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<br />-~ <br /> <br />Proceedings of the Se~'enth Federallnleragency Sedlmenta/ion Conference, March 25 fo 29. 2001. Reno. Nevada <br /> <br />. <br /> <br />USE OF ROTATING SIDE-SCAN SONAR TO MEASURE BEDLOAD <br /> <br />D. M. Rubin, Geologist, U.S. Geological Survey, Santa Cruz, California; <br />G. B. Tate, Geologist, U.S. Geological Survey', Menlo Park, California; <br />D. J. Topping, Hydrologist, U.S. Geological Survey, Reston, Virginia; and <br />R. A. Anima, Geologist, U.S. Geological Survey, Menlo Park, California <br />I Present address, Woods Hole Group, Redwood City, California <br /> <br />Abstract: Experiments in the 1960's demonstrated that the rate of sediment transport represented <br />by migrating bedforms gives a more accurate measure of bedload transport than rates predicted <br />from flow measurements. Rotating side-scan sonar can be used in the field to measure the rate of <br />bedform migration and to calculate bedload transport rates. A rotating side-scan sonar system <br />was deployed in the Colorado River in Grand Canyon for this purpose. For two sites where total <br />load was measured using a depth-integrating sampler, approximately 5% and 0.3% of the sand <br />transport was bedload involved in bedform migration; the other 95-99.7% occurred as suspended <br />load that bypassed the bedforms. <br /> <br />INTRODUCTION <br /> <br />. <br /> <br />In deep marine flows and in rivers with soft, sandy beds, measurements of bedload transport are <br />difficult to obtain using standard sampling techniques. Standard bedload samplers (such as <br />Helley-Smith, BL-84, or BLH-84 samplers) tend to dig into sandy beds and thus can yield overly <br />high measurements of bedload transport. This sampling problem illustrates the need to develop a <br />method for measuring bedload transport that does not involve direct contact with the bed. One <br />such method is to employ rotating side-scan sonar to measure the rate of bedload transport <br />represented by migrating bedforms. <br /> <br />Simons et al. (1965) tested a variety of bedload transport equations using experimental flume <br />data. They found that the rate of sediment transport represented by migrating bedforms gives a <br />more accurate measure of bedload transport than rates predicted from flow measurements. The <br />rate of sediment transport per unit width represented by a migrating bedform, called the <br />"bedform transport rate" by Rubin and Hunter (1982), is equal to the product of three terms: <br />bedform height, bedform migration speed, and a dimensionless shape factor (equal to 1/2 for <br />bedforms whose cross, section approximates triangles touching end-to-end). These values can be <br />measured relatively easily in the lab, but are difficult to measure in the field where bedforms are <br />larger and the water is usually deeper and more opaque. The bedload lransport rates calculated <br />using this approach are mean rates for the area over which bedfonn heights and migration rates <br />are sampled. In contrast, rates measured with a bedload sampler at a point on the bed are local <br />rates, which can be expected to vary from approximately zero in a dune trough to twice the mean <br />at a dune crest (Rubin and Hunter, 1982). <br /> <br />. <br /> <br />III - ] 39 <br />