of the average annual rainfall returns to the
atmosphere through evaporation and transpiration (Figure 2-6). The vegetation itself also
intercepts and slows the rainfall, reducing its erosive energy, reducing overland flow of runoff, and
allowing infiltration to occur. The root systems of plants also provide pathways for downward water
movement into the soil mantle.
Evapotranspiration (ET) varies tremendously with season and with type of vegetative cover. Trees
can effectively evapotranspire most, if not all, of the precipitation, that falls in summer rain showers.
Evapotranspiration dramatically declines during the winter season. During these periods, more
precipitation infiltrates and moves through the root zone, and the groundwater level rises.
Removing vegetation or changing the land type from woods and meadow to residential lawnscapes
reduces evapotranspiration and increases the amount of stormwater runoff.
Soil disturbance and compaction also increases stormwater runoff. Soils contain many small
openings called “macropores” that provide a mechanism for water to move through the soil,
especially under saturated conditions. When soil is disturbed (grading, stockpiling, heavy
equipment traffic, etc.) the soil is compacted, macropores are smashed and the natural soil
structure is altered. Soil permeability characteristics are substantially reduced.
Compaction can be measured by determining the bulk density of the soil. The more compacted the
soil is, the heavier it is by volume.
Heavy construction equipment can
compact soil so significantly that the
soil bulk density of lawn soil
approaches the bulk density of
concrete (Table 2-2 Ocean County,
New Jersey Soil Conservation
District, 2001; Hanks and
Lewandowski, 2003). The result is a
surface that is functionally impervious
because the water absorbing
capacity of the soil is so altered and
reduced.
As discussed in Chapters 5 and 6, comprehensive stormwater management focuses on preventing
an increase in stormwater runoff volume by protecting vegetation and soils, or minimizing
stormwater impacts by restoring vegetation and soils to reduce runoff volumes and the velocity of
runoff. Vegetation and soils are a critical component of the “water balance” and are an essential
part of better stormwater management.
2.2.3 Groundwater Recharge, Stream Base Flow, and First-Order Streams
Water moves through the soil until it is evapotranspired or reaches the groundwater table and
replenishes the aquifer. The actual movement of water through the sub-surface pathways is
complex, and less permeable soils, clay layers, and rock strata are often encountered. The water
moving through the soil is generally referred to as gravitational water or drainage water. Other
types of water in soil include capillary water and hygroscopic water. Capillary water is that water
held in soil pores by surface attraction (sometimes referred to as capillary action); this is the water
that is typically available to plants for uptake. Hygroscopic water is water that is tightly held by the
Table 2-2. Common Bulk Density Measurements
Undisturbed Lands
Forest & Woodlands
1.03 g/cc
Residential
Neighborhoods
1.69 to 1.97 g/cc
Golf Courses - Parks
Athletic Fields
1.69 to 1.97 g/cc
CONCRETE
2.2 g/cc

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soil particles and can only be removed by physical drying. Although capillary water does play an
important role in evaporation processes, gravitational water is of primary concern from a
stormwater management prospective.
The movement of gravitational water through the soil is influenced by a soils texture, structure,
layering and the presence of preferential flow pathways (macropores). Soil textures are defined by
the percentage of sand, silt and clay present in the soil. In general, the permeability and hydraulic
conductivity of a soil will decrease with decreasing textural grain size (i.e., gravitational water
moves more easily through sands than silts and clays). Soil texture also influences the shape of
the wetting front as water moves through a soil.
It has also been observed that there is a discontinuity of soil-water movement at the interface
between soils of different textures. This layering causes percolating water to concentrate at certain
points along the layer interface and then break into the layer interface in finger-like protrusions.
The significance is that even a change in soil texture within a vertical profile will cause a disruption
in the soil-water movement. This disruption often causes water to “back up” at the interface, which
can cause water to move laterally.
Soil structure also influences the movement of water through a soil. A disruption in the movement
of soil water will occur at the interface between soil layers of differing structures. While texture and
structure are certainly important to how water moves through soils, soil layering and the presence
of dominant flow paths (macropores) play the most significant role in defining how water moves
through the subsurface.
Soils form over time in response to their landscape position, climate, presence of organisms and
parent material. Soils that have formed in place from the weathering of their parent material,
usually form a typical profile with A, B and C horizons above bedrock. However, many soils form
from a combination of the weathering of parent materials and the deposition of transported soils
creating a more complex layering effect. In general, any interface between soil layers can slow the
downward movements of water through a soil profile and promote lateral flow. This is especially
true in sloping landscapes typical of most of Pennsylvania.
Restrictive soil layers within a soil profile also disrupt the vertical movement of soil-water and
promote the lateral movement of water through the soil. Restrictive soil layers include clay lenses,
fragipans or plow pans, for example. Fragipans are layers within a soil profile that have been
compressed as a result of some external influence (glaciation for example). This compressed layer
often causes water to perch above the fragipan and promotes lateral flow. Fragipans are
commonly found in colluvial and glacial soils. In addition, many soils in agricultural regions of
Pennsylvania contain “plow-pans” which are compressed layers of soil formed by the repeated
traversing by moldboard plows.
Soil water also follows preferential flow paths through the soil. Preferential flow paths include
pathways created by plant roots, worm or rodent burrows, cracks or voids in the soil resulting from
piping action caused by the lateral movement of soil-water. Preferential flow paths also form at the
soil rock interface and within rock structures.
The groundwater level rises and falls depending on the amount of rainfall/snowmelt and the time of
year. The water cycle illustration of Figure 2-6 estimates that approximately 12 inches of the 45
inches of average annual precipitation in this natural watershed system finds its way into the
groundwater table.

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A variety of processes can occur when precipitation falls on a natural soil surface. Hillslope
hydrology processes have been identified by Chorley (1978) and are systematically illustrated in
Figure 2-12. The flow processes illustrated here are only representative examples of the complex
interactions that occur in nature. Simplified descriptions of these processes follow:
1. Areas marked with a “1” are areas where the infiltration capacity of the soils exceeds the
rainfall rate. All rain falling on these areas infiltrates into the ground.
2. Areas labeled with a “2” identifies an area where the rainfall rate exceeds the surface
infiltration rate, and the excess rainfall becomes surface runoff (Hortonian surface runoff).
3. Areas marked with a “3” represents areas where the soil has become saturated and cannot
hold additional moisture; all rain falling on these areas immediately becomes surface runoff.
Saturation can occur as a result of various subsurface conditions. Areas marked “3a”
illustrates where a restricting layer (fragipans, clay lenses, etc.) limits the downward
movement of soil water creating a perched water table that reaches the ground surface.
Area “3b” identifies an area where water moving through the soil (through-flow) reaches the
surface as a spring or seep (return-flow); in these cases the surface in the vicinity of the
seep or spring becomes saturated.
4. The areas marked with a “4” represent areas of through-flow. Through-flow is the lateral
movement of water through the soil. Area “4a” illustrates through-flow along preferential
flow paths in unsaturated soils; area “4b” shows shallow surface flow (a common
occurrence in PA); and area “4c” illustrates through-flow in saturated areas.
5. Areas marked with a “5” represents an area of return-flow. Return-flow is water that has
moved through unsaturated or saturated subsurface areas and re-appears as surface flow
through springs or seeps.
6. The area labeled as “6” represents an area of deep percolation or groundwater recharge.
7. Area “7” points to a location where groundwater discharges to the stream (influent streams).
For effluent streams, water moves from the stream into the ground water table in these
areas. In some streams, both processes may occur during different times of the year.
(Brown/Fennessey/Petersen)
Figure 2-12 Components of hillslope hydrology (Adapted from Chorley [1978])

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Most of these flow processes occur within natural watersheds in Pennsylvania. The extent to
which one or more of these processes are active within a particular area is influenced by soil
characteristics, geology and topography or landscape position.
Eventually the groundwater table intersects the
land surface and forms springs, first order
streams and wetlands (Figure 2-5). This
groundwater discharge becomes stream base
flow and occurs continuously, during both wet
and dry periods. Much of the time, all of the
natural flow in a stream is from groundwater
discharge. In this sense, groundwater discharge
can be seen as the “life” of streams, supporting
all water-dependent uses and aquatic habitat.
First-order streams are defined as “that stream
where the smallest continuous surface flow
occurs” (Horton, 1945), and are the beginning of
the aquatic food chain that evolves and
progresses downstream (Figure 2-13). As the
link between groundwater and surface water,
headwaters represent the critical intersection
between terrestrial and aquatic ecosystems.
During periods of wet weather, the water table
may rise to near the ground surface in the vicinity of the stream. This higher ground water table
coupled with through-flow, return-flow and shallow subsurface flow result in an area of saturation in
the vicinity of the stream channel. As a result, this area saturates quickly during rain events; and
the larger the rain event, the more extensive the area of saturation may be. It is understood by
researchers that a significant amount of the surface runoff observed in streams during precipitation
events is generated from the saturated areas surrounding streams (Chorley, 1978; Hewlett and
Hibbert, 1967). The runoff generated from rainfall on saturated land areas is referred to as
saturation overland flow. Hydrologists understand that the watershed runoff process is a complex
integration of saturation overland flow and infiltration excess (Hortonian) overland flow (Troendle,
1985). Areas that generate surface runoff pulsate, shrink and expand in response to rainfall. This
concept on a watershed scale is consistent with the hillslope hydrologic processes.
Changes in land use cause runoff volumes to increase and groundwater recharge to decrease.
Wetlands and first order streams reflect changes in groundwater levels most profoundly, and this
reduced flow can stress or even eliminate the aquatic community. As the most hydrologically and
biologically sensitive elements of the drainage network, headwaters and first order streams warrant
special consideration and protection in stormwater management.
2.2.4 Stream Channel Changes
The shape of a stream channel, its width, depth, slope, and how it moves through the landscape, is
influenced by the amount of flow the stream channel is expected to carry. The stream channel
morphology is determined by the energy of stream flows that range from “low flow” to “bank full”.
The flow depths determine the energy in the stream channel, and this energy shapes the channel
itself. In an undeveloped watershed, bank full flow occurs with a frequency of approximately once
every 18 months. During larger flood events, the flow overtops the stream banks and flows into
the floodplain with much less impact on the shape of the stream channel itself.
Figure 2-13 Leaves and organic matter are
initially broken down by bacteria and
processed into food for higher organisms
downstream.

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In a developing watershed, the volume and rate of stormwater runoff increase during small storm
events and the stream channel changes to accommodate the greater flows. Because the stream is
conveying greater flows more often and for longer periods of time, the stream will try to
accommodate these larger flows by eroding stream banks or cutting down the channel bottom.
Since traditional detention basins do not manage small storms, these impacts are often most
pronounced downstream of detention basins.
Numerous studies have documented the link between altered stream channels and land
development. The Center for Watershed Protection (Article 19, Technical Note 115, Watershed
Protection Techniques 3(3): 729-734) states that land development influences both the geometry
(morphology) and stability of stream channels, causing downstream channels to enlarge through
widening and stream bank erosion. These physical changes, in turn, degrade stream habitat and
produce substantial increases in sediment loads resulting from accelerated channel erosion.
As the shape of the stream channel changes to accommodate more runoff, aquatic habitat is often
lost or altered, and aquatic species decline. Studies, such as US EPA’s
Urbanization and
Streams: Studies of Hydrologic Impacts
(1997), conclude that land development is likely to be
responsible for dramatic declines in aquatic life observed in developing watersheds. These stream
channel impacts have been observed even where conventional stormwater management is
applied.
The effects occur at many levels in the aquatic community. As the gravel stream bottom is covered
in sediment, the amount and types of microorganisms that live along the stream bottom decline.
The stream receives sediment from runoff, but additional sediment is generated as the stream
banks are eroded and this material is deposited along the stream bottom. Pools and riffles
important to fish and other aquatic life are lost, and the number and types of fish and aquatic
insects diminishes. Trees and shrubs along the banks are undercut and lost, removing important
habitat and decreasing natural shading and cooling for the stream.
The runoff from impervious surfaces is usually warmer than the stream flow, and can harm the
aquatic community. When the stream flow is comprised primarily of groundwater discharge, the
constant, cool temperature of the groundwater buffers the stream temperature. As the flow of
groundwater decreases and the amount of surface runoff increases, the temperature regime of the
stream changes. Runoff from impervious surfaces in the summer months can be much hotter than
the stream temperature, and in the winter months this same runoff can be colder. These changes
in temperature dramatically affect the aquatic habitat in the stream, ranging from the fish
community that the stream can support to the microorganisms that form the foundation of the food
chain. Important fungal communities can be lost altogether. It is apparent that increasing
impervious areas can lead to significant degradation of surface water by altering the entire aquatic
ecosystem.
2.2.5 Water Quality
Impervious surfaces and maintained landscapes generate pollutants that are conveyed in runoff
and discharged to surface waters. Many studies of pollutant transport in stormwater have
documented that pollutant concentrations show a distinct increase at the beginning of a flow
hydrograph referred to as the “first flush”. In fact, the particulate associated pollutants that are
initially scoured from the land surface and suspended in the runoff are observed in a stream or
river before the runoff peak occurs. These pollutants include sediment, phosphorus that is moving
with colloids (clay particles), metals, and organic particles and litter. Dissolved pollutants, however,

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may actually decrease in concentration during heavy runoff. These include nitrate, salts and some
synthetic organic compounds applied to the land for a variety of purposes.
Managing stormwater to minimize pollutant loading includes reducing the sources of these
pollutants as well as restoring and protecting the natural systems that are able to remove
pollutants. These include stream buffers, vegetated systems, and the natural soil mantle, all of
which can be put to use to remove pollutants from stormwater runoff.
Stormwater quantity and quality are inextricably linked and need to be managed together.
Although the most obvious impact of land development is the increased rate and volume of surface
runoff, the pollutants transported with this runoff comprise an equally significant impact.
Management strategies that address quantity
will in most cases address quality.
Stormwater runoff pollutants include sediment, organic detritus, phosphorus and nitrogen
forms, metals, hydrocarbons, and synthetic organics.
The increased stormwater runoff
brought on by land development scours both impervious and pervious land surfaces. Stormwater
runoff transports suspended and dissolved pollutants that were initially deposited on the land
surface. Hot spot impervious areas such as fueling islands, trash dumpsters, industrial sites, fast
food parking lots, and heavily traveled roadways contribute heavy pollutant loads to stormwater.
Many so-called pervious surfaces, such as the chemically maintained lawns and landscaped
areas, also add significantly to the pollutant load, especially where these pervious areas drain to
impervious surfaces, gutters and storm sewers. The soil compaction process applied to many land
development sites results in a vegetated surface that is close to impervious in many instances, and
produces far more runoff than the pre-development soil did. These new lawn surfaces are often
loaded with fertilizers that result in polluted runoff that degrades all downstream ponds and lakes.
The two physical forms of stormwater pollutants are particulates
and solutes
. One very
important distinction for stormwater pollutants is the extent to which pollutants are particulate in
form, or dissolved in the runoff as solutes. The best example of this comparison is the two
common fertilizers: Total phosphorus (TP) and nitrate (NO3-N). Phosphorus typically occurs in
particulate form, usually bound to colloidal soil particles. Because of this physical form, stormwater
management practices that rely on physical filtering and/or settling out of sediment particles can be
quite successful for phosphorus removal. In stark contrast, nitrate tends to occur in highly soluble
forms, and is unaffected by many of the structural BMPs designed to eliminate suspended
pollutants. As a consequence, stormwater management BMPs for nitrate may be quite different
than those used for phosphorous removal. Non-Structural BMPs (Chapter 5) may in fact be the
best approach for nitrate reduction in runoff.
Particulates:
Stormwater pollutants that move in association with or attached to particles include
total suspended solids (TSS), total phosphorus (TP), most organic matter (as estimated by COD),
metals, and some herbicides and pesticides. Kinetic energy keeps particulates in suspension and
some do not settle out as easily. For example, an extended detention basin offers a good method
to reduce total suspended solids, but is less successful with TP, because much of the TP load is
attached to fine clay particles that may take longer to settle out.
If the concentration of particulate-associated pollutants in stormwater runoff, such as TSS and TP,
is measured in the field during a storm event, a significant increase in pollutant concentration
corresponding to but not synchronous with the surface runoff hydrograph is usually observed
(Figure 2-14). This change in pollutant concentration is referred to as a “chemograph”, and has
contributed to the concept of a “first flush” of stormwater pollutants. In fact, the actual transport

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process of stormwater pollutants is somewhat more complex than “first flush” would indicate, and
has been the subject of numerous technical papers (Cahill et al, 1974: 1975; 1976; 1980; Pitt,
1985, 2002). To accurately measure the total mass of stormwater pollution transported during a
given storm event, both volume and concentration must be measured simultaneously, and a
double integration performed to estimate the mass conveyed in a given event. To fully develop a
stormwater pollutant load for a watershed, a number of storm events must be measured over
several years. The dry weather chemistry is seldom indicative of the expected wet weather
concentrations, which can be two or three orders of magnitude greater.
Because a major fraction of particulate associated pollutants is transported with the smallest
particles, or colloids, their removal by BMPs is especially difficult. These colloids are so small that
they do not settle out in a quiescent pool or basin, and remain in suspension for days at a time,
passing through a detention basin with the outlet discharge. It is possible to add chemicals to a
detention basin to coagulate these colloids to promote settling, but this chemical use turns a
natural stream channel or pond into a treatment unit, and subsequent removal of sludge is
required. A variety of BMPs have been developed that serve as runoff filters, and are designed for
installation in storm sewer elements, such as inlets, manholes or boxes. The potential problem
with all measures that attempt to filter stormwater is that they quickly become clogged, especially
during a major event. Of course, one could argue that if the filter systems become clogged, they
are performing efficiently, and removing this particulate material from the runoff. The major
problem then with all filtering (and to some extent settling) measures is that they require substantial
maintenance. The more numerous and distributed within the built conveyance system that these
BMPs are situated, the greater the removal efficiency, but also the greater the cost for operation
and maintenance.

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Figure 2-14. Chemograph of phosphorus and suspended solids in Perkiomen Creek (Cahill, 1993).
Solutes:
Dissolved stormwater pollutants generally do not exhibit any increase during storm event
runoff, and in fact may exhibit a slight dilution over a given storm hydrograph. Dissolved
stormwater pollutants include nitrate, ammonia, salts, organic chemicals, many pesticides and
herbicides, and petroleum hydrocarbons (although portions of the hydrocarbons may bind to
particulates and be transported with TSS). Regardless, the total mass transport of soluble
pollutants is dramatically greater during runoff because of the volume increase. In some
watersheds, the stormwater transport of soluble pollutants can represent a major portion of the
total annual discharge for a given pollutant, even though the absolute concentration remains
relatively constant. For these soluble pollutants, dry weather sampling can be very useful, and
often reflects a steady concentration of soluble pollutants that will be representative of high flow
periods.
Some dissolved stormwater pollutants can be found in the initial rainfall, especially in regions with
significant emissions from fossil fuel plants. Precipitation serves as a “scrubber” for the
atmosphere, removing both fine particulates and gases (NOX and SOX). Chesapeake Bay
scientists have measured rainfall with NO
3
concentrations of 1 to 2 mg/l, which could comprise a
significant fraction of the total input to the Bay. Other rainfall studies by NOAA and USGS have
resulted in similar conclusions. Impervious pavements can transport nitrate load, reflecting a mix
of deposited sediment, vegetation, animal wastes, and human detritus of many different forms.
Pollution prevention through use of Non-Structural BMPs is very effective. A variety of Structural
BMPs, including settling, filtration, biological transformation and uptake, and chemical processes

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can also be used. Stormwater related pollution can be reduced if not eliminated through
preventive Non-Structural BMPs (Chapter 5), but not all stormwater pollution can be avoided.
Many of the Structural BMPs (Chapter 6) employ natural pollutant removal processes as essential
elements. These “natural” processes tend to be associated with and rely upon both the existing
vegetation and soil mantle. Thus preventing and minimizing disturbance of site vegetation and
soils is essential to successful stormwater management.
Settling:
Particles remain suspended in stormwater as long as the energy of the moving water is
greater than the pull of gravity. In a natural stream, the stormwater that overflows the banks slows
and is temporarily stored in the floodplain, which allows for sediment settling, and the building of
the alluvium soils that comprise this floodplain. As runoff passes through any type of man-made
structure, such as a detention basin, the same process takes place, although not as efficiently as in
a natural floodplain. Where it is possible to create micro versions of runoff ponds (rain gardens),
distributed throughout a site, the same settling effect will result. The major issue with settling
processes is that the dissolved pollutant load is not subject to gravitational settling.
Filtration:
Another natural process is physical filtration. Filtration through vegetation and soil is by
far the most efficient way to remove suspended stormwater pollutants. Suspended particles are
physically filtered from stormwater as it flows through vegetation and percolates into the soil.
Runoff that is concentrated in swales, however, can exceed the ability of the vegetation to remove
particles. Therefore, it is important to avoid concentrated flows by slowing and distributing the
runoff over a broad vegetated area.
Stormwater flow through a relatively narrow natural riparian buffer of trees and herbaceous
understory growth has been demonstrated to physically filter surprisingly large proportions of larger
particulate-form stormwater pollutants. Both filter strip and grassed swale BMPs rely very much on
this surface filtration process as discussed in Chapter 6.
Biological Transformation and Uptake/Utilization:
This category includes an array of different
processes that reflect the remarkable complexity of different surface vegetative types, their varying
root systems, and their different needs and rates of transformation and utilization of different
“pollutants,” especially nutrients. An equally vast and complex community of microorganisms
exists below the surface within the soil mantle, and though more micro in scale, the myriad of
natural processes occurring within this soil realm is just as remarkable.
Phosphorus and nitrate are essential to plant growth and therefore are taken up through the root
systems of grasses, shrubs and trees. Nitrogen transformations are quite complex, but the muck
bottom of wetlands allows the important process of denitrification to occur and convert nitrates for
release in gaseous form. Nitrates in stormwater runoff passing through wetlands is removed and
used by wetland plants to build biomass. The caution in terms of a wetland or similar surface BMP
is that if the vegetation dies at the end of a growing season and the detritus is discharged from the
wetland, the net removal of nitrate is maybe less than expected. The guidance for BMP
applications is that if biological transformation processes are considered, care must be taken to
remove and dispose of the biomass produced in the process.
Chemical Processes:
Various chemical processes occur in the soil to remove pollutants from
stormwater. These include adsorption through ion exchange and chemical precipitation. Cation
Exchange Capacity (CEC) is a rating given to soil, that relates the soil organic content to its ability
to remove pollutants as stormwater infiltrates through the soil. Adsorption will increase as the total
surface area of soil particles and/or the amount of decomposed organic material increases. Clay
soils have better pollutant reduction performance than sandy soils, and their slower permeability

