A.
No discharge of toxic materials shall be permitted
into any stormwater management system. Where required by federal and
state regulation, the landowner or developer shall be responsible
for obtaining an NPDES permit for stormwater discharges.
A.
The stormwater performance standards contained in
this section are intended to implement the standards and criteria
contained in the Ohio River Watershed Stormwater Management Plan,
adopted and approved in accordance with the Pennsylvania Storm Water
Management Act. If there is any discrepancy between the provisions
of this section and the standards and criteria of the plan, or if
the watershed plan is subsequently amended, then the standards/criteria
of the amended watershed plan shall govern.
B.
Storm frequencies. Stormwater management facilities
on all development sites shall control the peak stormwater discharge
for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
storm frequencies. The Soil Conservation Service (SCS) 24-hour, Type
II Rainfall Distribution shall be used for analyzing stormwater runoff
for both pre- and post-development conditions. The twenty-four-hour-total
rainfall for these storm frequencies in the watershed is:
|
Storm Frequency
|
Rainfall Depth
(inches)
| |
|---|---|---|
|
2-year
|
2.6
| |
|
5-year
|
3.3
| |
|
10-year
|
3.9
| |
|
25-year
|
4.4
| |
|
50-year
|
4.75
| |
|
100-year
|
4.95
|
C.
Calculation methods.
(1)
Development sites. For the purpose of computing peak
flow rates and runoff hydrographs from development sites, calculations
shall be performed using one of the following: SCS publications, Technical
Release (TR) 55 or 20, HEC I or Penn State Runoff Model (PSRM), or
Modified Rational Method. Under special circumstances, other computation
methods may be used, subject to the approval of the municipality.
(2)
Stormwater collection/conveyance facilities. For the
purposes of designing storm sewers, open swales and other stormwater
runoff collection and conveyance facilities, the Rational Method or
other method as approved by the municipality may be applied. Rainfall
intensities for design should be obtained from the Pennsylvania Department
of Transportation rainfall charts.
(3)
Routing of hydrographs through detention/retention
facilities for the purpose of designing those facilities shall be
accomplished using the Modified-Pulse Method or other recognized reservoir
routing method, subject to the approval of the municipality.
(4)
Predevelopment conditions. Predevelopment conditions
shall be assumed to be those which exist on any site at the time prior
to the commencement of development activities. SCS runoff curve numbers
selected for use in the calculations shall accurately reflect existing
conditions, subject to the approval of the municipality. At its discretion,
the municipality may direct that hydrologic conditions for all areas
with pervious cover (i.e., fields, woods, lawn areas, pastures, cropland,
etc.) shall be assumed to be in "good" condition, and the lowest recommended
SCS runoff curve number (CN) shall be applied for all pervious land
uses within the respective range for each land use and hydrologic
soil group.
D.
Release rate percentage.
(1)
Definition: The release rate percentage of the predevelopment
peak rate of runoff that can be discharged from an outfall on the
site after development under the two-, five-, ten-, twenty-five-,
and fifty-year storm conditions. Under all circumstances, a 100% release
rate percentage shall be applied to the one-hundred-year frequency
storm, whether or not lower reduced release rate percentages are specified
for the two-, five-, ten-, twenty-five-, and fifty-year return frequency
storms. The assigned release rate percentage for each subarea applies
uniformly to all land development or alterations within the subarea.
A listing of the release rate percentage by subarea appears in Appendix
A of this chapter; the subareas are delineated on the Release Rate
Percentage Maps.
[Amended 4-14-2004 by Ord. No. 99]
(2)
Procedure for use.
(a)
Identify the specific subarea in which the development
site is located from the watershed map and obtain the subarea release
rate percentage from Appendix A and the appropriate Release Rate Percentage
Map.
(b)
Compute the pre- and post-development runoff
hydrographs for each stormwater outfall on the development site using
an acceptable calculation method for the two- five-, ten-, twenty-five-,
fifty- and one-hundred-year storms. Apply no on-site detention for
stormwater management but include any techniques to minimize impervious
surfaces and/or increase the time of concentration for stormwater
runoff flowing from the development site. If the post-development
peak runoff rate and the runoff volume are less than or equal to the
predevelopment peak runoff rate and volume, then additional stormwater
control shall not be required at that outfall.
(c)
If the post-development peak runoff rate and
volume are greater than the predevelopment peak runoff rate and volume,
then stormwater detention shall be required. The capacity of the detention
facility shall be calculated by multiplying the predevelopment rate
of runoff from the two-, five-, ten-, tweny-five- and fifty-year frequency
storm by the subarea release rate percentage and the predevelopment
runoff rate from the one-hundred-year storm event by 100% to determine
the maximum allowable releases from any detention facility.
E.
Exception areas.
(1)
Exceptions to the peak discharge standards will be
considered only in the following instances:
(a)
In the Ohio River Area Watershed: only in instances
where the discharge from the development site occurs directly to the
Ohio River, an adequately sized storm or combined sewer which discharges
directly into Ohio River, or through a properly sized and designed
regional stormwater detention facility.
(2)
The analyses of storm or combined sewers and/or regional
detention facilities as are necessary to demonstrate their adequacy
for the proposed discharges shall be completed by the applicant using
methods and procedures as directed by the municipality.
