[Ord. 705, 4/28/2011]
1.
Applicants proposing regulated activities in the Neshaminy Creek watershed that do not fall under the exemption criteria shown in § 26-506 shall submit a stormwater management (SWM) site plan consistent with the Neshaminy Creek Watershed SWM Plan to Warminster Township for review. The SWM criteria of this Part 5 shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved SWM site plan is required. No regulated activities shall commence until Warminster Township issues written approval of a SWM site plan, which demonstrates compliance with the requirements of this Part 5.
2.
SWM site plans approved by Warminster Township, in accordance with
Part 5D, shall be on-site throughout the duration of the regulated
activity.
3.
Warminster Township may, after consultation with the Department of
Environmental Protection (PADEP), approve measures for meeting the
state water quality requirements other than those in this Part 5,
provided that they meet the minimum requirements of, and do not conflict
with, state law, including but not limited to the Clean Streams Law.
4.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated, and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this Part 5 and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law. Various BMPs and their
design standards are listed in the Erosion and Sediment Pollution
Control Program Manual, No. 363-2134-008 (April 15, 2000), as amended
and updated.
6.
Impervious areas:
A.
The measurement of impervious areas shall include all of the impervious
areas in the total proposed development even if development is to
take place in stages.
B.
For development taking place in stages, the entire development plan
must be used in determining conformance with this Part 5.
C.
For projects that add 5,000 or more square feet of impervious area
to a parcel, the total impervious area on the parcel is subject to
the requirements of this Part 5. For projects that add less than 5,000
square feet of impervious area to a parcel, only the additional impervious
area on the parcel is subject to the requirements of this Part 5,
as specified in Table 106.1.[1]
7.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written approval
of the adjacent property owner(s) from the developer. Such stormwater
flows shall be subject to the requirements of this Part 5.
8.
All regulated activities shall include such measures as necessary
to:
A.
Protect health, safety, and property;
B.
Meet the water quality goals of this Part 5 by implementing measures
to:
(1)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(2)
Create, maintain, repair or extend riparian buffers.
(3)
Avoid erosive flow conditions in natural flow pathways.
(4)
Minimize thermal impacts to waters of this commonwealth.
(5)
Disconnect impervious surfaces (i.e., disconnected impervious
areas, DIAs) by directing runoff to pervious areas, wherever possible.
C.
To the maximum extent practicable, incorporate the techniques for
low-impact development practices (e.g., protecting existing trees,
reducing area of impervious surface, cluster development, and protecting
open space) described in the Pennsylvania Stormwater Best Management
Practices Manual, Pennsylvania Department of Environmental Protection
(PADEP) No. 363-0300-002 (2006).
9.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize the use of natural on-site infiltration features
while still meeting the other requirements of this Part 5.
10.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
11.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
hours and not more than 72 hours from the end of the design storm.
12.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/
13.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this Part 5 and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Stormwater Management Act.
14.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. 705, 4/28/2011]
Approvals issued and actions taken under this Part 5 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.
[Ord. 705, 4/28/2011]
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 Part 5 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 in Subsection C below. For small projects that propose 1,000 square feet or less of impervious surface, Subsection D should be used.
A.
Design storm method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 26-525, Subsection 1.
(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:
|
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 shown below. The calculated volume shall be either reused,
evapotranspired, or infiltrated through structural or nonstructural
means.
|
|
Soil-Cover Complex Method:
| |
|
Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S) where
| |
|
P = 2-year Rainfall (in)
| |
|
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
|
|
Q = Runoff (in)
|
|
Area = SWM Area (square feet)
|
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 inch runoff/12 inches) * impervious
surface (square feet)
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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 inch runoff/12 inches) * impervious
surface (square feet)
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(3)
Infiltration facilities should be designed to accommodate the
0.5 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
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 and a SWM site plan submission
is required for the regulated activity, the applicant shall determine
and display the total acreage of protected area where no disturbance
is proposed on the plan. 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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(2)
Calculate the volume controls provided through nonstructural
BMPs.