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rate has a positive effect. CEC values typically range from 2 to 60 milli-equivalents (meq) per 100
grams of soil. Coarse sandy soils have low CEC values and therefore are not especially good
stormwater pollutant removers. The addition of compost will greatly increase the CEC of sandy
soils. A value of 10 meq. is often considered necessary to accomplish a reasonable degree of
pollutant removal.
Section 2.3 References
Amandes, C. and Bedient, P., 1980,
Stormwater Detention in Developing Watersheds,
ASCE Journal of Hydraulics
Barnes, J. H., and W. D. Sevon, 1996.
The Geologic Story of Pennsylvania
, PA.
Geological Survey, Educational Series No. 4, Harrisburg, PA
Brown, Scott A., P.E., Larry A.J.Fennessey, Ph.D.,P.E., and Gary W. Petersen,Ph.D.,
CPSS
Understanding Hydrologic Process for Better Stormwater Management
Cahill, T. H., 1989.
The Use of Porous Paving for Groundwater Recharge in Stormwater
Management Systems
. Floodplain/Stormwater Management Symposium, State College,
PA, Oct., 1988.
Cahill, T. H., 1993.
Stormwater Management Systems, Porous Pavement System with
Underground Recharge Systems, Engineering Design Report
, Spring, 1993.
Cahill, 1997.
Land Use Impacts on Water Quality: Stormwater Runoff
. T. H. Cahill,
Chesapeake Bay Restoration and Protection: Townships in the Lead, Ches. Bay Local
Govt. Adv. Comm.
,
PA Assn of Twp Supv., Hbg., PA, Apr. 28, 1997.
Cahill, 1996.
Sustainable Watershed Management: Balancing Water Resources and Land
Use.
W.R. Horner, T.H. Cahill and J. McGuire, Hydrology and Hydrogeology of Urban and
Urbanizing Areas, Cahill Associates, West Chester, PA, April 21-24, 1996.
Cahill, et al., 1996.
Sustainable Watershed Management in Developing Watershed
. T. H.
Cahill, J. McGuire, W. R. Horner, Cahill Assoc., West Chester, PA Am. Environment
Congress, Anaheim, CA June 22-28, 1996.
Cahill, et al., 1996.
Sustainable Watershed Management at the Rapidly Growing Urban
Fringe
. T. H. Cahill, J. McGuire, W. R. Horner, and R. D. Heister, at Watershed, 96, Balt.
Conv. Ctr., Balt., MD. June 8-12, 1996.
Cahill, 1992.
Limiting Nonpoint Source Pollution from New Development in the New
Jersey Coastal Zone: Summary.
T.H. Cahill, W.R. Horner, J.S. McGuire, and C. Smith,
NJDEP; S. Whitney and S. Halsey, Cahill Assoc., West Chester, PA, September, 1992.
Cahill, 1992.
Structural and Nonstructural Best Management Practices for the
Management of Non-Point Source Pollution in Coastal Waters: A Cost-Effectiveness

Pennsylvania Stormwater Best Management Practices Manual
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Page 20 of 22
Comparison.
T.H. Cahill and W.R. Horner, The Coastal Society; Thirteenth International
Conference: Conference Proceedings Organizing for the Coast, Washington, D.C., April
1992.
Chorley, R.J. (1978).
The hillslope hydrologicCycle
, Hillslope Hydrology, Edited by M.J.
Kirby, Wiley, New York, NY, pp. 1-42
CWP, 2003.
Impacts of Impervious Cover on Aquatic Systems
, Center for Watershed
Protection, Ellicott City, MD
Emerson, C., Welty, C., Traver, R., 2005,
A Watershed—scale Evaluation of a System of
Stormwater Detention Basins,
ASCE Journal of Hydraulic Engineering
Fenneman, N. M., 1938. Physiography of Eastern United States
, McGraw-Hill Book Co.,
New York, NY.
Fennessey, Lawrence A.J.,Ph.D.,P.E. and Arthur C. Miller, Ph.D.,P.E. 2001.
Hydrologic
Processes During Non-Extreme Events in Humid Regions
Fennessey, Lawrence A.J., Ph.D.,P.E. and Richard H. Hawkins, Ph.D.,P.E. 2001.
The
NRCS Curve Number, a New Look at an Old Tool
Friedman, D.B., 2001. "Impact of Soil Disturbance During Construction on Bulk Density
and Infiltration in Ocean County, New Jersey." Ocean County Soil Conservation District.
Forked River, NJ
Friedman, D.B., "Developing an Effective Soil Management Strategy." Ocean County Soil
Conservation District. Forked River, NJ.
Godfrey, M. A., 1997.
Field Guide to the Piedmont
, The University of North Carolina
Press, Chapel Hill, NC.
Hanks, D & A. Lewandowski, 2003.
Protecting Urban Soil Quality: Examples for
Landscape Codes and Specifications
, USDA-NRCS, Toms River, NJ
Hewlett, J.D., and A.R. Hibbert (1967).
Factors Affecting the Response of Small
Watersheds to Precipitation in Humid Areas
, Forest Hydrology, Pergamon Oxford, pp. 275-
290
Konrad, C. P. & D. B. Booth, 2002.
Hydrologic Trends Associated with Urban
Development for Selected Streams in the Puget Sound Basin, Western Washington.
USGS Water Resources Investigation Report 02-4040 with Washington Dept. of Ecology,
Tacoma, Wash.
.
Kochanov, W. E., 1987.
Sinkholes and Related Karst Features Map of Lehigh County, PA
.
PA Geologic Survey, Open File Report 8701, Harrisburg, PA

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Kochanov, W. W., 1999.
Sinkholes in Pennsylvania
. PA Geological Survey, 4
th
Series,
Educational Series 11, Harrisburg, PA
Linsley, et al., 1992.
Water Resources Engineering, 4
th
Edition.
.
Maryland Department of the Environment, 2000.
Maryland Stormwater Design Manual
.
McCandless, T. L. and R. A. Everett, 2002.
Maryland Stream Survey: Bankfull Discharge
and Channel Characteristics of Streams in the Piedmont Hydrologic Region
, US Fish &
Wildlife Service, CBFO –S02-01, Annapolis, MD
McCandless, T. L., 2003. Maryland Stream Survey:
Bankfull Discharge and Channel
Characteristics of Streams in the Allegheny Plateau and the Valley and Ridge Hydrologic
Regions
, US Fish & Wildlife Service, CBFO-S01-01, Annapolis, MD
McCuen, Richard, 1979,
Downstream Effects of Stormwater Management Basins,
ASCE
Journal of Hydraulics
McCuen, R. and Moglen, G., 1988,
Multicriterion Stormwater Management Methods,
ASCE Journal opf Hydraulics
NOAA, 2001.
Tropical Cyclones and Inland Flooding
, US Dept. of Commerce, Natl.
Weather Service, Bohemia, NY
Ocean Co. SCD, 2001.
Impact of Soil Disturbance During Construction on Bulk Density
and Infiltration in Ocean County, NJ
. Ocean Co. Soil Consv. Dist., Schnabel Eng. Assoc.,
USDA-NRCS, Toms River, NJ
PADCNR, 2003.
Geologic Shaded Relief Map of Pennsylvania
, compiled by C. E. Miles,
PA Dept. of Conservation and Natural Resources, Harrisburg, PA
Pitt, Robert, 1985, 2002
Roberts, D. C., 1996. A
Field Guide to the Geology of Eastern North America
, Peterson
Field Guide, Houghton Mifflin, New York
Scotese, C. R., 1997.
Paleogeographic Atlas
, PALEOMAP Progress Report 90-0497,
Dept. of Geology, Univ. of Texas at Arlington, TX
Sorvig, K., 1993.
Porous Paving
. Landscape Architecture, Vol. 83, No. 2, Feb., 1993.
Wash., DC
Sloto, R. A., 1994.
Geology, Hydrology and Groundwater Quality of Chester County,
Pennsylvania, Chester Co
. Water Resources Auth., w/USGS, West Chester, PA

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Socolow, A. A., 1976.
Pennsylvania Geology Summarized
, Pa Dept. of Topographic and
Geologic Survey, Ed. Series No. 4, Harrisburg, PA
Sauer, L. J., 1998.
The Once and Future Forest
, Island Press, Washington, DC
Troendle, C.A. (1985)
Variable Source Area Models, Hydrologic Forecasting, M.G.
Anderson and T.P. Burt (ed.) John Wiley, New York
Viessman, W. and G. Lewis, 2003.
Introduction to Hydrology
, 5
th
Edition, Pearson Educ.,
Ltd., Upper Saddle River, New Jersey
Whitney, G. G., 1994.
From Coastal Wilderness to Fruited Plain: A History of
Environmental Change in Temperate North America from 1500 to the Present
, Cambridge
Univ. Press, New York

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Chapter 3

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Stormwater Management Principles
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Pennsylvania Stormwater Best Management Practices Manual
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Chapter 3 Stormwater Management Principles and Recommended Control Guidelines
3.1
Introduction……………………………………………………………………………………1
3.2
Recommended Site Control Guidelines………………………………………………….1
3.3
Recommended Volume Control Guidelines……………………………………………..2
3.3.1 Volume Control Criteria…………………………………………………………….4
3.3.2 Volume Control Alternatives………………………………………………………5
3.3.3 Control Guideline 1 (CG-1)…………………………………………………………6
3.3.4 Control Guideline 2 (CG-2)…………………………………………………………7
3.3.5 Retention and Detention Considerations……….………………………………7
3.4
Recommended Peak Rate Control Guideline……………………………………………8
3.5
Recommended Water Quality Control Guideline………………………………………..8
3.6
Stormwater Standards for Special Management Areas…………………..……………9

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3.1
Introduction
This Chapter provides guidance for municipalities striving to improve their stormwater management
programs. It presents stormwater management principles and recommends site control guidelines to
address volume, water quality and flow rate. These guidelines can serve as the basis for municipal
stormwater regulation. Pennsylvania laws and regulations do not directly manage stormwater at the
state level, although some state level management occurs through the Stormwater Management Act
and the NPDES permitting program. All municipalities, regardless of their specific setting, are
encouraged to enact the most comprehensive stormwater management ordinances possible. They
should also work with their watershed neighbors to integrate their individual municipal actions within the
watershed as a whole.
The guidelines established in this chapter reflect the ten basic principles of stormwater management
presented in the forward. The principles are listed below once more to emphasize their fundamental
importance as the foundation for the control guidelines that will follow.
1. Managing stormwater as a resource;
2. Preserving and utilizing existing natural features and systems;
3. Managing stormwater as close to the source as possible;
4. Sustaining the hydrologic balance of surface and ground water;
5. Disconnecting, decentralizing and distributing sources and discharges;
6. Slowing runoff down, and not speeding it up;
7. Preventing potential water quality and quantity problems;
8. Minimizing problems that cannot be avoided;
9. Integrating stormwater management into the initial site design process; and
10. Inspecting and maintaining all BMPs.
3.2
Recommended Site Control Guidelines
Site control guidelines are designed to meet water volume and water quality requirements and to follow
the ten principles previously listed. The control guidelines presented in this Chapter are comprehensive
are consistent with the Pennsylvania Comprehensive Stormwater Management Policy, and are
recommended to restore natural hydrology including velocity, current, cross-section, runoff volume,
infiltration volume, and aquifer recharge volume. Following the guidelines will help sustain stream base
flow and prevent increased frequency of damaging bank full flows. The guidelines also will help
prevent increases in peak runoff rates for larger events (2-year through 100-year) on both a site-by-site
and watershed basis. When applicable, Act 167 watershed plans may require additional rate controls
to reduce cumulative flooding impacts downstream.
The site control guidelines are:
Effective
— The morphologic impacts on streams from increased volumes of runoff during smaller
storms are prevented. The guidelines will be effective on a site-by-site basis, as well as on a
broader watershed-wide scale;
Proportional
— The stormwater controls will produce approximately the same post-development
stormwater discharge for all types of development in almost any location;

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Equitable
— The requirements are based on project characteristics rather than project location so
that physically similar projects will have similar storm water controls;
Flexible
— The diversity among Pennsylvania’s 2,566 municipalities is accommodated by the
guidelines. This diversity in physical conditions presents a major challenge that requires flexibility
to achieve a uniform stormwater management program across the state.
3.3
Recommended Volume Control Guidelines
Regardless of where land development occurs, the impervious surfaces, the changes in vegetation,
and the soil compaction associated with that development result in significant increases in runoff
volume. When the balance of a developed site is cleared of existing vegetation, graded, and re-
compacted, it produces an increase in runoff volume. While traditionally, if the original vegetation were
replaced with natural vegetation, the runoff characteristics would be considered to be equivalent to the
original natural vegetation. The disturbance and the compaction destroy the permeability of the natural
soil.
The relative increase in runoff volume varies with event magnitude (return period). For
example, the two-year rainfall of 3.27 inches/24 hours (SE PA) will result in an increase in runoff
volume of 2.6 inches from every square foot of impervious surface placed on well-drained HSG B soil in
woodland cover (Figure 3-1). For larger events, as the total rainfall increases, the net runoff also
increases, but less than proportionately. For example, total rainfall for the 100-year storm is twice the
rainfall for the 2-year storm (7.5 inches vs. 3.27 inches); however, the increase in runoff for the 100-
year storm is only 1.7 inches more than the runoff for the 2-year storm (4.3 – 2.6 inches). This pattern
holds true throughout the state.

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Runoff Volume Increase from Development
Difference Between Pervious Woodland (B Soil) and Impervious Surface
0.95
1.26
2.60
3.04
3.37
4.04
4.30
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1 inch
Rainfall
1.5 inch
Rainfall
2-yr Storm
(3.27")
5-yr Storm
(4.09")
10-yr Storm
(4.78")
50-yr Storm
(6.61")
100-yr
Storm (7.5")
Figure 3-1.
Runoff Volume Increase from Impervious Surfaces - B Soils.
Runoff (inches)
Runoff Values for the 1" and 1.5" storms generated
using the Small Storm Hydrology Methodology (Pitt,
1994) and runoff values for the storms generated using
the SCS Runoff Curve Number Method (CN-98 for
impervious and CN=60 for woods, B soils, Fair
Condition).
For a specific site, the net increase in runoff volume during a given storm depends on both the pre-
development permeability of the natural soil and the vegetative cover. Poorly drained soils result in a
smaller increase of runoff volume because the volume of pre-development runoff is already high.
Therefore, the amount of runoff resulting from development does not represent a large net increase.
Using the same rainfall values, Figure 3-2 illustrates that the two-year rainfall of 3.27 inches/24 hours
produces an increase of only 2.01 inches on a HSG C soil, while the better drained (B) soil in Figure
3-1 produces a 2.60-inch runoff volume increase. Thus a volume control guideline must be based on
the net change in runoff volume for a given frequency rainfall to be equitable throughout the state on
any given development site.

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Runoff Volume Increase from Development
Difference Between Pervious Woodland (C Soil) and Impervious Surface
0.95
1.13
2.01
2.26
2.43
2.77
2.89
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1 inch Rainfall
1.5 inch
Rainfall
2-yr Storm
(3.27")
5-yr Storm
(4.09")
10-yr Storm
(4.78")
50-yr Storm
(6.61")
100-yr Storm
(7.5")
Figure 3-2
. Runoff Volume Increase from Impervious Surfaces - C Soils
Runoff (inches)
Runoff Values for the 1" and 1.5" storms generated using the Small
Storm Hydrology Methodology (Pitt, 1994) and runoff values for
the storms generated using the SCS Runoff Curve Number Method
(CN-98 for impervious and CN=73 for woods, C soils, Fair
Condition).
Consideration of a volume control guideline has focused on providing stream channel protection and
water quality protection from the frequent rainfalls that comprise a major portion of runoff events in any
part of the state. On the basis of these factors, the 2-year event has been chosen as the stormwater
management design storm for Volume Control Guideline 1.
Regardless of the volume reduction goal desired, it is considered unreasonable to design any
stormwater BMP for greater than a 2-year event. The increase in runoff volume from the 100-year
rainfall after site development is so large that it is impractical to require management of the total
increase in volume. During such extreme events, the runoff simply overwhelms the natural and human-
made conveyance elements of pipes and stream channels. In practice, a BMP sized for the increase in
the 100-year runoff volume would be empty most of the time and would have a 1% probability of
functioning at capacity in any one year. Of course, large storms need to be managed in terms of
flooding and peak rate control, to the extent practicable.
3.3.1 Volume Control Criteria
A volume control guideline is essential to mitigate the consequences of increased runoff. To do this,
the volume reduction BMP must:

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1. Protect stream channel morphology;
2. Maintain groundwater recharge;
3. Prevent downstream increases in flooding; and
4. Replicate the natural hydrology on site before development to the greatest extent
possible.
Protect Stream Channel Morphology:
Increased volume of runoff results in an increase in the
frequency of bank full or near bank full flow conditions in stream channels. The increased presence of
high flow conditions in riparian sections has a detrimental effect on stream shaping, including stream
channel and overall stream morphology. Stream bank erosion is greatly accelerated. As banks are
eroded and undercut and as stream channels are gouged and straightened; meanders, pools, riffles,
and other essential elements of habitat are lost or diminished. Research has demonstrated that bank-
full stream flow typically occurs between the 1-year and the 2-year storm event (often around the 1.5-
year storm). Urbanization can cause the natural bankfull stream flows to occur far more often.
Strategies employed by the CG’s include a combination of volume reduction and extended detention to
reduce the bankfull flow occurances.
Maintain Groundwater Recharge:
Over 80 percent of the annual precipitation infiltrates into the soil
mantle in Pennsylvania’s watersheds under natural conditions. More than half of this is taken up by
vegetation and transpired. Part of this infiltrated water moves down gradient to emerge as springs and
seeps, feeding local wetlands and surface streams. The rest enters deep groundwater aquifers that
supply drinking water wells. Without groundwater recharge, surface stream flows and supplies of
groundwater for wells will diminish or disappear during drought periods. Certain land areas recharge
more groundwater than others; therefore, protecting the critical recharge areas is important in
maintaining the water cycle’s balance. In round numbers, an estimate of the annual water balance is:
surface water runoff, 20%; evapotranspiration (ET), 45%; groundwater recharge, 35%.
Prevent Downstream Increases in Runoff Volume and Flooding:
Although site-based rate control
measures may help protect the area immediately downstream from a development site, the increased
volume of runoff and the prolonged duration of runoff from multiple development sites can increase
peak flow rates and duration of flooding from runoff caused by relatively small rain events. Replicating
pre-development runoff volumes for small storms will usually substantially reduce the problem of
frequent flooding that plague many communities. Although control of runoff volumes from small storms
almost always helps to reduce flooding during large storms, additional measures are necessary to
provide adequate relief from the serious flooding that occurs during such events.
Replicate the Surface Water Hydrology On-site Before Development:
The objective for stormwater
management is to develop a program that replicates the natural hydrologic conditions of watersheds to
the maximum extent practicable. However, the very process of clearing the existing vegetation from the
site removes the single largest component of the natural hydrologic regime, evapotranspiration (ET).
Unless the ET component is replaced, the runoff increase will be substantial. Several of the BMPs
described in this manual, such as infiltration, tree planting, vegetated roof systems and rain gardens,
can help replace a portion of the ET function.
3.3.2 Volume Control Alternatives
While the volume control guideline alternatives are quite specific concerning the volume of runoff to be
controlled from a development site, they do not specify the methods by which this can be
accomplished. The selection of a BMP, or combination of BMPs, is left to the design process. But in all
instances, minimizing the volume increase from existing and future development is the goal. The BMPs

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described in this manual place emphasis on infiltration of precipitation as an important solution;
however, three methods are provided to reduce the volume of runoff from land development:
1.
Infiltration;
2.
Capture and Reuse; and
3.
Vegetation systems that provide ET, returning rainfall to the atmosphere.
It is anticipated that many of the stormwater management systems used in Pennsylvania will include
one or more of these methods, depending on specific site conditions that constrain stormwater
management opportunities
.
Inherent in these guidelines is the assumption that all soils allow some
infiltration. Where this is not possible, a vegetated roof, or bioretention combined with capture-and-
reuse systems, or other forms of runoff volume control will be necessary to achieve the required
capture and removal volumes.
For Regulated Activities equal or less than one acre that do not require design of stormwater storage
facilities, the applicant may select either Control Guideline 1 or Control Guideline 2 on the basis of
economic considerations, applicability and limitations of the analytic procedures and other factors.
Control Guideline 1 may require more complex and detailed analyses while providing a greater
opportunity to select stormwater controls that require fewer resources to construct and operate. For all
Regulated Activities larger than one acre and for all projects that require design of stormwater storage
facilities, Control Guideline 2 may not be used.
3.3.3 Volume Control Guideline 1
The Control Guideline 1 is applicable to any size of the Regulated Activity. Use of Control
Guideline 1 (CG-1) is recommended where site conditions offer the opportunity to reduce the
increase in runoff volume as follows:
Do not increase the post-development total runoff volume for all storms equal to or less
than the 2-year/24-hour event
.
Existing (pre-development) non-forested pervious areas must be considered meadow
(good condition) or its equivalent.
Twenty (20) percent of existing impervious area, when present, shall be considered
meadow (good condition) in the model for existing conditions for redevelopment.
The scientific basis for Volume Control Guideline 1 is as follows:
The 2-year event provides stream channel protection and water quality protection for the
relatively frequent runoff events across the state;
Volume reduction BMPs based on this standard will provide a storage capacity to help reduce
the increase in peak flow rates for larger runoff events;
In a natural stream system in Mid-Atlantic States, the bank full stream flow occurs with a period
of approximately 1.5 years. If the runoff volume from storms less than the 2-year event are not
increased, the fluvial impacts on streams will be reduced;
The 2-year storm is well defined and data are readily accessible for use in stormwater
management calculations.

