Scientific Investigations Report 2006-5079

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Prepared in cooperation with Environment Canada

Application of the Loop Method for Correcting Acoustic Doppler Current Profiler Discharge Measurements Biased by Sediment Transport

By David S. Mueller and Chad R. Wagner

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A systematic bias in discharge measurements made with an acoustic Doppler current profiler (ADCP) is attributed to the movement of sediment near the streambed—an issue widely acknowledged by the scientific community. This systematic bias leads to an underestimation of measured velocity and discharge. The integration of a differentially corrected Global Positioning System (DGPS) to track the movement of the ADCP can be used to avoid the systematic bias associated with a moving bed. DGPS systems, however, cannot provide consistently accurate positions because of multipath errors and satellite signal reception problems on waterways with dense tree canopy along the banks, in deep valleys or canyons, and near bridges. An alternative method of correcting for the moving-bed bias, based on the closure error resulting from a two-way crossing of the river, was investigated by the U.S. Geological Survey. The uncertainty in the measured mean moving-bed velocity caused by nonuniformly distributed sediment transport, failure to return to the starting location, variable boat speed, and compass errors were evaluated using both theoretical and field-based analyses. The uncertainty in the mean moving-bed velocity measured by the loop method is approximately 0.6 centimeters per second. Use of this alternative method to correct the measured discharge was evaluated using both mean and distributed correction techniques. Application of both correction methods to 13 field measurements resulted in corrected discharges that were typically within 5 percent of discharges measured using DGPS.

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Posted June 2006

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