1. BMP 5.4.1: Protect Sensitive and Special Value Features
      1. Stormwater Functions
      2. Potential Applications
      3. Water Quality Functions
      4. Key Design Elements
      5. Description
      6. Variations
      7. Applications
      8. Design Considerations
      9. Detailed Stormwater Functions
      10. Construction Issues
      11. Maintenance Issues
      12. Cost Issues

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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