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DRAINAGE CRITERIA MANUAL RAINFALL 4. ADJUSTMENTS FOR LARGER WATERSHEDS 4.1 Adjustments for Larger Watersheds As the watershed size increases, it is necessary to also increase the duration of the design storm and to adjust for its distribution. Whenever a duration of more than2-hours is required, it is recommended for the Denver region that the difference between the rainfall depth at 2-hour duration and the depth at greater than 2-hour duration be distributed uniformly at the end of the 2-hour storm. It is also recollll1ended that the design storm duration and the need for area adjustment be done in accordance with Table 4-1. Table 4-1 Recommended Area-Storm Duration Adjustments Watershed Area Square Mil es Less than 5.0 5.0 to 10.0 10.0 to 20.0 20.0 or larger Need for Area Adjustment No Yes Yes Yes Minimum Duration 2 hours 2 hours 3 hou rs 6 hours The NOAA Atlas (1) provides guidelines for adjusting the rainfall depths with increasing area. Area-depth adjustments are given in the Atlas for durations of 1-, 1-, 3-, 6- and 24-hours and Figure 4-1 was taken from the NOAA Atlas. The 15-minute curve was mathematically extrapolated by UDFCD from the information on Figure 4-1. The fast response times of urbanized watersheds and sharp rainstorm distribution gradients in the Denver area require adjustments of rainfall depths for durations that are less than i-hour. The area adjustment procedure can be tedious and time consuming; therefore Table 4-2 is prov; ded to ass is t the engi neer with the area-depth adjustment calculations. To adjust the design storm distribution to account for the averaging effects of larger watersheds, follow these 3 steps: Step 1 - Begin with the unadjusted design rainstorm Step 2 - On the basis of total watershed size select the appropriate column(s) of adjustment coefficients in Table 4-2. 5-1-84 . . .