(A) The Rational Method may only be used to calculate peak discharge rates for drainage areas of 50 acres or less. The Rational Method shall not be used to calculate the volume of storm water runoff or develop runoff hydrographs.
(B) The Rational Method shall be performed as follows:
Q=CIA
where:
Q = peak runoff quantity in cubic feet per second;
C = runoff coefficient, varying with the amount of imperviousness and other characteristics of the drainage area;
I = average intensity of precipitation in inches per hour, varying with frequency of storm occurrence, duration or concentration time, and area of the tributary watershed; and
A = area in acres of the tributary watershed.
(C) The runoff coefficient (C) must represent a composite of the surface condition tributary to the point under consideration. To determine the composite C-factor for the entire project site, a weighted average must be calculated based upon the percentages of the areas with different C-factors. The runoff coefficient is dependent upon the soil type and land use. The type and condition of the soil determines its ability to absorb precipitation. Reference may be made to the Louisville Metropolitan Sewer District Design Manual for a list of Rational Method runoff coefficients.
(D) (1) Rainfall intensity is the average rainfall rate in inches per hour, and is selected on the basis of design rainfall duration and design frequency of occurrence. The rainfall intensity should be obtained from rainfall intensity-duration-frequency (IDF) curves for the appropriate design storm.
(2) Since IDF curves have not been specifically developed for the city, then the IDF curves developed for Metro Louisville shall be used and may be referenced in the Louisville Metropolitan Sewer District Design Manual. The design duration is equal to the time of concentration (Tc) for the drainage area under consideration.
(E) The time of concentration (Tc) shall be determined by calculating the time for a particle of water to travel from the most hydrological remote point of the project area to the point of interest. The time of concentration to any point in a storm drainage system is a combination of the sheet flow (overland), the shallow concentrated flow, and the channel flow, which includes storm sewers. The Soil Conservation Service TR-55 Method for calculating the time of concentration shall be used. The minimum time of concentration shall not be less than ten minutes. Manning’s Equation should be used to estimate any in-pipe or channel travel.
(Ord. 012-118, passed 10-8-2012)