1. 5.1Introduction……………………………………………………………………………………...1
    2. 5.3Non-Structural BMPs and Stormwater Methodologi
    3. 5.4Protect Sensitive and Special Value Resources
    4. 5.5Cluster and Concentrate
      1. _
        1. _
          1. _
      2. 5.6Minimize Disturbance and Minimize Maintenance
        1. BMP 5.6.1Minimize Total Disturbed Area – Grading………
    5. 5.7Reduce Impervious Cover
    6. 5.8Disconnect/Distribute/Decentralize
      1. _
        1. _
          1. _
    7. Introduction
    8. The terms “Low Impact Development” and “Conserva
    9. From a developer’s perspective, these practices c
    10. 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 qualit
    11. As described in Chapter 4, identifying a site’s n
    12. Protect Sensitive and Special Value Features
    13. Cluster and Concentrate
      1. Minimize Disturbance and Minimize Maintenance
    14. Reduce Impervious Cover
    15. Disconnect/Distribute/Decentralize
      1. _
        1. _
          1. Description
          2. Variations
    16. Applications
      1. _
        1. _
          1. Design Considerations
          2. Detailed Stormwater Functions
          3. Construction Issues
          4. Maintenance Issues
          5. Cost Issues

Pennsylvania Stormwater
Best Management Practices
Manual
Chapter 5
Non-Structural BMPs
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 5
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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Pennsylvania Stormwater Best Management Practices Manual
Chapter 5
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.
Source: Growing Greener: Putting Conservation Into Local Codes; Natural Land Trusts, Inc. 1997
Figure 5.1-2. Growing Greener’s Conservation
Subdivision Design: Step One, Part Two – Identify
secondary conservation areas.
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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.
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
• 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.
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Applications
Figure 5.1-4. Steep slope development with woodland
removal
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
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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.
Figure 5.1-5. Example of steep slope development.
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.
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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.
Figure 5.1-6. Woodland removal for steep slope
development with retaining walls
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
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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.
Figure 5.2-1. Riparian buffer zones support various ecological functions.
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.
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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.
Figure 5.2-2. Riparian buffer zones (DJ Welsh, 1991).
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.
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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).
Figure 5.2-4. Three zone urban buffer system (Schueler, 1995 and
Metropolitan COG, 1995).
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
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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
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.
Figure 5.3-1 Protect natural drainage features
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.
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Applications
Use buffers to treat stormwater runoff.
Figure 5.3-2 Section of buffer utilization
Figure 5.3-3 Section of buffer utilization
Use natural drainage pathways instead of structural drainage systems
Figure 5.3-4 The natural surface can provide stormwater
drainage pathways
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Use natural drainage features to guide site design
Figure 5.3-5 Natural drainage features can guide the design
Others…
Figure 5.3-6
Natural surface depressions can temporarily store
stormwater.
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
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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
Possible
DNREC and Brandywine Conservancy, 1997)
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?
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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 Conservation 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
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).
Figure 5.5-1 Rural landscape of Pennsylvania
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).
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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 u
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).
sed
Figure 5.5-5 Example of Transfer of Development Rights
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).
s 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:
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
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
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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 r
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.
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.
ural
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
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.
Figure 5.6-1 Residential Area with Disturbance Minimized
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
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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.
Figure 5.6-2 Minimally Disturbed Development
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
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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)
Figure 5.6-3 Woodlands Protected through Minimum Disturbance Practices
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:
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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.
Figure 5.6-4 Convential Development Versus Low Impact Development
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.
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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.
Figure 5.7-2 Example of site development with extreme soil compaction on steep slope
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.
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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
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
Table 5.7-1: Narrow Residential Street Widths
State of New Jersey
State of Delaware
Boulder, Colorado
Figure 5.7-1 Reduced road width using adjacent pervious strips.
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Source
Residential Street Width
U.S. Fire Administration
18-20 ft.
