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DRAINAGE CRITERIA MANUAL RUNOFF f = f + (f. - f ) e-at o , 0 In which f = f. = , fo = e = a = t = (2-1) . infiltration rate (inches/hour) initial infiltration rate (inches/hour) final infiltration rate (inches/hour) natural logarithm base decay coefficient (l/second) time in seconds In developing Equation 2-1 Horton observed that infiltration is high early in the storm and eventually decays to a steady state constant value as the pores in the soi 1 become saturated. The coeffi ci ents and i nitia land final infiltration values are site specific and depend on the soils and vegetative cover complex. It is possible to develop these values for each site if sufficient rainfall/runoff observations are made. However, such an approach is not always practical. The Urban Drainage and Flood Control Di stri ct analyzed a cons i derab 1 e amount of rainfall/runoff data. On the basis of this analysis, the values in . Table 2-2 are recommended for use within the District in conjunction with the 1982 version of the CUHP. The Soil Conservation Service (SCS) Hydrologic Soil Groups C and D occur most frequently within the District; however areas of SCS Group A and B soils are fairly common. Consult SCS Soil surveys for appropriate soil classifications. TABLE 2-2 RECOMMENDED HORTON'S EQUATION PARAMETERS SCS Hydrologic Soil Group A B C D Infiltration (in/hr) Initial - fi Final.fo 5.0 1.0 4.5 0.6 3.0 0.5 3.0 0.5 Decay Coefficient 0.0007 0.0018 0.0018 0.0018 To calculate the amount of maximum infiltration depths that may occur at each time increment, it is necessary to integrate Equation 2.1 and calculate . the values for each time increment. Very little accuracy is lost if instead 5-1-84