1. Chapter 4Integrating Site Design and Stormwater Management

Pennsylvania Stormwater
Best Management Practices
Manual
Chapter 4
Integrating Site Design and Stormwater
Management
363-0300-002 / December 30, 2006

 
Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
Chapter 4 Integrating Site Design and Stormwater Management
4.1
A Recommended Site Design Procedure for Comprehensive
Stormwater Management…………………………………………………1
4.2
The Site Design Checklist for Comprehensive Stormwater
Management…………………………………………………………………3
4.3
Importance of Site Assessment………………………………………….7
4.3.1 Background Site Factors…………………………………………..7
4.3.2 Site Factors Inventory………………………………………………8
4.3.3 Site Factors Analysis……………………………………………….8
363-0300-002 / December 30, 2006

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
4.1
A Recommended Site Design Procedure for Comprehensive
Stormwater Management
Chapters 5 and 6 describe multiple Non-Structural and Structural BMPs that can be
used to achieve the Recommended Site Control Guidelines for comprehensive
stormwater management described in Chapter 3. Obviously, not all of these BMPs are
appropriate for all land development activities or every site. How can BMPs be selected
to maximize their performance? What is the optimal blend between Non-Structural and
Structural BMPs? How can stormwater management be best integrated into the site
planning process?
A flow chart depicting a Site Design Procedure For Comprehensive Stormwater
Management (Procedure) is set forth in Figure 4-1 (also referenced to the Checklist
Summary in Figure 4-2 which is discussed in Section 4.2 below). This procedure begins
with an assessment of the site and its natural systems and then proceeds to integrate
both Non-Structural and Structural BMPs in the formulation of a comprehensive
stormwater management plan. The intent of the planning process is to promote
development of stormwater management “solutions” which achieve the rigorous quantity
and quality standards set forth in Chapter 3. Some aspects of the procedure will not be
fully applicable in all land development cases. For example, Non-Structural BMPs may
be challenging to apply in those cases where higher densities/intensities are proposed
on the smallest of sites in already developed areas.
An essential objective of the Procedure is to maximize stormwater “prevention” through
use of Non-Structural BMPs (Chapter 5). Once prevention has been maximized, some
amount of stormwater peaking and volume control will likely remain to be managed.
These stormwater management needs should be met with an array of natural-system
based Best Management Practices (Vegetated Swales, Vegetated Filter Strips, etc.),
with the remaining stormwater management needs met with structural Best Management
Practices such as infiltration basins, trenches, porous pavement, wet basins, retention
ponds, constructed wetlands, and others presented in Chapter 6.
This Procedure, or a process similar to it, is an integral part of comprehensive
stormwater management and transcends the bounds of conventional stormwater
management that has existed in most Pennsylvania municipalities. Perhaps most
importantly, the Procedure involves the total site design process
. Conventional
stormwater management has usually been relegated to the final stages of the site design
and overall land development process, after most other building program issues have
been determined and accommodated. To the contrary, the Procedure places
stormwater management in the initial stages of site planning process, when the building
program is being fitted and tested on the site. In this way, comprehensive stormwater
management can be integrated effectively into the site design process.
363-0300-002 / December 30,
2006
1 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
363-0300-002 / December 30,
2006
2 of 9
MUNICIPAL
INPUTS
STORMWATER
CALCULATIONS
Soil
Infiltration-
based BMPs
PRE-SUBMISSION
MEETING
Township
Comprehensive
Plan, Act 167
Plan, Other
SLDO
Guidance
Zoning
Guidance
Design
Phase 1
PREVENTIVE
Site Analysis:
Constraints vs.