16 ft. (no on-street parking)
24 ft. (on-street parking)
Virginia State Fire Marshall
18 ft. minimum
24 ft. (no parking)
30 ft. (parking on one side)
36 ft. (parking on both sides)
20 ft. (fire truck access)
18 ft. (parking on one side)
26 ft. (parking on both sides)
(Adapted from Center for Watershed Protection, 1998)
Baltimore County, Maryland Fire Department
Prince George’s County, Maryland Department of
Environmental Resources
Portland, Oregon Office of Transportation
Table 5.7-2 Fire Vehicle Street Requirements
In residential neighborhoods, the perception of the need for large quantities of parking may lead
developers to provide on-street parking; residential land use will greatly influence the quantity needed.
Each on-street lane increases street impervious cover by 25%. Many communities require 2-2.5
parking spaces per residence. In single-lot neighborhoods, with both standard and reduced setbacks,
parking requirements can likely be met using private driveways and garages. In townhouse
communities, if on-street parking is required, providing one on-street space per residence is likely
sufficient. Urban settings will require the greatest use of on-street parking. However, continuous parking
lanes on both sides of the street, while common for all residential land uses, is often unnecessary.
When on-street parking is necessary, queuing lanes provide a parking system alternative that
minimizes imperviousness. Communities are using queuing lanes to narrow roads while also providing
two-way traffic access. In a queuing lane design, one traffic lane is used by moving traffic and the
parking lanes allow oncoming traffic to pull over and let opposite traffic pass (Center for Watershed
Protection, 1998). Figure 5.7-2 shows traditional and queuing lane designs.
Street Length
Numerous factors influence street length including clustering techniques (discussed in a separate
Chapter). As with street width, street length greatly impacts the overall imperviousness of a developed
site. While no one prescriptive technique exists for reducing street length, alternative street layouts
should be investigated for options to minimize impervious cover.
Cul-de-sacs
The use of cul-de-sacs introduces large areas of imperviousness into residential developments, with
some communities requiring the cul-de-sac radius to be as large as 50 to 60 feet. In most instances,
and in large radius cul-de-sac designs especially, the full area of the circle is neither necessary nor
utilized. When cul-de-sacs are necessary, two primary alternatives can reduce their imperviousness.
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Figure 5.9-2 Traditional Streets vs. Traffic Queuing (Portland, Oregon Office of Transportation, 1994)
The first alternative is to reduce the required radius of the cul-de-sac. Many jurisdictions have identified
required turnaround radii (shown in Table 5.7-3).
A second alternative is to incorporate a landscaped island into the center of the cul-de-sac. This design
approach provides the necessary turning radius, minimizes impervious cover, and provides an
aesthetic amenity to the community. In some instance, developments are placing bioretention cells
(discussed in Chapter 6) in the center of cul-de-sacs to not only reduce imperviousness, but also
provide a distributed method of treating stormwater runoff. Other cul-de-sac configurations have been
developed which reduce impervious area.
Cost Issues
Street Width
Costs for paving have been estimated to be approximately $15/yd
2
(Center for Watershed Protection,
1998). At this cost, for each one-foot reduction in street width, estimated savings are $1.67 per linear
foot of paved street. For example reducing the width of a 500-foot road by 5 feet would result in a
savings of over $4,100. This cost is exclusive of other construction costs including grading and
infrastructure.
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Street Length
In addition to pavement, costs for street lengths, including traditional curb and gutter and stormwater
management controls, are approximately $150 per linear foot of road (Center for Watershed Protection,
1998). Decreasing road length by 100 feet can produce a savings of $15,000. Simply factoring in
pavement costs at $15/yd
2
, a 100-foot length reduction in a 25-foot wide road would produce a savings
in excess of $4,000.
Source
Residential Street Width
Portland, Oregon Office of Transportation
35 ft. (with Fire Deaprtment Approval)
Buck County, Pennsylvania Planning Commission
38 ft. (outside turning radius)
Fairfax County, Virginia Fire and Rescue
45 ft.
Baltimore County, Maryland Fire Department
35 ft. (with Fire Deaprtment Approval)
Montgomery County, Maryland Fire Department
45 ft.
Prince George’s County, Maryland Fire Department
43 ft.
