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2.4 Flood Protection Measures There are several small reservoirs in the Roaring Fork watershed above Aspen and the Twin Lakes diversion for the purposes of irrigation and water supply. 1~he effect of these is insignificant during a flood event. In recent years the City of Aspen has attempted to control the potential for flood damage by trying to limit construction cn flood prone areas. No other form of floodplain management is practiced by the City of Aspe~. 3.0 ENGINEERING METHODS - - For the flooding sources studied :.n detail in the county, standard hydrologic and hydraulic study methods wer'e used to determine tc,e flood hazard data required for this study. Flood events of a magnitude which are expected to be equalled or exceeded once on the average during any 10-, 50-, 1.00-, or SOO-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance premium rates. 1'hese events, commonly termed the 1.0-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equalled or exceeded during any year. Although the recurrence interval represents the long term ~'~~,~rage pe!riod between floods of a specific magnitudE!, rare floods c~'uld 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 occurrence) in any 50-year period is approximately 40 percent: (4 in 10) r for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the county at: the time of completion of this study. Maps and flood elevati.ons lVi 11 be amended periodically ':0 reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency l:elationships for floods of the selected recurrence intervals for each Sitl:eam studi,?cl in detai 1 in the City of Aspen. Discharges on Roaring FOrk River, Castle Creek and Maroon Creek were determined using a regional approach based upon gage analysis, as described in Water Resources Council Bulletin 17A (Reference 5). The streamflow data were separated into snowmelt and rainfall pea~s and each type was analyzed separately. ~'he length of r€!co!:d for thE! aiJove streams ran'e'8 --3-