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<br />. <br /> <br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />of integrating Equation 2-1, the infiltration rate is calculated at the center <br />of each time increment. This "central" value can then multipl ied by the unit <br />time increment to estimate the infiltration depth. This was done for the four <br />SCS Hydrologic Soil Groups and the results are presented in Table 2-3. <br />Although Tables 2-2 and 2-3 provi de recommended values for va ri ous Horton <br />equation parameters, these recommendations are being made specifically for the <br />urbanized or urbanizing watersheds in the Denver metropolitan area and may not <br />be valid in different meteriologic and climatic regions. <br /> <br />2.4 Example - Effective Rainfall <br />That portion of rainfall that becomes runoff during or soon after a storm <br />is called effective rainfall. The abstractions from rainfall that determine <br />the effective precipitation are functions of infiltration, detention and <br />depression storage, intensity of rainfall, percent of imperviousness, etc. <br />An example of estimating the effective rainfall is presented below using <br />Table 2-4 as an aid in calculations. <br /> <br />Col. 1 For the design location select a rainfall time interval, usually 5 <br />minutes (see Section 4.4 of RUNOFF for further guidance). <br />Col. 2 After selecting a design frequency calculate the design storm using <br />the procedure described in RAINFALL Part of this Manual. If the watershed is <br />5 square miles or larger, first correct the design storm for area using the <br />procedures in RAINFALL part of this manual before entering the design storm in <br />this column. In this example we will use the design storm given in Table 3-2 <br />of the RAINFALL chapter of this manual. <br /> <br />Pervious Area, Columns 3 through 6 <br /> <br />Col. 3 Tabulate Increments of infiltration for each time period. In this <br />example we used Table 2-3 values for SCS Hydrologic Soil Group D. <br />Col. 4 The total pervious depression storage is determined using Table 2-1. <br />It appears as the total at the bottom of Column 4. For each time period the <br />depression storage in Col. 4 is found by subtracting infiltration, Col. 3, <br />from precipitation, Col. 2. If the result is negative, there is no excess for <br />this period, and Col. 4 and Col. 5 are zero. If the result is positive, the <br />amount is entered in Col. 4 as the depression storage for the time period. <br /> <br />5-1-84 <br />