(Adapted from Center for Watershed Protection, 1998)
Table 5.7-3: Example Cul-de-sac Turnaround Radii
Figure 5.7-3 Five Turnaround Options for the end of a Residential Street, (“Better Site Design: A Handbook
for Changing Development Rules in Your Community”, Center for Watershed Protection, August, 1998)
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BMP 5.7.2: Reduce Parking Imperviousness
Reduce imperviousness by minimizing imperviousness associated
with parking areas.
Water Quality Functions
TSS:
TP:
NO3:
Preventive
Preventive
Preventive
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Very High
Very High
Very High
High
Stormwater Functions
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Yes
Limited
Limited
.
Evaluate parking requirements considering average demand as
well as peak demand.
.
Consider the application of smaller parking stalls and/or compact
parking spaces.
.
Analyze parking lot layout to evaluate the applicability of
narrowed traffic lanes and slanted parking stalls.
.
Where appropriate, minimize impervious parking area by utilizing
overflow parking areas constructed of pervious paving materials.
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Description
Reducing parking imperviousness performs valuable stormwater functions in contrast to conventional or
baseline development: 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;
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
In commercial and industrial areas, parking lots comprise the largest percentage of impervious area.
Parking lot size is dictated by lot layout, stall geometry, and parking ratios. Modifying all or any of these
three aspects can serve to minimize the total impervious areas associated with parking lots.
Parking Ratios
Parking ratios express the specified parking requirements provided for a given land use. These
specified ratios are often set as minimum requirements. Many developers seeking to ensure adequate
parking provide parking in excess of the minimum parking ratios. Additionally, commercial parking is
often provided to meet the highest hourly demand of a given site, which may only occur a few times per
year. Excess parking is often rationalized by the desire to avoid potential complaints from patrons that
have difficulty finding parking. However, as shown in Table 5.7-4, average parking demand is generally
less than typical required parking ratios and therefore much less than parking provided in excess of
these ratios. The result of using typically specified parking ratios is parking capacity that is
underutilized.
Land Use
Parking Ratio
Average Parking Demand
Single Family Home
2 spaces per dwelling unit
1.1 spaces per dwelling unit
Shopping Center
5 spaces per 1,000 ft
2
of GFA
3.97 spaces per 1,000 ft
2
of GFA
Convenience Store
3.3 spaces per 1,000 ft
2
of GFA
Not available
Industrial
1 space per 1,000 ft
2
of GFA
1.48 spaces per 1,000 ft
2
of GFA
Medical/Dental Office
5.7 spaces per 1,000 ft
2
of GFA
4.11 spaces per 1,000 ft
2
of GFA
GFA – gross floor area, excluding storage and utility space
(Institute of Transportation Engineers, 1987; Smith, 1984; Wells, 1994)
Table 5.7-4 Example Minimum Parking Ratios
In residential neighborhoods, the perception of the need for large quantities of parking may lead
developers to provide on-street parking; residential land use will greatly influence the quantity needed.
Each on-street lane increases street impervious cover by 25%. Many communities require 2-2.5
parking spaces per residence. In single-lot neighborhoods, with both standard and reduced setbacks,
parking requirements can likely be met using private driveways and garages. In townhouse
communities, if on-street parking is required, providing one on-street space per residence is likely
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sufficient. Urban settings will require the greatest use of on-street parking. However, continuous parking
lanes on both sides of the street, while common for all residential land uses, is often unnecessary.
When on-street parking is necessary, queuing lanes (discussed in BMP 5.7.1) provide a parking system
alternative that minimizes imperviousness.
