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<br />concrete blocks were used to simulate stationary sand bars. Studies were <br /> <br />also made to a limited extent with moving bars in the upstream channel. <br /> <br />Sand bar lengths were first investigated. Successive increase in <br /> <br />lengths of the bars from 2 ft to 12 ft (model dimensions) did not effect <br /> <br />a difference in the flow distribution. The increases in length were <br /> <br />always parallel to the flow. Thereafter, bars of about 3-1/2 ft in length <br /> <br />were used. <br /> <br />The sand bars were enlarged laterally in the flume at various positions <br /> <br />upstream of the structure. For the case of sand bars extending above the <br /> <br />water surface, the effect of its presence in the channel was noticeable on <br /> <br />the water surface profile if the bars were close to the structure. When the <br /> <br />bar extended about 25 percent of the width from one side of the flume, its <br /> <br />presence within 300 ft of the flume at a unit discharge of z:j cfs per ft <br /> <br />(prototype) affected the water surface slightly. When the bar was placed <br /> <br />further upstream the effect was not noticeable and was greater when loca- <br /> <br />ted closer to the structure. <br /> <br />The effect of sand bars was also dependent upon the discharge. For <br /> <br />the same size bar and location within the channel, lower discharges did not <br /> <br />affect the water surface profile in the same manner. In prinCiple, the bars <br /> <br />must be sufficiently far upstream from the structure so that the flow beyond <br /> <br />the bar can fully diverge across the entire width of the channel. <br /> <br />Sand bars which did not extend above the water surface were also <br /> <br />studied. It was found that larger bars in lateral extent could be allowed <br /> <br />for a comparable effect on the water surface profile. At a distance of 300 f't <br /> <br />upstream from the structure, for instance, 40 percent lateral constriction <br /> <br />-31- <br /> <br />-', - "-. ~ <br />li ~.' .:;., .L v <br />