Sec. 21A-68. Design criteria.
(a) Stormwater Runoff Criteria. It is the policy of the Town of St. John to use the Rational Method and/or industry recognized computer programs to determine the peak rate of runoff for all design storms for drainage areas of two hundred (200) acres or less.
For drainage areas over two hundred (200) acres and for determining major storms, the method explained in “Urban Hydrology for Small Watersheds,” Technical Release No. 55, may be used to provide peak rates of runoff. T.R. 55 can be obtained from the Natural Resources Conservation Service Engineering Division, U.S. Department of Agriculture. Industry recognized computer programs can be used as well.
(b) Stormwater Runoff Information.
(1) Peak Rate of Runoff. The basic formula for the Rational Method is Q = CiA in which:
Q = Peak rate of runoff in cubic feet per second
C = Runoff coefficient, ratio of the maximum rate of runoff to the average rate of rainfall
A = The drainage area in acres
i = Rainfall intensity in inches per hour for a selected storm frequency and the time of concentration
(2) Runoff Coefficient. The table of runoff coefficients presents average values for use with the Rational Method. The table relates the coefficient to land use and average slope of the watershed.
(3) Rainfall Intensity. Rainfall frequency curves, relation of rainfall intensity versus time, based on Bulletin 71 or the latest published records of the National Weather Service, for the Chicago area, will be used for all design storms.
(4) Design Storm. The peak discharge to be computed for all storm drainage facilities used for collecting and transporting storm runoff is defined in the design criteria for those facilities.
(5) Major Storm. The major storm will be based on a storm with a return period of not less than fifty (50) years.