A. Stormwater runoff from all development sites shall be calculated using either the rational method or a soil cover complex methodology.
B. Any stormwater runoff calculations shall use generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 309-1 summarizes acceptable computation methods. 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 seventy-five (75) acres. The Rational Method is recommended for drainage areas under twenty-five (25) acres.
Table 309-1. Acceptable Computation Methodologies For Stormwater Management Plans
METHOD | 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 within limitations described in TR-55 |
HEC-1, HEC-HMS | US Army Corps of Engineers | Applicable where use of full hydrologic computer model is desirable or necessary |
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 sites less than 75 acres, or as approved by the Township engineer. |
Other methods | Varies | Other computation methodologies approved by the Township engineer. |
C. For the purposes of existing conditions flow rate determination for all subdivision and land development applications, undeveloped land and existing impervious surfaces shall be considered as “meadow” in good condition, unless the natural ground cover generates a lower curve number (CN) or Rational ‘C’ value (e.g. forest).
D. 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.
E. Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Ordinance using any hydraulic analysis technique or method accepted by the Township.
F. The design of any stormwater management facilities intended to meet the performance standards of this Ordinance shall be verified by routing the design storm hydrograph through these facilities using the Storage Indication Method. For drainage areas greater than twenty (20) acres in area, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
G. 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.
H. 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 formula 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 (2nd Edition). For design purposes the time of concentration may not be less than five (5) 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 drainage conveyance system.
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 fifty (50) feet over paved surfaces and one hundred and fifty (150) feet over unpaved surfaces. Maximum permitted sheet flow length shall be one hundred and fifty (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. 186, passed 9-12-2017)