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the 100-year flood (1 percent chance of annual exceedence) in any 50- year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect floodi~9 potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak dis- charge-frequency relationships for each flooding source studied by detailed methods affecting the community. For Plum Creek, East Plum Creek, West plum Creek, Hangmans Gulch, Sellers Gulch, Unnamed Tributary to Sellers Gulch, Carpenter Creek, Happy Canyon Creek, Newlin Gulch, Baldwin Gulch, Sulphur Gulch, Tallman Gulch. Bayou Gulch, and West Creek, the synthetic hydrograph method was used to obtain the peak discharge. The analyses were based on a storm duration of 24 hours and a Type II A distribution, as described in the SCS National Engineering Handbook (Reference 8). The amount of rainfall was obtained from a precipitation-frequency atlas (Reference 9), and areal adjust- ment was applied to convert the point precipitation values to average precipitation over the watershed area. Hydrolo9ic soil cover complexes and associated Runoff Curve Numbers were obtained from field investigations, a soil survey of Castle Rock (Refer- ence 10). an unpublished SCS study of the area, and land use and natural plant cover maps of Douglas County (References 3 and 11). Values of 10-, 50-, 100-, and 500-year peak discharges were ob- tained using the computer program developed by the SCS (Refer- ence 12). The computed peak discharges compare favorably with the peak discharges estimated using the USGS Technical Manual No.1, (Reference 13.). Discharge magnitudes for floods in Cherry Creek analyzed in this FIS were based on an analysis of stream gaging data at the USGS stream gages located approximately 2.5 miles south of Franktown and 6 miles northwest of Parker. This information was obtained from a COE Flood Plain Information report (Reference 14). Discharge-frequency relationships for the upstream and downstream limits of the study reach were developed using data from the Franktown and Parker stream gages, respectively. The presence of 32 flood-retarding structures in the Cherry Creek basin constructed by the SCS was taken into consideration. A continuous record of flows at stream gaging station No. 0670500, located on the South platte River at South Platte, is available from 1900 to the present. and stream gaging station No. 06701500, located below Cheesman Lake, has a continuous record of flows from 1925 to the present. 18