Opportunities
Sensitive Areas
Site Factors
Inventory
Background
Factors
Minimum Disturbance,
Minimum Maintenance
Concentration &
Clustering
Impervious Coverage
Disconnect, Distribute,
Decentralize
Source Control
SITE ANALYSIS
NON-STRUCTURAL BMPs STRUCTURAL BMPs
APPLICANT
SUBMISSION
Design Phase
2
MITIGATIVE
Restoration
BMPs
Runoff
Quality
BMPs
Volume
Reduction
BMPs
STORMWATER
MANAGEMENT
PLAN
F
F
i
i
g
g
u
u
r
r
e
e
4
4
-
-
1
1
R
R
e
e
c
c
o
o
m
m
m
m
e
e
n
n
d
d
e
e
d
d
p
p
r
r
o
o
c
c
e
e
d
d
u
u
r
r
e
e
s
s
f
f
o
o
r
r
c
c
o
o
m
m
p
p
r
r
e
e
h
h
e
e
n
n
s
s
i
i
v
v
e
e
s
s
t
t
o
o
r
r
m
m
w
w
a
a
t
t
e
e
r
r
m
m
a
a
n
n
a
a
g
g
e
e
m
m
e
e
n
n
t
t
.
.
S
S
I
I
T
T
E
E
P
P
L
L
A
A
N
N
N
N
I
I
N
N
G
G
A
A
N
N
D
D
D
D
E
E
S
S
I
I
G
G
N
N
P
P
R
R
O
O
C
C
E
E
D
D
U
U
R
R
E
E

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
Much of the information relied on for the Procedure is information already required to
satisfy other aspects of existing municipal land development ordinances. The Procedure
is intended to more effectively utilize this already-collected site data to generate better
stormwater management in the context of a markedly improved site plan. To the extent
that this information is not already being collected and assessed, the information needs
to be collected as part of the site design process.
4.2
The Site Design Checklist for Comprehensive Stormwater
Management
Coordinated with the Recommended Site Design Procedure for Comprehensive
Stormwater Management is a series of questions structured to facilitate and guide an
assessment of the site’s natural features and stormwater management needs. The Site
Design Checklist for Comprehensive Stormwater Management (Figure 4-2) is intended
to help facilitate the Procedure. The initial questions in the Checklist focus on Site
Analysis, including Background Site Features, a Site Factors Inventory, Site Factors
Analysis and Constraints and Opportunities. The checklist relates directly to the first
Non-Structural BMP category: Protect Sensitive and Special Value Features, which
include:
BMP 5.4.1
Protect Sensitive/Special Value features
BMP 5.4.2
Protect/conserve/enhance utilize riparian areas
BMP 5.4.3
Protect/utilize natural flow pathways in overall stormwater planning
and design
Because these first steps in the Procedure are so important, they are further discussed
below in Section 4.3 – “Importance of Site Assessment”.
The Procedure continues with potentially multiple cycles of “testing” and “fitting”
preventive
Non-Structural BMPs at the site. The Checklist provides questions designed
to identify the potential application of additional Non-Structural BMPs. Once Non-
Structural BMPs have been “maximized,” the Recommend Procedure then continues
with the testing/fitting of Structural BMPs, again facilitated by the Checklist questions.
This testing/fitting of Non-Structural and Structural BMPs can continue through several
cycles. At the completion of the Procedure, a comprehensive stormwater management
plan emerges, satisfying the Chapter 3 Recommended Site Control Guidelines. If the
Checklist questions are addressed thoroughly and the Procedure is fully and effectively
applied, the critical objective of managing stormwater comprehensively will be achieved
in a cost effective manner. The Procedure, though largely common sense, constitutes a
change from conventional engineering practice in many Pennsylvania municipalities.
363-0300-002 / December 30,
2006
3 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
Background Site Factors
Describe hydrologic
context and other natural elements
Chapter 93 stream use designation?
Special Protection Waters (EV, HQ)?
Fishery / Aquatic Life Use (WWF, CWF, TSF)?
Any Chapter 303d/impaired stream listing classifications?
Aquatic biota sampling?
Existing water quality sensitivities downstream (water supply source?)?
Location of any known downstream flooding?
Includes any Special Areas?
Such as Previously Mined AMD/AML areas?
Brownfields?