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3.3.4 Volume Control Guideline 2
Control Guideline 2 (CG-2) is independent of site constraints and should be used if CG-1 is not
followed
.
This method is not applicable to Regulated Activities greater than one (1) acre or for
projects that require design of stormwater storage facilities. For new impervious surfaces:
Stormwater facilities shall be sized to capture at least the first two inches (2”) of
runoff from all contributing impervious surfaces.
At least the first one inch (1.0”) of runoff from new impervious surfaces shall be
permanently removed from the runoff flow — i.e. it shall not be released into the
surface Waters of this Commonwealth.
Removal options include reuse,
evaporation, transpiration, and infiltration.
Wherever possible, infiltration facilities should be designed to accommodate
infiltration of the entire permanently removed runoff; however, in all cases at least
the first one-half inch (0.5”) of the permanently removed runoff should be
infiltrated.
The scientific basis for Volume Control Guideline 2 is as follows:
Groundwater recharge will be maintained;
The permanently removed volume will reduce the runoff;
The combined permanently removed volume and extended detention volume will provide water
quality protection by:
o
Capture / treatment of 95+/-% of the yearly water budget, and a higher volume of
pollutants (first flush);
o
Capture / treatment of 99+/-% of the yearly storm events from paved areas. Example:
for over 50 years of data on the Brandywine, 2.6 storms per year on average exceed 2”;
Volume reduction BMPs based on this standard will provide a storage capacity to reduce the
increase in peak flow rates;
In many of Pennsylvania’s natural streams, the bank full stream flow occurs with a period of
approximately 1.5 years. The combination of volume reduction and extended detention will
reduce the depth and frequency of flows for all events less than the 2-year event, therefore, the
fluvial impacts on streams will be reduced.
3.3.5 Retention and Detention Considerations
Infiltration areas should be spread out and located in the sections of the site that are most
suitable for infiltration.
In all cases, retention and detention facilities should be designed to completely drain water
quality volumes including both the permanently removed volume and the extended detention
volume over a period of time not less than 24 hours and not more than 72 hours from the end of
the design storm.

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3.4
Recommended Peak Rate Control Guideline
Peak rate control for large storms, up to the 100-year event, is essential to protect against immediate
downstream erosion and flooding. Most designs achieve peak rate control through the use of detention
structures. Peak rate control can also be integrated into volume control BMPs in ways that eliminate
the need for additional peak rate control detention systems. Non-Structural BMPs also can contribute
to rate control, as discussed in Chapters 5 and 8.
The recommended control guideline for peak rate control is:
Do not increase the peak rate of discharge for the 1-year through 100-year events (at
minimum); as necessary, provide additional peak rate control as required by applicable and
approved Act 167 plans.
Where Act 167 plans apply, hydrologic modeling may have been performed to provide the basis for
establishing more stringent release rate controls on sub-districts within the watershed. As volume
reduction BMPs are incorporated into stormwater management on a watershed basis, release rate
values will require re-evaluation. Use of the control guidelines will reduce or perhaps even eliminate
the increase in peak rate and runoff volume for some storms.
3.5
Recommended Water Quality Control Guideline
The volume control achieved through applying CG-1 and CG-2 may also remove a major fraction of
particulate associated pollutants from impervious surfaces during most storms. Pervious surfaces such
as “lawnscapes” subject to continuing fertilization may generate NPS pollutants throughout a major
storm, as may stream banks subjected to severe flows. While infiltration BMPs and landscape BMPs
are very effective in NPS reduction, if the volume control measures simply overflow during severe
storms then they will not achieve the control anticipated. Solutes will continue to be transported in
runoff throughout the storm, regardless of magnitude.
CG-1 will provide water quality control and stream channel protection as well as flood control protection
for most storms if the BMPs drain reasonably well and are adequately sized and distributed. CG-2 will
not fully mitigate the peak rate for larger storms, and will require the addition of secondary BMPs for
peak rate control. These secondary BMPs could also provide water quality control. In the event that
this secondary BMP is added to assure rate mitigation during severe storms, the incorporation of
vegetation could provide effective water quality controls.
The recommended control guideline for
total water quality control
is:
Achieve an 85 percent reduction in post-development particulate associated pollutant load (as
represented by Total Suspended Solids), an 85 percent reduction in post-development total
phosphorus loads, and a 50 percent reduction in post-development solute loads (as represented
by NO3-N), all based on post-development land use.
The recommended water quality control guideline is a set of performance-based goals. The guideline
does not represent specific effluent limitations but presents composite efficiency expectations that can
be used to select appropriate BMPs.
These reductions may be estimated based on the pollutant load for each land use type and the
pollutant removal effectiveness of the proposed BMPs, as shown in Chapters 5 and 6 and discussed in
Chapter 8. The inclusion of total phosphorus as a parameter is in recognition of the fact that much of

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the phosphorus in transit with stormwater is attached to the small (colloidal) particles, which are not
subject to gravity settlement in conventional detention structures, except over extended periods. With
infiltration or vegetative treatment, however, the removal of both suspended solids and total
phosphorus should be very high.
New impervious surfaces, such as rooftops, that produce relatively little additional pollutants can be left
out of the water quality impact site evaluation under most circumstances. Rainfall has some latent
concentration of nitrate (1 to 2 mg/l) as the result of air pollution, but it would be unreasonable to
require the removal of this pollutant load from stormwater runoff. The control of nitrate from new
development should focus on reduction of fertilizer applications rather than removal from runoff.
When the proposed development plan for a site is measured by type of surface (roof, parking lot,
driveway, lawn, etc.), an estimate of potential pollutant load can be made based on the volume of runoff
from those surfaces, with a flow-weighted pollutant concentration applied. The total potential non-point
source load can then be estimated for the parcel, and the various BMPs, both Structural and Non-
Structural, can be considered for their effectiveness. This method is described in detail in Chapter 8.
In general, the Non-Structural BMPs are most beneficial for the reduction of solutes, with Structural
BMPs most useful for particulate reduction. Because soluble pollutants are extremely difficult to
remove, prevention or reduction on the land surface, as achieved through Non-Structural BMPs
described in Chapter 5, are the most effective methods for reducing them.
3.6
Stormwater Standards for Special Management Areas
CG-1 and CG-2 may require modification, on a case-by-case basis, before they are applied to Special
Management Areas around the Commonwealth. Special Areas include highways and roads, existing
urban or developed sites, contaminated or brownfield sites, sites situated in karst topography, sites
located in public water supply protection areas, sites situated in High Quality or Exceptional Value
watersheds, sites situated on old mining lands, etc. These are areas where BMP application of any
type may be limited. Stormwater management for these Special Management Areas is discussed in
more detail in Chapter 7.

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Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
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Chapter 4 Integrating Site Design and Stormwater Management
4.1
A Recommended Site Design Procedure for Comprehensive
Stormwater Management…………………………………………………1
4.2
The Site Design Checklist for Comprehensive Stormwater
Management…………………………………………………………………3
4.3
Importance of Site Assessment………………………………………….7
4.3.1 Background Site Factors…………………………………………..7
4.3.2 Site Factors Inventory………………………………………………8
4.3.3 Site Factors Analysis……………………………………………….8

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4.1
A Recommended Site Design Procedure for Comprehensive
Stormwater Management
Chapters 5 and 6 describe multiple Non-Structural and Structural BMPs that can be
used to achieve the Recommended Site Control Guidelines for comprehensive
stormwater management described in Chapter 3. Obviously, not all of these BMPs are
appropriate for all land development activities or every site. How can BMPs be selected
to maximize their performance? What is the optimal blend between Non-Structural and
Structural BMPs? How can stormwater management be best integrated into the site
planning process?
A flow chart depicting a Site Design Procedure For Comprehensive Stormwater
Management (Procedure) is set forth in Figure 4-1 (also referenced to the Checklist
Summary in Figure 4-2 which is discussed in Section 4.2 below). This procedure begins
with an assessment of the site and its natural systems and then proceeds to integrate
both Non-Structural and Structural BMPs in the formulation of a comprehensive
stormwater management plan. The intent of the planning process is to promote
development of stormwater management “solutions” which achieve the rigorous quantity
and quality standards set forth in Chapter 3. Some aspects of the procedure will not be
fully applicable in all land development cases. For example, Non-Structural BMPs may
be challenging to apply in those cases where higher densities/intensities are proposed
on the smallest of sites in already developed areas.
An essential objective of the Procedure is to maximize stormwater “prevention” through
use of Non-Structural BMPs (Chapter 5). Once prevention has been maximized, some
amount of stormwater peaking and volume control will likely remain to be managed.
These stormwater management needs should be met with an array of natural-system
based Best Management Practices (Vegetated Swales, Vegetated Filter Strips, etc.),
with the remaining stormwater management needs met with structural Best Management
Practices such as infiltration basins, trenches, porous pavement, wet basins, retention
ponds, constructed wetlands, and others presented in Chapter 6.
This Procedure, or a process similar to it, is an integral part of comprehensive
stormwater management and transcends the bounds of conventional stormwater
management that has existed in most Pennsylvania municipalities. Perhaps most
importantly, the Procedure involves the total site design process
. Conventional
stormwater management has usually been relegated to the final stages of the site design
and overall land development process, after most other building program issues have
been determined and accommodated. To the contrary, the Procedure places
stormwater management in the initial stages of site planning process, when the building
program is being fitted and tested on the site. In this way, comprehensive stormwater
management can be integrated effectively into the site design process.

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Much of the information relied on for the Procedure is information already required to
satisfy other aspects of existing municipal land development ordinances. The Procedure
is intended to more effectively utilize this already-collected site data to generate better
stormwater management in the context of a markedly improved site plan. To the extent
that this information is not already being collected and assessed, the information needs
to be collected as part of the site design process.
4.2
The Site Design Checklist for Comprehensive Stormwater
Management
Coordinated with the Recommended Site Design Procedure for Comprehensive
Stormwater Management is a series of questions structured to facilitate and guide an
assessment of the site’s natural features and stormwater management needs. The Site
Design Checklist for Comprehensive Stormwater Management (Figure 4-2) is intended
to help facilitate the Procedure. The initial questions in the Checklist focus on Site
Analysis, including Background Site Features, a Site Factors Inventory, Site Factors
Analysis and Constraints and Opportunities. The checklist relates directly to the first
Non-Structural BMP category: Protect Sensitive and Special Value Features, which
include:
BMP 5.4.1
Protect Sensitive/Special Value features
BMP 5.4.2
Protect/conserve/enhance utilize riparian areas
BMP 5.4.3
Protect/utilize natural flow pathways in overall stormwater planning
and design
Because these first steps in the Procedure are so important, they are further discussed
below in Section 4.3 – “Importance of Site Assessment”.
The Procedure continues with potentially multiple cycles of “testing” and “fitting”
preventive
Non-Structural BMPs at the site. The Checklist provides questions designed
to identify the potential application of additional Non-Structural BMPs. Once Non-
Structural BMPs have been “maximized,” the Recommend Procedure then continues
with the testing/fitting of Structural BMPs, again facilitated by the Checklist questions.
This testing/fitting of Non-Structural and Structural BMPs can continue through several
cycles. At the completion of the Procedure, a comprehensive stormwater management
plan emerges, satisfying the Chapter 3 Recommended Site Control Guidelines. If the
Checklist questions are addressed thoroughly and the Procedure is fully and effectively
applied, the critical objective of managing stormwater comprehensively will be achieved
in a cost effective manner. The Procedure, though largely common sense, constitutes a
change from conventional engineering practice in many Pennsylvania municipalities.

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Background Site Factors
Describe hydrologic context and other natural elements
Chapter 93 stream use designation?
Special Protection Waters (EV, HQ)?
Fishery / Aquatic Life Use (WWF, CWF, TSF)?
Any Chapter 303d/impaired stream listing classifications?
Aquatic biota sampling?
Existing water quality sensitivities downstream (water supply source?)?
Location of any known downstream flooding?
Includes any Special Areas?
Such as Previously Mined AMD/AML areas?
Brownfields?
Source Water Protection areas
Urban Areas?
Carbonate/Limestone?
Slide Prone Areas
Other
Site Factors Inventory
Describe the size and shape of the site
Special constraints/opportunities?
Special site border conditions and adjacent uses?
Describe the existing developed features of the site, if any
Existing structures/improvements, structures to be preserved?
Existing cover/uses?
Existing impervious areas?
Existing pervious maintained areas?
Existing public sewer and water?
Existing storm drainage systems at/adjacent to site?
Existing wastewater, water systems onsite?
Describe important natural features of site
Existing hydrology (drainage swales, intermittent, perennial)?
Existing topography, contours, subbasins?
Soil series found on site and their Hydrologic Soil Group ratings?
Areas of vegetation (trees, scrub, shrub)?
Special Value Areas?
Wetlands, hydric soils?
Floodplains/alluvial soils?
High quality woodlands, other woodlands and vegetation?
Riparian buffers?
Naturally vegetated swales/drainageways?
Sensitive Areas?
Steep slopes?
Special geologic conditions (limestone?)?
Shallow bedrock (less than 2ft)?
High water table (less than 2ft)?
PNDI areas or species?
Site Factors Analysis
Characterize the constraint-zones at site
Avoid development on or near special and sensitive natural features
Characterize the opportunity-zones at site
Location of well-draining soils
Location and quality of existing vegetation
Has a Potential Development Area been defined?
Does building program fit the constraints and opportunities of natural features?
(Figure 4-2) Checklist summary for use with Site Planning and Design Procedure
SITE ANALYSIS
BACKGROUND SITE CONDITIONS

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Township Comprehensive Plan and Zoning guidance
Guidance in Comprehensive Plan?
Existing Zoning District?
Total number of units allowed?
Type of units?
Density of units?
Any allowable options?
Township SLDO guidance and options
Performance standards for neo-traditional, village, hamlet planning?
Reduce building setbacks?
Curbs required?
Street width, parking requirements, other impervious requirements?
Cut requirements?
Grading requirements?
Landscaping requirements?
Township SLDO/stormwater requirements
Peak rate and design storms?
Total runoff volume?
Water quality provisions?
Methodological requirements?
Maintenance requirements?
Is applicant submission complete? Fully responsive to municipal zoning/
SLDO requirements?
Are municipal zoning/SLDO requirements inadequate?
Is useful interaction at sketch plan or even pre-sketch plan phases occurring?
Lot Concentration and Clustering
Reduce individual lot size?
Concentrate/cluster uses and lots?
Configure lots to avoid critical natural areas ?
Configure lots to take advantage of effective mitigative stormwater practices?
Orient built structures to fit natural topography?
Minimize site disturbance (excavation / grading) at site?
Minimize site disturbance (excavation / grading) for each lot?
Minimum Disturbance/Maintenance
Define disturbance zones for site?
Protect maximum total site area from development disturbance?
Protect naturally sensitive and special areas from disturbance?
Minimize total site compaction?
Maximize zones of open space and greenways?
Consider re-forestation and re-vegetation opportunities?
Impervious Coverage Reduction
Reduce road widths? Lengths?
Utilize turnarounds? Cul-de-sacs with vegetated islands?
Reduce driveway length and width?
Reduce parking ratios?
Reduce parking sizes?
Examine potential for shared parking?
Utilize porous surfaces for applicable parking features (overflow)?
Design sidewalks for single-side street movement?
Disconnect/Distribute/Decentralize
Rooftop disconnection?
Existing downgradient yard area opportunities?
Existing downgradient vegetated areas/woods?
Disconnection from storm sewers/street gutters?
Front/side yard opportunities?
Space for vegetated swales, rain gardens, etc.?
Source Control
Provisions for street sweeping? Other?
BACKGROUND SITE CONDITIONS
SITE DESIGN: NON-STRUCTURAL BMPs
DESIGN PHASE 1: PREVENTIVE
MUNICIPAL INPUTS

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Volume/Peak Rate Through Infiltration
Porous Pavement with Infiltration Beds?
Infiltration Basins?
Infiltration Trenches?
Rain Garden/Bioretention?
Dry Wells/Seepage Pits?
Vegetated Swales?
Vegetated Filter Strips?
Infiltration Berm/Retentive Grading?
Volume/Peak Rate Reduction
Vegetated Rooftops?
Capture & Reuse:
Cisterns?
Rain Barrels?
Other?
Runoff Quality/Peak Rate Reduction
Constructed wetland?
Wet pond/retention basin?
Dry extended detention basin?
Water quality filters: Constructed and Other
Sand and sand/peat?
Multi-chamber catch basins and inlets?
Other types?
Other
Level Spreaders?
Special Detention Storage: Parking Lots, Other
Site Restoration for Stormwater
Riparian Buffer Restoration?
Landscape Restoration
Soil Amendment/Restoration
Protocols
Soil Testing
Site Infiltration
Iterative Process Occurring Throughout Planning and Design Practices to Max out
Non-Structural and Structural Practices
Use acceptable methods, such as Soil Cover Complex Method (TR-55) for calculations
Do not use Weighted Curve Numbers!
Strive to:
Minimize the pre to post development increase in Curve Numbers
Maximize post-development Time of Concentration
Assume "conservative" pre-development cover conditions (i.e., Curve Numbers) such as
"Meadow Good" or "Woods" for all pre-development pervious areas?
Respect natural sub-areas in the design and engineering calculations
Strive To Achieve Standards of Comprehensive Stormwater Management
No increase in volume of runoff, pre to post development, for up to the 2-yr storm
No reduction in total volume of recharge, for up to the 2-yr storm
No increase in peak rate of runoff, small to large storms
No increase in pollutant loading
Has There Been Thorough Approach To Use of Both Non-Structural and Structural BMP's?
If not, what non-structurals and structurals might be used?
Should the building program be modified?
What Related Benefits Are Being Achieved Through The Use of BMPs?
PLAN
SITE DESIGN: STRUCTURAL BMPs
STORMWATER METHODOLOGY AND CALCULATIONS
DEVELOP COMPREHENSIVE STORMWATER MANAGEMENT PLAN
DESIGN PHASE 1: MITIGATIVE
SOTRMWATER CALCULATIONS

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4.3
Importance of Site Assessment
Comprehensive stormwater management begins with a thorough assessment of the site
and its natural systems. Site assessment includes inventorying and evaluating the
various natural resource systems which define each site and pose both problems and
opportunities for stormwater management. Resources include the full range of natural
systems such as water quantity, water quality, floodplains and riparian areas, wetlands,
soils, geology, vegetation, and more. Natural systems range in scale from resources of
areawide importance on a macro scale, down to micro- and site-specific detail.
4.3.1 Background Site Factors
Broader system characteristics should be described, including State Chapter 93 stream
classifications, presence of Special Protection Waters, stream order (i.e., 1
st
order, 2
nd
order, etc.), source water supply designations, 303d/TMDL/Impaired Stream
designations, flooding history, and other information that provides an understanding of
how a particular site is functioning within its watershed context. More specific questions
would include:
Does the site drain to special waterbodies with special water quality needs?
Determine if the site ultimately flows into a reservoir or other water body where
special water quality sensitivities exist, such as use as a water supply source.
Determine if a special fishery exists.
Determine if the site is linked to a special habitat system, such as delineated in
the Pennsylvania Natural Diversity Inventory. For both water quality and
temperature reasons, approaches and practices that achieve a higher order of
protection may become especially important.
Are there known downstream flooding problems?
Determine if a stream system to which the site discharge is currently experiencing
flooding problems. This is especially important where urbanization already has occurred
and where hydrology already has been altered. Unfortunately, the existing FEMA
mapping and related studies do not adequately assess this issue. County agencies and
municipal offices may be able to indicate anecdotally the extent to which downstream
flooding is already a problem or has the potential to become a problem if substantial
additional development is projected. Greater care should be taken in both floodplain
management as well as stormwater management if problems exist or are anticipated.
Does the site discharge to 1st, 2nd, 3rd order streams?
Another important question relates to the site’s location within its watershed. Sites
located near the base of watersheds pose less of a threat to the hydrologic
characteristics of the watershed system. Sites located farther up the watershed are
potentially more problematic when additional stormwater is generated. Perhaps even
more critical, sites located within headwaters must be managed most carefully in terms
of stormwater to maintain pre-development infiltration and groundwater recharge rates.