Parking Spaces and Lot Layout
Parking spaces are comprised of five impervious components (Center for Watershed Protection, 1998):
1. The parking stall;
2. The overhang at the stall’s edge;
3. A narrow curb or wheel stop;
4. The parking aisle that provides stall access; and
5. A share of the common impervious areas (e.g., fire lanes, traffic lanes).
Of these, the parking space itself accounts for approximately 50% of the impervious area, with stall
sizes ranging from 160 to 190 ft2. Several measures can be taken to limit parking space size. First,
jurisdictions can review standard parking stall sizes to determine their appropriateness. A typical stall
dimension may be 10 ft by 18 ft, much larger than needed for many vehicles; while the largest SUVs
are wider, the great majority of SUVs and vehicles are less than 7 ft providing opportunity for making
stalls slightly narrower and shorter. In addition, typical parking lot layout includes parking aisles that
accommodate two-way traffic and perpendicularly oriented stalls. The use of one-way isles and angled
parking stalls can reduce impervious area.
Jurisdictions can also stipulate that parking lots designate a percentage of stalls as compact parking
spaces. Smaller cars comprise 40% or more of all vehicles and compact parking stalls create 30% less
impervious cover than average-sized stalls (Center for Watershed Protection, 1998). This is currently
an underutilized practice that has potential to reduce the total area of parking lots.
Figure 5.7-4 (“Conservation Design for Stormwater Management”, DNREC, 1997)
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Parking Lot Design
Because of parking ratio requirements and the desire to accommodate peak parking demand, even
when it occurs only occasionally throughout the year, parking lots often provide parking capacity
substantially in excess of average parking needs. This results in vast quantities of unused impervious
surface.
A design alternative to this scenario is to provide designated overflow parking areas. The primary
parking area, sized to meet average demand, would still be constructed on impervious pavement to
meet local construction codes and American with Disabilities Act requirements. However, the overflow
parking area, designed to accommodate increased parking requirements associated with peak
demand, would be constructed on pervious materials (e.g., permeable pavers, grass pavers, gravel).
This design approach focused on average parking demand will still meet peak parking demand
requirements while reducing impervious pavement.
Figure 5.10-2 Overflow parking using permeable pavers
Cost Issues
Estimates for parking construction range from $1,200 to $1,500 dollars per space (Center for
Watershed Protection, 1998). For example, assuming a cost of $1,200 per parking space, reducing the
required parking ratio for a 20,000 ft
2
shopping center from 5 spaces per 1,000 ft
2
to 4 spaces per 1,000
ft
2
would represent a savings of $24,000.
Parking lots incorporating pervious overflow areas may not present cost savings, as permeable paving
products are generally more expensive than traditional asphalt. However, the additional costs may be
offset by reduced curb and gutter and stormwater management costs.
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Figure 5.7-5 Parking Stall Dimensions (Schueler, 1997)
References
Center for Watershed Protection, 1998
Center for Watershed Protection, 1995
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5.8 Disconnect/Distribute/Decentralize
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BMP 5.8.1: Rooftop Disconnection
Minimize stormwater volume by disconnecting
roof leaders and directing rooftop runoff to
vegetated areas to infiltrate.
Key Design Elements
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Limited
Limited
Limited
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
High
High
High
Low
Water Quality Functions
TSS:
TP:
NO3:
30%
0%
0%
.
Stormwater collection systems.
.
Redirect rooftop overland flow to minimize rapid transport to
conveyance structures and impervious areas, such as ditches and
roadways.
.
Direct runoff to vegetated areas designed to receive stormwater.
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Description
Traditionally, building codes have encouraged the rapid conveyance of rooftop runoff away from
building structures. It is not uncommon for municipal codes to specify minimum slopes which serve to
accelerate overland flow onto and across yards and lawns, directed ever more rapidly toward streets
and gutters. Concerns pertaining to surface ponding of rooftop stormwater and potential ice formation
on sidewalks and driveways are the main drivers of these lot requirements (Center for Watershed
Protection, 1998). These requirements, stemming from a convention of rapid transmission of
stormwater, serve to discourage on-site treatment of rooftop stormwater. This trend is further
exacerbated in northern latitudes where icing concerns are paramount and, consequently, where
downspouts may be connected directly to the stormwater collection system.
Disconnecting roof leaders from conventional stormwater conveyance systems allows rooftop runoff to
be collected and managed on site. Rooftop runoff can be directed to designed vegetated areas
(discussed in Chapter 6) for on-site storage, treatment, and volume control. This BMP offers a
distributed, low-cost method for reducing runoff volume and improving stormwater quality through:
• Increasing infiltration and evapotranspiration.
