(a) Stormwater runoff from all development sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 4 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval of the municipality.
Table 4: Acceptable Computation Methodologies for Swm Site Plan | ||
Method | Developed by | Applicability |
TR-20 (or commercial computer package based on TR-20) | USDA NRCS | Applicable where use of full hydrology computer model is desirable or necessary |
TR-55 (or commercial computer package based on TR-55) | USDA NRCS | Applicable for land development plans where limitations described in TR-55 |
HEC-1/HEC-HMS | U.S. Army Corps of Engineers | Applicable where use of a full hydrologic computer model is desirable or necessary |
Rational Method (or commercial computer package based on Rational Method) | Emil Kuichling (1889) | For sites up to five acres, or as approved by the municipality and/or Municipal Engineer |
Other methods | Varies | Other computation methodologies approved by the municipality and/or Municipal Engineer |
(b) All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms. Rainfall depths shall be according to NOAA Atlas 14 values consistent with a partial duration series, or the latest version of the PennDOT Drainage Manual (PDM Publication 584), whichever is greater. When stormwater calculations are performed for routing procedures or water quality functions, the duration of rainfall shall be 24 hours.
(c) The following criteria shall be used for peak rate runoff calculations:
(1) For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
A. Wooded sites shall use a ground cover of “woods in good condition”. Portions of a site having more than one viable tree measuring a diameter at breast height (DBH) of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.
B. The undeveloped portion of the site including agriculture, bare earth and fallow ground shall be considered as “meadow in good condition”, unless the natural ground cover generates a lower curve (CN) number or Rational V value (i.e., woods) as listed in Tables D-1 or D-2 in Appendix D of this chapter.
(2) For redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.
(d) All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584), whichever is greater. Times of concentration shall be calculated based on the methodology recommended in the respective model used. Times of concentration for channel and pipe flow shall be computed using Manning’s equation.
(e) Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table D-1 in Appendix D of this chapter.
(f) Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table D-2 in Appendix D of this chapter.
(g) Hydraulic computations to determine the capacity of pipes, culverts and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114 HEC 15). Values for Manning’s roughness coefficient (n) shall be consistent with Table D-3 in Appendix D.
(h) 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.
(i) The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using an acceptable method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality 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. 928, passed 6-18-2014)