Runoff quantities shall be computed for the watersheds within the parcel under development. The quantity of runoff which is generated as the result of a given rainfall intensity may be calculated as follows:
(A) For areas up to and including 200 acres, the Rational Method may be used: The peak rate of runoff (Q) in cubic feet per second is computed as Q = CIA;
Where:
C = runoff coefficient, representing the characteristics of the drainage area and defined as the ratio of runoff to rainfall;
I = average intensity of rainfall in inches per hour for a duration equal to the time of concentration (tc) for a selected rainfall frequency; and
A = tributary drainage area in acres.
(B) Guidance for selection of the runoff coefficients is to be found in appropriate design manuals. Rainfall intensity shall be determined from the rainfall frequency curves found in standard design manuals for this region or from data shown. The time of concentration (tc) to be used shall be the sum of the inlet time and flow time in the drainage facility from the most remote part of the drainage area to the point under consideration. The flow time in the storm sewers may be estimated by the distance in feet divided by velocity of flow in feet per second. The velocity shall be determined by the Manning Formula.
(C) Other methods of determining runoff may be used upon approval of the town staff. Computer programs may be used and computer printouts submitted for drainage calculations provided details of the program and the assumptions made by that program are submitted with the calculations and approved by the town staff.
(Ord. passed 10-11-2004)