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2.4 Flood Protection Measures The possibility of future flood damage in the upper Cherry Creek area has been somewhat reduced by the construction of 32 flood- water-retarding structures. The SCS completed construction of these structures in 1965 as part of an overall plan (Reference 4). These structures were designed for an agricultural community with design floods having a 25-year recurrence interval. 3.0 ENGINEERING METHODS " For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although ,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 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 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 out to establish the peak discharge-frequency relationships for each flooding source studied in detail affecting the community. 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 distribu- tion, as described in the SCS National Engineerinq Handbook (Reference 5). The amount of rainfall was obtained from Volume III of the Precipitation-Frequency 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 inves- tigations, a soil survey of Castle Rock (Reference 7), an 7