A.
For any of the activities regulated by this chapter, unless preparation
of a stormwater management (SWM) site plan is specifically exempted,
the preliminary or final approval of subdivision and/or land development
plans, the issuance of any building or occupancy permit, the commencement
of any earth disturbance activity may not proceed until the property
owner or applicant or his/her agent has received written approval
from the Municipality of a SWM site plan that demonstrates compliance
with the requirements of this chapter, and a written approval of an
adequate erosion and sediment (E&S) control plan from the Municipality
or County Conservation District when required.
B.
SWM site plan approved by the Municipality shall be on site throughout
the duration of the regulated activity.
C.
The Municipality may, after consultation with the Department of Environmental
Protection (DEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law including but not limited to the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, 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 chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. DEP regulations require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pa. Code § 102.4(b). In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction activities permit is required for regulated earth disturbance activities. A copy of the erosion and sediment control plan and any required permit, as required by DEP regulations, shall be available on the project site at all times. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual), No. 363-2134-008 (April 15, 2000), as amended and updated. However, the Municipality may require E&S controls for projects with lesser areas of earth disturbance.
F.
Impervious areas:
G.
Stormwater discharges onto adjacent property shall not be created,
increased, decreased, relocated, or otherwise altered without written
notification of the adjacent property owner(s). Such stormwater discharges
shall be subject to the requirements of this chapter.
H.
No regulated activities shall commence until the Municipality issues
written approval of an SWM site plan, which demonstrates compliance
with the requirements of this chapter.
I.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety and property;
(2)
Meet the water quality goals of this chapter by implementing
measures to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques
for low-impact development practices described in the Pennsylvania
Stormwater Best Management Practices Manual (BMP Manual).
J.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
K.
Infiltration BMPs should be dispersed on site, made as shallow as
practicable, and located to maximize use of natural onsite infiltration
features while still meeting the other requirements of this chapter.
L.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
M.
The design storm precipitation amounts to be used in the analysis
of peak rates of discharge shall be those from the upper limits of
the 90% confidence intervals for the twenty-four-hour precipitation
events in 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
(NWS), Hydrometeorological Design Studies Center (HDSC), Silver Spring,
Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
N.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Storm Water Management Act.
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.
A.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the Municipality.
B.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire drainage area contributory to the infiltration BMP
has achieved final stabilization.
The design of all regulated activities should include the following
to minimize stormwater impacts: (See Subappendix B-3 for a Nonstructural
Project Design Checklist.)[1]
A.
The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain as
much as possible the natural hydrologic regime of the site.
B.
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
C.
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
[1]
Editor's Note: Appendixes A through J are on file in
the Township offices.
A.
Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this chapter as listed in § 142-106:
(1)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones such as clay
layers.
(b)
An infiltration rate sufficient to accept the additional stormwater
load (stormwater load is the quantity above the pre-project condition
quantity) and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
(2)
The size of the infiltration facility shall be based upon the
following volume criteria:
(a)
Where practicable and appropriate the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to 1.0
inch of runoff (I) over all proposed impervious surfaces.
(b)
The Rev required shall be computed as:
|
Rev = (1/12) * (I)
|
|
Where:
| ||||
|---|---|---|---|---|
|
Rev
|
=
|
Recharge Volume (cubic feet)
| ||
|
I
|
=
|
Impervious Area within the limits of earth disturbance (square
feet)
| ||
|
An asterisk (*) in equations denotes multiplication.
| ||||
B.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified person, and at a minimum address
soil permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(1)
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.
(2)
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.
(3)
Design the infiltration structure for the required recharge
volume (Rev) based on field determined capacity
at the level of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
qualified person, it must be demonstrated to the Municipality that
the soils are conducive to infiltrate on the lots identified.
(5)
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.
The low-impact development practices provided in the BMP Manual
shall be utilized for all regulated activities to the maximum extent
practicable. Water volume controls shall be implemented using the
Design Storm Method in Subsection A or the Simplified Method in Subsection
B below. For regulated activity areas 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 on applicability of the
analytical procedures associated with each methodology, and other
factors. All regulated activities greater than one acre must use the
Design Storm Method.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
The post-development total runoff volume for all storms equal
to or less than the two-year, twenty-four-hour storm event shall not
be increased.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below 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 at least the first two inches
of runoff from all new impervious surfaces. [Note: An asterisk (*)
in equations denotes multiplication.]
|
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
|
(2)
At least the first one inch of runoff from 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. Removal
options include reuse, evaporation, transpiration, and infiltration.
|
Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
|
(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
A.
