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4.2 . 1/ R.a t lonal ~':e t hod- --'--~- For basins that are not complex and have 200 acres or less, it is recoI:lITlended that the design storm runoff be analyzed by the Rational Method. This method was introduced in 1889, and is used in most engi- neering offices in the United States. Even though it has frequently come under academic criticism for its si:mplicity, no other practical drainage design method has been evolvE'd to a level of general acceptance by the practicing engineer. The Rational Method, if judiciously applied, can produce satisfactory results on small ,,'atersheds. 4.2.1 Rational Formula The Rational Method is based on the Rational Formula: Q = CIA (Eq. 4.2.1-1) ..-here: Q = maximu:n rate of runoff, in cubic feet per second C runoff coefficient, which is the ratio between the maximum rate of runoff from the area and the average rate of rainfall intensi ty, in inches per: hou!', for the period of maxirr.uJD. rainfall of a given frequency of occurrence having a duration equal to the time of concentI'ation I = average intensity of rainfall in inches per hour for a duration equal to time of concentration (usClally time required for water to flow from most remote poirll: of area to point being investi- gated) A = area, in acres 4.2.2 Basic assumptions 1. The conputed maximum rate of ru"off to the design point is a function of the average rainfall rate during the time of concentration to that point. 2. The madmum rate of rainfall occurs during the time of concen- tration, and the design rainfall depth during the time of concentration is converted to the average rainfall intensity for the timEI of concen- tration. 3. The ma:l{irnurn runoff rate acc.urH ,.:hen the entire are.a is con- tributing flow. ~ ,Y This sec tion '.as adapted from Wr ight'.McLaughlin Engineers, 1969 (Urban Storm Jrainage Criteria Manual, Vol. I, Denver Regional Council of Go.,ernments). LCS-WM Manual 4.2---1 April 1979