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<br />F®-0'7-1995 09 38 FROM ~ REGION 8 NTFQ7 tD • 913034930213 P. 09 <br />i <br />computer model {Reference 13). Flood profiles were drawn show- <br />~ ing computed water-surface elevations to an accuracy of 0.5 <br />foot for floods of the selected recurrence intervals, <br />Between approximately 850' downstream of Railroad Avenue and <br />West First Street, the 10-year flood elevation is higher than <br />the 50- and 100-year flood elevations. This is a result of <br />gravel mining in the area which has created levees that contain <br />the frequent flows and the effect of the narrow channel during <br />these frequent flows is a large backwater during the 10-year <br />flood. During the 50-, lOD-, and 500-year floods, flows Overtop <br />the levees upstream of the area and flow into the flood overflow <br />channel, and the overbook flow is not restricted by the levees <br />creating less backwater effect from the channel, The 10-year <br />flood profile is not shown in this area. For similar reasoning, <br />the 50-year flood elevation cpnputed at cross-section Hon the <br />flood Profiles is slightly higher then the 100-year flood <br />elevation. The 50-year flood profile is therefore drawn to <br />coincide with the 100-year flood profile in this area. <br />Roughness coefficients(Manning's "n°) were estimated by field <br />inspection. Roughness valves for the main channel of the Big <br />Thompson River ranged from 0.03 to 0.04; roughness valves for <br />the flood plain ranged from 0.04 to O.DB. <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 {N6VD), formerly referred to as Sea Level Datum <br />of 1929. Locations of the elevation reference marks used in <br />this study are shown on the maps. <br />The hydraulic analyses for this study are based only on the <br />effects of unobstructed flow. Therefore, the flood elevations, <br />as shown nn the profiles, are considered valid only if hydraulic <br />structures remain unobstructed. <br />FLOOD PLAIN MANAGEMENT APPLICATIONS <br />The National Flood Insurance Program entourages state and local <br />governments to adopt sound flood plain management programs. There- <br />fore, each Flood Insurance Study includes a flood boundary map <br />designed to assist comno,~nities in developing sound flood plain <br />management measures. <br />4.1 Flood Boundaries <br />In order to provide a national standard without regional dis- <br />crimination, the 1D0-year flood has been adopted by FETM as the <br />base flood for purposes of flood plain rrenagement measures.. <br />The 500-year flood is employed to indicate additional areas of <br />flood risk in the community. For each stream studied in detail, <br />the boundaries of the 100-year and 500-year floods have been <br />delineated using the flood elevations determined at each cross <br />1 <br />