Source Water Protection areas
Urban Areas?
Carbonate/Limestone?
Slide Prone Areas
Other
Site Factors Inventory
Describe the size and shape
of the site
Special constraints/opportunities?
Special site border conditions and adjacent uses?
Describe the existing developed features of the site, if any
Existing structures/improvements, structures to be preserved?
Existing cover/uses?
Existing impervious areas?
Existing pervious maintained areas?
Existing public sewer and water?
Existing storm drainage systems at/adjacent to site?
Existing wastewater, water systems onsite?
Describe important
natural features of site
Existing hydrology (drainage swales, intermittent, perennial)?
Existing topography, contours, subbasins?
Soil series found on site and their Hydrologic Soil Group ratings?
Areas of vegetation (trees, scrub, shrub)?
Special Value Areas?
Wetlands, hydric soils?
Floodplains/alluvial soils?
High quality woodlands, other woodlands and vegetation?
Riparian buffers?
Naturally vegetated swales/drainageways?
Sensitive Areas?
Steep slopes?
Special geologic conditions (limestone?)?
Shallow bedrock (less than 2ft)?
High water table (less than 2ft)?
PNDI areas or species?
Site Factors Analysis
Characterize the constraint-zones at site
Avoid development on or near special and sensitive natural features
Characterize the opportunity-zones at site
Location of well-draining soils
Location and quality of existing vegetation
Has a Potential Development Area been defined?
Does building program fit the constraints and opportunities of natural features?
(Figure 4-2) Checklist summary for use with Site Planning and Design Procedure
SITE ANALYSIS
BACKGROUND SITE CONDITIONS
363-0300-002 / December 30,
2006
4 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
Township Comprehensive Plan and Zoning guidance
Guidance in Comprehensive Plan?
Existing Zoning District?
Total number of units allowed?
Type of units?
Density of units?
Any allowable options?
Township SLDO guidance and options
Performance standards for neo-traditional, village, hamlet planning?
Reduce building setbacks?
Curbs required?
Street width, parking requirements, other impervious requirements?
Cut requirements?
Grading requirements?
Landscaping requirements?
Township SLDO/stormwater requirements
Peak rate and design storms?
Total runoff volume?
Water quality provisions?
Methodological requirements?
Maintenance requirements?
Is applicant submission complete? Fully responsive to municipal zoning/
SLDO requirements?
Are municipal zoning/SLDO requirements inadequate?
Is useful interaction at sketch plan or even pre-sketch plan phases occurring?
Lot Concentration and Clustering
Reduce individual lot size?
Concentrate/cluster uses and lots?
Configure lots to avoid critical natural areas ?
Configure lots to take advantage of effective mitigative stormwater practices?
Orient built structures to fit natural topography?
Minimize site disturbance (excavation / grading) at site?
Minimize site disturbance (excavation / grading) for each lot?
Minimum Disturbance/Maintenance
Define disturbance zones for site?
Protect maximum total site area from development disturbance?
Protect naturally sensitive and special areas from disturbance?
Minimize total site compaction?
Maximize zones of open space and greenways?
Consider re-forestation and re-vegetation opportunities?
Impervious Coverage Reduction
Reduce road widths? Lengths?
Utilize turnarounds? Cul-de-sacs with vegetated islands?
Reduce driveway length and width?
Reduce parking ratios?
Reduce parking sizes?
Examine potential for shared parking?
Utilize porous surfaces for applicable parking features (overflow)?
Design sidewalks for single-side street movement?
Disconnect/Distribute/Decentralize
Rooftop disconnection?
Existing downgradient yard area opportunities?
Existing downgradient vegetated areas/woods?
Disconnection from storm sewers/street gutters?
Front/side yard opportunities?
Space for vegetated swales, rain gardens, etc.?
Source Control
Provisions for street sweeping? Other?