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4.3.2 Site Factors Inventory
Site-specific factors that influence comprehensive stormwater management include the
following items:
How does site size and shape affect stormwater management?
As site size increases, the ability to use a variety of Non-Structural and Structural BMPs
increases. Comprehensive stormwater management, especially through site planning
and the use of Non-Structural BMPs, can reduce space requirements at a site and offer
greater BMP flexibility. Oddly shaped sites can also be better adapted with BMPs set
forth here, given their wide variety of shapes and sizes.
What are the important natural features characterizing the site?
At the heart of the comprehensive stormwater management procedure is an
understanding of the natural systems characterizing each site. Existing vegetation and
soil have tremendous importance and are the key to understanding land development
impacts on natural systems. Careful accounting of existing vegetation is an important
prerequisite for comprehensive stormwater management, followed closely by soils
mapping for permeability ratings, and natural pre-development surface flow patterns.
Critical site features, such as wetlands, floodplains, riparian areas, natural drainage
ways, special habitat areas, special geological formations (e.g., carbonate), steep
slopes, shallow depth to water table, shallow depth to bedrock, and other factors should
be inventoried and understood. Critical areas include those with special positive
functions that can be translated into real economic value or benefit. Elimination or
reduction of these functions through the land development process leads to real
economic losses. These special value areas, including wetlands and floodplains and
riparian areas, should be conserved and protected during land development. Critical
natural areas also include sensitive areas, such as steep slopes, shallow bedrock, high
water table areas, and other constraining features, where encroachment by land
development typically creates unnecessary or unanticipated problems. Care must be
taken to avoid these potential pitfalls.
4.3.3 Site Factors Analysis
Identify site factors that constrain comprehensive stormwater management, and identify
site factors that can be viewed as opportunities.
How is the site constrained?
Determine where buildings, roads, and other disturbance should be avoided and why.
Where are the zones of site “opportunity,” in terms of stormwater management?
Determine where most infiltration occurs in terms of vegetation and in terms of soils.
Both constraints and opportunities are grounded in the natural systems present at the
site. Constraints and opportunities are not necessarily simple opposites of one another.
For example, certain types of critical natural areas should be viewed as constraints in
terms of direct land disturbance and building construction, yet also provide significant
opportunity in terms of stormwater management, quantity and quality. Woodlands,
which should be protected from direct land development, provide excellent opportunity
for stormwater management, provided that the correct approaches and practices are

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used. Vegetated riparian buffers should not be disturbed for building and road
construction yet they can be used carefully with level spreading devices to receive
diffuse stormwater runoff. Soils with maximum permeabilities at the site should not be
made impervious with buildings and roads, but used for stormwater management where
feasible. Conversely, buildings and other impervious areas should be located on those
portions of a site with least
permeable soils. Site opportunities for volume control can
typically be defined in terms of vegetation types that minimize runoff, as well as soil
types with maximum permeabilities.

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Chapter 5
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Chapter 5 Non-Structural BMPs
5.1
Introduction……………………………………………………………………………………...1
5.2
Non-Structural Best Management Practices………………………………………………1
5.3
Non-Structural BMPs and Stormwater Methodological Issues………………………..3
5.4
Protect Sensitive and Special Value Resources
BMP 5.4.1
Protect Sensitive/Special Value Features………………………….7
BMP 5.4.2
Protect/Conserve/Enhance Riparian Areas………………………13
BMP 5.4.3
Protect/Utilize Natural Flow Pathways in Overall Stormwater
Planning and Design…………………………………………………21
5.5
Cluster and Concentrate
BMP 5.5.1
Cluster Uses at Each Site; Build on the Smallest
Area Possible………………………………………………………….29
BMP 5.5.2
Concentrate Uses Area wide through Smart
Growth Practices……………………………………………………...37
5.6
Minimize Disturbance and Minimize Maintenance
BMP 5.6.1
Minimize Total Disturbed Area – Grading………………………...49
BMP 5.6.2
Minimize Soil Compaction in Disturbed Areas…………………..57
BMP 5.6.3
Re-Vegetate and Re-Forest Disturbed Areas, Using Native
Species………………………………………………………………….63
5.7
Reduce Impervious Cover
BMP 5.7.1
Reduce Street Imperviousness…………………………………….71
BMP 5.7.2
Reduce Parking Imperviousness…………………………………..77
5.8
Disconnect/Distribute/Decentralize
BMP 5.8.1
Rooftop Disconnection………………………………………………85
BMP 5.8.2
Disconnection from Storm Sewers………………………………..89
5.9
Source Control
BMP 5.9.1
Streetsweeping………………………………………….….…………95

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Chapter 5 Comprehensive Stormwater Management: Non-Structural BMPs
5.1
Introduction
The terms “Low Impact Development” and “Conservation Design” refer to an environmentally sensitive
approach to site development and stormwater management that minimizes the effect of development
on water, land and air. This chapter emphasizes the integration of site design and planning techniques
that preserve natural systems and hydrologic functions on a site through the use of Non-Structural
BMPs. Non-Structural BMP deployment is not a singular, prescriptive design standard but a
combination of practices that can result in a variety of environmental and financial benefits. Reliance
on Non-Structural BMPs encourages the treatment, infiltration, evaporation, and transpiration of
precipitation close to where it falls while helping to maintain a more natural and functional landscape.
The BMPs described in this chapter preserve open space and working lands, protect natural systems,
and incorporate existing site features such as wetlands and stream corridors to manage stormwater at
its source. Some BMPs also focus on clustering and concentrating development, minimizing disturbed
areas, and reducing the size of impervious areas. Appropriate use of Non-Structural BMPs will reflect
the ten “Principles” presented in the Foreword to this manual, and will be an outcome of applying the
procedures described in Chapter 4.
From a developer’s perspective, these practices can reduce land clearing and grading costs, reduce
infrastructure costs, reduce stormwater management costs, and increase community marketability and
property values. Blending these BMPs into development plans can contribute to desirability of a
community, environmental health and quality of life for its residents. Longer term, they sustain their
stormwater management capacity with reduced operation and maintenance demands.
Conventional land development frequently results in extensive site clearing, where existing vegetation
is destroyed, and the existing soil is disturbed, manipulated, and compacted. All of this activity
significantly affects stormwater quantity and quality. These conventional land development practices
often fail to recognize that the natural vegetative cover, the soil mantle, and the topographic form of the
land are integral parts of the water resources system that need to be conserved and kept in balance,
even as land development continues to occur.
As described in Chapter 4, identifying a site’s natural resources and evaluating their values and
functional importance is the first step in addressing the impact of stormwater generated from land
development. Where they already exist on a proposed development site, these natural resources
should be conserved and utilized as a part of the stormwater management solution. The term “green
infrastructure” is often used to characterize the role of these natural system elements in preventing
stormwater generation, infiltrating stormwater once it’s created, and then conveying and removing
pollutants from stormwater flows. Many vegetation and soil-based structural BMPs are in fact “natural
structures” that perform the functions of more “structural” systems (e.g., porous pavement with
recharge beds). Because some of these “natural structures” can be designed and engineered, they are
discussed in Chapter 6 as structural BMPs.
5.2
Non-Structural Best Management Practices
This Manual differentiates BMPs based on Non-Structural (Chapter 5) and Structural (Chapter 6)
designations. Non-Structural BMPs take the form of broader planning and design approaches – even
principles and policies – which are less “structural” in their form, although non-structural BMPs do have

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very important physical ramifications. An excellent example would be “reducing imperviousness” (see
BMPs 5.9 and 5.10 below) by reducing road width and/or reducing parking ratios. In this way, a
proposed building program can be accommodated but with reduced stormwater generation. These
non-structural BMPs can be applied over an entire site and are not fixed and designed at one location.
Virtually all of the Non-Structural BMPs set forth in this Chapter of the manual share this kind of site-
wide policy characteristic. Structural BMPs, on the other hand, are decidedly more locationally specific
and explicit in their physical form.
Sometimes called Low Impact Development or Conservation Design techniques, Non-Structural BMPs
are not always markedly different from Structural BMPs. In fact, some of the BMPs described in
Chapter 6, such as Vegetated Swales and Vegetated Filter Strips, are largely based in natural systems
and are intended to function as they would have prior to disturbance. Nevertheless, such BMPs can be
thought of as natural structures, which are designed to mitigate any number of stormwater impacts:
peak rates, total runoff volumes, infiltration and recharge volumes, non-point source water quality
loadings and temperature increases.
Perhaps the most defining distinction for the Non-Structural BMPs set forth in this chapter is their ability
to prevent
stormwater generation and not just mitigate stormwater-related impacts once these problems
have been generated. Prevention can be achieved by developing land in ways other than through use
of standard or conventional development practices. Prevention and Non-Structural BMPs go hand in
hand and can be contrasted with Structural BMPs that provide mitigation of those stormwater impacts,
which cannot be prevented and/or avoided.
Several major “areas” of preventive Non-Structural BMPs have been identified in this manual:
Protect Sensitive and Special Value Features
Cluster and Concentrate
Minimize Disturbance and Minimize Maintenance
Reduce Impervious Cover
Disconnect/Distribute/Decentralize
Source Control
More specific Non-Structural BMPs have been identified for each of these generalized areas to better
define and improve implementation of each of these areas. This list of specific BMPs will be refined
and expanded as these stormwater management practices become more common throughout
Pennsylvania.
A uniform format has been developed for the BMPs presented in Chapters 5 and 6 of this manual. It
provides as many engineering details as possible, facilitated through diagrams, graphics and pictures.
There are constant tradeoffs that must be made between providing a more complete explanation for the
countless variations which can be expected to emerge across the state versus the need to be concise
and user friendly.
The uniform format has been applied to all of the Non-Structural BMPs included in Chapter 5, to
encourage recognition that these Non-Structural techniques are every bit as essential as the
techniques presented in Chapter 6 Structural BMPs.
One of the most challenging technical issues considered in this manual involves the selection
of BMPs that have a high degree of NPS reduction or removal efficiency. In the ideal, a BMP
should be selected that has a proven NPS pollutant removal efficiency for all pollutants of
importance, especially those that are critical in a specific watershed (as defined by a TMDL or

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other process). Both Non-Structural BMPs in Chapter 5 and Structural BMPs in Chapter 6 are
rated in terms of their anticipated pollutant removal performance or effectiveness. The initial
BMP selection process analyzes the final site plan and estimates the potential NPS load, using
Appendix A. The targeted reduction percentage for representative pollutants (such as 85%
reduction in TSS and TP load and 50% reduction in the solute load) is achieved by a suitable
combination of Non-Structural and Structural BMPs. This process is described in more detail
in Chapter 8.
5.3
Non-Structural BMPs and Stormwater Methodological Issues
The methodological approach set forth in Chapter 8 provides a variety of straightforward and
conservative ways to take credit for applying Non-Structural BMPs, provided that the “specifications”
defined for each BMP in Chapter 5 are properly followed.
Because so many of the Non-Structural BMPs seem so removed from the conventional practice of
stormwater engineering, putting these BMPs into play may be a challenge. Many of these Non-
Structural BMPs ultimately require a more sophisticated approach to total site design. Some of the
Non-Structural BMPs don’t easily lend themselves to stormwater calculations as conventionally
performed. How do we get stormwater credit for applying any of these techniques? Taking BMPs 5.6.1
and 5.6.2 again as examples, minimizing impervious cover by reducing road width or impervious
parking area directly translates into reduced stormwater volumes and reduced stormwater rates of
runoff. Site planners and designers will also recognize that many of the other Non-Structural BMPs,
such as clustering of uses, conserving existing woodlands and other vegetative cover, and
disconnecting impervious area runoff flows, all translate into reduced stormwater volume and rate
calculations. As such, these BMPs are self-crediting.

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5.4 Protect Sensitive and Special Value Resources

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BMP 5.4.1: Protect Sensitive and Special Value Features
To minimize stormwater impacts, land development should avoid
affecting and encroaching upon areas with important natural
stormwater functional values (floodplains, wetlands, riparian areas,
drainageways, others) and with stormwater impact sensitivities
(steep slopes, adjoining properties, others) wherever practicable.
This avoidance should occur site-by-site and on an area wide basis.
Development should not occur in areas where sensitive/special
value resources exist so that their valuable natural functions are not
lost, thereby doubling or tripling stormwater impacts. Resources
may be weighted according to their functional values specific to
their municipality and watershed context.
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Potential Applications
Residential:
Commercial: Ultra
Urban: Industrial:
Retrofit:
Highway/Road:
Yes Yes
Yes Yes
Yes Yes
Very High
Very High
Very High
Very High
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
Key Design Elements
.
Identify and map floodplains and riparian area
.
Identify and map wetlands
.
Identify and map woodlands
.
Identify and map natural flow pathways/drainage ways
.
Identify and map steep slopes
.
Identify and map other sensitive resources
.
Combine for Sensitive Resources Map (including all of the
above)
.
Distinguish between including Highest Priority Avoidance Areas
and Avoidance Areas
.
Identify and Map Potential Development Areas (all those areas
not identified on the Sensitive Resources Map)
.
Make the development program and overall site plan conform to
the Development Areas Map to the maximum; minimize
encroachment on Sensitive Resources.

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Description
A major objective for stormwater-sensitive site planning and design is to avoid encroachment upon,
disturbance of, and alteration to those natural features which provide valuable stormwater functions
(floodplains, wetlands, natural flow pathways/drainage ways) or with stormwater impact sensitivity
(steep slopes, historic and natural resources, adjoining properties, etc.) Sensitive Resources also
include those resources of special value (e.g., designated habitat of threatened and endangered
species that are known to exist and have been identified through the Pennsylvania Natural Diversity
Inventory or PNDI). The objective of this BMP is to avoid harming Sensitive/Special Value Resources
by carefully identifying and mapping these resources from the initiation of the site planning process and
striving to protect them while defining areas free of these sensitivities and special values (Potential
Development Areas). BMP 5.4.2 Protect/Conserve/Enhance Riparian Areas and BMP 5.6.2 Minimize
Soil Compaction in Disturbed Areas build on recommendations included in this BMP.
Variations
• BMP 5.4.1 calls for actions both on the parts of the municipality as well as the individual
landowner and/or developer. Pennsylvania municipalities may adopt subdivision/land
development ordinances which require that the above steps be integrated into their respective
land development processes. A variety of models are available for municipalities to facilitate
this adoption process, such as through the PADCNR
Growing Greener
program.
Figure 5.1-1. Growing Greener’s Conservation
Subdivision Design: Step One, Part One – Identify
primary conservation areas.
Figure 5.1-2. Growing Greener’s Conservation
Subdivision Design: Step One, Part Two – Identify
secondary conservation areas.
Source: Growing Greener: Putting Conservation Into Local Codes; Natural Land Trusts, Inc. 1997

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• The above steps use the
Growing Greener
Primary Conservation Areas and Secondary
Conservation Areas designations and groupings. Identify and map the essential natural
resources, including those having special functional value and sensitivity from a stormwater
perspective, and then avoid developing (destroying, reducing, encroaching upon, and/or
impacting) these areas during the land development process. Additionally, it is possible that
Primary and Secondary can be defined in different ways so as to include different resources.
• Definition of the natural resources themselves can be varied. The definition of Riparian Buffer
Area varies. Woodlands may be defined in several ways, possibly based on previous
delineation/definition by the municipality or by another public agency. It is important to note
here that Wooded Areas, which may not rank well in terms of conventional woodland definitions,
maintain important stormwater management functions and should be included in the
delineation/definition. Intermittent streams/swales/natural flow pathways are especially given to
variability. Municipalities may not only integrate the above steps within their subdivision/land
development ordinances, but also define these natural resource values as carefully as possible
in order to minimize uncertainty.
• The level of rigor granted to Priority Avoidance and Avoidance Areas may be made to vary in a
regulatory manner by the municipality and functionally by the owner and/or developer. A
municipal ordinance may prohibit and/or otherwise restrict development in Priority Avoidance
Areas and even Avoidance Areas. All else being equal, the larger the site, the more restrictive
these requirements may be.
Figure 5.1-3. Growing Greener’s Conservation Subdivision
Design: Step One, Part Three – potential development areas.
Source: Growing Greener: Putting Conservation Into Local Codes; Natural Land Trusts, Inc. 1997

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Applications
A number of communities across
Pennsylvania have adopted ordinances that
require natural resources to be identified,
mapped, and taken into account in a multi-
step process similar to the Growing Greener
program. These include:
BUCKS COUNTY
Milford Township SLDO (Sep. 2002)
CHESTER COUNTY
London Britain Township (1999)
London Grove Township (2001)
Newlin Township (1999)
North Coventry Township (Dec. 2002)
Wallace Township (1994)
West Vincent Township (1998)
MONTGOMERY COUNTY
Upper Salford Township (1999)
MONROE COUNTY
Chestnuthill Township (2003)
Stroud Township SLDO (2003)
YORK COUNTY
Carroll Township (2003)
BMP 5.4.1 applies to all types of development in all types of municipalities across Pennsylvania,
although variations as discussed above allow for tailoring according to different development
density/intensity contexts.
Design Considerations
Not applicable.
Detailed Stormwater Functions
Impervious cover and altered pervious covers translate into water quantity and water quality impacts as
discussed in Chapter 2 of this manual. Additional impervious area may further eliminate or in some
way reduce other natural resources that were having especially beneficial functions.
Water quality concerns include all stormwater pollutant loads from impervious areas, as well as all
pollutant loads from the newly created maintained landscape (i.e., lawns and other). Much of this load
is soluble in form (especially fertilizer-linked nitrogen forms). Clustering as defined here, and combined
with other Chapter 5 Non-Structural BMPs, minimizes impervious areas and the pollutant loads related
to these impervious areas. After Chapter 5 BMPs are optimized, “unavoidable” stormwater is then
directed into BMPs as set forth in Chapter 5, to be properly treated. Similarly, for all stormwater
pollutant load generated from the newly-created maintained landscape, clustering as defined here, and
Figure 5.1-4. Steep slope development with woodland
removal.

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combined with other Chapter 5 Non-Structural BMPs, minimizes pervious areas and the pollutant loads
related to these pervious areas, thereby reducing the opportunity for fertilization and other chemical
application. Water quality prevention accomplished through Non-Structural BMPs in Chapter 5 is
especially important because Chapter 6 Structural BMPs remain poor performers in terms of
mitigating/removing soluble pollutants that are especially problematic in terms of this pervious
maintained landscape. See Appendix A for additional documentation of the water quality benefits of
clustering.
See Chapter 8 for additional volume reduction calculation work sheets, additional peak rate reduction
calculation work sheets, and additional water quality mitigation work sheets.
Construction Issues
Clearly, application of this BMP is required from the
start of the site planning and development process.
In fact, not only must the site developer embrace
BMP 5.4.1 from the start of the process, the BMP
assumes that the respective municipal officials have
worked to include clustering in municipal codes and
ordinances, as is the case with so many of these
Chapter 5 Non-Structural BMPs.
Maintenance Issues
As with all Chapter 5 Non-Structural BMPs, maintenance issues are of a different nature and extent,
when contrasted with the more specific Chapter 6 Structural BMPs. Typically, the designated open
space may be conveyed to the municipality, although most municipalities prefer not to receive these
open space portions, including all of the maintenance and other legal responsibilities associated with
open space ownership. In the ideal, open space reserves ultimately will merge to form a unified open
space system, integrating important conservation areas throughout the municipality. These open space
segments may exist dispersed and unconnected. For those Pennsylvania municipalities that allow for
and enable creation of homeowners associations or HOA’s, the HOA may assume ownership of the
open space. The HOA is usually the simplest solution to the issue.
In contrast to some of the other long-term maintenance responsibilities of a new subdivision and/or land
development (such as maintenance of streets, water and sewers, play and recreation areas, and so
forth), the maintenance requirements of “undisturbed open space” by definition should be minimal. The
objective is conservation of the natural systems, including the natural or native vegetation, with little
intervention and disturbance. Nevertheless, some legal responsibilities must be assumed and need to
be covered.
Cost Issues
Clustering is beneficial from a cost perspective in several ways. Development costs are decreased
because of less land clearing and grading, less road construction (including curbing), less sidewalk
construction, less lighting and street landscaping, potentially less sewer and water line construction,
potentially less stormwater collection system construction, and other economies.
Figure 5.1-5. Example of steep slope development.

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Clustering also reduces post construction costs. A variety of studies from the landmark
Costs of Sprawl
study and later updates have shown that delivery of a variety of municipal services such as street
maintenance, sewer and water services, and trash collection are more economical on a per person or
per house basis when development is clustered. Even services such as police protection are made
more efficient when residential development is clustered.
Additionally, clustering has been shown to positively
affect land values. Analyses of market prices of
conventional development over time in contrast with
comparable cluster developments (where size, type,
and quality of the house itself is held constant) have
indicated that clustered developments with their
proximity to permanently protected open space
increase in value at a more rapid rate than
conventionally designed developments, even though
clustered housing occurs on considerably smaller
lots than the conventional residences.
Specifications
Clustering is not a new concept and has been defined, discussed, and evaluated in many different
texts, reports, references and sources detailed in the References for BMP 5.5.1
Figure 5.1-6. Woodland removal for steep slope
development with retaining walls.

