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.
The green infrastructure and low impact development practices
provided in the PA BMP Manual shall be utilized for all regulated
activities wherever possible. Runoff volume controls shall be implemented
using the design storm method in subsection A of this section or the
simplified method in subsection B of this section. 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 §
174-18A.
(1) Post
development total runoff cannot 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:
To calculate the runoff volume (cubic feet) for existing site
conditions (predevelopment) 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 of this chapter is available to
guide a qualified professional and/or an applicant to calculate the
stormwater runoff volume.
Soil Cover Complex Method:
Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S)
|
Where:
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P
|
=
|
Two-year rainfall (inches)
|
S
|
=
|
(1,000/CN) - 10; the potential maximum retention (including
initial abstraction, Ia)
|
Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12
|
Where:
|
|
|
Q
|
=
|
Runoff (inches)
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Area
|
=
|
Stormwater management area (square feet)
|
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)
(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 this 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)
(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 subsections (C)(5) and (6) of this section, at least the first
one-half 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 hours
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 =
Total Site Area (for both pre- and postdevelopment conditions)
minus Protected Area
Natural resource areas should be calculated based upon the municipality’s own natural resource protection ordinance requirements in the zoning ordinance. See Table B-2 in Appendix B of this chapter for guidance to assess the total protected area. For additional reference, see Chapter
5, Section 5.4.1 of the PA BMP Manual.
(2) Calculate
the volume controls provided through nonstructural BMPs. Table B-5
in Appendix B of this chapter 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 (feet3)
|
minus
|
Nonstructural Volume Control (feet3)
|
=
|
Structural Volume Requirement (feet3)
|
(4) Calculate
the volume controls provided through structural BMPs. Table B-6 in
Appendix B of this chapter 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 of this chapter 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 manmade 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 174-17-1. 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 174-17-1 below:
Table 174-17-1. Peak Rate Runoff Control Standards by Stormwater
Management Districts in the Neshaminy Creek Watershed (Includes Little
Neshaminy Creek)
|
---|
District
|
Design Storm Post development (Proposed Conditions)
|
Design Storm Predevelopment (Existing Conditions)
|
---|
A
|
2-year
|
1-year
|
5-year
|
5-year
|
10-year
|
10-year
|
25-year
|
25-year
|
50-year
|
50-year
|
100-year
|
100-year
|
B
|
2-year
|
1-year
|
5-year
|
2-year
|
10-year
|
5-year
|
25-year
|
10-year
|
50-year
|
25-year
|
100-year
|
50-year
|
C
|
2-year
|
2-year
|
5-year
|
5-year
|
10-year
|
10-year
|
25-year
|
25-year
|
50-year
|
50-year
|
100-year
|
100-year
|
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 of this chapter) 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, and four other maps with zoomed-in extents, are included
in Appendix D of this chapter. 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. On-site drainage facilities shall
be designed to safely convey flows from an undisturbed area of the
property through the developed portion of the site.
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 174-17-1 and subsections
A through
E of this section; 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.