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the recurrence interval represents the long-term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approxi- mately 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 flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood eleva- tions will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied by detailed methods affecting the community. Discharge magnitudes for floods on Cherry Creek analyzed in this report were based on an analysis of stream gaging data at the u.s. Geological Survey (USGS) stream gages located approximately 2.5 miles south of Franktown and 6 miles northwest of Parker. This information was obtained from a u.s. Army Corps of Engineers (COE) Flood Plain Information report (Reference 10). Because there are no gaging stations on the streams in the immediate Castle Rock area, the synthetic hydrograph method was used to obtain the peak rates of runoff for East Plum Creek, Hangmans Gulch, Sellers Gulch, and Unnamed Tributary to Sellers Gulch. The analyses were based on a storm duration of 24 hours and a Type II A distribution, as described in the SCS National Enqineerinq Handbook (Reference 11). The amount of rainfall was obtained from a precipitation frequency atlas (Reference 12), and areal adjustment was applied to convert the point precipitation values to average precipitation over the watershed area. Hydrologic soil cover complexes and associated Runoff CUrve Numbers were obtained from field investigations, a soil survey of Castle Rock (Reference 5), an unpublished SCS study of the area, and land use and natural plant cover maps of Douglas County, Colorado (References 4 and 6, respectively). Values of 10-, 50-, 100-, and sOO-year peak discharges were obtained using the TR-20 computer program developed by the SCS (Reference 13). The computed peak discharges for the streams studied in detail compare favorably with the peak discharges estimated using Tech- nical Manual No.1, developed by the USGS (Reference 14). The discharges have been reviewed by the USGS, SCS, and CWCB, and have been found to be reasonable and acceptable for the study area. 10