• Increasing filtration.
• Decreasing stormwater runoff volume.
• Increasing stormwater time of concentration.
Variations
In addition to directing rooftop runoff to vegetated areas, runoff may also be discharged to non-
vegetated BMPs, such as dry wells, rain barrels, and cisterns for stormwater retention and volume
reduction. With proper design, this rooftop water can be used for lawn watering, gardening, toilet
flushing and fire protection.
Applications
Routing rooftop runoff to naturally vegetated areas will reduce runoff volume and peak discharge, as
well as improve water quality by slowing runoff, allowing for filtration, and providing opportunity for
infiltration and evapotranspiration. The use of pervious areas for rooftop discharge has the ability to
reduce the quantity of site stormwater runoff and improve the quality of the stormwater that does
discharge from the site. Alternatives for disconnecting roof leaders and the use of vegetated areas
should consider the following issues (Prince George’s County Department of Environmental Protection,
1997; Maryland Department of the Environment, 1997).
• Encourage shallow sheet flow through vegetated areas, using flow spreading and leveling
devices if necessary.
• Direct roof leader flow into BMPs designed specifically to receive and convey rooftop runoff.
• Direct flows into stabilized vegetated areas, including on-lot swales and bioretention areas.
• Rooftop runoff may also be directed to on-site depression storage areas.
• Runoff from industrial roofs and similar uses should not be directed to vegetated areas, if there
is reason to believe that pollutant loadings will be elevated.
• Limit the contributing rooftop area to a maximum of 500 ft2 per downspout.
• Flow from roof leaders should not contribute to basement seepage.
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Figure 5.8-1 Examples of Directly Connected Impervious Areas (Roesner, ASCE, 1991)
Careful consideration should be given to the design of vegetated collection areas. Concerns pertaining
to basement seepage and water-soaked yards are not unwarranted, with the potential arising for
saturated depressed areas and eroded water channels. The proper design and use of bioretention
areas, infiltration trenches, and/or dry wells will reduce or eliminate the potential of surface ponding and
facilitate functioning during cold weather months.
Maintenance of the planted areas would be required, but would be limited. Routine maintenance would
include a biannual health evaluation of the vegetation and subsequent removal of any dead or diseased
vegetation plus mulch replenishment, if included in the design. This maintenance can be incorporated
into regular maintenance of the site landscaping. If the vegetated area is located in a residential
neighborhood, the maintenance responsibility could be delegated to the residents. The use of native
plant species in the vegetated area will reduce fertilizer, pesticide, water, and overall maintenance
requirements.
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Cost Issues
Construction cost estimates for vegetated areas should be similar or in line with that of conventional
landscaping. If bioretention areas are incorporated into the site, their costs are slightly more than costs
required for conventional landscaping. Commercial, industrial, and institutional site costs range
between $10 and $40 per square foot, based on the design of the bioretention area and the control
structures included. These costs, however, can potentially be offset by the reduced costs of
conventional stormwater management systems that otherwise would be required, if it were not for the
reduction achieved through the application of this BMP.
References
Prince George’s County Department of Environmental Protection, 1997
Maryland Department of the Environment, 1997
Center for Watershed Protection, 1998
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BMP 5.8.2: Disconnection from Storm Sewers
Minimize stormwater volume by
disconnecting impervious roads and
driveways and directing runoff to grassed
swales and/or bioretention areas to infiltrate.
Water Quality Functions
TSS:
TP:
NO3:
30%
0%
0%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
High
High
High
Low
Stormwater Functions
Residential:
Commercial: Ultra
Urban: Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Limited
Limited
Limited
Limited
Key Design Elements
Potential Applications
.
Disconnect road and driveways from stormwater collection
systems.
.
Redirect road and driveway runoff into grassed swales or other
vegetated systems designed to receive stormwater.
.
Eliminate curbs/gutters/conventional collection and conveyance.
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Description
Impervious roads and driveways account for a large percentage of post-development imperviousness.
