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I I I I I I I I I I I I I I I I I I I 5 May 1973 -. Rainfall amounts ranging to as much as 6 inches fell on the plains east of the Rocky Mountains beginning on 5 May. The Lefthand Creek basin received amounts ranging from 2 to 3 inches. Runoff from this general storm contributed to considerable flooding on the South Platte River. HYDROLOGIC AND HYDRAULIC DETERMINATIONS Flood Characteristics Floodflows on Lefthand Creek result from heavy rainfall associated with cloudburst thunderstorms during the spring and summer months. This rainfall may occasionally be augmented by melting snows, These rainfall flood events are characterized by high peak flows of relatively short duration. Hydrologic Analysis Discharge values for the 10-, 50-, 100-, and 500-year floods were developed for Lefthand Creek by the U.S. Army Corps of Engineers, Omaha Di strict us ing the Environmental Protect ion Agency's Storm Water Management Model (SWMM) (Reference 2). Rainfall values for the 10-, 50-,' and 100-y~ar; 1-, 3-, and 6-hour events were obtained from "Precipitation Frequency Atlas of the Western United States, Volume III, Colorado" (Reference 8), The peak discharge values on Lefthand Creek below James Creek were computed by applying the 6-hour rainfall to the entire basin. Rainfall depths for the 10-, 50~, 100-, and 500-year storms are shown in Tables 2 thru 5. Table 2 Rainfall Depths for l-Hour Storm Duration at Various Frequencies - Lefthand Creek Reach - Upstream of Spring Gulch Frequency Depth (Inches) 1.3 1.8 2,1 2.8 10-year 50-year 100-year 500-year 5