The stormwater performance standards contained in this section
are intended to implement the standards and criteria contained in
the Erie County Stormwater Management Plan adopted and approved in
accordance with the Act. If there is any discrepancy between the provisions
of this section and the standards and criteria of the plan, or if
the County's Stormwater Management Plan is subsequently amended,
then the standards/criteria of this section shall control if they
are more stringent than those in the plan and those of the plan or
amended watershed plan shall govern if they are more stringent than
those in this chapter.
A. Storm frequencies/rainfall values. Stormwater management facilities
on all development sites shall control the peak stormwater discharge
for the two-, ten-, twenty-five-, fifty- and one-hundred-year storm
frequencies. The twenty-four-hour total rainfall for these storm frequencies
in the Township are:
|
Return Interval
(year)
|
24-hour Rainfall Total
(inches)
|
---|
|
1
|
2.13
|
|
2
|
2.56
|
|
10
|
3.71
|
|
25
|
4.46
|
|
50
|
5.09
|
|
100
|
5.76
|
B. Calculation methodology.
(1) All calculations shall be consistent with the guidelines set forth
in the BMP Manual, as amended herein.
(2) Runoff calculation. The NRCS twenty-four-hour, Type II Rainfall Distribution
shall be used for analyzing stormwater runoff for both pre- and postdevelopment
conditions.
(3) 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: Soil-Cover-Complex (SCS) Method, Technical
Release TR-55 or 20, or the HECI or Penn State Runoff Model (PSRM)
or Stormwater Management Model (SWMM). Under special circumstances,
the Township may approve use of other computation methods.
(4) The Rational Method shall not be used to calculate runoff volume.
(5) Peak flow rates.
(a)
The NRCS Rainfall-Runoff Method is recommended for design of
stormwater management facilities and where stormwater runoff must
be taken into consideration. As to the model applicability: there
are no size limitations as to NRCS TR-20 or HEC-HMS and SWMM; other
models shall be as preapproved by the Township. Other methods upon
approval by the Township may be applied.
(b)
The NRCS antecedent runoff condition II (ARC II, previously
AMC II) must be used for all simulations. Use of continuous simulation
models that vary the ARC is not permitted for stormwater management
purposes.
(c)
Rainfall intensities for design should be obtained from those sources authorized in §
116-12K above.
(d)
The Rational Method may be used for design of conveyance facilities
only. Extreme caution should be used by the design professional if
the watershed has more than one main drainage channel. If the watershed
is divided so that hydrologic properties are significantly different
in one versus the other, if the time of concentration exceeds 60 minutes,
or if stormwater runoff volume is an important factor. The combination
of Rational Method hydrographs based on timing is prohibited.
(e)
For comparison of peak flow rates, flows shall be rounded to
a tenth of a cubic foot per second (cfs).
(6) Routing of hydrographs through detention/retention facilities for
the purpose of designing those facilities shall be accomplished using
the Modified-Plus Method or other recognized reservoir routing method
subject to Township's approval.
(7) HEC-HMS Model Calibrated and/or SWMM modeling is acceptable.
(8) The Township may require that computed existing runoff rates be reconciled
with field observations, conditions and site history. If the designer
can substantiate, through actual physical calibration, that more appropriate
runoff and time of concentration values should be utilized at a particular
site, then appropriate variations may be made upon review and recommendation
of the Township.
(9) Runoff coefficients.
(a)
Rational Method (as authorized): use Table D-1 (Appendix D).
(b)
NRCS Rainfall-Runoff Method: use Table D-2 (Appendix D). Curve
numbers (CN) should be rounded to tenths for use in hydrologic models
as they are a design tool with statistical variability. For large
sites, CNs should realistically be rounded to the nearest whole number.
(10)
Time of concentration.