The applicant shall comply with the following water quality requirements
of this article.
(1)
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The recharge volume computed under § 142-403 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration best management practices (BMPs). The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of
stormwater runoff from the developed areas of the site.
B.
The following calculation formula is to be used to determine the
water quality storage volume (WQv) in cubic
feet:
|
WQv = [(P)*(Rv)*(A)]/12
|
|
Where:
| ||||
|
WQv
|
=
|
Water quality volume (acre-feet)
| ||
|
P
|
=
|
1 inch
| ||
|
A
|
=
|
Area of the project contributing to the water quality BMP (acres)
| ||
|
Rv
|
=
|
0.05 + 0.009(I) where I is the percent of the area that is impervious
surface (Impervious surface/A)*100)
| ||
|
An asterisk (*) in equations denotes multiplication.
| ||||
C.
For a noninfiltrating BMP, release of water can begin at the start
of the storm (i.e., the invert of the water quality orifice is at
the invert of the facility). The design of the facility shall provide
for protection from clogging and unwanted sedimentation.
A.
If a perennial or intermittent stream passes through the site, the
applicant shall create a riparian buffer extending a minimum of 50
feet to either side of the top-of-bank of the channel. The buffer
area shall be established and maintained in an undisturbed state.
This buffer area may be maintained as a meadow with minimal mowing
of the grassed area, or as a forested buffer, being planted with appropriate
native vegetation (refer to Appendix B of the BMP Manual for plant
lists). If the applicable rear or side yard setback is less than 50
feet, the buffer width may be reduced to 25% of the setback to a minimum
of 10 feet. If an existing buffer is legally prescribed (i.e., deed,
covenant, easement, etc.) and it exceeds the requirements of this
chapter, the existing buffer shall be maintained. This buffer requirement
is for perennial or intermittent streams and does not include lakes
or wetlands.
B.
Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
(1)
In addition to the control of water quality volume (in order
to minimize the impact of stormwater runoff on downstream stream bank
erosion), the primary requirement is to design a BMP to detain the
proposed conditions two-year, twenty-four-hour storm event to the
existing conditions one-year flow using the SCS Type II distribution.
Additionally, provisions shall be made (such as adding a small orifice
at the bottom of the outlet structure or a sand filter) so that the
proposed conditions one-year, twenty-four-hour storm event takes at
least 24 hours to drain from the facility from a point when the maximum
volume of water from the one-year, twenty-four-hour storm event is
captured (i.e., the maximum water surface elevation is achieved in
the facility). Release of water can begin at the start of the storm
(i.e., the invert of the water volume control or [or water quality]
orifice is at the invert of the facility).
(2)
The minimum orifice size in the outlet structure to the BMP
shall be three inches in diameter where possible, and a trash rack
shall be installed to prevent clogging. On sites with small drainage
areas contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake. It is recommended that the design,
to accommodate maintenance, include a sand or porous media filter.
A.
Area P.
(1)
The Pennypack Creek Watershed has been divided into stormwater
management districts as shown on the Management District Map (Figure
409.1P).[1] Portions of Abington Township are in Districts A and B.
(a)
In addition to the requirements specified in Table 409.1 below, the erosion and sedimentation control (§ 142-403), the nonstructural project design (§ 142-404), the groundwater recharge (§ 142-405), the water volume control (§ 142-406), and the stream bank erosion (§ 142-408) requirements shall be implemented.
(b)
Standards for managing runoff from each subarea in the Pennypack
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year storm events are shown in Table 409.1P. Development
sites located in each of the management districts must control proposed
condition runoff rates to existing condition runoff rates for the
design storms in accordance with Table 409.1P.
|
Table 409.1P
| ||||
|---|---|---|---|---|
|
Peak Rate Control Standards by Stormwater Management District
in the Pennypack Creek Watershed
| ||||
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
A
|
2-year
|
Reduce to
|
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
|
Reduce to
|
1-year
| |
|
5-year
|
2-year
| |||
|
10-year
|
5-year
| |||
|
25-year
|
10-year
| |||
|
50-year
|
25-year
| |||
|
100-year
|
50-year
| |||
[1]
Editor's Note: Said figure is included as an attachment to this chapter.
(2)
General. Proposed condition 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 (Figure 409.1P).[2]
[2]
Editor's Note: Said figure is included as an attachment to this chapter.
