(A) Stormwater runoff from all sites shall be calculated using either the Rational Method or soil cover complex method.
(B) Any stormwater runoff calculations shall use generally accepted calculation technique that is based on the NRCS soil-cover complex method. The table in division (D) below summarizes acceptable computation methods. The appropriate method must be selected by the applicant based on the individual limitations and suitability of each method for a particular site. The Rational Method may be used to estimate peak discharges from drainage areas that contain less than 75% acres and shall be used for drainage areas under 25% acres.
(C) All calculations using the soil-cover complex method shall use the appropriate design rainfall depths for the various return period storms according to the region for which they are located as presented in Table B-1 in Appendix B of this chapter. If a hydrologic computer model, such as HEC-1 or HEC-HMS, is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The S-curve shown in Figure B-1 of Appendix B shall be used for the rainfall distribution.
(D) 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-4 in Appendix B. For the purposes of existing conditions flow rate determination for all subdivisions and land developments, all undeveloped land and 20% of existing impervious surface shall be considered as “meadow” in good condition, unless the natural ground cover generates a lower curve number or Rational C value (e.g., forest), as listed in Table B-4 in Appendix B.
Method | Method Developed by | Applicability |
HEC-1, HEC-HMS | US Army Corps of Engineers | Applicable where use of full hydrologic computer model is desirable or necessary. |
Other methods | Varies | Other computation methodologies approved by the Township Engineer. |
PSRM | Penn State University | Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1. |
Rational Method (or commercial computer package based on Rational Method) | Emil Kuichling (1889) | Applicable for sites less than 75 acres, or as approved by the Township Engineer. |
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 within limitations described in TR-55. |
(C) 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. (Refer Table B-8 of Appendix B.)
(D) Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any hydraulic analysis technique or method accepted by the township.
(E) The design of stormwater management facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the storage indication method. For drainage areas greater than 20 acres, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
(F) The township has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the design professional engineer can substantiate through actual physical calibration that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Township Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.
(G) The
TIME OF CONCENTRATION (Tc) is the time required for water to flow from the hydraulically most remote point of the drainage area to the point of interest (design point). Use of the Rational Method requires calculation of a Tc for each design point within the drainage basin. Travel time estimation for the Rational Method shall be based on NRCS Technical Release No. 55 (Second Edition). For design purposes, the time of concentration may not be less than five minutes. TRAVEL TIME (Tt) is the time it takes runoff to travel from one location to another in a watershed (subreach) and is a component of time of concentration. Tc is computed by summing all the travel times for consecutive components of the stormwater management conveyance system.
(H) Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow or some combination of these. Sheet flow rates shall be calculated using the NRCS TR-55 (1986) variation of the kinematic wave equation. Sheet flow length may not exceed 50 feet over paved surfaces and 150 feet over unpaved surfaces. Maximum permitted sheet flow length shall be 150 feet, unless site specific conditions exist (that can be demonstrated) that warrant an increase of the sheet flow length. Under no circumstances shall sheet flow length exceed 300 feet. Shallow concentrated flow time and open channel flow time shall be calculated using standard engineering methodologies.
(Ord. 239, passed 10-14-2020)