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BMP 5.4.2: Protect /Conserve/Enhance Riparian Areas
The Executive Council of the Chesapeake Bay Program
defines a Riparian Forest Buffer as "an area of trees, usually
accompanied by shrubs and other vegetation, that is adjacent
to a body of water and which is managed to maintain the
integrity of stream channels and shorelines, to reduce the
impact of upland sources of pollution by trapping, filtering and
converting sediments, nutrients, and other chemicals, and to
supply food, cover, and thermal protection to fish and other
wildlife."
Potential Applications
Residential:
Commercial: Ultra
Urban: Industrial:
Retrofit:
Highway/Road:
Yes Yes
Yes Yes
Yes Yes
Key Design Elements
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Medium
Medium
Low/Med.
Very High
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
.
Linear in Nature
.
Provide a transition between aquatic and upland environments
.
Forested under natural conditions in Pennsylvania
.
Serve to create a "Buffer" between development and aquatic
environment
.
Help to maintain the hydrologic, hydraulic, and ecological integrity
of the stream channel.
.
Comprised of three "zones" of different dimensions:
.
Zone 1
: Adjacent to the stream and heavily vegetated
under ideal conditions (Undisturbed Forest) to
shade stream and provide aquatic food sources.
.
Zone 2
: Landward of Zone 1 and varying in width,
provides extensive water quality improvement.
Considered the Managed Forest.
.
Zone 3
: Landward of Zone 2, and may include BMPs
such as Filter Strips.
There are two components to Riparian Buffers to be considered in the development process:
1. Protecting, maintaining, and enhancing existing Riparian Forest Buffers.
2. Restoring Riparian Forest Buffers that have been eliminated or degraded by past practices.

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BMP 5.4.2 focuses on protection, maintenance, and enhancement of existing Riparian Forest Buffers.
Restoration of Riparian Forest Buffers is treated in Chapter 6 as a Structural BMP.
Detailed Stormwater Functions
Riparian Corridors are vegetated ecosystems along a waterbody that serve to buffer the waterbody
from the effects of runoff by providing water quality filtering, bank stability, recharge, rate attenuation
and volume reduction, and shading of the waterbody by vegetation. Riparian corridors also provide
habitat and may include streambanks, wetlands, floodplains, and transitional areas. Functions can be
identified and sorted more specifically by Zone designation:
Zone 1
: Provides stream bank and channel stabilization; reduces soil loss and sedimentation/nutrient
and other pollution from adjacent upslope sheet flow; roots, fallen logs, and other vegetative debris
slow stream flow velocity, creating pools and habitat for macroinvertebrates, in turn enhancing
biodiversity; decaying debris provides additional food source for stream-dwelling organisms; tree
canopy shades and cools water temperature, critical to sustaining certain macroinvertebrates, as well
as critical diatoms, which are essential to support high quality species/cold water species. Zone 1
functions are essential throughout the stream system, especially in 1st order streams.
Figure 5.2-1. Riparian buffer zones support various ecological functions.

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Zone 2
: Removes, transforms, and stores nutrients, sediments, and other pollutants flowing as sheet
flow as well as shallow sub-surface flow. A healthy Zone 2 has the potential to remove substantial
quantities of excess nitrates through root zone uptake. Nitrates customarily can be significantly
elevated when adjacent land uses are agricultural or urban/suburban. Healthy vegetation in Zone 2
slows surface runoff while filtering sediment and particulate bound phosphorus. Total nutrient removal
is facilitated through a variety of complex processes: long-term nutrient storage through microbe
uptake, denitrification through bacterial conversion to nitrogen gases and additional microbial
degradation processes.
Zone 3
: Provides the first stage in managing upslope runoff so that runoff flows are slowed and evenly
dispersed into Zone 2. Some physical filtering of pollutants may be accomplished in Zone 3 as well as
some limited amount of infiltration.
Design Considerations/Variations
Although this manual refers frequently to the Chesapeake Bay Program’s Riparian Handbook, many
different sources of guidance have been developed in recent years. Not all of these are exactly
comparable in terms of their recommendations and specifications. To some extent these variations
relate to different land use development contexts.
Riparian Forest Buffer Zone widths should be adjusted according to site conditions and type of upslope
development. Variation in standards (see Specifications below) should vary with the function to be
performed by the forested buffer. In undisturbed forested areas where minimal runoff is expected to be
occurring, standards can be made more flexible than in agricultural contexts where large quantities of
natural vegetation have been removed and significant quantities of runoff are expected. In addition to
factors related to technical need, practical and political factors also must be considered. In urbanized
settings where hundreds, if not thousands of small lots may abut riparian areas and already intrude into
potential forested buffer zones, buffer standards must be practicable.
Figure 5.2-2. Riparian buffer zones (DJ Welsh, 1991).

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Figure 5.2-3. Riparian buffer zone functions.
Lastly, confusion has emerged
between the concept of
floodplain and riparian forest
buffer. In many cases,
mapped and delineated
floodplain may overlap and
even largely coincide with
riparian forest buffer zones.
On the other hand, mapped
100-year floodway/floodplain
may not coincide with the
forest buffer due to either very
steep topography or very
moderate slopes. A second
important clarification is that
floodplain ordinances typically
manage use to prevent flood
damage, which contrasts to
riparian forest buffer regulation
which manages clearing and
grading actions in the zones, specifically for environmental reasons.
Construction Issues
Riparian Forest Buffer Protection should be defined and included in municipal ordinances, including
both the zoning ordinance and subdivision and land development ordinance (SALDO). The Riparian
Forest Buffer should be defined and treated from the initial stages of the land development process,
similar to floodplain, wetland or any other primary conservation value. It is the municipality’s
responsibility to determine a fair and effective riparian forest buffer program, balancing the full range of
water resource and watershed objectives along with other land use objectives. A fair and effective
program should evolve for all municipal landowners and stakeholders. State-supported River
Conservation Plans, Act 167 Stormwater Management Plans, and other planning may contribute to this
effort.
Whether a respective municipality has included riparian forest buffers in its ordinances or not,
landowners/developers/applicants should include riparian forest buffers in their site plans from the
initiation of the site planning process. If standards and guidelines have been set forth by the
municipality or by other relevant planning group, these standards and guidelines should be followed. If
none of these exist, standards recommended in this manual should be followed.
The ease of accommodating a riparian forest buffer can be expected to vary based on intensity of land
use, zoning at the site and size of the parcel. Holding all other factors constant, as site size decreases,
the challenges posed by riparian zone accommodation can be expected to increase. As sites become
extremely small, reservation of site area for riparian forest buffer may become problematic, thereby
requiring riparian forest buffer modification in order to accommodate a reasonable building program for
the site. Zoned land use intensity is another factor to be considered. As this intensity increases and
specifications for maximum building area and impervious area and total disturbed area are allowed to
grow larger, reserving site area for the riparian forest buffer becomes more challenging. Riparian forest
buffer programs need to be sensitive to these constraints.

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All of these factors should be reviewed and integrated by the municipality as the riparian forest buffer
program is being developed.
Cost Issues
Costs of riparian forest buffer establishment are not significant, defined in terms of direct development.
In these cases, costs can be reasonably defined as the lost opportunity costs of not being able to use
acreage reserved for the riparian forest buffer in the otherwise likely land use. A likely land use might
be defined in terms of zoned land use. Depending upon the zoning category provisions and the degree
to which a riparian forest buffer’s Zone 1 or Zone 2 or Zone 3 might be able to be included as part of a
land development plan or as part of yard provisions for lots in a residential subdivision acreage included
within the riparian forest buffer may or may not be able to be included as part of the development. If
riparian acreage must be totally subtracted, then it’s fair value should be assessed as a cost. If riparian
forest buffers can be credited as part of yards (though still protected), then that acreage should not be
considered to be a cost. Any one-time capital cost can be viewed alternatively as an annualized cost.
To the extent that the riparian forest buffer coincides with the mapped and regulated floodplain, where
homes and other structures and improvements should not be located, then attributing any lost
opportunity costs exclusively to riparian forest buffers is not reasonable. The position can be argued
that any riparian forest buffer area, which is included within floodplain limits, should not be double-
counted as a riparian forest buffer cost. Alternatively, any riparian forest buffer area that extends
beyond the floodplain could be assigned a cost.
Lost opportunity costs can be expected to vary depending upon land use. Alternative layouts, including
reduced lot size configurations, may be able to provide the same or close to the same number of units
and the same level of profitability.
Over the long-term, some modest costs are required for periodic inspection of the riparian forest buffer
plus modest levels of maintenance. Generally, the buffers require very little in the way of operating and
maintenance costs.
If objective cost-benefit analysis were to be undertaken on most riparian forest buffers, results would be
quite positive, demonstrating that the full range of environmental and non-environmental benefits
substantially exceeds costs involved. Protection of already existing vegetated areas located adjacent
to streams, rivers, lakes, and other waterways is of tremendous importance, given their rich array of
functional benefits.
Stormwater Management Calculations
Stormwater calculations in most cases for Volume Control and Recharge and Peak Rate will not be
affected dramatically. See Chapter 8 for more discussion relating to Water Quality.

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Specifications
The Chesapeake Bay Program’s Riparian Handbook provides an in-depth discussion of establishing
the proper riparian forest buffer
width, taking into consideration:
1. existing or potential value of
the resource to be protected,
2. site, watershed, and buffer
characteristics,
3. intensity of adjacent land use,
and
4. specific water quality and/or
habitat functions desired.
(Handbook, p. 6-1)
At the core of the scientific basis for
riparian forest buffer establishment
are a variety of site-specific factors,
including: watershed condition,
slope, stream order, soil depth and
erodibility, hydrology, floodplains,
wetlands, streambanks, vegetation
type, and stormwater system, all of
which are discussed in the
Handbook. Positively, this body of
scientific literature has expanded
tremendously in recent years and provides excellent support for effective buffer management. The
downside is that this scientific literature now exceeds quick and easy summary. Fortunately, this
Handbook and many additional related references are available online without cost (given the
comprehensiveness of the Handbook itself, it is recommended that the reader start here).
Zone 1:
Also termed the “streamside zone,” this zone “…protects the physical and ecological integrity
of the stream ecosystem. The vegetative target is mature riparian forest that can provide shade, leaf
litter, woody debris, and erosion protection to the stream. The minimum width is 25 feet from each
streambank (approximately the distance of one or two mature trees from the streambank), and land use
is highly restricted….” (Handbook, p. 11-8)
Zone 2:
Also termed the “middle zone,” this zone”…extends from the outward boundary of the
streamside zone and varies in width depending on stream order, the extent of the 100-year flood plain,
adjacent steep slopes, and protected wetland areas. The middle zone protects key components of the
stream and provides further distance between upland development and the stream. The minimum
width of the middle core is approximately 50 feet, but it is often expanded based on stream order, slope
of the presence of critical habitats, and the impact of recreational or utility uses. The vegetative target
for this zone is also mature forest, but some clearing is permitted for stormwater management Best
Management Practices (BMPs), site access, and passive recreational uses….” (Handbook, p. 11-8)
Zone 3:
Also termed the “outer zone,” this zone “…is the ‘buffer’s buffer.’ It is an additional 25-foot
setback from the outward edge of the middle zone to the nearest permanent structure. In many urban
situations, this area is a residential backyard. The vegetative character of the outer zone is usually turf
or lawn, although the property owner is encouraged to plant trees and shrubs to increase the total width
Figure 5.2-4. Three zone urban buffer system (Schueler, 1995 and
Metropolitan COG, 1995).

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of the buffer… The only significant restrictions include septic systems and new permanent structures.”
(Handbook, p. 11-9)
The Handbook also provides more detailed specifications for riparian forest buffers (Appendix 1), as
developed by the USDA’s Forest Service.

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BMP 5.4.3: Protect/Utilize Natural Flow Pathways in Overall Stormwater

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Planning and Design
Identify, protect, and utilize the site’s natural drainage
features as part of the stormwater management system.
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
No
Yes
Yes
Yes
Key Design Elements
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/Med.
Low
Med./High
Medium
Water Quality Functions
TSS:
TP:
NO3:
30%
20%
0%
.
Identify and map natural drainage features (swales, channels,
ephemeral streams, depressions, etc.)
.
Use natural drainage features to guide site design
.
Minimize filling, clearing, or other disturbance of drainage
features
.
Utilize drainage features instead of engineered systems
whenever possible
.
Distribute non-erosive surface flow to natural drainage features
.
Keep non-erosive channel flow within drainage pathways
.
Plant native vegetative buffers around drainage features

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Description
Most natural sites have identifiable drainage features such as swales, depressions, watercourses,
ephemeral streams, etc. which serve to effectively manage any stormwater that is generated on the
site. By identifying, protecting, and utilizing these features a development can minimize its stormwater
impacts. Instead of ignoring or replacing natural drainage features with engineered systems that
rapidly convey runoff downstream, designers can use these features to reduce or eliminate the need for
structural drainage systems. Naturally vegetated drainage features tend to slow runoff and thereby
reduce peak discharges, improve water quality through filtration, and allow some infiltration and
evapotranspiration to occur. Protecting natural drainage features can provide for significant open
space and wildlife habitat, improve site aesthetics and property values, and reduce the generation of
stormwater runoff. If protected and used properly, natural drainage features generally require very little
maintenance and can function effectively for many years.
Variations
Natural drainage features can also be made more effective through the design process. Examples
include constructing slight earthen berms around natural depressions or other features to create
additional storage, installing check dams within drainage pathways to slow runoff, and planting
additional native vegetation.
Figure 5.3-1 Protect natural drainage features

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Applications
Use buffers to treat stormwater runoff.
Use natural drainage pathways instead of structural drainage systems
Figure 5.3-2 Section of buffer utilization
Figure 5.3-3 Section of buffer utilization
Figure 5.3-4 The natural surface can provide stormwater
drainage pathways

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Use natural drainage features to guide site design
Others…
Design Considerations
1. IDENTIFICATION OF NATURAL DRAINAGE FEATURES.
Identifying and mapping natural
drainage features is generally done as part of a comprehensive site analysis. This process is an
integral part of site design and is the first step for many of the non-structural BMPs described in this
Chapter.
2. NATURAL DRAINAGE FEATURES GUIDE SITE DESIGN.
Instead of imposing a two-dimensional
‘paper’ design on a particular site, designers can use natural drainage features to steer the site layout.
Drainage features can be used to define contiguous open space/undisturbed areas as well as road
alignment and building placement. The design should minimize disturbance to natural drainage
features and crossings of them. Drainage features that are to be protected should be clearly shown on
Figure 5.3-5 Natural drainage features can guide the design
Figure 5.3-6
Natural surface depressions can temporarily store
stormwater.

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all construction plans. Methods for protection, such as signage and fencing, should also be noted on
applicable plans.
3. UTILIZE NATURAL DRAINAGE FEATURES.
Natural drainage features should be used in place of
engineered stormwater conveyance systems wherever possible. Site designs should use and/or
improve natural drainage pathways to reduce or eliminate the need for stormwater pipe networks. This
can reduce costs, maintenance burdens, disturbance/earthwork related to pipe installation, and the size
of other stormwater management facilities. Natural drainage features should be protected from any
increased runoff volumes and rates due to development. The design should prevent the erosion and
degradation of natural drainage features through the use of upstream volume and rate control BMPs.
Level spreaders, erosion control matting, re-vegetation, outlet stabilization and check dams can also be
used to protect natural drainage features, where appropriate.
4. NATIVE VEGETATION.
Natural drainage pathways should be provided with native vegetative
buffers and the features themselves should include native vegetation where applicable. If drainage
features have been previously disturbed, they can be restored with native vegetation and buffers.
Detailed Stormwater Functions
Volume Reduction Calculations
Protecting/utilizing natural drainage features can reduce the volume of runoff in several ways.
Reducing disturbance and maintaining a natural cover can significantly reduce the volume of runoff
through infiltration and evapotranspiration. This will be self-crediting in site stormwater calculations
through lower runoff coefficients and/or higher infiltration rates. Utilizing natural drainage features can
reduce runoff volumes because natural drainage pathways allow infiltration to occur, especially during
smaller storm events. Encouraging infiltration in natural depressions also reduces stormwater
volumes. Employing strategies that direct non-erosive sheet flow onto naturally vegetated areas can
allow considerable infiltration. See Chapter 8 for volume reduction calculation methodologies.
Peak Rate Mitigation Calculations
Protecting/utilizing natural drainage features can reduce the anticipated peak rate of runoff in several
ways. Reducing disturbance and maintaining a natural cover can significantly reduce the runoff rate.
This will be self-crediting in site stormwater calculations through lower runoff coefficients, higher
infiltration rates, and longer times of travel. Using natural drainage features can lower discharge rates
significantly by slowing runoff and increasing on-site storage.
Water Quality Improvement
Protecting/utilizing natural drainage features can improve water quality through filtration, infiltration,
sedimentation, and thermal mitigation. See Chapter 8 for Water Quality Improvement methodologies.
Construction Issues
1. At the start of construction, natural drainage features to be protected should be flagged/fenced
with signage as shown on the construction drawings.
2. Non-disturbance and minimal disturbance zones should be strictly enforced.
3. Natural drainage features must be protected from excessive sediment and stormwater loads
while their drainage areas remain in a disturbed state.

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Maintenance Issues
Natural drainage features that are properly protected/utilized as part of site development should require
very little maintenance. However, periodic inspections and maintenance actions (if necessary) are
important. Inspections should assess erosion, bank stability, sediment/debris accumulation, and
vegetative conditions including the presence of invasive species. Problems should be corrected in a
timely manner. If native vegetation is being established it may require some support – watering,
weeding, mulching, replanting, etc. – during the first few years. Undesirable species should be
removed and desirable replacements planted if necessary.
Protected drainage features on private property should have an easement, deed restriction, or other
legal measure to prevent future disturbance or neglect. DEP has worked with the Pennsylvania Land
Trust Association (PALTA) to develop an easement template with guiding commentary for permanently
protecting forest riparian buffers. The model is tailored to protect a relatively narrow ribbon of land
along a waterway or lake. Presumably, the riparian buffers will most often comprise lands of severely
limited development potential and the landowner will not be seeking a charitable federal income tax
deduction.
In preparing the model, it was also assumed that landowners would be receiving no more than a
nominal sum for placing the restrictive covenants on their land. To promote landowner donation, the
model was drafted to be as brief as possible while providing core protections to forest riparian buffers.
The model with guiding commentary is available at
http://conserveland.org/model_documents/#riparian
PALTA is now offering landowners who use this model a grant of up to $6000 to cover associated costs
such as attorney’s fees.
Cost Issues
Protecting/utilizing natural drainage features generally results in a significant construction cost savings.
Protecting these features results in less disturbance, clearing, earthwork, etc. and requires less re-
vegetation. Utilizing natural drainage features can reduce the need and size of costly, engineered
stormwater conveyance systems. Together, protecting and utilizing drainage features can reduce or
eliminate the need for stormwater management facilities (structural BMPs), lowering costs even more.
Design costs may increase slightly due to a more thoughtful, site-specific design.
Specifications
Not applicable

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5.5 Cluster and Concentrate

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BMP 5.5.1: Cluster Uses at Each Site; Build on the Smallest Area

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Possible
As density is held constant, lot size is reduced,
disturbed area is decreased, and undisturbed open
space is increased.
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes*
Limited
Limited
Yes
No
Stormwater Functions
*Depending on site size, constraints and
other factors.
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Very High
Very High
Very High
Very High
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
.
Reduce total site disturbance/total site maintenance and increase
undisturbed open space by clustering proposed uses on a total site
basis through moving uses closer together (i.e., reducing lot size)
and/or through stacking uses (i.e., building vertically), even as
amount of use (i.e., gross density) is held constant as per existing
zoning (or any other gross density determination). As density is
held constant (Example A), lot size is reduced, disturbed area
decreases, and undisturbed open space increases (Example B).
.
Per lot values/prices may decline marginally; however,
development costs also decrease.
.
Cluster provisions may/may not be allowed by municipal zoning;
if no zoning exists, ability to cluster may not be clear (lacking
zoning, has the municipality in any way set standards for site uses,
gross densities of these uses, etc.?).
.
Pending answers to above questions, have lot sizes been
reduced to the minimum, given proposed uses? Given existing
ordinance provisions? Given other development feasibility factors
such as public water/sewer vs. on-site water and sewer and
others?
.
Is the applicant maximizing clustering as much as possible
legally?
.
Is the applicant maximizing clustering functionally within municipal
ordinance limits?
DNREC and Brandywine Conservancy, 1997)

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Description
See Key Design Elements.
Variations
Clustering can be mandated by a municipality as the so-called by-right provision of the zoning
district, rather than allowed as a zoning option.
Density bonus with reduced lot size. In some cases, when lot size is reduced, gross density
allowed at the site may be increased, in order to balance what might be lesser
values/profitability from smaller lots (Example C). Extent of bonus density is variable, becoming
larger as lot size reduction increases (net effect is to always reduce net disturbed area); density
bonuses may be made to increase as total undisturbed open space provisions are increased
(e.g., for every 10 percent increase in undisturbed open space being provided, density is
allowed to increase by 5 percent, and so forth; Example D).
Extreme Clustering in the form of the Growing Greener 4-Step Design Process which includes:
Step 1: Map of Primary and Secondary Conservation Areas; Step 2: Map of Potential
Development Area with Yield Plan, calculated as per allowed gross density; Step 3: Map of
Street and Trail Connection; Step 4: Map of Lot Lines
Applications
Residential Clustering:
Example A, shown in Figure 5.4-1: The kind of subdivision most frequently created in
Pennsylvania is the type which blankets the development parcel with house lots and
pays little attention to designing around the special features of the property. In this
example, the house placement avoids the primary conservation areas, but disregards
the secondary conservation features. Such a sketch can provide a useful estimate of a
site's capacity to accommodate new houses at the base density allowed under zoning-
and is therefore known as a "Yield Plan."
Figure 5.4-1 Conventional Development, (Source: Growing Greener: Putting
Conservation Into Local Codes. Natural Lands Trust, Inc., 1997)

