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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 SOO-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 SOO-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. Peak flows on the Colorado River and Alkali Creek were based on records from USGS stream gaging stations and on the results of a regional peak flow-frequency analysis (Reference 2). Peak discharge values on the Crystal River were adopted from a regional analysis performed by the COE (Reference 3). Both snowmelt and rainfall flood peaks were analyzed to define floodflow-frequency. Regional drainage area-mean peak flow relationships, regional standard elevations, and regional skew coefficients were developed for both flood types. This information was adjusted using statistics developed from Crystal River USGS stream gage records. Floodflow-frequency curves for rainfall and snowmelt floods were developed at selected sites using the adjusted information. These curves were then combined statistically to generate composite flow- frequency curves. A separate review by the study contractor (Reference 4) of existing methods recommended using the COE regional analysis. . Peak flows of the Roaring Fork River; Cattle, Fourmile, Threemile, Mitchell, and Government Creeks; and Hubbard, Helmer, and Ramsey Gulches were based on annual peak flow data for snowmelt and rain- fall floods. 8