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and the Lincolnwood SUbdivision drainage system. Both facilities are adequate for frequent storms, but have a history of debris plugging during severe floods. 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. The 10-, 50-, 100-, and SOO-year peak discharges for the Colorado and Roaring Fork Rivers were previously approved by the Colorado Water Conservation Board as a part of the Glenwood Springs Flood Plain Information Report (Reference 2). The hydrologic analyses for Threemile and Mitchell Creeks were based on annual peak-flow data for snowmelt and rainfall floods. Annual peak flows for rainfall and snowmelt floods have been published by the U.S. Geological survey (USGS) (Reference 7). Additional annual peak flows for rainfall and snowmelt floods were obtained from inspection of the USGS stripcharts. Frequency analyses were made on both peak-flow data sets. The two frequency curves were assumed to represent independent events, and a composite flow frequency curve was developed for each gaging. A regional regression analysis was then made to determine peak flow as a function of drainage area and return period (Reference 8). 13