FLOOD10843 - Laserfiche WebLink

Home Browse Search

2.4 Flood Protection Measures There are several small lakes and reservoirs in Pi tkin County, mostly located within the upper watersheds of the Roaring Fork River. The Twin Lakes diversion reservoir controls flows from only a IS-square-mile drainage area and is intended to provide storage for irrigation and water supply. The above measures are insufficient for flood protection. A few temporary or localized flood protection works, including berms, levees, and bank protection works, are scattered near developed areas. The Fryingpan and Roaring Fork Ri vers exist in deep channels. No authorized flood-control structures are in the local study area, and none are under investigation. The only substantial structure that affects the flow of the Fryingpan River is the Ruedi Dam, located approximately 17 miles east of the Town of Basalt. The Ruedi Dam is part of the Fryingpan-Arkansas Project to divert water from the Colorado River basin to the Arkansas Ri ver basin. The dam was designed for an inflow design flood of 17,500 cis at a I5-day volume of 110,000 acre-feet. The probable maximum discharge is 5, S40 cfs from the spillway, and 1,810 cfs from the outlet structure. The total probable maximum discharge from the outlet structure and spillway is 7,350 cfs, approximating the SOQ-year flood in the Town of Basalt (Reference 8). A SOD-foot reach of the Roaring Fork River, located approximately 800 feet upstream of North Mill Street in the City of Aspen, was widened in December 1984. Current regulations in Pitkin County limit construction in floodprone areas, thus contributing to reduction in future flood potential. No other floodplain management measures are in effect in Pitkin County. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the conununi ty, 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 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 SOQ-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 which equals or exceeds the 100-year flood (I-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10}, and, for any gO-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported 8