Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
Volume controls will mitigate increased runoff impacts, protect
stream channel morphology, maintain groundwater recharge, and contribute
to water quality improvements. Stormwater runoff volume control methods
are based on the net change in runoff volume for the two-year storm
event.
Volume controls shall be implemented using the Design Storm
Method in Subsection A or the Simplified Method in Subsection B below.
For regulated activities equal to or less than one acre, this chapter
establishes no preference for either methodology; therefore, the applicant
may select either methodology on the basis of economic considerations,
the intrinsic limitations of the procedures associated with each methodology,
and other factors. All regulated activities greater than one acre
must use the Design Storm Method.
A. Design Storm Method (any regulated activity): This method requires detailed modeling based on site conditions. For modeling assumptions refer to §
28A-305A.
(1)
Postdevelopment total runoff should not be increased from predevelopment
total runoff for all storms equal to or less than the two-year-twenty-four-hour-duration
precipitation.
(2)
The following applies in order to estimate the increased volume
of runoff for the two-year-twenty-four-hour-duration precipitation
event:
(a)
To calculate the runoff volume (cubic feet) for existing site
conditions (predevelopment) and for the proposed developed site conditions
(postdevelopment), it is recommended to use the Soil Cover Complex
Method as shown on the following page. The calculated volume shall
be either reused, evapotranspired, or infiltrated through structural
or nonstructural means. Other means of calculating the runoff volume
may be used if approved in advance by the Township Engineer.
Soil Cover Complex Method:
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Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S) where:
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P = 2-year Rainfall (in)
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S = (1000/CN) - 10
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Step 2: Runoff Volume (Cubic Feet) = Q x Area x
1/12
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Q = Runoff (in) (from Step 1)
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Area = SWM Area (sq ft), i.e., the square foot area
of the regulated activity
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Step 3: Required Volume = Postdev, Runoff Volume
- Predev. Runoff Volume
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(b)
Once runoff volume is calculated for the site under both existing
and proposed developed conditions, the difference between the two
site conditions is the increase in runoff volume from predevelopment
conditions to postdevelopment conditions for the two-year-twenty-four-hour
precipitation event.
(c)
The calculated volume shall be captured, reused, evapotranspired,
or infiltrated through nonstructural or structural means.
B. Simplified Method (regulated activities less than or equal to one
acre):
(1)
Stormwater facilities shall capture the runoff volume from at
least the first two inches of runoff from all new impervious surfaces.
Volume (cubic feet) = (2" runoff/12 inches) * impervious surface
(sq ft)
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(2)
At least the first inch of runoff volume from the new impervious
surfaces shall be permanently removed from the runoff flow —
i.e., it shall not be released into the surface waters of the commonwealth.
The calculated volume shall be either reused, evapotranspired or infiltrated
through structural or nonstructural means.
Volume (cubic feet) = (1" runoff/12 inches) * impervious surface
(sq ft)
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(3)
Infiltration facilities should be designed to accommodate the
first half inch of the permanently removed runoff.
(4)
No more than one inch of runoff volume from impervious surfaces
shall be released from the site. The release time must be over 24
to 72 hours.
C. Stormwater control measures: The applicant must demonstrate how the
required volume is controlled through stormwater best management practices
(BMPs), which shall provide the means necessary to capture, reuse,
evaporate, transpire or infiltrate the total runoff volume.
(1)
If natural resources exist on the site, the plan shall indicate
the total acreage of protected area where no disturbance is proposed.
The acreage of the protected area should be subtracted from the total
site area and not included in the stormwater management site area
acreage used in determining the volume controls.
Stormwater Management Site Area = Total Site Area (for both
pre and post development conditions) - Protected Area
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Natural resource areas should be calculated based upon the municipality's
own natural resource protection ordinance. If no ordinance exists,
See Table B-2 in Appendix B for guidance to assess the total protected area. For additional reference see Chapter 5, Section 5.4.1, of the Pennsylvania BMP Manual.
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(2)
Calculate the volume controls provided through nonstructural
BMPs. Table B-5 in Appendix B is recommended as guidance.
