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.
The green infrastructure and low-impact development practices
provided in the PA BMP Manual shall be utilized for all regulated
activities wherever possible.
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 that do not
require hydrologic routing to design the stormwater facilities, 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 and those that require hydrologic routing to
design the stormwater facilities must use the Design Storm Method.
A. The Design Storm Method (CG-1 in the PA BMP Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions. For modeling assumptions, refer to §
375-305.
(1)
Post-development total runoff cannot be increased from pre-development
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 (pre-development) and for the proposed developed site conditions
(post-development), it is recommended to use the Soil Cover Complex
method as shown below. The calculated volume shall be either reused,
evapotranspired, or infiltrated through structural or nonstructural
means. Runoff volume must be calculated for each land use type and
soil. The use of a weighted CN value for volume calculations is not
acceptable. Table B-3 in Appendix B is available to guide a qualified
professional and/or an applicant to calculate the stormwater runoff
volume.
Soil Cover Complex Method:
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Step 1: Runoff (in) = Q = (P-0.2S)2/(P + 0.8S)
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Where:
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P
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=
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Two-year rainfall (inches)
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S
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=
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(1,000/CN) — 10; the potential maximum retention (including
initial abstraction, la)
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Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12
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Where:
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Q
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=
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Runoff (inches)
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Area
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=
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Stormwater management area (square feet)
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B. The Simplified Method (CG-2 in the PA BMP Manual) is independent
of site conditions and should be used if the Design Storm Method is
not followed. This method is not applicable to regulated activities
greater than one acre or for projects that require design of stormwater
storage facilities. For new impervious surfaces:
(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 inches runoff/12 inches)* impervious
surface (square feet)
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(2)
At least the first one inch of runoff volume from 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 removed through reuse, evapotranspiration,
or infiltration through structural or nonstructural means.
Volume (cubic feet) = (1 inch runoff/12 inches)* impervious
surface (square feet)
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(3)
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases where soils are suitable for infiltration based on the criteria of §
375-303C(5) and
(6), at least the first 1/2 inch of the permanently removed runoff should be infiltrated.
(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 applicant who is
required to submit a SWM site plan shall determine 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 =
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Total Site Area (for both pre and post development conditions)
minus Protected Area
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Natural resource areas should be calculated based upon the municipality's own natural resource protection ordinance requirements in § 450-38. See Table B-2 in Appendix B for guidance to assess the protected area. For additional reference, see Chapter 5 Section 5.4.1 of the PA 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 )
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Nonstructural
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Structural Volume
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Volume Control (feet3
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minus
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Volume Control (feet3)
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=
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Requirement (feet3)
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(4)
Calculate the volume controls provided through structural BMPs.
Table B-6 in Appendix B is recommended as guidance. See PA 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, seasonally high water table,
or other limiting zone.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.2 inches/hour 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 inch/hour, the BMP design should use 0.2 inch/hour
to ensure 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.
(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 by
a qualified design professional which at minimum address 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.
(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,
is 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. All of New Britain
Borough is located with District A. Development sites must control
proposed development conditions runoff rates to existing conditions
runoff rates for the design storms in accordance with Table 304.1
below:
Table 304.1
Peak Rate Runoff Control Standards in the Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
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District
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Design Storm Post-development (Proposed Conditions)
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Design Storm Pre-development (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. 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.
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 is included in Appendix D.
D. 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.
E. Site areas. The stormwater management site area is the only area
subject to the management district criteria. Non-impacted areas or
nonregulated activities bypassing the stormwater management facilities
would not be subject to the management district criteria.
F. 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 Borough.
(1)
Meet the full requirements specified by Table 304.1 and §
375-304A through
E; 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.