(a)
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. The following
time of concentration (Tc) computational methodologies
shall be used unless another method is preapproved by the Township:
[1]
Predevelopment: NRCS's Lag Equation:
|
Time of Concentration = Tc = [(Tlag"/0.6)* 60] (minutes)
|
|
Where:
|
|
|
Tlag
|
=
|
Lag time (hours)
|
|
|
L
|
=
|
Hydraulic length of watershed (feet)
|
|
|
Y
|
=
|
Average overland slope of watershed (percent)
|
|
|
S
|
=
|
Maximum retention in watershed as defined by:
S = [(1,000/CN) - 10]
|
|
|
CN
|
=
|
NRCS curve number for watershed
|
[2]
Postdevelopment commercial, industrial or other areas with large
impervious areas (> 20% impervious area): NRCS Segmental Method. The
length of sheet flow shall be limited to 100 feet. Tc for channel and pipe flow shall be computed using Manning's
equation.
(b)
Additionally, the following provisions shall apply to calculations
for time of concentration:
[1]
The postdevelopment Tc shall never be
greater than the predevelopment Tc for any
watershed or subwatershed. This includes when the designer has specifically
used swales to reduce flow velocities. In the event that the designer
believes that the postdevelopment Tc is greater,
it will still be set by default equal to the predevelopment Tc for modeling purposes.
[2]
The minimum Tc for any watershed shall
be five minutes.
[3]
The designer must provide computations for all predevelopment
Tc paths. A five-minute Tc cannot be assumed for predevelopment.
[4]
Undetained fringe areas (areas that are not tributary to a stormwater
facility but where a reasonable effort has been made to convey runoff
from all new impervious coverage to best management practices) may
be assumed to represent the predevelopment conditions for purposes
of Tc calculation.
C. Predevelopment conditions. Predevelopment conditions shall be assumed
to be those which existed on any site at the time prior to the commencement
of any development activities. Where a site has been developed previously
and is proposed to be redeveloped, the predevelopment coefficient
for the developed portion of the site shall be based upon an assumed
50% native land cover (i.e., grass, trees, etc.) and 50% developed
land cover (e.g., buildings, pavement, etc.).
(1) NRCS runoff curve numbers selected for use in the calculations shall
accurately reflect existing conditions subject to the approval of
the Township.
(2) 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 NRCS 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.
(1) Definition. The release rate percentage defines the percentage of
the predevelopment peak rate of runoff that can be discharged from
an outfall on the site after development under the one-, two-, ten-,
twenty-five- and fifty-year storm conditions. Under all circumstances,
a one-hundred-percent release rate percentage shall be applied to
the one-hundred-year frequency storm, whether or not lower reduced
release rate percentage are specified for the one-, two-, 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
Map attached hereto and made a part of Appendix A. This Appendix was
established under the County of Erie's 1996 Act 167 Plan and
is being retained to better protect the public health and safety.
(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 the information in Appendix A.
(b)
Compute the pre- and postdevelopment runoff hydrographs for
each stormwater outfall on the development site using an acceptable
calculation method for the one-, two-, 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.
(c)
If the postdevelopment peak runoff rate is greater than the
predevelopment peak runoff rate, or if such runoff exceeds release
rates established in this chapter, then stormwater detention shall
be required. The capacity of the detention facility shall be calculated
by multiplying the predevelopment rate of runoff from the one-, two-,
ten-, twenty-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 basin.
E. Exception areas. Exceptions to the peak discharge standards will
be considered only in the following instances:
(1) In the Lake Erie Area Watershed: only in instances where the discharge
from the development site occurs directly to Lake Erie, an adequately
sized storm or combined sewer which discharges directly into Lake
Erie, 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 Township.
On all lands within Millcreek Township, the postdevelopment
peak discharge rates shall be in accordance with the release rate
map for the Lake Erie Watershed as set forth in Appendix A of this
chapter. See also §
116-13D.
In developments not subject to this chapter under §
116-3, a grading plan must be submitted to and approved by the Township Engineer prior to issuance of any building or local street access permit.
No person shall modify or alter any approved and/or implemented stormwater management or grading plan without the prior approval of the Board of Supervisors. This prohibition shall extend to the developer and to all successors and assigns, including grantees, owners, lessees, occupants and their contractors and/or agents. Any violation of this section shall be subject to the remedies and penalties set forth in Article
XIII, Prohibited Acts, Enforcement Procedures and Remedies, of this chapter.