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<br />All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). <br />Elevation reference marks and the descriptions of the marks used in this study are shown on <br />the maps. <br /> <br />Flood elevations for streams studied by approximate methods in Green Mountain Falls were <br />determined from drainage-area flood depth relationships (Reference 21) and topographic <br />maps (Reference 42). <br /> <br />Inadequate channel capacities of Sutherland Creek downstream of Cross Section A, coupled <br />with the inadequate culvert capacity under the U.S. Highway 24 Access Road, result in <br />breakouts of flow greater than the lO-year flood. Breakout flows occur in both the east and <br />west overbanks. The lack of any defined flow channel results in a complete flow pattern <br />which consists of overland flow and flow along U.S. Highway 24. These flows, joined with <br />the breakout flows from Fountain Creek, cannot be modeled with a one-dimensional <br />backwater model. Therefore, no elevations have been determined for the downstream portion <br />of Sutherland Creek and the flows along U.S. Highway 24 and Manitou Avenue. Therefore, <br />the flood hazard area has been analyzed by approximate methods. <br /> <br />The approximate elevations for the upper portions of Williams Canyon and Beckers Lane <br />Tributary were developed using cross sections taken from a USGS topographic map enlarged <br />to a scale of I :4,800, with a contour interval of 40 feet (Reference 42) and the USACE HEC- <br />2 computer program (Reference 29), which generates a topwidth for each section based on <br />cross section area, drainage area, tOO-year discharge, slope, and roughness values. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound floodplain management programs. <br />Therefore, each Flood Insurance Study provides lOO-year flood elevations and delineations of the 100- <br />and SOO-year floodplain boundaries and lOO-year flood way to assist communities in developing <br />floodplain management measures. <br /> <br />4. I Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent annual chance <br />(lOO-year) flood has been adopted by FEMA as the base flood for floodplain management <br />purposes. The 0.2-percent annual chance (SOO-year) flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied by detailed methods, the 100- <br />and SOO-year floodplain boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the boundaries were interpolated <br />using topographic maps at scales of 1:240; 1:1,200; 1:2,400; I :4,800; I :6,000; and 1:24,000, <br />with contour intervals of 2, 4, S, and 20 feet (References S, 30-32, 34-40, and 43-S0). <br /> <br />The 100- and SOO-year floodplain boundaries are shown on the Flood Insurance Rate Map. <br />On this map, the lOO-year floodplain boundary corresponds to the boundary of the areas of <br />special flood hazards (Zones A, AE, AH, AO, A99, V, and VEl; and the SOO-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood hazards. In cases where <br />the 100- and SOO-year floodplain boundaries are close together, only the lOO-year floodplain <br />boundary has been shown. Small areas within the floodplain boundaries may lie above the <br />flood elevations but cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />35 <br />