Use of Acoustic Doppler Instruments for Measuring Discharge in Streams with Appreciable Sediment Transport

Acoustic Doppler instruments measure the velocity of water relative to the instrument (relative water velocity). To obtain the true water velocity in a stream when the instrument is mounted onto a boat, the instrument must accurately measure the speed and direction of the boat and correct the relative water velocity for the boat motion. Boat speed and direction usually are measured by means of bottom tracking. Bottom tracking uses acoustic pulses to measure the boat velocity relative to the streambed, similar to those used to measure the water velocity. This technique can be accurate and is resistant to errors in the internal compass of the instrument; however, streams often transport fine sediments and sand as suspended load near the streambed or as bed load. During flood flows streams may transport appreciable volumes of larger sediments. The acoustic Doppler instrument measures a “moving bed” when the transported sediment causes a Doppler shift in the bottom-tracking pulses. This moving-bed condition will cause the instrument to measure an upstream boat velocity greater than the true boat velocity. When this boat velocity is used to correct the relative water velocity, it results in water-velocity measurements that are biased low. This moving-bed condition is commonly encountered in the Mississippi and Missouri Rivers when using acoustic instruments with frequencies greater than 600 kilohertz and can occur in most streams during higher flows. The best method of compensating for boat velocity on streams where a moving-bed condition occurs is to use a differentially corrected global positioning system (DGPS). When using DGPS to measure the boat velocity any errors in the acoustic instrument’s internal compass become important sources of error in the resulting velocity and discharge measurements. Data collected by use of instruments and software manufactured by RD Instruments, Inc. 2 indicate that with proper calibration and data-collection techniques, accurate discharges can be measured with DGPS as the boatvelocity reference. Conversely, these data indicate that appreciable errors will result if the compass is not properly calibrated and (or) proper data-collection techniques are not followed. Although proper data-collection technique is always important, measurements