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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RAINFALL <br /> <br />Step 3 - Multiply each incremental design storm depth by an adjustment ~ <br />factor for that time increment. <br />4.2 Example: Area Adjustment of Design Storm <br />Adjust the 2-hour, la-year design storm developed for Example 3.2 for a <br />watershed area of 11.0 square miles. First convert the 2-hour storm into a <br />3-hour storm as reconrnended by Table 4-1. Do this by subtracting the 2-hour <br />rainfall total in Table 3-2 from the 3-hour depth given in Section 2.3: <br /> <br />1.99 - 1.84 = 0.15 inches <br /> <br />Divide 0.15 inches by 12 (i.e. 12 time increments of 5-minutes each in <br />the I-hour past the 2-hour storm): <br /> <br />0.15 / 12 = 0.0125-inches/5-minutes <br /> <br />Use O.02-inches for each 5-minute increment between 125 and 135 minutes <br />of the design storm and O.Ol-inches for each 5-minutes for the remainder of <br />the 3-hour storm. ~ <br /> <br />Next, set up a Table 4-3 using the area adjustment factors for lO-year <br />design rainfall and 10 to 20 square mile area. Calculate the area adjusted <br />design storm using the steps outlined in Section 4.1. <br /> <br />~ <br />