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DRAINAGE CRITERIA MANUAL . Step 9. . Step 10. Step 11. Step 12. . Step 13. 5-1-84 RUNOFF W75% ahead of Q = 0.45 x 11.2 P = 5.0 minutes *Note: Less than 0.6 T = 9.6 minute; therefore p use 7.4 minutes and 5.0 minutes at W50% and W75% respectively. If the 7.4 minutes were greater than 0.6 T , the alternate p procedure would have been used. Sketch the unit hydrograph (Figure 4-6) using Tp' Qp' W50%, W75% and the portions of W50% and W75% ahead of Qp determined above. Needed Unit Hydrograph Volume: (243 acres) x (1 inch/12 inch/foot) = 20.2 ac. ft. Pl animeter the uni t hydrograph to obta in its volume. Adjust recession leg of the hydrograph until 20.2 ac. ft. (:t5%) is obtained. Volume of the first trial was 17.2 ac. ft. which is 15% to low. The unit hydrograph was revised as shown in Figure 4-6 and a volume of 20.3 ac. ft. was measured which is within the required 5% accuracy. Obtain the design effective precipitation values for from a table similar to Table 2-5. To illustrate the process of calculating a storm hydrograph, we will use in this example the Effective Precipitation from Table 2-5 even though it was calculated using a different value of imperviousness than is used in this example. Set up Table 4-1. Multiply the precipitation value at the top of Column 3 by each of the unit hydrograph ordinates and put in Column 3 for the corresponding time. Next multiply the precipitation value in Column 4 by each of the unit hydrograph ordinates and place in col umn 4 lagged by one time increment from the corresponding unit hydrograph time. Proceed to multiply each of the precipitation values times the unit hydrograph ordinates, each time lagging the new hydrograph by one more time unit. Column 25 is the design storm hydrograph obtained by summing horizontally the individual hydrographs in Column 3 through 24. Note that in this example, time zero is the beginning of excess