Laserfiche WebLink
<br />2.4 Flood protection Measures <br /> <br />There are no flood control structures within the City of Rifle. <br /> <br />Rifle Gap Dam is located approximately 5 miles upstream of the <br />City of Rifle along Rifle Creek. The dam and reservoir are operated <br />to supply irrigation water and provide no flood control protection. <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 />10-, 50-, 100-, or 500-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance rates. These events, commonly termed the 10-, 50-, 100-, and <br />500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equaled or exceeded during any year. Although the recurrence <br />interval represents the long term average period between floods of a <br />specific magnitude, rare floods could occur at short intervals or even <br />within the same year. The risk of experiencing a rare flood increases <br />when periods greater than 1 year are considered. For example, the risk <br />of having a flood which equals or exceeds the 100-year flood (1 percent <br />chance of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 in 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect <br />flooding potentials based on conditions existing in the community at the <br />time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for each flooding source studied in detail <br />affecting the community. <br /> <br />Peak flows on the Colorado River were based on records from USGS <br />stream-gaging stations (Reference 3). <br /> <br />peak flows for Hubbard, Helmer, and Ramsey Gulches, and Government <br />Creek were based on annual peak flow data for snowmelt and rainfall <br />floods. Annual peak flows for rainfall and snowmelt floods have <br />been published by the USGS (Reference 4). Additional annual peak <br />flows were obtained from inspection of the USGS stripcharts. <br />Frequency analyses were made on both peak flow data sets. The two <br />frequency curves were assumed to represent independent events, and <br />a composite flow frequency curve was developed for each gaging. A <br />regional regression analysis was then made to determine peak flow <br />as a function of drainage area and return period (Reference 5). <br /> <br />6 <br />