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<br /> <br />20 <br /> <br /> <br />. <br /> <br />study area as they were in 1950-51 are shown on plate 2. Since <br />that date, there has been additional construction on the flood <br />plain, principally for small industries and business establishments, <br />but these buildings are not shown. <br />Four highways and one railroad cross the valley in the study <br />area. The highways are normal to the general direction of uncon- <br /> <br /> <br />stricted flood flow, but the railroad is at an angle of about <br /> <br /> <br />45 degrees with the valley as a whole. Parts of one or both <br /> <br /> <br />approach fills to the bridges are lower than the embankments at <br /> <br /> <br />the bridges at all five crossings. <br /> <br /> <br />A levee, designed to protect a part of the left flood plain, <br /> <br /> <br />has been constructed along the left bank of the main channel <br /> <br /> <br />between Sunset Drive and the railroad. It is composed of valley <br /> <br /> <br />alluvium, principally coarse gravel. In the vicinity of the <br /> <br /> <br />railroad bridge, the top width of the levee is about 20 feet, and <br /> <br /> <br />it is 3 or 4 feet higher than the general level of the left flood <br /> <br /> <br />plain. <br /> <br /> <br />The geometry of the bridges is shown in table 2, p. 21. <br /> <br /> <br />Recent Erosion <br /> <br /> <br />There is evidence that fairly recent erosion has occurred at <br /> <br /> <br />Hover Road and in the channel downstream. The left upstream wing- <br /> <br /> <br />wall of the bridge has been undermined and is partially collapsed, <br /> <br /> <br />and the elevation of the streambed under the bridge is substantially <br /> <br /> <br />lower than it was when the bridge was built. The position of the <br />