(3)
District A is shown as the green area on Figure 409.1P Area P Management District Watershed Map.[3]
[3]
Editor's Note: Said figure is included as an attachment to this chapter.
(4)
District B is shown as the blue area on Figure 409.1P Area P Management District Watershed Map.[4]
[4]
Editor's Note: Said figure is included as an attachment to this chapter.
(5)
District C is shown as the pink area on Figure 409.1P Area P Management District Watershed Map.[5]
[5]
Editor's Note: Said figure is included as an attachment to this chapter.
(6)
District boundaries. The boundaries of the stormwater management
districts are shown on an official map that is available for inspection
at the municipal and County Planning offices. A copy of the official
map at a reduced scale is included as Figure 409.1P.[6] 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 drainage
plan.
[6]
Editor's Note: Said figure is included as an attachment to this chapter.
(7)
Sites located in more than one district. For a proposed development
site located within two or more stormwater management districts, the
peak discharge rate from any subarea shall meet the management district
criteria in which the discharge is located.
(8)
Off-site areas. Off-site areas that drain through a proposed
development site are not subject to release rate criteria when determining
allowable peak runoff rates. However, on-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
(9)
Site areas. Where the site area to be impacted by a proposed
development activity differs significantly from the total site area,
only the proposed impact area utilizing stormwater management measures
shall be subject to the management district criteria. In other words,
unimpacted areas bypassing the stormwater management facilities would
not be subject to the management district criteria.
(10)
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 Abington Township:
(a)
Meet the full requirements specified by Table 409.1P and § 142-409A(1) through 5; or
(b)
Reduce the total impervious surface on the site by at least
20% based upon a comparison of existing impervious surface to proposed
impervious surface.
B.
Area T.
(1)
The Tookany/Tacony-Frankford Watershed has been divided into
stormwater management districts as shown on the Management District
Map (Figure 409.1T).[7] Portions of Abington Township are in Districts A and B.
(a)
In addition to the requirements specified in Table 409.1T below, the erosion and sedimentation control (§ 142-403), the nonstructural project design (§ 142-404), the groundwater recharge (§ 142-405), the water quality (§ 142-407), and the stream bank erosion (§ 142-408) requirements shall be implemented.
(b)
Standards for managing runoff from each subarea in the Tookany/Tacony-Frankford
Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
storm events are shown in Table 409.1T. Development sites located
in each of the management districts must control proposed condition
runoff rates to existing condition runoff rates for the design storms
in accordance with Table 409.1T.
|
Table 409.1T
| ||||
|---|---|---|---|---|
|
Peak Rate Control Standards by Stormwater Management District
in the Tookany/Tacony-Frankford Creek Watershed
| ||||
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
A
|
2-year
|
Reduce to
|
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
|
Reduce to
|
1-year
| |
|
5-year
|
2-year
| |||
|
10-year
|
5-year
| |||
|
25-year
|
10-year
| |||
|
50-year
|
25-year
| |||
|
100-year
|
100-year
| |||
[7]
Editor's Note: Said figure is included as an attachment to this chapter.
(2)
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 (Figure 409.1T)[8] and this section of this chapter.
[8]
Editor's Note: Said figure is included as an attachment to this chapter.
(3)
District boundaries. The boundaries of the stormwater management
districts are shown on an official map that is available for inspection
at the municipal and County Planning offices. 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 drainage plan.
(4)
Sites located in more than one district. For a proposed development
site located within two or more stormwater management districts, the
peak discharge rate from any subarea shall meet the management district
criteria in which the discharge is located.
(5)
Off-site areas. Off-site areas that drain through a proposed
development site are not subject to release rate criteria when determining
allowable peak runoff rates. However, on-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
(6)
Site areas. Where the site area to be impacted by a proposed
development activity differs significantly from the total site area,
only the proposed impact area utilizing stormwater management measures
shall be subject to the management district criteria. In other words,
unimpacted areas bypassing the stormwater management facilities would
not be subject to the management district criteria.
(7)
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 Abington:
(a)
Meet the full requirements specified by Table 409.1T and § 142-409B(1) through (6); or
(b)
Reduce the total impervious surface on the site by at least
20% based upon a comparison of existing impervious surface to proposed
impervious surface.
C.
Area W.
(1)
The Wissahickon Creek Watershed has been divided into stormwater
management districts as shown on the Management District Map (Figure
409.1W).[9]
(a)
The peak rate requirements specified in Table 409.1W below shall
be implemented in addition to all other applicable requirements.
