[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.
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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 = (1,000/CN) – 10, the potential maximum retention (including
initial abstraction, Ia)
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Step 2: Runoff volume (cubic feet) = Q x Area x 1/12
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Q = Runoff (in)
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Area = SWM Area (square feet)
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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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Natural resource areas should be calculated based upon Warminster Township's environmental and natural resource protection standards found in Chapter 27, Zoning, and Chapter 22, Subdivision and Land Development. 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.
(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. 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. 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.
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.
(3)
Proposed deciduous or evergreen trees must be selected, planted, maintained, and replaced in accordance with the requirements of §
22-523 of Chapter
22, Subdivision and Land Development.
(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.
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)
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Design Storm Predevelopment
(Existing Conditions)
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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 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.
F. Criteria for redevelopment sites. For redevelopment sites, meet the
full requirements specified by Table 304.1 and §
26-524, Subsections
A through
D.
[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.