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<br />2.4 Flood Protection Measures <br /> <br />There are several small lakes and reservoirs in Pitkin County, <br />mostly located within the upper watersheds of the Roaring Fork <br />River. The Twin Lakes diversion reservoir controls flows from <br />only a 15-square-mile drainage area and is intended to provide <br />storage for irrigation and water supply. The above measures are <br />insufficient for flood protection. A few temporary or localized <br />flood protection works, including berms, levees, and bank protec- <br />tion works, are scattered near developed areas. <br /> <br />The Fryingpan and Roaring Fork Rivers exist in deep channels. No <br />authorized flood-control structures are in the local study area, <br />and none are under investigation. <br /> <br />The only substantial structure that affects the flow of the <br />Fryingpan River is the Ruedi Darn, located approximately 17 miles <br />east of Basalt. The Ruedi Darn is part of the Fryingpan-Arkansas <br />Project to divert water from the Colorado River basin to the <br />Arkansas River basin. The dam was designed for an inflow design <br />flood of 17,500 cfs at a 15-day volume of 110,000 acre-feet. The <br />probable maximum discharge is 5,540 cfs from the spillway and <br />1,810 cfs from the outlet structure. The total probable maximum <br />discharge from the outlet structure and spillway is 7,350 cfs, <br />approximating the 500-year flood in the Town of Basalt <br />(Reference 6). <br /> <br />A SOO-foot reach of the Roaring Fork River, located approximately <br />800 feet upstream of North Mill Street in Aspen, was widened in <br />December 1984. <br /> <br />Current regulations in Basalt limit construction in flOOd-prone <br />areas, thus contributing to reduction in future flood potential. <br />No other flood plain management measures are in effect in Pitkin <br />County. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />IO-, 50-, 100-, or SOQ-year period (recurrence interval) have been <br />selected as having special significance for flood plain management and <br />for flood insurance rates. These events, commonly termed the 10-, 50-, <br />100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, <br />respectively, of being equaled or exceeded during any year. Although <br />the recurrence interval represents the long-term average period between <br />floods of a specific magnitude, rare floods could occur at short <br /> <br />6 <br />