(b)
Standards for managing peak rates of runoff from each subarea
in the Wissahickon Creek Watershed for the two-, five-, ten-, twenty-five-,
fifty-, and one-hundred-year storm events are shown in Table 409.1W.
Development sites located in each of the management districts must
control proposed condition runoff rates to existing condition runoff
rates for the design storms in accordance with Table 409.1W.
|
Table 409.1W
| ||||
|---|---|---|---|---|
|
Peak Rate Control Standards by Stormwater Management District
in the Wissahickon Creek Watershed
| ||||
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
A
|
2-year
|
Reduce to
|
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
|
Reduce to
|
1-year
| |
|
5-year
|
2-year
| |||
|
10-year
|
5-year
| |||
|
25-year
|
10-year
| |||
|
50-year
|
25-year
| |||
|
100-year
|
50-year
| |||
[9]
Editor's Note: Said figure is included as an attachment to this chapter.
(2)
General. Proposed condition 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 (Figure 409.1W).[10]
[10]
Editor's Note: Said figure is included as an attachment to this chapter.
(3)
District boundaries. The boundaries of the stormwater management
districts are shown on an official map that is available for inspection
at the municipal and County Planning offices. A copy of the official
map at a reduced scale is included as Figure 409.1W.[11] 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 drainage
plan.
[11]
Editor's Note: Said figure is included as an attachment to this chapter.
(4)
Sites located in more than one district. For a proposed development
site located within two or more stormwater management districts, the
peak discharge rate from any subarea shall meet the management district
criteria in which the discharge is located.
(5)
Off-site areas. Off-site areas that drain through a proposed
development site are not subject to release rate criteria when determining
allowable peak runoff rates. However, on-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
(6)
Site Areas. Where the site area to be impacted by a proposed
development activity differs significantly from the total site area,
only the proposed impact area utilizing stormwater management measures
shall be subject to the management district criteria. In other words,
unimpacted areas bypassing the stormwater management facilities would
not be subject to the management district criteria.
(7)
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 Abington Township:
(a)
Meet the full requirements specified by Table 409.1W and § 142-409C(1) through (6); or
(b)
Reduce the total impervious surface on the site by at least
20% based upon a comparison of existing impervious surface to proposed
impervious surface.
(8)
Stormwater control measures which increase storage or infiltration
volume, and which are not associated with new land development or
redevelopment activity that increases runoff volume above existing
levels, are exempt from the peak rate requirements of this chapter,
so long as peak outflow is not increased.
A.
Stormwater runoff from all development sites with a drainage area
of greater than 200 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS soil cover complex
method. The qualified person must consult with the Municipality to
gain approval of design methods prior to design.
(1)
Table 410-1 summarizes acceptable computation methods and the
method selected by the qualified person shall be based on the individual
limitations and suitability of each method for a particular site.
The Municipality may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than 200 acres.
The Soil Cover Complex Method shall be used for drainage areas greater
than 200 acres.
|
Table 410-1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|
Method
|
Method Developed by
|
Applicability
| |
|
TR-20 or WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55 or WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
Rational Method or commercial computer package based on Rational
Method
|
Emil Kuichling (1889)
|
For sites less than 200 acres and with times of concentration
less than 60 minutes (tc< 60 min), or as approved by the Municipality
and/or Municipal Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Municipality
and/or Municipal Engineer
| |
|
*
|
Note: Successors to the above methods are also acceptable.
|
B.
If a hydrologic computer model such as HydroCAD or HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NRCS
Type II.
C.
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow," unless the natural
ground cover generates a lower curve number or Rational 'C'
value (i.e., forest), as listed in Table E-1 or E-2 in Appendix E
of this chapter.[1]
[1]
Editor's Note: Appendixes A through J are on file in
the Township offices.
D.
All calculations using the Rational Method shall use rainfall intensities
from the NOAA 14 Precipitation-Frequency Atlas of the United States
(2004, revised 2006). 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 or replaced
from time to time by NRCS). Times-of-concentration for channel and
pipe flow shall be computed using flow velocities as determined by
Manning's equation. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
G.
The Manning equation is preferred for 1-D, gradually-varied, open
channel flow. In other cases, appropriate, applicable methods should
be applied; however, early coordination with the Municipality is necessary.
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the Municipality.
I.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than 200 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The Municipality 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.