These surfaces influence stormwater runoff volume and quality by facilitating the rapid transport of
stormwater and collecting pollutants from atmospheric deposition, automobile leaks, and additional
sources. Considered a source of more potentially damaging pollution than rooftops, roads and
driveways contribute toxic chemicals, oil, and metals to stormwater runoff.
Conventional stormwater management has involved the rapid removal and conveyance of stormwater
from these surfaces. The result of this management system has been increased runoff volume,
decreased time of concentration, and greater pollutant mobility. Distributed stormwater management
through the use of vegetated swales and bioretention areas (discussed in Section 6.4.8 and 6.4.5) can
reduce the volume of stormwater runoff while providing on-site treatment and pollutant removal,
providing:
• Increased infiltration and evapotranspiration.
• Increased filtration.
• Decreased stormwater runoff volume.
• lncreased stormwater time of concentration.
Variations
A variety of alternatives exist for
redirecting road and driveway
runoff away from stormwater
collection systems. In addition to
vegetated swales, infiltration
trenches or bioretention areas may
be utilized. Curbing may be
eliminated entirely or selectively
eliminated, as shown in Figure 5.8-
2. The choice of BMP will depend
upon site-specific characteristics
including soil type, slope, and
stormwater volume.
Figure 5.8-2 Example of Concrete Road Edging and Corner Curb (Roesner, ASCE, 1991)
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Applications
Routing road and driveway runoff to vegetated swales will reduce runoff volume and peak discharge, as
well as improve water quality by slowing runoff, allowing for filtration, and providing opportunity for
infiltration and evapotranspiration. Most importantly, in contrast to conventional systems where roads
and driveways are connected directly to the stormwater collection and conveyance system, vegetated
swales offer the potential for pollutant reductions (see additional discussion in Section 6.8). When
stormwater enters the stormwater system directly from road and driveways surfaces, a large variety of
pollutants are introduced into the stormwater and eventually the receiving stream. These pollutants
include toxic chemicals, oil, metals, and large particulate matter.
The use of vegetated swales, while slowing runoff discharge and permitting infiltration, also allows for
pollutant reduction facilitated by the soil media complex and plant uptake. Thus, vegetated swales used
in this manner serve a range of functions, intercepting runoff, reducing stormwater volume, and
retaining and reducing pollutants. Proper design and implementation still allows stormwater to be
quickly removed from road and driveway surfaces alleviating concerns over standing water.
The suitability of vegetated swales depends on land use, soil type, imperviousness of the contributing
watershed, and dimensions and slope of the vegetated swale system. Use of natural low-lying areas is
encouraged and natural drainage courses should be preserved and utilized.
Maintenance of the vegetated swale should include providing sufficient capacity of the channel and
maintaining a dense, healthy vegetated cover. Maintenance activities should include periodic mowing
(with plantings never cut shorter than the design flow depth), weed control, watering during drought
conditions, reseeding of bare areas, and clearing of debris and blockages.
Cost Issues
See discussion in Chapter 6.4.8. Vegetated swale construction costs are estimated at approximately
$0.25 per ft2. By including design costs, this estimated cost increases to $0.50 per ft2, allowing
vegetated swales to compare favorably with other stormwater management practices.
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5.9 Source Control
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BMP 5.9.1: Streetsweeping
Use of one of several modes of sweeping equipment (e.g.,
mechanical, regenerative air, or vacuum filter sweepers) on a
programmed basis to remove larger debris material and
smaller particulate pollutants, preventing this material from
clogging the stormwater management system and washing
into receiving waterways/waterbodies.
Water Quality Functions
TSS:
TP:
NO3:
85%
85%
50%
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/None
Low/None
Low/None
High
Stormwater Functions
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
Yes
Yes
Yes
Yes
Key Design Elements
Potential Applications
.
Use proper equipment; dry vacuum filters demonstrate optimal
results, significantly better than mechanical and regenerative air
sweeping, though move slowly and are most costly
.
Develop a proper program; vary sweeping frequency by street
pollutant load (a function of road type, traffic, adjacent land uses,
other factors); sweep roads with curbs/gutters
.
