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 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 §
173-16A.
(1) Post-development total runoff shall not be increased from predevelopment
total runoff for all storms equal to or less than the two-year twenty-four-hour
duration precipitation.
[Amended 7-6-2016 by Ord.
No. 402]
(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. Table B-3 in Ordinance
Appendix B is available to guide a qualified professional and/or
an applicant to calculate the stormwater runoff volume. 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
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=
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2-year Rainfall (in)
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S
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=
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(1000/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
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=
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Runoff (in)
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Area
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=
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SWM Area (square ft)
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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.
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Volume (cubic feet) = (2 inches 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 inch 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 applicant is required
to submit a SWM site plan which shall determine the total acreage
of protected area where no disturbance is proposed.
(a)
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 =
{Total Site Area (for both pre and post development conditions)
– Protected Area}
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(b)
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.
(2) Calculate the volume controls provided through nonstructural BMPs.
Table B-5 in Ordinance 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
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Nonstructural
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Structural Volume
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Volume Control (ft3)
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–
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Volume Control (ft3)
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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 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 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 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 addresses soil permeability,
depth to bedrock, depth to seasonally high water table 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 sub-grade
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 two.
(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.
(f)
All runoff to an infiltration or extended detention subsurface
basin that utilizes a stone bed must either be pretreated or the basin's
storage volume increased to account for the loss of volume in the
stone bed due to sediment accumulation. This loss should be based
upon the expected life of the basin. If pretreatment is needed, it
must remove 50% of the total suspended solids in the runoff from the
basin's maximum design storm.
(g)
Groundwater mounding may occur beneath stormwater management
structures designed to infiltrate stormwater runoff. Concentrating
recharge in a small area can cause groundwater mounding that affects
the basements of nearby homes and other structures. A groundwater
mounding analysis must be performed to determine whether or not the
underlying aquifer will be able to manage the infiltration loading
proposed without raising the groundwater to within two feet of the
infiltration surface or affecting nearby structures. A simplified
spreadsheet was developed by USGS to solve the Hantush Analytical
Equation, which can be used to calculate groundwater mounding. The
documentation and spreadsheet can be found in the USGS publication
Simulation of Groundwater Mounding Beneath Hypothetical Stormwater
Infiltration Basin, also available at http://pubs.usgs.gov/sir/2010/5102/.
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 Delaware River South Watershed.
A. Standards for managing runoff from each subarea in the Delaware River
South Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year design storms are shown in Table 173-15.1. 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 173-15.1.
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Table 173-15.1
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Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Delaware River South Watershed
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District
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Design Storm Postdevelopment
(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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50-year
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100-year
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100-year
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C*
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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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*
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In District C, development sites which can discharge directly
to the Delaware River South main channel or major tributaries or indirectly
to the main channel through an existing stormwater drainage system
(i.e., storm sewer or tributary) may do so without control of postdevelopment
peak rate of runoff greater than the five-year storm. Sites in District
C will still have to comply with the groundwater recharge criteria,
the water quality criteria, and stream bank erosion criteria. If the
postdevelopment runoff is intended to be conveyed by an existing stormwater
drainage system to the main channel, assurance must be provided that
such system has adequate capacity to convey the flows greater than
the two-year predevelopment peak flow or will be provided with improvements
to furnish the required capacity. When adequate capacity in the downstream
system does not exist and will not be provided through improvements,
the postdevelopment peak rate of runoff must be controlled to the
predevelopment peak rate as required in District A provisions (i.e.,
ten-year postdevelopment flows to ten-year predevelopment flows) for
the specified design storms.
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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 chapter.
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 the Township of Lower Makefield:
(1) Meet the full requirements specified by Table 173-15.1 and §
173-15A through
F; 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.
H. "No-harm" option. For any proposed development site not located in a provisional direct discharge district, the applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. The no-harm option is used when an applicant can prove that the postdevelopment hydrographs can match predevelopment hydrographs, or if it can be proved that the postdevelopment conditions will not cause increases in peaks at all points downstream. Proof of "no-harm" would have to be shown based upon the following downstream impact evaluation, which shall include a downstream hydraulic capacity analysis consistent with §
173-15H, to determine if adequate hydraulic capacity exists. The land applicant shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1) The downstream impact evaluation shall include hydrologic and hydraulic
calculations necessary to determine the impact of hydrograph timing
modifications due to the proposed development upon a dam, highway,
structure, natural point of restricted stream flow, or any stream
channel section, established with the concurrence of the municipality.
(2) The evaluation shall continue downstream until the increase in flow
diminishes due to additional flow from tributaries and/or stream attenuation.
(3) The peak flow values to be used for downstream areas for the design
return period storms (two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year) shall be the values from the calibrated model for
the Delaware River South Watershed. These flow values can be obtained
from the watershed plan.
(4) Applicant-proposed runoff controls that would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove "no-harm," except in conjunction with proposed capacity improvements for the problem areas consistent with §
173-15H.
(5) Financial considerations shall not constitute grounds for granting
a no-harm exemption.
(6) Capacity improvements may be provided as necessary to implement the
no-harm option which proposes specific capacity improvements to provide
that a less stringent discharge control would not create any harm
downstream.
(7) Any no-harm justifications shall be submitted by the applicant as part of the drainage plan submission per Article
IV.
I. Downstream hydraulic capacity analysis. Any downstream capacity hydraulic
analysis conducted in accordance with this chapter shall use the following
criteria for determining adequacy for accepting increased peak flow
rates:
(1) Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the Department of Environmental Protection's Erosion and Sediment
Pollution Control Program Manual.
(2) Natural or man-made channels or swales must be able to convey increased
twenty-five-year return period runoff without creating any hazard
to persons or property.
(3) Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area must be designed in accordance
with the Department of Environmental Protection's Chapter 105
regulations (if applicable) and, at minimum, pass the increased twenty-five-year
return period runoff.
J. Regional detention alternatives. For certain areas within the study
area, it may be more cost-effective to provide one control facility
for more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective applicants. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined on a case-by-case
basis using the hydrologic model of the watershed consistent with
protection of the downstream watershed areas. Hydrologic model refers
to the calibrated model as developed for the stormwater management
plan. It is a requirement that, even if regional basins are proposed
for the water quantity control, that the water quality, streambank
erosion and recharge criteria be accomplished on site or as close
to the source of the runoff as possible.