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respecti vely, of being equaled or exceeded during any year. Al though 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 (l-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and for any 90-year period, the ri sk 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 elevations will be amended periodically to reflect future changes. ". 3.1 Hydrologic Analyses Hydrologic analyses were carried discharge-frequency relationships for in detail affecting the community. out to establish the peak each flooding source studied For Sulphur and Tallman Gulches, the synthetic hydrograph method was used to obtain the peak rates of runoff. The analyses were based on a storm duration of 24 hours and a Type II A distribution, as described in the SCS Nlltional Engineering Handbook (Reference 5). The amount of rainfall was obtained from volume III of the Precioitation-Frequencv Atlas of the Western United States (Reference 6), and an 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associated Runoff Curve Numbers were obtained from field investigations, a soil survey of Castle Rock (Reference 7), an unpublished SCS study of the area, and land use and natural plant cover maps of Douglas County, Colorado (References 8 and 9, respectively). Values of 10, 50-, 100-, and 500-year peak discharges were obtained using the SCS TR-20 computer program (Reference 10). The computed peak discharges compare favorably with the peak discharges estimated using USGS Technical Manual No.1 (Reference 11). Discharge magnitudes for Cherry Creek floods were based on an analysis of stream-gaging data at the USCS stream gages located approximately 2.5 miles south of Franktown and 6 miles northwest of Parker. This information was obtained from a COE Floodplain Information Report (Reference 12). 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 32 flood-retarding structures in the Cherry Creek basin constructed by the SCS were taken into consideration. Using two UDFCD reports (References 13 and 14), drainage area curve weas developed for streams approximate methods. This method was used for Tributary and Oak Gulch. a discharge- studied by Jordan Road 7