Develop a proper program; restrict parking when sweeping to
improve removal.
.
Develop a proper program; seasonal variation for winter
applications as necessary.
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Description
National Urban Runoff Program (NURP) studies from the 1980’s reported generally very poor results
from street sweeping. In some cases, results suggested that water quality effects of conventional
mechanical street sweeping programs were actually negative. This is possibly explained by the fact that
the superficial sweeping accomplished by mechanical sweepers removes a “crust” of large, coarser
debris on many surfaces and exposes the finer particles to upcoming storm events. These particles are
then washed into receiving water bodies. However, new street sweeping technology (see discussion
below) has dramatically improved street sweeping performance. While these new street sweeping
technologies are considerably more costly than previous street sweeping
technologies, their pollutant
reduction performance compares quite favorably to other
pollutant reduction BMPs. Streetsweeping can actually be
quite cost effective in terms of water quality performance.
Variations
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Variations in street sweeping relate primarily to differences in
equipment but also relate to important aspects of the street
sweeping programs, such as frequency of street sweeping,
use of regulations such as parking prohibitions, and other
program factors.
Equipment -
Mechanical broom:
use of mechanical brooms/brushes with conveyor belts. Designed to remove
standard road debris, using various types of circulating brushes that sweep material onto conveyors
and then into bins. Some machines apply water to reduce dust. Includes the Elgin Pelican (3-
wheel) and Eagle (4-wheel), Athey;s Mobile (3- and 4-wheel) and Schwarze M-series. Stormwater
reports that the vast bulk of sweepers in use in the US are of this type. These sweepers are least
expensive and vary in cost from (approximately $60,000 in 2002, according to Stormwater
magazine).
Figure 5.13-1 Vacuum Filter Street sweeper
Regenerative air:
compressed air is directed onto the road surface, loosening fine particles that
are then vacuumed. Includes Elgin’s Crosswind J, Mobile’s RA730 series, Schwarze’s A-series,
Tymco sweepers. About twice as expensive as mechanical sweepers ($120,000 in 2002, according
to Stormwater
magazine).
Vacuum filter:
vacuum assisted small-micron particle sweepers, either wet or dry. Dry vacuum
includes mechanical broom sweeping with a vacuum (Elgion’s GeoVac and Whirlwind models and
Schwarze’s EV-series particulate management); this technology works well even in cold weather
conditions. Wet vacuum uses water dust suppression with scrubbers that apply water to pavement;
particles are suspended, and then vacuumed. Four to 5 times as expensive as mechanical
sweepers, according to Stormwater
magazine in 2002. Equipment has been constrained by slow
driving speeds (max of 25 mph).
Tandem sweeping:
using two machines, surfaces are mechanically swept and then vacuumed.

Applications
Streets weeping programs vary by sweeping frequency that in turn depends on several other factors.
Certainly the most obvious factor is the intensity of the roadway and its expected pollutant load – the
greater the traffic intensity, the greater the pollutant load. Other factors such as frequency and intensity
of rainfall also affect desired street sweeping frequency. Sutherland and Jelen (1997), measuring
sediment load reduction, found very high pollutant load reduction with weekly or greater sweeping
frequencies in the Portland area with relatively frequent rainfall events.
Another factor to consider in street sweeping programs is “wash-on” or material that washes onto
impervious areas from upgradient/upstream pervious surfaces. Obviously if large amounts of sediment
and related-pollutants wash onto the paved surfaces during storm events themselves, street sweeping
is going to be relatively ineffective. The Center for Watershed Protection maintains that as site
imperviousness itself increases and as the imperviousness of upgradient watershed areas increases,
potential for wash-on decreases and potential effectiveness of street sweeping increases (Article 121,
Center for Watershed Protection
Technical Note 103 from Watershed Protection Techniques 3(1)
, pp.
601-604).
