|Tour Stop #12: Acoustic Measurement of Suspended Sediment Load|
R. W. Derrow, II
R. A. Kuhnle
C. V. Alonso
Measurement of the rate of sand in transport as suspended load in streams is difficult and expensive. Studies have demonstrated that samples over the entire flow depth at several locations across the cross section are needed to adequately determine the mean sand concentration at a given stream cross section for a given flow rate. Currently the best way to obtain sand concentration data is for two persons to be on site during the runoff event with the necessary sampling equipment. Manual sampling, however, is expensive, and adequate coverage of the range of flows is often difficult due to the uncertainty of predicting runoff events.
The goal of this project is to develop an automatic sampler to measure depth integrated sand concentrations. The use of acoustic backscattering has promise to allow noninvasive measurements of suspended sand concentration profiles with a spatial resolution of ~1 cm, a temporal resolution of ~0.1 sec, and to yield information on the size distribution of the suspended sediment. Much progress has been made on this problem by researchers at Proudman Oceanographic Institute, UK. Our work will be expand on previous studies to allow sand concentration measurements to be made in stream environments with larger variations in sediment sizes, greater sediment concentrations, and greater fine sediment concentrations than previous studies.
A suspended sediment calibration tunnel (SSCT) has been constructed and is currently being used to develop and calibrate an acoustic suspended sediment sampler. Initial results have demonstrated the concept of this sampler using an acoustic frequency of 1 MHz with unisize glass beads of diameter 0.65 mm. In later experiments frequencies of 5, 2.25, and 1 MHz will be used. Future experiments in the SSCT will vary ranges in sediment size and the size distribution. Following the experiments in the SSCT, suspended sand concentrations will be measured under alluvial conditions in flume experiments. A field version of the suspended sediment sampler will then be tested on the Goodwin Creek Experimental Watershed.
Key Words:Suspended sediment sampler, Acoustic backscattering, Sand transport
Publications:Derrow, 11, R. W., and Kuhnle, R. A., Preliminary analysis of NCPA-NSL suspended sediment calibration system. Proceedings of the 6th Federal Interagency Sedimentation Conference, Las Vegas, Nevada, p. V-58 - V-65,1996.