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<br />2.3 Principal Flood Problems <br /> <br />Clear Creek flows through Silver Plume in a narrow channel with relatively steep slopes. <br />Large flows through the city are characterized by very high velocities and unstable <br />water-surface elevations. Bridges appear to be the greatest constraint on passage of <br />floodflows. <br /> <br />Flooding in Silver Plume is primarily a function of the spring snowmelt, or possibly <br />snowmelt in conjunction with rain-on-snow. Summer rainstorms do no appear to be the <br />major cause of flooding, although heavy hailstorms occasionally cause drifts of hail that <br />block drainage paths and create some flooding. <br /> <br />A search of newspaper articles and interviews with local citizens revealed that there has <br />been no significant flooding inside the corporate limits of Silver Plume. The only flooding <br />occurred in the vicinity of Cherokee and Gay Streets, when the bridge was overtopped. <br />No definite information as to year, depth, quantity of flow, or exact limits of flooding was <br />available. <br /> <br />2.4 Flood Protection Measures <br /> <br />No flood protection measures are presently in effect in the Town of Silver Plume. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, standard hydrologic and <br />hydraulic study methods were used to determine the flood-hazard data required for this study. <br />Flood events of a magnitude that are expected to be equaled or exceeded once on the average <br />during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having <br />special significance for floodplain management and for flood insurance rates. These events, <br />commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent <br />chance, respectively, of being equaled or exceeded during any year. Although the recurrence <br />interval represents the long-term, average period between floods of a speciflc magnitude, rare <br />floods could occur at short intervals or even within the same year. The risk of experiencing a rare <br />flood increases when periods greater than 1 year are considered. For example, the risk of having <br />a flood that equals or exceeds the lOO-year flood (I-percent chance of annual exceedence) in any <br />50-year period is approximately 40 percent (4 in 10); for any 90-year period, th~ risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based <br />on conditions existing in the community at the time of completion of this study. Maps and flood <br />elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge-frequency relationships <br />for each flooding source studied by detailed methods affecting the community. <br /> <br />For Clear Creek, floodflow-frequency data were based on a regional hydrologic analysis <br />in Manual for Estimating Flood Characteristics of Natural-Flow Streams in Colorado <br />(Reference 4). Values of the 10-,50-, 100- and 500-year peak discharges were obtained <br />by suing a series of regression equations from which flood characteristics were estimated <br />from measurable physical and climatic parameters of drainage basins. <br /> <br />3 <br />