(3)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
|
Required
Volume Control (ft3)
|
—
|
Nonstructural
Volume Control (ft3)
|
=
|
Structural Volume
Requirement (ft3)
|
(4)
Calculate the volume controls provided through structural BMPs. 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. Infiltration BMPs shall be constructed on soils that have
the following characteristics:
a.
The minimum separation between the bottom of the infiltration
BMPs and the top of bedrock and/or seasonally high water table and/or
groundwater (limiting zones) shall be:
1)
For runoff from impervious surfaces associated with residential
land use and/or pervious surface runoff associated with both residential
uses and nonresidential uses: a minimum depth of 36 inches between
the intended bottom of facility and limiting zones.
2)
For runoff from impervious surface associated with nonresidential
uses: a minimum depth of 36 inches between the bottom of facility
and limiting zones. The minimum required separation between the limiting
zone may be increased at the Township's discretion if project specific
conditions, such as anticipated contaminants, dictate greater prevention
of groundwater contamination.
3)
For rooftop runoff: a minimum depth of 24 inches between the
intended bottom of the facility and limiting zones.
b.
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.25 inch/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 0.5 inch/hour, the BMP design should use 0.25 inch/hour
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.
(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. Field testing guidelines
are identified in Appendix A.[1] 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 testing data at the elevation of the proposed
infiltration surface or zone to determine the appropriate percolation
rate and/or 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 Warminster Township 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 Warminster Township.
[1]
Editor's Note: Appendix A of Ord. 705 (Part 5 of this chapter) is identical to Appendix B, Soil Testing Requirements, of Ord. 691 (codified as Ch. 26, Part 4). Refer to Appendix B included as an attachment to this chapter.
D.
Small project stormwater management rate control requirements. This
section applies to only small projects (residential and nonresidential)
that propose less than 1,000 square feet of new impervious surface.
(1)
Newly planted deciduous trees can reduce runoff volume by six
cubic feet. Newly planted evergreen trees can reduce runoff volume
by 10 cubic feet.
(2)
Projects that proposed between zero square feet and 500 square
feet of new impervious surface are required to plant trees that would
reduce runoff volume by 10 cubic feet. Projects that proposed between
501 square feet and 1,000 square feet of new impervious surface are
required to plant trees that would reduce runoff volume by 20 cubic
feet.
(4)
Deciduous or evergreen trees planted to meet the requirements of this section shall be subject to the requirements of Part 5G and be covered by an operations and maintenance agreement with Warminster Township in accordance with § 26-565.
(5)
If an applicant demonstrates the required number of plantings
can not be accommodated on the property, the applicant is required
to pay a fee-in-lieu of landscaping in an amount as specified in the
Warminster Township Fee Schedule.
[Ord. 705, 4/28/2011]
Peak rate controls for large storms, up to the one-hundred-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
one-hundred-year design storms are shown in Table 304.1. Development
sites must control proposed development conditions runoff rates to
existing conditions runoff rates for the design storms in accordance
with Table 304.1.
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Table 304.1
Peak Rate Runoff Control Standards In The Neshaminy Creek
Watershed
(includes Little Neshaminy Creek)
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|---|---|
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
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2-year
|
1-year
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5-year
|
2-year
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10-year
|
5-year
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25-year
|
10-year
|
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50-year
|
25-year
|
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100-year
|
50-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 in this section of this Part 5.
C.
Runoff volume standard. Post-development stormwater runoff volume
being discharged from any regulated activity shall not exceed predevelopment
stormwater runoff volume being discharged for up to the two-year design
storm for each watershed or design point on the site.
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. The stormwater management
site area includes on-site areas that are not proposed to be disturbed,
but drain to a proposed stormwater management facility. Nonimpacted
areas or nonregulated activities bypassing the stormwater management
facilities would not be subject to the management district criteria.
[Ord. 705, 4/28/2011]
1.
The following criteria shall be used for runoff calculations:
A.
For development sites not considered redevelopment, the ground cover
used to determine the existing conditions runoff volume and flow rate
shall be as follows:
(1)
For the wooded portion of sites use a ground cover of "woods
in good condition." An area is classified as wooded if a continuous
canopy of trees covers an area greater than 1/4 acre.
