Stormwater runoff from all development sites shall be calculated using the rational method, modified rational method, or a soil cover complex methodology.
A. Any stormwater runoff calculations involving drainage areas greater than 200 acres, including on- and off-site areas, shall use generally accepted calculation technique that is based on the NRCS soil cover complex method. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site.
B. The Township may allow the use of the rational method or modified rational method to estimate peak discharges from drainage areas that contain less than 200 acres. When using the rational method, an ascending and descending limb factor of three and seven shall be used, respectively.
C. All calculations consistent with this Chapter using the soil cover complex method shall use the appropriate design rainfall depths. If a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater runoff calculations, then the duration of rainfall shall be 24-hours. The SCS Rainfall Type II curve shall be used for the rainfall distribution. Those projects proposing infiltration structures shall utilize this method.
D. When routing a detention basin, the rational method or SCS method may be used to determine peak rates through the primary outlet structure. Only the SCS Method may be used to determine the peak water surface elevation during the 100-year, 24-hour design storm, dictating the height of the embankment.
E. Underground storage facilities that solely receive stormwater and are designed to capture and infiltrate the entire 100-year, 24-hour SCS runoff volume may have their drainage areas removed from the overall post-development drainage area(s).
F. For the purposes of pre-development flow rate determination, undeveloped land, including disturbed areas, shall be considered as “meadow” in good condition, unless the natural ground cover generates a lower curve number or rational “C” value (i.e., forest), as listed in Tables 1 and 2, respectively.
G. All calculations using the rational method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods. 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). Time of concentration for channel and pipe flow shall be computed using Manning’s equation.
H. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table 1.
I. Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table 2.
J. Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations such as the capacity of open channels, pipes, and storm sewers. Values for Manning’s roughness coefficient (n) shall be consistent with Table 3.
K. 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 either manual methods or computerized routing. Routing shall be based upon the modified PULS method; other routing methodologies shall be subject to the approval of the Municipal Engineer.
L. The stormwater collection system shall be designed using the peak discharge computed using the rational formula.
(Ord. 2022-01, 7/25/2022)