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Example B, shown in Figure 5.4-2: Density-neutral with Pre-existing Zoning; 18 lots; Lot
Size Range: 20,000 to 40,000 sq. ft.; 50% undivided open space
Example C, shown in Figure 5.4-3: Enhanced Conservation and Density; 24 lots; Lot
Size Range: 12,000 to 24,000 sq. ft.; 60% undivided open space
Example D, shown in Figure 5.4-4: Hamlet or Village; 36 lots; Lot Size Range: 6,000 to
12,000 sq. ft.; 70% undivided open space
Figure 5.4-2 Clustered Development, (Source: Growing Greener: Putting
Conservation Into Local Codes. Natural Lands Trust, Inc., 1997)
Figure 5.4-3 Modest Density Bonus, (Source: Growing
Greener: Putting
Conservation Into Local Codes.
Natural Lands Trust, Inc., 1997)
Figure 5.4-4 Hamlet or Village, (Source: Growing
Greener: Putting Conservatio
n Into Local Codes.
Natural Lands Trust, Inc., 1997)

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Non-Residential Clustering:
Conventional Development
Preferred Vertical Neo-Traditional Development
Design Considerations
Objectives:
Maximize open space, especially when it includes sensitive areas (primary and secondary).
Maximize access to open space.
Maximize sense of place design qualities.
Balance infrastructure needs (sewer, water, roads, etc.)
Clustering should respond to a variety of site considerations. This BMP discussion assumes that
proper and effective work has been undertaken by the municipality to determine the proper site by site
land uses and the proper densities/intensities of these land uses. The question is then:
how can X
amount of Y uses be best clustered at a particular site
?
Detailed Stormwater Functions
Clustering, as defined here, is self-reinforcing. Clustering reduces total impervious areas, including
street lengths and total paved area and is likely to link with other BMPs, as defined in this Chapter,
including reduced imperviousness, reduced setbacks, reduced areas for drives and walkways, and so
forth. All of this directly translates into reduced volumes of stormwater being generated and reduced
peak rates of stormwater being generated, thereby benefiting stormwater planning. Additionally,
clustering translates into reduced disturbance and increased preservation of the natural landscape and
natural vegetative land cover, which further translates into reduced stormwater runoff, volume and
peak. To the extent that this clustering BMP also involves increased vertical development, net site roof
area and impervious area is reduced, holding number of units and amount of square footage of a use
constant. In all cases, density bonuses, if utilized, should be scrutinized to make sure that additional
density allowed is more than balanced by additional open space being provided, including further
reductions in street lengths, other impervious surfaces, other disturbed areas, and so forth.
Water quality is affected by non-point source pollutant load from impervious areas, as well as the
pollutant load from the newly created maintained landscape, much of which is soluble in form
(especially fertilizer-linked nitrogen forms). Clustering, alone and when combined with other Chapter 5
Non-Structural BMPs, minimizes impervious areas and the pollutant loads related to these impervious
areas. Similarly, clustering minimizes pollutant loads from lawns and other mowed areas. After
Chapter 5 BMPs are optimized, “unavoidable” stormwater is then directed into BMPs as set forth in
Chapter 6, to be properly treated. Chemical pollution prevention accomplished through Non-Structural
BMPs is especially important because Structural BMPs remain poor performers in terms of
mitigating/removing soluble pollutants that are especially problematic in terms of this pervious
maintained landscape. See Appendix A for additional documentation of the water quality benefits of
clustering.
See Chapter 8 for volume reduction calculation work sheets, peak rate reduction calculation work
sheets, and water quality mitigation work sheets.
Construction Issues
Application of this BMP clearly is required from the start of the site planning and development process.
Not only must the site owner/builder/developer embrace BMP 5.5.1 Cluster Uses at Each Site from the

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start of the process, the respective municipal officials must have included clustering in municipal codes
and ordinances, as is the case with so many of these Chapter 5 Non-Structural BMPs. Any areas to be
protected from development must be clearly marked in the field prior to the beginning of construction.
Maintenance Issues
As with all Chapter 5 BMPs, maintenance issues are of a different nature and extent then the more
specific Chapter 6 Structural BMPs. Typically, the primary issue is “who takes care of the open
space?” Legally, the designated open space may be conveyed to the municipality, although most
municipalities prefer not to receive these open space portions, including all of the maintenance and
other legal responsibilities associated with open space ownership. Ideally, open space reserves will
merge to form a unified open space system, integrating important conservation areas throughout the
municipality and beyond. In reality, these open space segments may exist dispersed and unconnected
for a considerable number of years. For those Pennsylvania municipalities that allow for and enable
creation of homeowners associations or HOA’s, the HOA, may assume ownership of the open space.
The HOA is usually the simplest solution to the “who takes care of the open space” question.
In contrast to some of the other long-term maintenance responsibilities of a new subdivision and/or land
development (such as maintenance of streets, water and sewers, play and recreation areas, etc.), the
maintenance requirements of “undisturbed open space” should be minimal. The objective here is
conservation of the natural systems already present, with minimal intervention and disturbance.
Nevertheless, invariably some legal responsibilities must be assumed and need to be covered.
Cost Issues
Clustering is beneficial from a cost perspective in several ways. Costs to build a single-family
residential development is less when clustered than when not clustered, holding the home type and all
other relevant infrastructure constant. Costs are decreased because of less land clearing and grading,
less road construction (including curbing), less sidewalk construction, less lighting and street
landscaping, potentially less sewer and water line construction, potentially less stormwater collection
system construction, and similar savings.
Clustering also reduces post construction costs. A variety of studies from the landmark
Costs of Sprawl
study and later updates have shown that delivery of a variety of municipal services such as street
maintenance, sewer and water services, and trash collection are more economical on a per person or
per house basis when development is clustered. Even services such as police protection are made
more efficient when residential development is clustered.
Additionally, clustering has been shown to positively affect land values. Analyses of market prices over
time of conventional development in contrast with comparable residential units in clustered
developments have indicated that clustered developments with their proximity to permanently protected
open space increase in value at a more rapid rate than conventionally designed developments, even
though clustered housing occurs on considerably smaller lots than the conventional residences.
Specifications
Clustering is not a new concept and has been defined, discussed, and evaluated in many different
texts, reports, references, sources, as set forth below.

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References
Arendt, Randall. Fall, 1991. “Cluster Development, A Profitable Way to Some Open Space.” In
Land
Development
.
Arendt, Randall. 1994.
Rural by Design
. Washington D.C.: Planners Press.
Brandywine Conservancy, Environmental Management Center. 2003.
Transfer of Development
Rights: A Flexible Option for Redirecting Growth in Pennsylvania
. Chadds Ford, PA.
Chesapeake Bay Program and Redman/Johnston Associates. 1997.
Beyond Sprawl: Land
Management Techniques to Protect the Chesapeake Bay, A Handbook for Local Governments.
Delaware Department of Natural Resources and Environmental Control and the Brandywine
Conservancy. 1997.
Conservation Design for Stormwater Management: A Design Approach to
Reduce Stormwater Impacts from Land Development.
Dover, DE
Delaware Riverkeeper. 2001.
Stormwater Runoff: Lost Resource of Community Asset?
Washington
Crossing, PA
Gottsegen, Amanda Jones. 1992.
Planning for Transfer of Development Rights: A Handbook for New
Jersey Municipalities
. Burlington County Board of Chosen Freeholders.
Greenbelt Alliance. 1996. “Factsheet: Urban Growth Boundaries.”
Hampton Roads Planning District Commission, 1992.
Vegetative Practices for Nonpoint Source
Pollution Prevention Management
.
Herson-Jones, Lorraine M. 1995.
Riparian Buffer Strategies for Urban Watersheds.
Metropolitan
Washington Council of Governments.
Lincoln Institute of Land Policy. 1995.
Alternatives to Sprawl.
Washington DC.
Maryland Office of Planning. 1995.
Managing Maryland’s Growth: Transfer of Development Rights.
Mauer, George. 1996.
A Better Way to Grow.
Chesapeake Bay Foundation.
National Association of Home Builders. 1982.
Cost Effective Site Planning.
Washington D.C.
Pennsylvania Environmental Council. 1992.
Guiding Growth: Building Better Communities and
Protecting our Countryside, A Planning and Growth Management Handbook for Pennsylvania
Municipalities. Philadelphia, PA
Porter, Douglas R. et al. 2000.
The Practice of Sustainable Development.
The Urban Land Institute.
Washington, D.C.
Report of the Pennsylvania 21
st
Century Commission, 1998.
Regional Plan Association and New York City Department of Environmental Protection, 1996
.
Managing Watersheds: Combining Water Quality Protection and Community Planning.
New York,
NY.

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Schueler, Thomas R. and Heather K. Holland. 2000.
The Practice of Watershed Protection:
Techniques for Protecting our Nation’s Streams, Lakes, Rivers and Estuaries.
Center for
watershed Protection Ellicott City, MD
Terrene Institute and the US Environmental Protection Agency. 1996.
A Watershed Approach to Urban
Runoff: Handbook for Decisionmakers.
Washington DC.
US Environmental Protection Agency. 1993.
Guidance Specifying Management Measures for Sources
of Nonpoint Pollution in Coastal Waters
840-B-92-002.

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BMP 5.5.2: Concentrate Uses Area wide through Smart Growth

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Practices
On a municipal, multi-municipal or areawide basis, use of "smart growth" planning techniques, including
neo-Traditional/New Urban planning principles, to plan and zone for concentrated development
patterns can accommodate reasonable growth and development. These practices direct growth to
areas or groups of parcels in the municipality that are most desirable and away from areas or groups of
parcels that are undesirable. BMP 5.5.2 can be thought of as Super Clustering that transcends the
reality of the many different large and small parcels that exist in most Pennsylvania municipalities.
Clustering parcel by parcel simply cannot accomplish the growth management that is so essential to
conserve special environmental and cultural values and protect special sensitivities. These smart
growth techniques include but are not limited to, transfer of development rights (TDR), urban growth
boundaries, effective agricultural zoning, purchase of development rights (PDR) by municipalities,
donation of conservation easements by owners, limited development and bargain sales by owners, and
other private sector landowner options. "Desirability" is defined in terms of environmental, historical
and archaeological, scenic and aesthetic, "sense of place," and quality of life sensitivities and values.
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
Limited
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Very High
Very High
Very High
Very High
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
.
Establish baseline growth and development context for the
municipality or multi-municipal area (how much of what by when
and where, using decade increments, plus ultimate build out).
.
On macro level (defined as municipality-wide, multi-municipality-
wide, areawide), define criteria for growth "desirability"
(opportunities) and "undesirability" (constraints) on a multi-site
and/or municipality-wide and/or areawide basis.
.
Apply these "desirability" and "undesirability" criteria.
.
Contrast baseline growth and development (first step) with third
step; highlight problems.
.
Apply smart growth techniques as needed to re-form "business
as usual" future to max out "desirability" and "undesirability"
performance. Techniques include: transfer of development rights
(TDR), urban growth boundaries, effective agricultural zoning,
purchase of development rights (PDR), donation of conservation
easements by owners, limited development and bargain sales by
owners, and other private sector landowner options.

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Variations
Because of the broadness of this BMP and its macro scale, variations in this BMP can be substantial.
Variations include: 1) how areas deemed to be desirable for growth are defined, whether clusters,
hamlets, villages, towns and/or cities; 2) how areas deemed undesirable for growth are defined
(conserving natural resources, agricultural lands and other vital resources); and 3) how any of this is
made to happen and what blend of smart growth techniques can be applied (where and when) to
implement 1 and 2.
1. Defining Desirable Growth – Opportunities for Growth: Clusters, Hamlets, Villages, Towns
and Cities
The vision for growth and development can take many different forms and can vary substantially
depending upon the respective municipality, group of municipalities, or area. Rural areas (Figure 5.5-1)
striving to preserve their rural character can concentrate development through adherence to building
onto or even creating Hamlets and Villages. If adjacent communities exist, development can be
directed into the town or at the town edge (Figure 5.5-2). Clustering (see BMP 5.5.1) on a site-by-site
basis is superior from a site perspective but yields a pattern that is less than optimal from a multi-site or
area wide perspective (Figure 5.5-3). However, this overall pattern is vastly preferable to the business
as usual approach across many different sites comprising the entire area (Figure 5.5-4).
Areas already developed and urbanized are likely to define appropriate in-fill development and re-
development at higher densities. Multiple community planning sources with specific community
building standards and specifications are available for reference. The importance of careful
definition of growth zones and the performance standards that define these growth zones cannot be
overemphasized. Often this BMP has been driven by environmental conservation objectives such
as saving the undesirable growth areas (Sending Zones in TDR parlance) as discussed below but
every bit as much care must be taken in defining and planning the desirable growth areas
(Receiving Zones).
Figure 5.5-1 Rural landscape of Pennsylvania

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Figure 5.5-2 Use of TDR to protect rural landscapes and direct development into the Town or Town Edge
Figure 5.5-3 Site clustering provides a partial open space network, though less than that
provided by TDR
Figure 5.5-4 Large lot zoning ignores natural and cultural resource values.

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2. Defining Undesirable Growth Areas – Constraints: High Value Watershed Areas, Agricultural
Areas, Eco-Sensitive Habitat Areas, Headwaters, and Stream Designations
Criteria used by a municipality or area for managing development may be expected to vary to some
extent. Municipalities may include special watershed areas, which have Pennsylvania Code
Chapter 93 Special Protection Waters designation (Exceptional Value and High Quality), as well as
critical headwater (first order streams) portions of watersheds. Source Water Protection zones may
exist, including areas of especially important groundwater recharge, or habitat areas where the
Pennsylvania Natural Diversity Inventory (PNDI) indicates especially important species presence.
Also, important wetlands, floodplains and other natural features may exist. Prime Agricultural
Lands and Agricultural Security Districts may be deserving of conservation. Areas may be
especially sensitive due to rugged topography or steep slopes. Areas may be sensitive due to
richness of historical and archaeological and even scenic values. All of these important values are
likely to extend well beyond individual parcel boundaries and require smart growth area wide growth
management techniques.
3. Mixing and Matching Smart Growth Techniques: Public and Private
If a municipality consists of only a handful of enormous parcels where BMP 5.5.1 Clustering can
work together to achieve the areawide “desirable growth” and “undesirable growth” patterns for the
entire municipality as described above, BMP 5.5.2 would be made unnecessary. Such is usually
not the case. A municipality may decide to use all or most of the smart growth techniques
discussed here. A municipality may decide that “less is more” and try to achieve its objectives with
the most simple growth management program possible, using the fewest techniques. The blend of
public techniques versus private techniques is also important. Most of what is involved here entails
public sector management action, such as zoning ordinance provisions. A few municipalities in
Pennsylvania (West Marlborough, Chester County) have achieved municipality-wide success
through private landowner actions, such as voluntary donation of conservation easements to
conservancies and land trusts.
The optimal blend of smart growth techniques is not easily determined. Each technique has pros
and cons, in terms of technical effectiveness, ease of implementation, political and socioeconomic
implications, and integration with the local culture. Municipalities may decide to hire a local
planning consultant (contact the Pennsylvania Planning Association for additional references), or
may decide to consult with a free or low cost information resource such as the Pennsylvania
Environmental Council or 10,000 Friends of Pennsylvania. The direct state government agency
contact is the Pennsylvania Department of Community and Economic Development. These
organizations and agencies offer a variety of planning resources by providing information on smart
growth techniques and their potential usefulness in any one particular municipal setting. The
organizations’ respective websites should be consulted for more detailed information.

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Applications
Transfer of Development Rights (TDR)
Transfer of Development Rights (TDR, see Figure 5.5-5)
is allowed as an option in Pennsylvania under the
Municipalities Planning Code. TDR creates an overlay
(Sending Zone) in the zoning ordinance where property
owners are allowed to sell development rights for
properties where growth is deemed to be less than
desirable for any number of reasons. In a second
created overlay zone (Receiving Zone), these
development rights that have been purchased may be
used to increase development density, above the
maximum baseline or conventional zoned density. TDR
has been in existence for some years and has been used
by a relatively small number of Pennsylvania
municipalities, although it has been used more widely in
New Jersey and several other states. Although TDR is created in the municipal zoning ordinance, all
TDR transactions or transfers of development rights may occur within the private sector, between
Sending Zone owners and Receiving Zone purchasers or developers. TDR has been used in
Buckingham Township (Bucks County), West Bradford and West Vincent Townships (Chester County),
Manheim and Warwick Townships (Lancaster County).
Growth Boundaries:
Growth Boundaries (Urban Growth Boundaries, see Figure 5.5-6) are based on the concept that
infrastructure such as public road systems and public water and wastewater treatment systems have a
powerful growth inducing and growth shaping influence
on an area wide basis. By controlling the location and
timing of this infrastructure through municipal or public
sector action, municipalities can encourage development
in certain areas and discourage development in others.
Growth Boundaries define where municipalities will
directly and indirectly encourage, and even provide
infrastructure services, significantly increasing zoned
densities. Areas lacking such infrastructure services are
zoned at significantly decreased densities. The State of
Oregon has been a leading advocate of Growth
Boundaries. Lancaster County for some years has been
applying Growth Boundary principles in its
comprehensive planning (go to their website to the
annual Growth Tracking reports which document how
their planning is achieving Growth Boundary objectives).
Effective Agricultural Zoning:
Large lot zoning (usually defined as zoning that requires average lot size to be greater than 2 acres per
lot) has been rejected by Pennsylvania courts as exclusionary and unacceptable. However, very large
minimum lot size to maintain existing agricultural uses has been deemed to be acceptable by
Pennsylvania courts and is being practiced throughout Pennsylvania, especially in intensive agricultural
communities in southcentral Pennsylvania (e.g., multiple municipalities in Adams, Berks, Chester,
Lancaster, York, etc.). Effective agricultural zoning may take the form of a specified mapped zoning
category with a minimum lot size of 10,15, 20, or 25 acres (this varies). Sliding scale agricultural
Figure 5.5-6 Example of Urban Growth Boundary
Figure 5.5-5 Example of Transfer of Development Rights

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zoning is a popular variation, where additional lots to be created and subdivided are a function of the
size of the total agricultural tract (though gross density remains very low). The intent is to allow a small
number of lots to be created over time, possibly for family members or for agricultural workers, but to
keep the functioning farms as intact as possible without residential subdivision or any other
development intrusion. The concept here is that the so-called “highest and best use of the land” is
agricultural use, which will be best maintained through protection of the farming community and through
this very low-density zoning. Application of Agricultural Zoning has been restricted to areas where
agriculture can be defined explicitly, typically in the presence of prime farmland soils, intensive
agricultural activity, formation of Agricultural Security Districts, or other indicators of important
agricultural activity. Obviously, this smart growth technique has limited application in terms of a growth
management technique.
Purchase of Development Rights:
Similar to TDR, the concept of Conservation Easements hinges on the notion that development rights
for any particular property can be defined and separated from a property. These development rights
can then be purchased and in a sense retired from the open market. The Pennsylvania Farmland
Preservation Program, which purchases development rights from existing agricultural owners and
allows farmers to continue their ownership and their agricultural activities, has become one of the most
successful agricultural preservation programs in the country. This program is highly competitive and
obviously limited to agricultural properties and contexts. The Farmland Preservation Program is a
priority of the current administration, will continue to be funded, and has been reinforced in several
counties with county-funded farmland preservation programs in order to stretch the state dollars.
Some counties (Bucks, Chester, Montgomery Counties) and municipalities (North Coventry, East
Bradford, Pennsbury, Solebury, West Vincent and others) have enacted special open space and
recreation acquisition programs. They are funded in various ways (bond issues, real estate taxes,
small payroll taxes) to purchase additional county-owned and municipality-owned lands, for use as
active and passive recreation as well as open space conservation. These efforts can be used in
conjunction with TDR programs, whereby a municipality funds a revolving fund-supported land
development bank which purchases development rights from vulnerable and high priority properties in
Sending Zones. It later sells these development rights (Warwick Township in Lancaster County has
done this) to Receiving Zone developers.
Conservation Easements (Donation and Purchase): Brandywine Conservancy, Natural Lands
Trust, Western Pennsylvania Conservancy, Others
Similar to TDR, the concept of Conservation Easements hinges on the notion that development rights
for any particular property can be defined and separated from a property. These development rights
can then be donated to an acceptable organization to support the public’s health, safety and welfare, in
the form of a conservation easement which restricts the owner’s ability to develop the property in
perpetuity, regardless of municipal zoning. Historically, a major incentive for these conservation
easement donations has been the major tax benefits afforded such donations. Organizations such as
the Brandywine Conservancy, Natural Lands Trust, the Western Pennsylvania conservancy and many
others have protected thousands of acres of otherwise developable property in Pennsylvania through
privately donated conservation easements, with absolutely no public expenditure of funds.
Brandywine’s 30,000 acres of conservation easements in the Brandywine Creek Watershed is an
excellent case in point. Municipalities such as West Marlborough Township in Chester County have
large portions of their jurisdictions permanently conserved as the result of this Conservation Easement
program. Conservation Easements also can be purchased by a conservation organization or
government agency. National organizations such as the Nature Conservancy, the Trust for Public
Land, the Land Trust Alliance, and others are active in Pennsylvania and are excellent sources of
technical information relating to this smart growth technique. In parts of Pennsylvania, these larger

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organizations are helping fledgling local land trusts form and begin their important work of land
conservation.
Bargain Sale/Limited Development Options:
A variation on the donation of development rights through conservation easements is a “bargain sale,”
where a portion of the development rights value is donated (in the manner described above) but the
property owner still enjoys a return on his/her property. In any number of development-pressured
municipalities in Pennsylvania, fair market value for a large 100-acre farm to be developed as single-
family residences or some other use may reach 2 or 3 million dollars. The owner, beyond tax benefits,
may need a monetary settlement, though not in the order of 2 to 3 million dollars. In such cases, a
defined “bargain sale” might be arranged if a source of funds can be located to provide a partial
financial settlement for the owner. The owner benefits from an approved donation of the remainder of
the value that can reduce the owner’s tax bill. The property is conserved.
A further variation would be a limited development option wherein a substantially reduced development
program is developed which conserves much if not most of the property in question. An existing
farmstead or homestead is retained and the property owner may even retain this farmstead/homestead.
A much smaller number of lots surrounded by open space is carefully created; these lots typically
command a considerably higher value than would be the case for a conventional subdivision. A large
amount of open space is created and protected through a conservation easement, which may be
donated as well, providing further tax benefit. The outcome is that the property owner, after taxes, may
be almost as well off after a Limited Development approach to the property than would be the case with
a complete conventional “as of right” approach to development. If the Limited Development concept
has been prepared carefully, total property disturbance can be substantially reduced.
Sustainable Watershed Management and Water-Based Zoning: Green Valleys Association and
the Brandywine Conservancy
Design Considerations:
Objectives for BMP 5.5.2 resemble BMP 5.5.1, although they must be understood as municipality-wide,
rather than just site-wide:
Maximize open space, especially sensitive areas (primary and secondary) and areas of
special value.
Maximize “sense of place” design qualities where growth is desirable.
Balance infrastructure needs (sewer, water, roads, etc.) and use infrastructure to shape
desirable growth
BMP 5.5.2 relies on application of smart growth techniques. The specific optimal blend of these smart
growth techniques should respond to a variety of municipality characteristics and considerations. This
BMP discussion assumes that proper and effective work has been undertaken by the municipality to
determine the proper land uses and the proper densities/intensities of these land uses, municipality-
wide. The question is then: how can these uses – this future development - be best planned within the
municipality, achieving the best and most livable communities for the future, even as disruption to the
natural landscape is minimized?