BACKGROUND SITE CONDITIONS
SITE DESIGN: NON-STRUCTURAL BMPs
DESIGN PHASE 1:
PREVENT
I
VE
MUNICIPAL INPUTS
363-0300-002 / December 30,
2006
5 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
Volume/Peak Rate Through Infiltration
Porous Pavement with Infiltration Beds?
Infiltration Basins?
Infiltration Trenches?
Rain Garden/Bioretention?
Dry Wells/Seepage Pits?
Vegetated Swales?
Vegetated Filter Strips?
Infiltration Berm/Retentive Grading?
Volume/Peak Rate Reduction
Vegetated Rooftops?
Capture & Reuse:
Cisterns?
Rain Barrels?
Other?
Runoff Quality/Peak Rate Reduction
Constructed wetland?
Wet pond/retention basin?
Dry extended detention basin?
Water quality filters: Constructed and Other
Sand and sand/peat?
Multi-chamber catch basins and inlets?
Other types?
Other
Level Spreaders?
Special Detention Storage: Parking Lots, Other
Site Restoration for Stormwater
Riparian Buffer Restoration?
Landscape Restoration
Soil Amendment/Restoration
Protocols
Soil Testing
Site Infiltration
Iterative Process Occurring Throughout Planning and Design Practices to Max out
Non-Structural and Structural Practices
Use acceptable methods, such as Soil Cover Complex Method (TR-55) for calculations
Do not use Weighted Curve Numbers!
Strive to:
Minimize the pre to post development increase in Curve Numbers
Maximize post-development Time of Concentration
Assume "conservative" pre-development cover conditions (i.e., Curve Numbers) such as
"Meadow Good" or "Woods" for all pre-development pervious areas?
Respect natural sub-areas in the design and engineering calculations
Strive To Achieve Standards of Comprehensive Stormwater Management
No increase in volume of runoff, pre to post development, for up to the 2-yr storm
No reduction in total volume of recharge, for up to the 2-yr storm
No increase in peak rate of runoff, small to large storms
No increase in pollutant loading
Has There Been Thorough Approach To Use of Both Non-Structural and Structural BMP's?
If not, what non-structurals and structurals might be used?
Should the building program be modified?
What Related Benefits Are Being Achieved Through The Use of BMPs?
PL
AN
SITE DESIGN: STRUCTURAL BMPs
STORMWATER METHODOLOGY AND CALCULATIONS
DEVELOP COMPREHENSIVE STORMWATER MANAGEMENT PLAN
DESIGN PHASE 1:
M
I
T
I
GAT
I
VE
SOTRMWATER CALCULATIONS
363-0300-002 / December 30,
2006
6 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
4.3
Importance of Site Assessment
Comprehensive stormwater management begins with a thorough assessment of the site
and its natural systems. Site assessment includes inventorying and evaluating the
various natural resource systems which define each site and pose both problems and
opportunities for stormwater management. Resources include the full range of natural
systems such as water quantity, water quality, floodplains and riparian areas, wetlands,
soils, geology, vegetation, and more. Natural systems range in scale from resources of
areawide importance on a macro scale, down to micro- and site-specific detail.
4.3.1 Background Site Factors
Broader system characteristics should be described, including State Chapter 93 stream
classifications, presence of Special Protection Waters, stream order (i.e., 1
st
order, 2
nd
order, etc.), source water supply designations, 303d/TMDL/Impaired Stream
designations, flooding history, and other information that provides an understanding of
how a particular site is functioning within its watershed context. More specific questions
would include:
Does the site drain to special waterbodies with special water quality needs?
Determine if the site ultimately flows into a reservoir or other water body where
special water quality sensitivities exist, such as use as a water supply source.
Determine if a special fishery exists.
Determine if the site is linked to a special habitat system, such as delineated in
the Pennsylvania Natural Diversity Inventory. For both water quality and
temperature reasons, approaches and practices that achieve a higher order of
protection may become especially important.
Are there known downstream flooding problems?