Lastly, pollutant loads being contributed by the rainfall itself, or wetfall (such as total solids, total
nitrogen, chemical oxygen demand, extractable copper) will not be reduced or removed through street
sweeping by definition. For example, research performed by the Metropolitan Washington Council of
Governments found that 34 percent of total nitrogen, 24 percent of total solids, and 18 percent of COD
occurred as wetfall (Urban Runoff in the Washington Metropolitan Area, 1983. Final Report:
Washington DC Area Urban Runoff Project. USEPA Nationwide Urban Runoff Program, MWCOG
Washington DC).
In general, the greater the traffic on a roadway and the greater the number of vehicles using a parking
area, the greater the pollutant loads. The greater the pollutant loads, the greater the potential
effectiveness of street sweeping. Winter road applications affect street sweeping programs
Cost Issues
Costs of street sweeping include capital costs of purchasing the equipment, annual costs of
maintenance, annual costs of operation, plus costs of disposal of the material that is collected.
According to the US Environmental Protection Agency’s
Preliminary Data Summary of Urban Storm
Water Best Management Practices
(August 1999, EPA-821-R-99-012), street sweeper costs are quite
variable. A mechanical sweeper with $75,000 purchase price and a 5-year life cycle was found to cost
$30 per curb mile (Finley, 1996 and SWRPC, 1991), while a vacuum street sweeper purchased at
$150,000 and having an 8-year life cycle cost $15 per curb mile (Satterfield, 1996 and SWRPC, 1991).
Further comparisons were made by the EPA, including the effects of varying frequency of sweeping
(USEPA, 1999).
The point is that although mechanical sweepers are less expensive than vacuum sweepers, their
economic life is shorter than vacuum sweepers. If pollutant removal effectiveness is included in the
comparison, vacuum sweepers yield substantially better cost effectiveness in most cases.
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Pollutant Removal Performance
Although pollutant removal performance for street sweeping will vary with the frequency of the street
sweeping program, evaluations are demonstrating remarkably high pollutant removal, especially if the
program includes weekly street sweeping. The Center for Watershed Protection reports one recent
study with 45-65 percent removal of total suspended solids, 30-55 percent total phosphorus, 35-60
percent total lead, 25-50 percent total zinc, and 30-55 percent total copper (Kurahashi & Associates,
Inc. 1997.
Port of Seattle, Stormwater Treatment BMP Evaluation
). In
Street Sweeping for Pollutant
Removal
(Montgomery County Department of Environmental Protection, Montgomery County,
Maryland, February 2002), additional pollutant removal effectiveness data is reported from studies
performed by the Center for Watershed Protection (
Watershed Treatment Model,
2001). Total
suspended solids reduction ranged from 5 percent (major road) and 30 percent (residential street) for
mechanical sweepers to 22 and 64 percent respectively for regenerative air and 79 to 78 percent
respectively for vacuum sweepers. For nitrogen, mechanical sweeper pollutant removal was 4 and 24
percent removal for major roads and residential streets, regenerative air was 18 and 51 percent, and
vacuum 53 and 62 percent. In summary, although pollutant removal performance for new mechanical
sweepers has improved considerably over those of the past generation, the new vacuum technology is
significantly better than either mechanical or even regenerative air sweepers and achieves a level of
pollutant removal that is frequently better than all other BMPs.
References
Center for Watershed Protection, 2001.
Watershed Treatment Model.
Center for Watershed Protection,
Article 121: Technical Note 103 from Watershed Protection
Techniques 3(1)
, pp. 601-604
Finley, 1996 and SWRPC, 1991
Kurahashi & Associates, Inc. 1997.
Port of Seattle, Stormwater Treatment BMP Evaluation.
Montgomery County Department of Environmental Protection,
2002. Street Sweeping for Pollutant
Removal,
Montgomery County, MD.
Satterfield, 1996 and SWRPC, 1991
Sutherland and Jelen, 1997.
USEPA, 1999.
Preliminary Data Summary of Urban Storm Water Best Management Practices
Urban Runoff in the Washington Metropolitan Area, 1983. Final Report: Washington DC Area Urban
Runoff Project. USEPA Nationwide Urban Runoff Program, MWCOG Washington DC
363-0300-002 / December 30, 2006
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