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<br />The dam was built for recreational purposes and not flood control. <br />For that reason, the effect of the dam to reduce flood peaks <br />is negligible. However, the Ralph White Reservoir seems to have <br />had a positive effect on reducing the ice jam problem downstream <br />in Craig. <br /> <br />There is no flood plain management in effect in Craig. <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 SOD-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-, <br />and SOD-year floods, have aID, 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 <br />40 percent (4 in 10), and, for any 90-year period, the risk increases <br />to approximately 60 percent (6 in 10). The analyses reported herein <br />reflect flooding potentials based on conditions existing in the community <br />at the time of completion of this study. Maps and flood elevations <br />will be 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 discharges for floods of the 10-, SO-, 100-, and SOO-year <br />recurrence intervals were obtained from previous studies conducted <br />by the COE (References 3 and 4). <br /> <br />In the COE study, the flood discharges were computed by the use <br />of the HEC-l rainfall-runoff computer model (Reference S) and <br />a detailed statistical analysis of stream gage snowmelt data in <br />the study area. <br /> <br />For Fortification Creek and Cedar Mountain Gulch, the reductions <br />in the 100- and SOO-year discharges throughout the city are due <br />to sheetflow diversions from the channel. <br /> <br />9 <br />