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<br />or near the streambed when the present bridge was constructed, <br /> <br />probably in the 1920's. <br /> <br />Flood Profiles <br />The profile of the thalweg (a line following the lowest part <br />of the valley) and flood profiles along the thalweg for the main <br />channel for the 25- and 50-year floods are shown in figure 8. The <br />distance used for stationing of the profiles (shown in plan on plate 2) <br />is measured along the thalweg in thousands of feet downstream from <br />Hover Road. Elevations along the thalweg were determined by field <br />surveys in 1961 and 1962. <br />Areas of Inundation and Depths of Flooding <br />Estimated boundaries of areas subject 'to inundation by the 25- <br />and 50-year floods are shown on plate 2. The arrows show the general <br />direction of flow of the 50-year flood. <br />Water surfaces of the 25-year flood will be practically level <br />at cross-sections normal to the thalweg. Therefore, depths of in- <br />undation for the 25-year flood can be estimated by subtracting the <br />elevation of the ground at any point along a cross section normal to <br />the thalweg from the elevation of the 25-year flood profile where it <br />intersects the cross section. Ground elevations should be determined <br />by field survey, rather than by interpolation between contours on the <br />base map, because of the approximations inherent in the contour <br />elevations shown on the map, and because of the possibility of changes <br />in topography after the map was compiled. <br /> <br />23 <br /> <br />