(2)
The undeveloped portion of the site, including agriculture,
bare earth, and fallow ground shall be considered as "meadow in good
condition," unless the natural ground cover generates a lower curve
number (CN) or Rational "c" value (i.e., woods).
B.
For development and redevelopment sites, the ground cover used to
determine the existing conditions runoff volume and flow rate for
the developed portion of the site shall be based upon actual land
cover conditions, except that 20% of the impervious surface area shall
be considered meadow in the model for existing conditions.
2.
Stormwater runoff peak discharges from all development sites with
a drainage area greater than one acre shall be calculated using a
generally accepted calculation technique that is based on the NRCS
Soil-Cover Complex Method. Table 305.1 summarizes acceptable computation
methods. The method selected by the design professional shall be based
on the individual limitations and suitability of each method for a
particular site. Warminster Township may allow the use of the Dekalb
Rational Method (Q=CIA) to estimate peak discharges from drainage
areas that contain one acre of less, where:
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Q
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=
|
Peak flow rate, cubic feet per second (CFS)
|
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C
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=
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Runoff coefficient, dependent on land use/cover
|
|
I
|
=
|
Design rainfall intensity, inches per hour
|
|
A
|
=
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Drainage area, acres.
|
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Table 305.1
Acceptable Computation Methodologies For Stormwater Management
Plans
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|---|---|---|
|
Method
|
Method Developed By
|
Applicability
|
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TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Dekalb Rational Method (or commercial computer package based
on Rational Method)
|
Emil Kuichling (1889)
|
For sites 1 acre or less, or as approved by Warminster Township
and/or Township Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by Warminster Township
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3.
All calculations consistent with this Part 5 using the Soil-Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14, Volume 2, Version
3, rain data corresponding to the Doylestown rain gage for the precipitation
depth data and the partial duration time series using the upper bound
of the ninety-percent confidence interval as follows:
|
Precipitation Frequency
(inches)
| |
|---|---|
|
Design Storm
(years)
|
24 Hours
|
|
1
|
2.94
|
|
2
|
3.54
|
|
5
|
4.45
|
|
10
|
5.20
|
|
25
|
6.29
|
|
50
|
7.21
|
|
100
|
8.21
|
|
This data may also be directly retrieved from the NOAA Atlas
14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. If a hydrologic
computer model, such as PSRM or HEC-1/HEC-HMS is used for stormwater
runoff calculations, then the duration of rainfall shall be 24 hours.
|
4.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods from NOAA Atlas 14, Volume 2, Version 3, rain data
corresponding to the Doylestown rain gage for the precipitation intensity
data and the partial duration time series using the upper bound of
the 90% confidence interval as follows:
|
Precipitation Intensity
(inches/hour)
| ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
|
Design Storm
(years)
|
5 Min.
|
10 Min.
|
15 Min.
|
30 Min.
|
60 Min.
|
120 Min.
|
3 Hrs.
|
6 Hrs.
|
12 Hrs.
|
24 Hrs.
|
|
1
|
4.45
|
3.55
|
2.96
|
2.03
|
1.27
|
0.76
|
0.56
|
0.35
|
0.21
|
0.12
|
|
2
|
5.29
|
4.24
|
3.55
|
2.45
|
1.54
|
0.92
|
0.67
|
0.42
|
0.26
|
0.15
|
|
5
|
6.23
|
4.99
|
4.21
|
2.99
|
1.92
|
1.15
|
0.85
|
0.53
|
0.32
|
0.19
|
|
10
|
6.91
|
5.53
|
4.66
|
3.37
|
2.20
|
1.33
|
0.98
|
0.61
|
0.38
|
0.22
|
|
25
|
7.72
|
6.15
|
5.20
|
3.85
|
2.56
|
1.57
|
1.16
|
0.73
|
0.46
|
0.26
|
|
50
|
8.29
|
6.61
|
5.58
|
4.20
|
2.84
|
1.76
|
1.30
|
0.83
|
0.53
|
0.30
|
|
100
|
8.86
|
7.04
|
5.93
|
4.54
|
3.13
|
1.95
|
1.45
|
0.94
|
0.60
|
0.34
|
A.
