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 control requirements shall be applied in both the Neshaminy
Creek and Skippack Creek Watersheds. 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 §
242-16A.
(1) Post-development 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
(post-development), it is recommended to use the Soil Cover Complex
Method as shown following this subsection. 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.
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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 post-development 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.
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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.
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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.
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Stormwater Management Site Area =
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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 Ordinance Appendix B for guidance to assess the total 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.
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Required Volume Control (ft3) –
Nonstructural Volume Control (ft3) = Structural
Volume Requirement (ft3)
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(4) Calculate the volume controls provided through structural BMPs. Table B-6 in Ordinance
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 Ordinance
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 by
a qualified design professional 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 one-hundred-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. Projects
in the Skippack Creek Watershed (white/uncolored portion of the Watershed
Map) shall use the criteria for "District A" of the Neshaminy Creek.
A. Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year design storms are shown in Table 2. 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 2.
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Table 2
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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 Post-development
(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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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 (Ordinance
Appendix D) and in this section of the Ordinance.
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 Ordinance
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 Hatfield Township:
(1) Meet the full requirements specified by Table 2 and §§
242-15A through
242-15F; 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.