[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.
(1) 
Detention basins.
(2) 
Roof top storage.
(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.
(8) 
Routed flow over grass.
(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.