Times of concentration for overland flow shall be calculated using
the methodology presented in Chapter 3 of Urban Hydrology for Small
Watersheds, NRCS, TR-55 (as amended from time to time by NRCS). Times
of concentration for channel and pipe flow shall be computed using
Manning's equation and the following roughness coefficients:
|
Table 305.2
Roughness Coefficients (Manning's "n") For Overland Flow
Source: U.S. Army Corps of Engineers, HEC-1 Users Manual
| ||
|---|---|---|
|
Surface Description
|
n
| |
|
Dense growth
|
0.4 — 0.5
| |
|
Pasture
|
0.3 — 0.4
| |
|
Lawns
|
0.2 — 0.3
| |
|
Bluegrass sod
|
0.2 — 0.5
| |
|
Short grass prairie
|
0.1 — 0.2
| |
|
Sparse vegetation
|
0.05 — 0.13
| |
|
Base clay-loam soil (eroded)
|
0.01 — 0.03
| |
|
Concrete/asphalt
| ||
|
Very shallow depths (less than 1/4 inch)
|
0.10 — 0.15
| |
|
Small depths (1/4 inch to several inches)
|
0.05 — 0.10
| |
|
Table 305.3
Roughness Coefficients (Manning's "n") For Channel Flow
| ||
|---|---|---|
|
Reach Description
|
n
| |
|
Natural stream, clean, straight, no rifts or pools
|
0.03
| |
|
Natural stream, clean, winding, some pools or shoals
|
0.04
| |
|
Natural stream, winding, pools, shoals, stony with some weeds
|
0.05
| |
|
Natural stream, sluggish deep pools and weeds
|
0.07
| |
|
Natural stream or swale, very weedy or with timber underbrush
|
0.10
| |
|
Concrete pipe, culvert or channel
|
0.012
| |
|
Corrugated metal pipe
|
0.012 — 0.027(1)
| |
|
High density polyethylene (HDPE) pipe
| ||
|
Corrugated
|
0.021 — 0.029(2)
| |
|
Smooth lined
|
0.012 — 0.020(2)
| |
|
(1)
|
Depending upon type, coating and diameter
|
|
(2)
|
Values recommended by the American Concrete Pipe Association,
check manufacturer's recommended value.
|
5.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil-Cover Complex Method shall be based on TR-55.
6.
Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table 305.4.[1]
[1]
Editor's Note: Table 305.4, Rational Runoff Coefficients, is included as an attachment to this chapter.
7.
Runoff from proposed sites graded to the subsoil will not have the
same runoff conditions as the site under existing conditions because
of soil compaction, even after top-soiling or seeding. The proposed
condition "CN" or "C" shall increase by 5% to better reflect proposed
soil conditions.
8.
The Manning equation is preferred for one-dimensional, gradually-varied,
open channel flow. In other cases, appropriate, applicable methods
should be applied, however, early coordination with the municipality
is necessary.
9.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part 5 using the generally
accepted hydraulic analysis technique or method of Warminster Township.
10.
The design of any stormwater detention facilities intended to meet
the performance standards of this Part 5 shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than one acre in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. Warminster Township may approve the use
of any generally accepted full hydrograph approximation technique
that shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
[Ord. 705, 4/28/2011]
The design of BMPs used to manage stormwater impacts from regulated activities, to meet state water quality requirements, and to otherwise meet the purposes of this Part 5 shall be in accordance with the design requirements in §§ 22-518, 22-520, 22-521, 22-522, 22-523, and 22-529 of Chapter 22, Subdivision and Land Development.
[Ord. 705, 4/28/2011]
1.
Hot spots.
A.
The use of infiltration BMPs is prohibited on hot spot land use areas,
such as vehicle fueling stations, public works storage areas, recycling
facilities, fleet storage areas, facilities that make or store hazardous
materials, etc.
B.
Stormwater runoff from hot spot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
2.
West nile guidance requirements. All wet basin designs shall incorporate
biologic controls to deter the breeding of mosquitoes.
