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<br />this, it was determined that the 2-foot contour map had datum that <br />was, on the average, 1.5 feet high (it was 1. 5 feet higher than <br />the mean sea level datum of 1929). <br /> <br />The CWCB field surveys were used to <br />elevations in the areas where there was <br />by the Town of Georgetown. Dredging <br />downstream from the 15th Street Bridge <br />downstream from the Rose Street Bridge. <br /> <br />determine channel bottom <br />recent dredging undertaken <br />was done on Clear Creek <br />and on South Clear Creek <br /> <br />Locations of selected cross-sec tions used in the hydraulic <br />analyses are shown on the Flood Profiles (Exhibit 1). <br /> <br />Roughness coefficients (Manning I s "nil values) were selected for <br />the HEC-2 program. They were obtained by the CWCB staff field <br />survey. Roughness values for Clear Creek varied from 0.035 to <br />0.055 for the channel and from 0.030 to 0.090 for the overbank <br />areas. Roughness vslues for South Clear Creek varied from 0.040 <br />to 0.050 for the channel and from 0.035 to 0.080 for the overbank <br />areas. <br /> <br />The bridge capacities were analyzed by a procedure in which <br />bridges were modaled using the HEC-2 computer program with the <br />computer calculations being checked by hand calculations. Ten <br />percent blockage of bridge openings due to debris was assumed. <br /> <br />A starting water-surface elevation was established from the <br />elevation of Georgetown Lake. This elevation was derived from the <br />overflow rating curve for the spillway at the lake. <br /> <br />Flood profiles were drawn showing the computed water-surface <br />elevations for floods of the selected recurrence intervals. <br /> <br />The hydraulic analyses for this study are based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation Reference Marks (ERMs) used in <br />this study are shown on the maps; the descriptions of the marks <br />are presented in ERMs (Exhibit 3). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and <br />plain management programs. <br /> <br />local governments <br />Therefore, each <br /> <br />to adopt sound flood- <br />Flood Insurance Study <br /> <br />7 <br />