Development of a Simple Loop Method for Correcting Acoustic Doppler Current Profiler Discharge Measurements Biased by Sediment Transport

A negative bias in discharge measurements made with an acoustic Doppler current profiler (ADCP) is attributed to the movement of sediment near the streambed and is an issue widely acknowledged by the scientific community. 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, is presented. The uncertainty in the mean moving-bed velocity measured by the loop method is shown to be approximately 0.6 cm/s. Use of this method to correct the measured discharge is evaluated. For the 13 field measurements presented, the loop method resulted in corrected discharges that were within 5 percent of discharges measured utilizing DGPS to compensate for moving-bed conditions.