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<br />2.4 Flood Protection Measures <br /> <br />There are no significant structural flood protection measures In <br />the Town of Parachute, Colorado. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods 1n the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magnitude which are expected to be equaled or exceeded once on the <br />average during any 10-, 50-, lOO-, or 500-year period (recurrence <br />interval) have been selected as having special significance for <br />floodplain management and for flood insurance rates. These events, <br />commonly termed the lO-, 50-, 100-, and 500-year floods, have a 10, 2, 1 <br />and 0.2 percent chance, respectively, of being equaled or exceeded <br />during any year. Although the recurrence interval represents the long- <br />term, average period between floods of a specific magnitude, rare floods <br />could occur at short intervals or even within the same year. The risk of <br />experiencing a rare flood increases when periods greater than 1 year are <br />considered. For example, the risk of having a flood which equals or <br />exceeds the 100-year flood (l percent chance of annual exceedence) in <br />any 50-year period is approximately 40 percent (4 in 10); for any <br />90-year period, the ri sk increases to approximately 60 percent (6 in <br />10). The analyses reported herein reflect flooding potentials based on <br />conditions existing in the community at the time of completion of this <br />study. Maps and flood elevations will be amended periodically to <br />reflect future changes. <br /> <br />3.l Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establ ish peak discharge- <br />frequency relationships for each flooding source studied by <br />detailed methods affecting the community. <br /> <br />Peak discharges for Parachute Creek developed by the SCS are <br />derived partially "ith the use of the SCS TR-20 computer program <br />which simulates rainfall flood peaks. Output from the program for <br />l6 different watersheds tributary to the Colorado River between <br />DeBeque and Glenwood Springs were plotted and a regional curve of <br />drainage area versus peak discharge and return frequency was <br />developed. Regional curves were also drawn for snowmelt flood <br />events derived from stream gage data from eight gages in the area. <br />The simulated rainfall curves and the snowmelt curves were then <br />combined using a standard probability equation into one set of <br />curves which was applied to several tributaries in the area, <br />including Parachute Creek. <br /> <br />Regional stream <br />discharges for <br />River as well. <br />Roaring Fork <br />differentiating <br /> <br />gage analyses were used by the COE to derive flood <br />the indicated return frequencies for the Colorado <br />Data from eight gages on the Colorado River, the <br />River and Plateau Creek were used without <br />between rainfall and snowmelt peaks. <br /> <br />5 <br />