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I I I I I I I I I I I I I I I I I I I HYDRDLOGIC AND HYDRAULIC DETERMINATIONS A discharge probability analysis was made at the U.S. Geological Survey stream gage on Lefthand Creek listed as "near Boulder." This gage is located below the confluence of Sixmile Canyon with Lefthand Creek. The analytical methods presented in Bulletin No.1 published by the Water Resources Council (Reference 9) were used. Adjustments for the length of record were made on logarithmic probability paper using Beard's table (Reference 10) of expected probability adjustments for various lengths of record. The results of this analysis were used as guides in the calibration of the SWMM model. Discharges for the area below James Creek were determined by the calibrated 6-hour rainfall SWMM model. By trial and error, it was found that the I-hour storm produced the highest discharges on Lefthand Creek from near Ward to upstream of Spring Gulch. From Spring Gulch to upstream of James Creek, a 3-hour storm was used to link the I-hour and 6-hour reaches together. A I-hour storm was used to produce the discharges for James Creek, Little James Creek, and Lefthand Creek near Ward. Table 2 contains the rainfall values used in this analysis. A uniform infiltration rate of 1 inch per hour was used for the Lefthand Creek Basin. Detention storage was 0.3 inch for the 6-hourrainfall, 0.4 inch for 3-hour rainfall, and 0.5 inch for the I-hour rainfall. Flood Characteristics Flood flows 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 The hydrologic analysis for Upper Lefthand Creek, James Creek, and Little James Creek was performed by the U.S. Army Corps of Engineers, Omaha District and consisted of determining the magnitude of the 10-, 50-, 100-, and SOO-year floods (Reference 6). This was accomplished by use of the Environmental Protection Agency's Storm Water Management Model (SWMM) (Reference 7). Upper Lefthand Creek, James Creek, and Little James Creek were a portion of a larger SWMM model for the Lefthand Creek Basin which itself was a portion of a SWMM model to determine flood flow frequencies in the St. Vrain Creek 8asin. Rainfall values for the 10-, 50-, and 100-year; 1-, 3-, and 6-hour events were obtained from "Precipitation Frequency Atlas of the Western United States, Atlas 2, Volume III, Colorado," published by the National Oceanic and Atmospheric Administration (NOAA) 1973 (Reference 8). The SOO-year values were esti- mated by extrapolating the selected Atlas values on semilogarithmic paper. Adjustments to rainfall depth were based on the depth-area relationship presented in the NOAA publication. 7