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Detailed Stormwater Functions
Concentrating growth, as defined here, is self-reinforcing from a stormwater management perspective –
in terms of peak rate reduction, runoff volume reduction, and nonpoint source load reduction.
Concentrating growth reduces total impervious areas and is likely to link with other BMP’s in this
Section, including reduced imperviousness, reduced setbacks, reduced areas for drives and walkways,
etc. All of this directly translates into reduced volumes of stormwater being generated and reduced
peak rates of stormwater being generated, thereby benefiting stormwater planning. Additionally,
concentrating growth translates into reduced disturbance and increased preservation of the natural
landscape and natural vegetative land cover, which further translates into reduced stormwater runoff.
To the extent that this BMP also involves increased vertical development, net site roof area and
impervious area is reduced, holding number of units and amount of square footage of a use constant.
In all cases, density bonuses, if utilized in Receiving Zones, should be scrutinized to make sure that
additional density allowed is more than balanced by additional open space being provided, including
further reductions in street lengths, other impervious surfaces, other disturbed areas, and so forth. If
properly implemented, these smart growth techniques such as TDR and Growth Boundaries will almost
always translate into reduced total disturbed area and reduced total impervious area, even more
dramatically than non-structural techniques such as clustering.
Documentation of the positive water quality effects of area wide growth concentration, holding total
growth and development constant, is provided by the City of Olympia’s (Washington)
Impervious
Surface Reduction Study: Final Report 1995
. Holding population projected to 2015 constant, two
dramatically different scenarios of land development (a baseline pattern of low density unconcentrated
development reflecting recent development trends versus a concentrated pattern of increased density
development in and near existing developed areas) were defined. These were mapped (Figure 5.5-7)
and tested for a variety of stormwater-related impacts (total impervious area, total disturbed area,
stormwater generation, non-point source pollutant generation). The analysis results indicated that the
concentrated development scenario significantly reduced total impervious area. This was due to
significant reductions in impervious
surfaces being created in outlying rural
and low density areas and more
efficient utilization of impervious
surfaces already created in areas of
existing development. Other studies
focusing on concentrated growth
patterns have similarly confirmed
these relationships and further
documented a reduction in total
disturbed areas created, stormwater
being generated, and total non-point
source pollutant loads being
generated.
As stated above in BMP 5.5.1, water
quality issues include all the non-point
source pollutant load from impervious
areas, a well as all the pollutant load from the newly created maintained landscape (i.e., lawns and
other), much of which is soluble in form (especially fertilizer-linked nitrogen forms). Concentrating
growth as defined in BMP 5.5.2, and combined with other Chapter 5 Non-Structural BMP’s, minimizes
impervious areas and the pollutant loads related to these impervious areas. After Chapter 5 BMP’s are
optimized, “unavoidable” stormwater is then directed into BMP’s as set forth in Chapter 6, to be
Figure 5.5-7 Dispersed versus Concentrated Development at the Regional Scale,
(Source: “Impervious Surface Reduction Study”, City of Olympia, 1995)

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properly treated. Similarly, for all that non-point source pollutant load generated from the newly-created
maintained landscape and combined with other Chapter 5 Non-Structural BMP’s, minimizes pervious
areas and the pollutant loads related to these pervious areas, thereby reducing the opportunity for
fertilization and other chemical application. Prevention of water quality degradation accomplished
through Non-Structural BMP’s in Chapter 5 is especially important because Chapter 6 Structural BMP’s
remain poor performers in terms of mitigating/removing soluble pollutants that are especially
problematic in terms of this pervious maintained landscape. See Appendix A for additional
documentation of the water quality benefits of clustering.
See Chapter 8 for additional volume reduction calculation work sheets, additional peak rate reduction
calculation work sheets, and additional water quality mitigation work sheets.
Construction Sequence
Application of this BMP must be undertaken by the municipality and must precede the start of any
individual site planning and development process. In most cases, the municipality must take action in
its comprehensive plan and then in its zoning and SLDO to incorporate the optimal blend of these smart
growth techniques in their respective municipal planning and growth management program (the
proactive municipality may act further to program for use of conservation easements, creation of a local
land trust, and the like). At the same time, the site owner/builder/developer may elect to embrace
options set forth in BMP 5.5.2 Concentrate Uses Area wide from the start of the process. Use of
conservation easement donation, bargain sale or limited development all require careful consideration
by the site owner/builder/developer from the beginning of the site development process.
Maintenance Issues
Very few maintenance problems or issues are generated by BMP 5.5.2. Because most of these smart
growth techniques are preventive in nature and in fact translate into maximum retention of undisturbed
open space and the natural features contained within this open space, typically in private ownership,
specific maintenance requirements as defined in a conventional manner are extremely limited, if not
nonexistent.
Cost Issues
According to Delaware’s recent
Conservation Design for Stormwater Management: A Design Approach
to Reduce Stormwater Impacts from Land Development
, application of the municipality-wide or
areawide smart growth techniques will require some additional costs. Application of an optional TDR
program or Growth Boundary program could cost a municipality in technical planning fees, including
incorporation into the comprehensive plan and zoning ordinance (other costs may be required as well).
Although it is hard to specifically document, a program of structural BMP’s which mitigate adverse
impacts of land development and achieve the same level of water resource (quantity and quality)
performance throughout the municipality and its respective watershed areas becomes much more
difficult to achieve, and much more expensive when all development and all lots are tallied. Prevention
is simply much more cost effective.
Furthermore, BMP 5.5.2’s preventive smart growth techniques, when fully applied, achieve a level of
performance that exceed even the best structural BMP’s. This clearly demonstrates why non-structural
BMP’s are important for all Pennsylvania watersheds, but especially important for Special Protection
Waters where High Quality and Exceptional Value designations call for extremely high levels of water
resource protection. In these cases, significant amounts of development watershed-wide, even

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assuming use of Chapter 6 structural BMP’s, may fail to provide the water resource protection which is
needed to sustain special Protection Waters’ values over the long-term.
Specifications
BMP 5.5.2 is not a new concept and has been defined, discussed, and evaluated in many different
texts, reports, references, sources, as set forth below. More specifications for clustering can be found
in references that are included in above discussions.

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5.6
Minimize Disturbance and Minimize Maintenance

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BMP 5.6.1: Minimize Total Disturbed Area - Grading
Without changing the building program, you can reduce site grading, removal
of existing vegetation (clearing and grubbing) and total soil disturbance. This
eliminates the need for re-establishment of a new maintained landscape for
the site and lot-by-lot, by modifying the proposed road system and other
relevant infrastructure as well as the building location and elevations to better
fit the existing topography.
Water Quality Functions
TSS:
TP:
NO3:
40%
0%
0%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
High
High
High
High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Yes
Limited
Limited
.
Identify and avoid special value and environmentally sensitive
areas
.
Minimize overall disturbance at the site
.
Minimize disturbance at the individual lot level
.
Maximize soil restoration to restore permabilities
.
Minimize construction-traffic locations
.
Minimize stockpiling and storage areas

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Description
This Non-Structural BMP assumes that the special value and sensitive resource areas have been
identified on a given development parcel and have been protected, and that clustering and area wide
concentration of uses also have been considered and included in the site design. All of these BMPs
serve to reduce site grading and to minimize disturbance/minimize maintenance. This BMP specifically
focuses on how to minimize the grading and overall site disturbance required to build the desired
program while maximizing conservation of existing site vegetation.
Reduction of site disturbance by grading can be accomplished in several ways. The requirements of
grading for roadway alignment (curvature) and roadway slope (grade) frequently increase site
disturbance throughout a land development site and on individual lots. Most land development plans
are formulated in 2-dimensional plan, based on the potential zoned density, and seldom consider the
constraints presented by topographic variation (slope) on the site. The layout and design of internal
roadways on a land development site with significant topographic variation (slope) can result in
extensive earthwork and vegetation removal (i.e., grading). Far less grading and a far less disruptive
site design can be accomplished if the site design is made to better conform with the existing
topography and land surface, where road alignments strive to follow existing contours as much as
possible, varying the grade and alignment criteria as necessary to comply with safety limits.
Site design criteria have evolved in municipalities to make sure that developments meet safety
standards (sight distance, winter icing, and so forth) as well as certain quality or appearance standards.
A common perception among municipal officials is that little deviation should be allowed in order to
maintain the integrity of the community. In fact, roadway design criteria should be made flexible in
order to better fit a given parcel and achieve a more “fluid” roadway alignment. The avoidance of
sensitive site features, such as important woodlands,
may be facilitated through flexible roadway layout.
Additionally, rigorous parcel criteria (front footage,
property setbacks, etc.) often add to this “plane
geometry” burden. Although the rectilinear grid layout
is the most efficient in terms of maximizing the number
of potential lots created at a development site, the end
result is a “cookie cutter” pattern normally found in
residential sites and the “strip” development found in
most highway commercial districts, all of which are apt
to translate into significant resource loss.
From the perspective of a single lot, the municipally-
required conventional lot layout geometry can also
impose added earthwork and grading that could be
avoided. Lot frontage criteria, yard criteria, and driveway criteria force the placement of a structure in
the center of every lot, often pushed well back from the roadway. Substantial terracing of the lot with
added grading and vegetation removal is required in many cases. Although the intent of these
municipal requirements is to provide privacy and spacing between units, the end result is often totally
cleared, totally graded lots, which can be visually monotonous. Configuring lots in a rectilinear shape
may optimize the number of units but municipalities should require that the site design in total should be
made to fit the land as much as possible.
Municipal criteria that impose road geometry are usually contained within the subdivision and land
development ordinance (SALDO), while densities, lot and yard setbacks, and minimum frontages are
usually contained in the zoning ordinance. Variations in these land development standards should be
Figure 5.6-1 Residential Area with Disturbance Minimized

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accepted by the local government where appropriate, which should modify their respective ordinances.
Municipalities should consider being more flexible without compromising public safety in terms of:
Road vertical alignment criteria (maximum
grade or slope).
Road horizontal alignment criteria (maximum
curvature)
Road frontage criteria (lot dimensions)
Building setback criteria (yards dimensions)
Related Non-Structural BMPs, such as road width
dimensions, parking ratios, impervious surface
reduction, chemical maintenance of newly created
landscapes, and others are discussed as separate
BMPs in this Chapter, though are all substantially
interrelated.
Detailed Stormwater Functions
Volume Reduction Calculations:
Minimizing Total Disturbed Area can reduce the volume of
runoff in several ways. Reducing disturbance and maintaining a natural cover can significantly
reduce the anticipated volume of runoff through increased infiltration and increased
evapotranspiration. This practice will be self-crediting in site stormwater calculations through lower
runoff coefficients and/or higher infiltration rates. Minimizing Total Disturbed Area can reduce
anticipated runoff volumes because undisturbed areas of existing vegetation allow more infiltration
to occur, especially during smaller storm events. Furthermore, employing strategies that direct non-
erosive sheet flow onto naturally vegetated areas can allow considerable infiltration to occur and
can be coupled with level spreading devices (see Chapter 6) and possibly other BMPs to more
actively manage stormwater that cannot be avoided. In other words, Minimizing Total Disturbed
Area/Maintained Area through Reduced Site Grading (Designing with the Land) not only prevents
increased stormwater generation (a volume and peak issue), but also offers an opportunity for
managing stormwater generation that cannot be avoided. See Chapter 8 for volume reduction
calculation methodologies.
Peak Rate Mitigation Calculations:
Minimizing Total Disturbed Area/Maintained Area through
Reduced Site Grading (Designing with the Land) can reduce the peak rate of runoff in several ways.
Reducing disturbance and maintaining a natural cover can significantly reduce the runoff rate. This
will be self-crediting in site stormwater calculations through lower runoff coefficients, higher
infiltration rates, and longer times of travel. Minimizing Total Disturbed Area/Maintained Area
through Reduced Site Grading (Designing with the Land) can lower discharge rates significantly by
slowing runoff and increasing on-site storage.
Water Quality Improvement:
Minimizing Total Disturbed Area can improve water quality
preventively by reducing construction phase sediment-laden runoff. Water quality benefits also by
maximizing preservation of existing vegetation at a site (e.g., meadow, woodlands) where post-
construction maintenance including application of fertilizers and pesticides/herbicides is avoided.
Given the high rates of chemical application which have been documented at newly created
maintained areas for both residential and non-residential land uses, eliminating the opportunity for
chemical application is important for water quality – perhaps the most effective management
technique. In terms of water quality mitigative functions, Minimizing Total Disturbed Area provides
filtration and infiltration opportunities, assuming that undisturbed areas are being used to manage
Figure 5.6-2 Minimally Disturbed Development

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stormwater generated elsewhere on the development site, as well as thermal mitigation. See
Chapter 8 for Water Quality Improvement methodologies.
Design Considerations
During the initial conceptual design phase of a land development project, the applicant’s design
engineer should provide the following information, ideally through development of a Minimum
Disturbance/Minimum Maintenance Plan:
1. Identify and Avoid Special Value/Sensitive Areas (see BMP 5.4.1)
Delineate and avoid environmentally sensitive areas (e.g., Primary and Secondary Conservation
areas, as defined in BMP 5.4.1); delineation of Woodlands, broadly defined to include areas of
immature and mixed tree growth, is especially important; configure the development program on the
balance of the parcel (i.e., Development Areas as discussed in BMP 5.4.1).
2. Minimize Disturbance at Site
Modify road alignments (grades, curvatures, etc.), lots, and building locations to minimize grading,
earthwork, overall site disturbance, as necessary to maintain safety standards. Minimal disturbance
design shall allow the layout to best fit the land form without significant earthwork. The limit of
grading and disturbance should be designated on the plan documentation submitted to the
municipality for review/approval, and should be physically designated at the site during construction
by flagging, fencing, or other methods.
3. Minimize Disturbance at Lot
Limit lot grading to roadways and building footprints. Municipalities should establish Minimum
Disturbance/Minimum Maintenance Buffers, designed to be rigorous but reasonable in terms of
current feasible site construction practices. These standards may need to vary with the type of
development being proposed and the context of that development (the required disturbance zone
around a low density single-family home can be expected to be less than disturbance necessary for
a large commercial structure), given the necessity for use of different types of construction
equipment and the realities of different site conditions. For example, the U.S. Green Building
Council’s Leadership in Energy & Environmental Design Reference Guide (Version 2.0 June 2001)
specifies the following:
Figure 5.6-3 Woodlands Protected through Minimum Disturbance Practices

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…limit site disturbance including earthwork and clearing of vegetation to
40 feet
beyond
the building perimeter,
5 feet
beyond the primary roadway curbs, walkways, and main utility
branch trenches, and
25 feet
beyond pervious paving areas that require additional staging
areas in order to limit compaction in the paved area…”
Municipalities in New Jersey’s Pinelands Preservation Zone for years have supported ordinances
where limits are more restrictive than the LEED footages (e.g., clearing around single-family homes
is reduced to 25 feet). Again, such requirements can be made to be flexible with special site factors
and conditions. The limit of grading and disturbance should be designated on the plan
documentation submitted to the municipality for review/approval, and should be physically
designated at the lot during construction by flagging, fencing or other marking techniques.
4. Maximize Soil Restoration
Where construction activity does require grading and filling and where compaction of soil can be
expected, this disturbance should be limited. Soil treatments/amendments should be considered
for such disturbed areas to restore permeability. If the bulk density is not reduced following fill,
these areas will be considered semi-impervious after development and runoff volumes calculated
accordingly.
5. Minimize Construction Traffic Areas
Areas where temporary construction traffic is allowed should be clearly delineated and limited.
These areas should be restored as pervious areas following development through a required soil
restoration program.
Figure 5.6-4 Convential Development Versus Low Impact Development

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6. Minimize Stockpiling and Storage Areas
All areas used for materials storage during construction should be clearly delineated with the
surface maintained, and subject to a soil restoration program following development. For low-
density developments, the common practice of topsoil stripping might be unnecessary and should
be minimized, if not avoided.
Construction Issues
Most of the measures discussed above are part of the initial concept site plan and site design process.
Only those measures that restore disturbed site soils are related to the construction and post-
construction phase, and may be considered as avoidance of impacts.
Cost Issues
Cost avoidance as a result of reduced grading and earthwork should benefit the developer. This BMP
is considered to be self-crediting, given the benefits resulting from reduced costs. Cost issues include
reduced grading and related earthwork (see Site Clearing and Strip Topsoil and Stockpile below), as
well as reduced costs involved with site preparation, fine grading, and stabilization.
Calculation of reduced costs is difficult due to the extreme variation in site factors that will affect costs
(amount of grading, cutting/filling, haul distances for required trucking, and so forth). Some relevant
costs factors are as follows (as based on R.S. Means,
Site Work & Landscape Cost Data
, 2002):
Site Clearing
Cut & chip light trees to 6” diameter
$2,900/acre
Grub stumps and remove
$1,400/acre
Cut & chip light trees to 24” diameter
$9,700/acre
Grub stumps and remove
$5,600/acre
Strip Topsoil and Stockpile
Ranges from $0.52 to $1.78 / cy because of Dozer horse power, and ranges from ideal to
adverse conditions
Assuming 8” of topsoil, the price per sq. yd. is $0.12 – $0.40
Assuming 8” of topsoil, the price per acre is $560 – $1,936
Site Preparation, Fine Grading, Seeding
Fine grading w/ seeding $2.33 /sq. yd.
Fine grading w/ seeding $11,277 /acre
In sum, total costs appear to approximate $20,000 per acre and could certainly exceed that figure in
more challenging sites. Reducing graded and disturbed acreage clearly translates into substantial cost
reductions.
Stormwater Management Calculations
No calculations are applicable for this BMP.

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Specifications
The modification of road geometry is a site-specific issue, but in general any criteria that will result in
significant earthwork should be reconsidered and evaluated.

