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I I I I I I I I I I I I I I I I I I I Platte River Basin) (NOAA, 1973). One-hour depths for the 5-, 10-, and 50-year storm events were estimated from Figure 6 in the NOAA Atlas (NOAA, 1973). Six-hour duration rainfall depths were estimated for the 2-, 5-, 10-, 50-, and lOO-year storm events by superimposing the study area onto the rainfall isopluvial maps provided in the NOAA Atlas (Figures 20-25) (NOAA, 1973). Three-hour duration rainfall depths were estimated using equation 4 in the NOAA Atlas, which is a function of the I-hour and 6-hour rainfall depths (NOAA, 1973). The 1-, 3-, and 6-hour rainfall depths for the 500-year storm event were determined by plotting the 2-, 5-, 10-, 50-, and l00-year depths on log-probability paper and extrapolating. A summary table of the 1-, 3-, and 6-hour depths is shown in Table 3-3. Table 3-3 Summary of Unadjusted Rainfall Depths Unadjusted Rainfall Depths (inches) Storm Event I-Hour 3-Hour 6-Hour 2-Year 1.0 1.2 1.4 5-Year 1.3 1.7 2.0 100Year 1.7 2.0 2.2 50-Year 2.3 2.7 3.0 l00-Year 2.6 3.1 3.4 500-Year 3.3 3.9 4.2 3.5.3 Hyetographs A total of 24 hyetographs were developed for six recurrence intervals (2-, 5-, 10-, 50-, 100-, and 500-year), each having four area adjustments as shown in Table 3-2. These hyetographs are provided in Appendix I. The methods used to create the hyetographs are shown in Appendix I and briefly described below. For area adjustment 1, 2-hour duration hyetographs were created by distributing the I-hour rainfall depths according to Table 3-1 of the Rainfall chapter of the USDCM (UDFCD, 1969). No area adjustments were applied. For area adjustment 2, 3-hour duration hyetographs were created by adding rainfall exceeding the 2-hour storm depth uniformly over the extended duration. The 3-hour hyetographs were then adjusted according to the 10 to 20 square mile area adjustments shown in Table 4-2 of the Rainfall chapter of the USDCM (UDFCD, 1969). DENIOOI5186.WP5 3-4