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2.4 Flood Protection Measures See Section 10.1. 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 that are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or SOa-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10-, 2-, 1-, and O.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 that equals or exceeds the lOO-year flood (1-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); for any gO-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 conununity 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 peak discharge frequency relationships for each flooding source studied in detail affecting the community. The peak discharges for the 10-, 50-, 100-, and SOa-year floods for Coal Creek used in this study were developed by the SCS in a report entitled "Flood Hazard Analyses, Coal Creek and Rock Creek, Boulder and Weld Counties, Colorado" (Reference 2). The SCS used synthetic rainfall-runoff procedures, as described in the SCS National Enqineering Handbook (Reference 5), and the TR-20 computer program for flood routing (Reference 6) to establish the selected discharges along the stream. Peak discharge-drainage area relationships for Coal Creek are shown in Table 1. Analyses were carried out to establish the peak elevation-frequency relationships for each flooding source studied in detail. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of sources studied were carried out to provide estimates of the selected recurrence intervals. flooding flood from the elevation 4