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BMP 5.6.2: Minimize Soil Compaction in Disturbed Areas
Minimizing Soil Compaction and Ensuring Topsoil Quality is the
practice of enhancing, protecting, and minimizing damage to soil
quality caused by land development.
Image Source: “Developing an Effective Soil Management Strategy: Healthy Soil Is At the Root
Of Everything”, Ocean County Soil Conservation District
Water Quality Functions
TSS:
TP:
NO3:
30%
0%
0%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Very High
Very High
High
Very High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
.
Protecting disturbed soils areas from excessive compaction
Yes
during construction
.
Minimizing large cleared areas and stockpiling of topsoil
.
Using quality topsoil
.
Maintaining soil quality after construction
.
Reducing the Site Disturbance Area through design and
construction practices
.
Soil Restoration for areas that are not adequately protected or
have been degraded by previous activities (Section 6)

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Description:
Soil is a physical matrix of weathered rock particles and organic matter that supports a complex
biological community. This matrix has developed over a long time period and varies greatly within the
state. Healthy soils, which have not been compacted, perform numerous valuable stormwater
functions, including:
Effectively cycling nutrients
Minimizing runoff and erosion
Maximizing water-holding capacity
Reducing storm runoff surges
Adsorbing and filtering excess nutrients, sediments, pollutants to protect surface and
groundwater
Providing a healthy root environment and creating habitat for microbes, plants, and animals
Reducing the resources needed to care for turf and landscape plantings
Once natural soils are overly compacted and permeability is drastically reduced, these functions are
lost and can never be completely restored (Hanks and Lewandowski, 2003). In fact, the runoff
response of vegetated areas with highly compacted soils closely resembles that of impervious areas,
especially during large storm events (Schueler, undated). Therefore this BMP is intended to prevent
compaction or minimize the degree and extent of compaction in areas that are to be “pervious”
following development.
Although erosion and sediment control practices are equally important to protect soil, this BMP differs
from them in that it is intended to reduce the area of soil that experiences excessive compaction during
construction activities.
Applications
This BMP can be applied to any land development that has existing areas of relatively healthy soil and
proposed “pervious” areas. If existing soils have already been excessively compacted, Soil Restoration
is applicable (Chapter 6).
Figure 5.7-1 Example of development with site compaction of soils

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Design Considerations
Early in the design phase of a project, the designer should develop a soil management plan based on
soil types and existing level of disturbance (if any), how runoff will flow off existing and proposed
impervious areas, areas of trees and natural vegetation that can be preserved, and tests indicating soil
depth and quality. The plan should clearly show the following:
1. Protected Areas.
Soil and vegetation disturbance is not allowed. Protection of healthy, natural
soils is the most effective strategy for preserving soil functions. Not only can the functions be
maintained but protected soil organisms are also available to colonize neighboring disturbed
areas after construction.
2. Minimal Disturbance Areas.
Limited construction disturbance occurs - soil amendments may
be necessary for such areas to be considered fully pervious after development. Areas to be
vegetated after development should be designated Minimal Disturbance Areas.
3. Construction Traffic Areas.
Areas where construction traffic is allowed - if these areas are to
be considered fully pervious following development, a program of Soil Restoration will be
required.
4. Topsoil Stockpiling and Storage Areas.
These areas should be protected and maintained and
are subject to Soil Restoration (including compost and other amendments) following
development.
5. Topsoil Quality and Placement.
Soil tests are recommended. Topsoil applied to disturbed
areas should meet certain parameters as shown in Appendix C. Adequate depth (4” minimum
for turf, more for other vegetation), organic content (5% minimum), and reduced compaction
(1400 kPa maximum) are especially important (Hanks and Lewandowski, 2001). To allow water
to pass from one layer to the other, topsoil must be “bonded” to the subsoil when it is reapplied
to disturbed areas.
The first two areas (Protected and Minimal Disturbance) should be made as large as possible, identified
by signage, and fenced off from construction traffic. Construction Traffic Areas should be as small as
practicable.
Figure 5.7-2 Example of site development with extreme soil compaction on steep slope

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Detailed Stormwater Functions
Volume Reduction Calculations
Minimizing Soil Compaction and Ensuring Topsoil Quality can reduce the volume of runoff by
maintaining soil functions related to stormwater and thereby increasing infiltration and
evapotranspiration. This can be credited in site stormwater calculations through lower runoff
coefficients and/or higher infiltration rates. See Chapter 8 for volume reduction calculation
methodologies.
Peak Rate Mitigation Calculations
Minimizing Soil Compaction and Ensuring Topsoil Quality can reduce the rate of runoff by
maintaining soil functions related to stormwater. This can be credited in site stormwater
calculations through lower runoff coefficients, higher infiltration rates, and/or longer times of travel.
See Chapter 8 for peak rate calculation methodologies.
Water Quality Improvement
Minimizing Soil Compaction and Ensuring Topsoil Quality can improve water quality through
infiltration, filtration, chemical and biological processes in the soil, and a reduced need for fertilizers
and pesticides after development. See Chapter 8 for Water Quality Improvement methodologies.
Construction Issues
1. At the start of construction, Protected and Minimal Disturbance Areas must be identified with
signage and fenced as shown on the construction drawings.
2. Protected and Minimal Disturbance Areas should be strictly enforced.
3. Protected and Minimal Disturbance Areas should be protected from excessive sediment and
stormwater loads while upgradient areas remain in a disturbed state.
4. Topsoil storage areas should be maintained and protected at all times. When topsoil is
reapplied to disturbed areas it must be “bonded” with the subsoil. This can be done by
spreading a thin layer of topsoil (2 to 3 inches), tilling it into the subsoil, and then applying the
remaining topsoil. Topsoil must meet certain requirements as detailed in Appendix C.
Maintenance Issues
Sites that have minimized soil compaction properly during the development process should require
considerably less maintenance than sites that have not. Landscape vegetation will likely be healthier,
have a higher survival rate, require less irrigation and fertilizer, and even look better.
Some maintenance activities such as frequent lawn mowing can cause considerable soil compaction
after construction and should be avoided whenever possible. Planting low-maintenance native
vegetation is the best way to avoid damage due to maintenance.
Protected Areas on private property could have an easement, deed restriction, or other legal measure
to prevent future disturbance or neglect.

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Cost Issues
Minimizing Soil Compaction and Ensuring Topsoil Quality generally results in a significant construction
cost savings. Minimizing soil compaction can reduce disturbance, clearing, earthwork, the need for Soil
Restoration, and the size and extent of costly, engineered stormwater management systems. Ensuring
topsoil quality can significantly reduce the cost of landscaping vegetation (higher survival rate, less
replanting) and landscaping maintenance.
Design costs may increase slightly due to a more thoughtful, site-specific design.
Specifications
Soil Restoration specifications can be found in Chapter 6.
References
Hanks, D. and Lewandowski, A.
Protecting Urban Soil Quality: Examples for Landscape Codes and
Specifications
. USDA-NRCS, 2003.
Ocean County Soil Conservation District.
Impact of Soil Disturbance during Construction on Bulk
Density and Infiltration in Ocean County, New Jersey
. 2001. Available at
http://www.ocscd.org/publications.shtml
as of May 2004.
Schueler, T. “The Compaction of Urban Soils,” Technical Note #107 from
Watershed Protection
Techniques
. 3(2): 661-665, undated.

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BMP 5.6.3: Re-Vegetate and Re-Forest Disturbed Areas, Using

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Native Species
Sites that require landscaping and re-vegetation
should select and use vegetation (i.e., native
species) that does not require significant
chemical maintenance by fertilizers,
herbicides, and pesticides.
Image: Rose Mallow, Bowman’s Hill Wildflower Preserve,
www.bhwp.org
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
50%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/Med.
Low/Med
Low/Med.
Very High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Yes
Yes
Limited
.
Preserve all existing high quality plant materials and soil mantle
wherever possible
.
Protect these areas during construction
.
Develop Landscape Plan using native species
.
Reduce landscape maintenance, especially grass mowing
.
Reduce or eliminate chemical applications to the site, wherever
possible
.
Reduce or eliminate fertilizer and chemical-based pest control
programs, wherever possible

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Description of BMP
Minimum Disturbance/Minimum Maintenance is comprised of two distinct steps, neither of which
involves structural BMPs. The first step is to preserve existing vegetation on the development site as
defined in BMP 5.6.1, so as to minimize the need for landscaping and re-vegetation. This BMP
emphasizes the second step - the selection and use of vegetation that does not require significant
chemical maintenance by fertilizers, herbicides and pesticides. Implicit in this BMP is the assumption
that native species have the greatest tolerance and resistance to pests and require less fertilization and
chemical application than non-native species. Landscape architects specializing in the local plant
community usually are able to identify a variety of species that meet these criteria.
The production of biomass, such as grass clippings, is a significant pollutant source for water quality (if
this biomass is not removed, over time this biomass decays and is converted to additional nutrient
sources which add to the water quality problem). Native grasses and other herbaceous materials that
do not require mowing are preferred. Because the selection of such materials begins at the concept
design stage, where lawns are avoided or eliminated and landscaping species selected, this Non-
Structural BMP can generally result in a site with reduced runoff volume and rate, as well as significant
nonpoint source load reduction/prevention.
A native landscape may take several forms in Pennsylvania, ranging from re-establishment of
woodlands to re-establishment of meadow. It should be noted that as this native landscape grows and
matures, the positive stormwater benefits relating to volume control and peak rate control increase and
these landscapes become much more effective in reducing runoff volumes than maintained landscapes
such as lawns.
The elimination of traditional lawnscapes as a site design element can be an extremely difficult BMP to
implement, given the extent to which the traditional lawn as an essential landscape design feature is
embedded in current national culture.
Additional information relating to native species and their use in landscaping is available through
PADCNR and its website: http://www.dcnr.state.pa.us/forestry/wildplant/native.aspx
Detailed Stormwater Functions
Volume Reduction Calculations
and
Peak Rate Calculations
are not affected substantially by this
BMP - at least in the short term. In the longer term, as species grow and mature, the runoff volume
production of more mature native species can reasonably be expected to be lower than a
conventionally maintained landscape (especially the conventionally mowed lawn). Native species are
customarily strong growers with stronger and denser root and stem systems, thereby generating less
runoff. If the objective is re-vegetation with woodland species, the longer-term effect is a significant
reduction in runoff volumes, with increases in infiltration, evapotranspiration, and recharge, when
contrasted with a conventional lawn planting. Peak rate reduction also is achieved. Similarly, meadow
re-establishment is also more beneficial than a conventional lawn planting, although not so much as the
woodland landscape. Again, these benefits are long term in nature and will not be forthcoming until the
species have had an opportunity to grow and mature (one advantage of the meadow is that this
maturation process requires considerably less time than a woodland area).
Water Quality Improvement
Minimizing Disturbance/Minimizing Maintenance through Use Native Species for Landscaping and Re-
Vegetation can improve water quality preventively by minimizing application of fertilizers and
pesticides/herbicides. Given the high rates of chemical application which have been documented at

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newly created maintained areas for both residential and non-residential land uses, eliminating the
opportunity for chemical application is important for water quality – perhaps the most effective
management technique. Of special importance here is the reduction in fertilization and nitrate loadings.
For example, Delaware’s
Conservation Design for Stormwater Management
lists multiple studies,
which document high fertilizer application rates, including both nitrogen and phosphorus, in newly
created landscapes in residential and non-residential land developments. Expansive lawn areas in low
density single-family residential subdivisions as well as large office parks – development which has and
continues to proliferate in Pennsylvania municipalities - typically receives intensive chemical
application, both fertilization and pest control, which can exceed application rates being applied to
agricultural fields. Avoidance of this nonpoint pollutant source is an important water quality objective.
See Chapter 8 for Water Quality Improvement methodologies.
Design Considerations
Native species is a broad term. Different types of native species landscapes may be created, from
meadow to woodland areas, obviously requiring different approaches to planting. In terms of woodland
areas, Delaware’s
Conservation Design for Stormwater Management
states, “…a mixture of young
trees and shrubs is recommended…. Tree seedlings from 12 to 18 inches in height can be used, with
shrubs at 18 to 24 inches. Once a ground cover crop is established (to offset the need for mowing),
trees and shrubs should be planted on 8-foot centers, with a total of approximately 430 trees per acre.
Trees should be planted with tree shelters to avoid browse damage in areas with high deer populations,
and to encourage more rapid growth.” (p.3-50). As tree species grow larger, both shrubs and ground
covers recede and yield to the more dominant tree species. The native tree species mix of small
inexpensive saplings should be picked for variety and should reflect the local forest communities.
Annual mowing to control invasives may be necessary, although the quick establishment of a strong-
growing ground cover can be effective in providing invasive control. Native meadow planting mixes
also are available. A variety of site design factors may influence the type of vegetative community,
which is to be planned and implemented. In so many cases, the “natural” vegetation of Pennsylvania’s
communities is, of course, woodland.
Native species plantings can achieve variation in landscape across a variety of characteristics, such as
texture, color, and habitat potential. Properly selected mixes of flowering meadow species can provide
seasonal color; native grasses offer seasonal variation in texture. Seed production provides a food
source and reinforces habitat. In all cases, selection of native species should strive to achieve species
variety and balance, avoiding creation of single-species or limited species “monocultures” which pose
multiple problems. In sum, many different aspects of native species planting reinforce the value of
native landscaping, typically increasing in their functional value as species grow and mature over time.
Maintenance Issues
Although many conventional landscape management requirements are made unnecessary with this
BMP, Using Native Species for Landscaping and Re-Vegetation can be expected to require some level
of management – especially in the short term immediately following installation. Woodland areas
planted with a proper cover crop can be expected to require annual mowing in order to control
invasives. Application of a carefully selected herbicide around the protective tree shelters/tubes may
be necessary, reinforced by selective cutting/manual removal, if necessary. This initial maintenance
routine is necessary for the first 2 to 3 years of growth and may be necessary for up to 5 years until tree
growth and tree canopy begins to form, naturally inhibiting weed growth. Once shading is adequate,
growth of invasives and other weeds will be naturally prevented, and the woodland becomes self-
maintaining. Review of the new woodland should be undertaken intermittently to determine if
replacement trees should be provided (some modest rate of planting failure is typical). Meadow

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management is somewhat more straightforward; a seasonal mowing may be required, although care
must be taken to make sure that any management is coordinated with essential reseeding and other
important aspects of meadow re-establishment.
Construction Issues
During the initial conceptual design phase of a project, the design engineer should develop a Minimum
Disturbance/Minimum Maintenance concept plan that includes the following:
Areas of Existing Vegetation Being Preserved
Areas to Be Re-Vegetated/Landscaped by Type (i.e., Native Species Woodland, Meadow, etc.
plus Non-Native Conventional Areas)
A landscape maintenance plan that avoids/minimizes mowing and other maintenance, except
for limited areas of high visibility, special needs, etc.; specific landscape areas not to receive
fertilization and other chemical applications should be identified in plan documentation
This information needs to appear on the plan drawings and receive municipal review and approval.
Existing Vegetation Being Preserved must be flagged or fenced in the field. In terms of specific
construction sequencing, all plantings including native species should be installed during the final
construction phase of the project. Because native species plantings are likely to have a less “finished”
appearance than conventionally landscaped areas, additional field identification for these areas through
flagging or fencing similar to Existing Vegetation Being Preserved should be considered.
Cost Issues
BMP 5.6.3 cost implications are minimal during construction. Seeding for installation of a conventional
lawn is likely to be less expensive than planting of a “cover” of native species, although when
contrasted with a non-lawn landscape, “natives” often are not more costly than other non-native
landscape species. In terms of woodland creation, somewhat dated (1997) costs have been provided
by the
Chesapeake Bay Riparian Handbook: A Guide for Establishing and Maintaining Riparian Forest
Buffers
:
$860/acre trees with installation
$1,600/acre tree shelters/tubes and stakes
$300/acre for four waterings on average
Current values may be considerably higher, well over $3,000/acre for installation costs. Costs for
meadow re-establishment are lower than those for woodland, in part due to the elimination of the need
for shelters/tubes. Again, such costs can be expected to be greater than installation of conventional
lawn (seeding and mulching), although the installation cost differences diminish when conventional
lawn seeding is redefined in terms of conventional planting beds.
Cost differentials grow greater when longer term operating and maintenance costs are taken into
consideration. If lawn mowing can be eliminated, or even reduced significantly to a once per year
requirement, substantial maintenance cost savings result, often in excess of $1,500 per acre per year.
If chemical application (fertilization, pesticides, etc.) can be eliminated, substantial additional savings
result with use of native species. These reductions in annual maintenance costs resulting from a native
landscape re-establishment very quickly outweigh any increased installation costs that are required at
project initiation. Unfortunately, because developers pay for the installation costs and longer term

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reduced maintenance costs are enjoyed by future owners, there is reluctance to embrace native
landscaping concepts.
Stormwater Management Calculations
See Chapter 8 for calculations.
References
Bowman’s Hill Wildflower Preserve, Washington Crossing Historic Park, PO Box 685, New Hope, PA
18938-0685, Tel (215) 862-2924, Fax (215) 862-1846, Native plant reserve, plant sales, native
seed, educational programs, www.bhwp.org
Morris Arboretum of the University of Pennsylvania; 9414 Meadowbrook Avenue, Philadelphia, PA
19118, Tel (215) 247-5777, www.upenn.edu/morris, PA Flora Project Website: Arboretum and
gardens (some natives), educational programs, PA Flora Project, www.upenn.edu/paflora
Pennsylvania Department of Conservation and Natural Resources; Bureau of Forestry; PO Box 8552,
Harrisburg, PA 17105-8552, Tel (717)787-3444, Fax (717)783-5109, Invasive plant brochure; list of
native plant and seed suppliers in PA; list of rare, endangered, threatened species.
Pennsylvania Native Plant Society, 1001 East College Avenue, State College, PA 16801
www.pawildflower.org
Western Pennsylvania Conservancy; 209 Fourth Avenue, Pittsburgh, PA 15222, Tel (412) 288-2777,
Fax (412) 281-1792, www.paconserve.org

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5.7
Reduce Impervious Cover

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BMP 5.7.1: Reduce Street Imperviousness
Reduce impervious street areas by
minimizing street widths and lengths
.
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Very High
Very High
Very High
Medium
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Yes
Limited
Limited
.
Evaluate traffic volume and on-street parking requirements.
.
Consult with local fire code standards for access requirements.
.
Minimize pavement by using alternative roadway layouts,
restricting on-street parking, minimizing cul-de-sac radii, and using
permeable pavers.

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Description
Reducing impervious street areas performs valuable stormwater functions, in contrast to conventional
or baseline development. Some of these functions are increasing infiltration, decreasing stormwater
runoff volume, increasing stormwater time of concentration, improving water quality by decreasing the
pollutant loading of streams, improving natural habitats by decreasing the deleterious effects of
stormwater runoff and decreasing the concentration and energy of stormwater. Imperviousness greatly
influences stormwater runoff volume and quality by facilitating the rapid transport of stormwater and
collecting pollutants from atmospheric deposition, automobile leaks, and additional sources. Increased
imperviousness alters an area’s hydrology, habitat structure, and water quality. Stream degradation has
been witnessed at impervious levels as low as 10-20% (Center for Watershed Protection, 1995).
Applications
Street Width
Streets comprise the largest single component of imperviousness in residential design. Universal
application of high-volume, high-speed traffic design criteria results in many communities requiring
excessively wide streets. Coupled with the perceived need to provide both on-street parking and
emergency vehicle access, the end result of these requirements is residential streets that may be 36
feet or greater in width (Center for Watershed Protection, 1998).
The American Society of Civil Engineers (ASCE) and the American Association of State Highway and
Transportation Officials (AASHTO) recommend that low traffic volume roads (less than 50 homes or
500 daily trips) can be as narrow as 22 feet. PennDot Pub. 70 gives a range of 18-22 foot width for low
volume local roads. Some municipalities have reduced their lowest trafficable residential roads to 18
feet or less. Higher volume roads are recommended to be wider. Table 5.7-1 provides sample road
widths from different jurisdictions.
The desire for adequate emergency vehicle access, notably fire trucks, also leads to wider streets.
While it is perceived that very wide streets are required for fire trucks, some local fire codes permit
roadway widths as narrow as 18 feet (as shown in Table 5.7-2). Concerns also exist about other
vehicles and maintenance activities on narrow streets. School buses are typically nine feet wide from
mirror to mirror; Prince George’s and Montgomery Counties in Maryland require only a 12-foot driving
lane for buses (Center for Watershed Protection, 1998). Similarly, trash trucks require only a 10-½ foot
driving lane, as they are a standard width of nine feet (Waste Management, 1997; BFI, 1997). In some
cases, road width for emergency vehicles may be added through use of permeable pavers for roadway
shoulders (see Figure 5.7-1).
Snow removal on narrower streets is readily accomplished with narrow, 8-foot snowplows. Restricting
parking to one side of the street allows accumulated snow to be piled on the other side. Safety
concerns are also cited as a justification for wider streets, but increased vehicle-pedestrian accidents
on narrower streets are not supported by research. The Federal Highway Administration states that
narrower streets reduce vehicle travel speeds, decreasing the incidence and severity of accidents.
Higher density developments require wider streets, but alternative layouts can minimize street widths.
For example, in instances where on-street parking is desired, impervious pavement is used for the
travel lanes and permeable pavers are placed on the road apron for the parking lanes. The width of
permeable pavers is often the width of a standard parking lane (six to eight feet). This design approach
minimizes impervious area while also providing an infiltration and recharge area for the impervious
roadway stormwater (Prince George’s County, Maryland, 2002).

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Jurisdiction
Residential Street Pavement
Width
Maximum Daily Traffic
(trips/day)
20 ft. (no parking)
0-3,500
28 ft. (parking on one side)
0-3,500
12 ft. (alley)
---
21 ft. (parking on one side)
---
Howard County, Maryland
24 ft. (parking not regulated)
1,000
Charles County, Maryland
24 ft. (parking not regulated)
---
Morgantown, West Virginia
22 ft. (parking on one side)
---
20 ft.
150
20 ft. (no parking)
350-1,000
22 ft. (parking on one side)
350
26 ft. (parking on both sides)
350
26 ft. (parking on one side)
500-1,000
12 ft (alley)
---
16-18 ft. (no parking)
200
20-22 ft. (no parking)
200-1,000
26 ft. (parking on one side)
200
28 ft. (parking on one side)
200-1,000
(Cohen, 1997; Bucks County Planning Commission, 1980; Center for Watershed Protection, 1998)
Bucks County, Pennsylvania