1. Stormwater runoff from all development sites shall be calculated using a generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 226-1 summarizes acceptable computation methods and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular development site. The Township may allow the use of the rational method after consultation with the Township Engineer to estimate peak discharges from drainage areas on a case by case basis.

2. All calculations consistent with this Part using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms according to the region in which they are located as presented in Table B-1 in Appendix B. If a hydrologic computer model such as PSRM or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be twenty-four (24) hours. The SCS “S” curve shown in Figure B-1 of Appendix B shall be used for the rainfall distribution.

3. For the purposes of existing conditions flow rate determination, undeveloped land shall be considered as “meadow” in good condition, unless the natural ground cover generates a lower curve number or Rational “C” value, as listed in Table B-2 or B-3 in Appendix B.

4. All calculations using the rational method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods from the design storm curves from the PennDOT design rainfall curves (1986) (Figures B-2 to B-4). 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 Manning’s equation.

5. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table B-2 in Appendix B.

6. Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table B-3 in Appendix B.

7. The design professional shall consider that the runoff from the development sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions, even after topsoiling or seeding. The design professional may increase his proposed condition “CN” or “c” to better reflect proposed soil conditions.

8. Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning’s roughness coefficient (n) shall be consistent with Table B-4 in Appendix B.

9. Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Part using any generally accepted hydraulic analysis technique or method.

10. The design of any stormwater detention facilities intended to meet the performance standards of this Part shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. The design storm hydrograph shall be computed using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. The Township 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. (Ord. 181, 10/11/2006, §306)