[Ord. 7-98, 9/1/1998, § 201]
1. Method of Computation.
A. The design of stormwater detention facilities intended to meet the
performance standards of this chapter shall be verified by routing
a full design storm hydrograph through the proposed facility to estimate
outflows. Peak discharge and runoff volumes shall be computed using
the Soil Cover Complex Method as set forth in the latest edition of
"Urban Hydrology for Small Watersheds," Technical Release No. 55,
as published by Soil Conservation Service, or its successors, or by
any other method approved by the Township Engineer.
B. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
C. Impacts of postactivity stormwater flows to downstream areas and
stormwater conveyance systems shall be evaluated. Critical points
downstream of the proposed activity shall be analyzed and peak pre
and postactivity stormwater flows shall be estimated and compared
to the hydraulic capacity of the existing conveyance system.
2. Stormwater Conveyance.
A. Calculation Methodology.
(1)
For drainage areas less than 20 acres, peak stormwater discharges may be computed using the Rational Method as set forth in § 10.4 of the Pennsylvania Department of Transportation's "Design Manual," Part
2, "Highway Design," Publication 13, January, 1990 edition or by any other method approved by the Township Engineer.
(2)
For drainage areas equal to or more than 20 acres, peak stormwater
discharge shall be computed using the Soil Cover Complex Method as
set forth in the latest revision of "Urban Hydrology for Small Watersheds,"
Technical Release No. 55, as published by the Soil Conservation Service,
or its successors, or by any other method approved by the Township
Engineer.
(3)
Manning's Equation shall be used for hydraulic computations
and to determine the capacity of open channels, culverts and storm
sewers.
(4)
Swales and ditches shall be designed in such a manner that nonerosive
velocities will not be exceeded in open channels, culverts and storm
sewers.
B. Upgrading the Existing Stormwater Conveyance System. The applicant
may be required to upgrade existing down stream stormwater conveyance
systems to provide adequate capacity to accommodate post activity
stormwater flows.
3. Maintenance of Natural Drainage Ways. All natural streams, channels,
swales, drainage systems and/or areas of surface water concentration
shall be maintained in their existing condition unless an alteration
is approved by Township. All encroachment activities shall comply
with the requirements of chapter 105 (Water Obstructions and Encroachments)
of Title 25, Rules and Regulations of the Pennsylvania Department
of Environmental Protection.
4. Methods of Stormwater Runoff Detention and Control.
A. The following is a listing of detention and control methods which
may be utilized in stormwater management systems, if appropriate.
The choice of control techniques is not limited to the ones appearing
on this list.
(3)
Parking lot and street ponding.
(4)
Seepage pits, seepage trenches or other infiltration structures.
(5)
Porous pavement and concrete lattice block surfaces.
(6)
Grassed channels and vegetated strips.
(7)
Cisterns and underground reservoirs.
(9)
Decreased impervious area coverage.
B. The use of other control methods which meet the criteria in this
section will be permitted when approved by the Township Engineer.
Various combinations of methods should be tailored to suit the particular
requirements of the type of development and the topographic features
of the project area.
C. Stormwater management facilities shall be capable of safely conveying runoff from the post development one-hundred-year storm while providing one foot of freeboard in the management facility. This overflow shall not be utilized during the design storms listed in §
26-202, Subsection
1A. The Township may require fences, covers or other items to limit access.
5. Design. The applicant is urged to consult the publications listed
in the Appendix to this chapter for aid in design of control methods.
The Appendix is not a part of this chapter.
[Ord. 7-98, 9/1/1998, § 202]
1. Stormwater Rate.
A. Stormwater runoff from any activity covered by this chapter shall
be controlled such that after the activity, the land disturbed will
generate, at a maximum, no greater peak flow than prior to the activity
for a two year, ten-year and twenty-five-year, twenty-four-hour storm,
considered individually. Further, reduction in the magnitude of postactivity
peak flows may be required to prevent exceeding the capacity of existing
downstream conveyance systems.
B. Runoff Conveyance Systems. Storm sewers and other stormwater conveyance
systems shall be able to convey peak postactivity stormwater flows
from a ten-year design storm drainage area of less than 20 acres and
from a twenty-five-year design storm for drainage areas in excess
of 20 acres. In addition, provisions for safe conveyance of stormwater
flows from a one-hundred-year storm through the activity area shall
be provided (generally over the top of the conveyance system). Roadway
cross culverts shall be able to convey stormwater flows from a twenty-five-year
design storm for drainage areas of less than 20 acres and fifty-year
design storm for drainage areas of 20 acres or more. A minimum of
one foot of freeboard shall be provided below lowest point of the
roadway profile.
2. Erosion and Sedimentation. All land disturbance activities shall
be conducted in such a way as to minimize accelerated erosion and
resulting sedimentation. Measures to control erosion and sedimentation
shall at a minimum meet the standards of the Conservation District
and Chapter 102 (Erosion Control) of Title 25, Rules and Regulations
of the Pennsylvania Department of Environmental Resources.