Determine if a stream system to which the site discharge is currently experiencing
flooding problems. This is especially important where urbanization already has occurred
and where hydrology already has been altered. Unfortunately, the existing FEMA
mapping and related studies do not adequately assess this issue. County agencies and
municipal offices may be able to indicate anecdotally the extent to which downstream
flooding is already a problem or has the potential to become a problem if substantial
additional development is projected. Greater care should be taken in both floodplain
management as well as stormwater management if problems exist or are anticipated.
Does the site discharge to 1st, 2nd, 3rd order streams?
Another important question relates to the site’s location within its watershed. Sites
located near the base of watersheds pose less of a threat to the hydrologic
characteristics of the watershed system. Sites located farther up the watershed are
potentially more problematic when additional stormwater is generated. Perhaps even
more critical, sites located within headwaters must be managed most carefully in terms
of stormwater to maintain pre-development infiltration and groundwater recharge rates.
363-0300-002 / December 30,
2006
7 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
4.3.2 Site Factors Inventory
Site-specific factors that influence comprehensive stormwater management include the
following items:
How does site size and shape affect stormwater management?
As site size increases, the ability to use a variety of Non-Structural and Structural BMPs
increases. Comprehensive stormwater management, especially through site planning
and the use of Non-Structural BMPs, can reduce space requirements at a site and offer
greater BMP flexibility. Oddly shaped sites can also be better adapted with BMPs set
forth here, given their wide variety of shapes and sizes.
What are the important natural features characterizing the site?
At the heart of the comprehensive stormwater management procedure is an
understanding of the natural systems characterizing each site. Existing vegetation and
soil have tremendous importance and are the key to understanding land development
impacts on natural systems. Careful accounting of existing vegetation is an important
prerequisite for comprehensive stormwater management, followed closely by soils
mapping for permeability ratings, and natural pre-development surface flow patterns.
Critical site features, such as wetlands, floodplains, riparian areas, natural drainage
ways, special habitat areas, special geological formations (e.g., carbonate), steep
slopes, shallow depth to water table, shallow depth to bedrock, and other factors should
be inventoried and understood. Critical areas include those with special positive
functions that can be translated into real economic value or benefit. Elimination or
reduction of these functions through the land development process leads to real
economic losses. These special value areas, including wetlands and floodplains and
riparian areas, should be conserved and protected during land development. Critical
natural areas also include sensitive areas, such as steep slopes, shallow bedrock, high
water table areas, and other constraining features, where encroachment by land
development typically creates unnecessary or unanticipated problems. Care must be
taken to avoid these potential pitfalls.
4.3.3 Site Factors Analysis
Identify site factors that constrain comprehensive stormwater management, and identify
site factors that can be viewed as opportunities.
How is the site constrained?
Determine where buildings, roads, and other disturbance should be avoided and why.
Where are the zones of site “opportunity,” in terms of stormwater management?
Determine where most infiltration occurs in terms of vegetation and in terms of soils.
Both constraints and opportunities are grounded in the natural systems present at the
site. Constraints and opportunities are not necessarily simple opposites of one another.
For example, certain types of critical natural areas should be viewed as constraints in
terms of direct land disturbance and building construction, yet also provide significant
opportunity in terms of stormwater management, quantity and quality. Woodlands,
which should be protected from direct land development, provide excellent opportunity
for stormwater management, provided that the correct approaches and practices are
363-0300-002 / December 30,
2006
8 of 9

Pennsylvania Stormwater Best Management Practices Manual
Chapter 4
363-0300-002 / December 30, 2006
9 of 9
used. Vegetated riparian buffers should not be disturbed for building and road
construction yet they can be used carefully with level spreading devices to receive
diffuse stormwater runoff. Soils with maximum permeabilities at the site should not be
made impervious with buildings and roads, but used for stormwater management where
feasible. Conversely, buildings and other impervious areas should be located on those
portions of a site with least
permeable soils. Site opportunities for volume control can
typically be defined in terms of vegetation types that minimize runoff, as well as soil
types with maximum permeabilities.

Back to top