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