Laserfiche WebLink
<br />a scale of 1 inch equals 100 feet with a contour interval of 2 feet (Reference 8). <br />Bridges and culverts were field surveyed in September 1995. Stream bed <br />elevations taken from the topographic mapping were adjusted according to field <br />survey data obtained at selected locations to more accurately follow the actual <br />stream bed profile. <br /> <br />Cross sections for the backwater analysis were field surveyed and located at close <br />intervals above and below bridges and culverts in order to compute the <br />significant backwater effects of these structures in the urbanized areas. <br /> <br />Cross section data were taken from a 2-foot contour map of the Town of <br />Georgetown (Reference 13). Building areas were "blocked out" of the cross <br />sections to determine the effective flow through the floodplain. This cross <br />section data became input data for the USACE HEC-2 step-backwater computer <br />program (Reference 14). <br /> <br />A comparison of bridge cross section elevations with those obtained from the 2- <br />foot contour map led to the conclusion that there was a significant problem with <br />the datum of one of the two sources. Further study of the problem was thus <br />undertaken. Two other sources of elevation data were used in this analysis: a 1- <br />foot contour map of the vicinity of Georgetown Lake (Reference 15), and two <br />field surveys of selected cross sections along Clear Creek performed in May <br />1979 by Hayes and Soucie. A total of 60 points from the three described sources <br />were compared with the corresponding points on the 2-foot contour map. From <br />this, it was determined that the 2-foot contour map had datum that was, on the <br />average, 1.5 feet higher than the National Geodetic Vertical Datum of 1929 <br />(NGVD29) elevations. <br /> <br />The CWCB field surveys were used to determine channel bottom elevations in <br />the areas where there was dredging undertaken by the Town of Georgetown. <br />Dredging was done on Clear Creek downstream from the 15th Street Bridge and <br />on South Clear Creek downstream from the Rose Street Bridge. <br /> <br />Locations of selected cross sections used in the hydraulic analyses are shown on <br />the Flood Profiles (Exhibit 1) and on the FIRM. <br /> <br />Water Surface Elevations <br /> <br />Water-surface profiles for all streams previously studied in detail were developed <br />using the USACE HEC-2 backwater computer program (Reference 14) and <br />supplemented by manual calculations. Manual calculations were primarily used <br />to determine the division of flood flows at bridge sections and to verify water- <br />surface elevations near bridge sections and to verify water-surface elevations <br />near critical depth. Flood profiles were drawn showing computed water-surface <br />elevations to an accuracy of 0.5 foot for floods of the selected recurrence <br />intervals (Exhibit 1). <br /> <br />9 <br />