(3)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
Required Volume Control (ft3)
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—
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Nonstructural Volume Control (ft3)
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=
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Structural Volume Requirement (ft3)
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(4)
Calculate the volume controls provided through structural BMPs.
Table B-6 in Appendix B is recommended as guidance. See Pennsylvania BMP Manual Chapter
6 for description of the BMPs.
(5)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on the suitability of soils and site
conditions (see Table B-6 in Appendix B for a list of infiltration BMPs). Infiltration BMPs shall
be constructed on soils that have the following characteristics:
(a)
A minimum soil depth of 24 inches between the bottom of the
infiltration BMPs and the top of bedrock or seasonally high water
table.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.2 inch/hour (in/hr) should be utilized and for acceptable rates
a safety factor of 50% should be applied for design purposes (e.g.,
for soil which measured 0.4 in/hr, the BMP design should use 0.2 in/hr
to insure safe infiltration rates after construction).
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate runoff volume within three days (72 hours) from the start
of the design storm. Subsurface systems shall be designed to infiltrate
the stored volume in 48 hours or less with a minimum infiltration
rate and percolation rate of greater than 0.2 inch per hour.
(6)
Soils. A soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. All regulated
activities are required to perform a detailed soils evaluation (deep
hole testing and infiltration testing) by a certified professional
soil scientist (CPSS), certified professional soil classifier (CPSC),
or a sewage enforcement officer (SEO) which at minimum addresses soil
permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(a)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(b)
Provide field tests such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes. The Township Engineer
shall observe all field testing of soils.
(c)
Design the infiltration structure based on field determined
capacity at the level of the proposed infiltration surface and based
on the safety factor of 50%.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the municipality that the soils are conducive to infiltrate on
the lots identified.
(e)
An impermeable liner will be required in detention basins where
the possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
Peak rate controls for large storms, up to the 100-year event,
are essential in order to protect against immediate downstream erosion
and flooding. The following peak rate controls have been determined
through hydrologic modeling of the Neshaminy Creek watershed.
A. Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
100-year design storms are shown in Table 304.1. Development sites
located in each of the management districts must control proposed
development conditions runoff rates to existing conditions runoff
rates for the design storms in accordance with Table 304.1.
Table 304.1
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Peak Rate Runoff Control Standards by Stormwater Management
Districts In The Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
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District
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Design Storm Postdevelopment (Proposed Conditions)
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Design Storm Predevelopment (Existing Conditions)
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A
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2-year
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1-year
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5-year
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5-year
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10-year
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10-year
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25-year
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25-year
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50-year
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50-year
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100-year
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100-year
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B
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2-year
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1-year
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5-year
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2-year
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10-year
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5-year
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25-year
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10-year
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50-year
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25-year
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100-year
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50-year
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C
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2-year
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2-year
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5-year
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5-year
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10-year
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10-year
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25-year
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25-year
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50-year
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50-year
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100-year
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100-year
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B. General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff from existing conditions
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix D) and in this section of the chapter.
C. District boundaries. The boundaries of the stormwater management
districts are shown on official maps and are available for inspection
at the municipal office and county planning offices. A copy of the
map at a reduced scale, and four other maps with zoomed-in extents
are included in Appendix D. The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most accurate data required) provided as part of the SWM site
plan.
D. Sites located in more than one district. For a proposed development
site located within two or more stormwater management district category
subareas, the peak discharge rate from any subarea shall meet the
management district criteria for the district in which the discharge
is located.
E. Off-site areas. When calculating the allowable peak runoff rates,
developers do not have to account for runoff draining into the subject
development site from an off-site area. On-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
F. Site areas. The stormwater management site area is the only area
subject to the management district criteria. Nonimpacted areas or
nonregulated activities bypassing the stormwater management facilities
would not be subject to the management district criteria.
G. Alternate criteria for redevelopment sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by the municipality:
(1)
Meet the full requirements specified by Table 304.1 and §
28A-304A through
F; or
(2)
Reduce the total impervious surface on the site by at least
20% based upon a comparison of existing impervious surface to proposed
impervious surface.