Field Testing of An Ultrasonic Acoustic Attenuation System For Measuring Fine Suspended Sediments and Algal Movements

The use of ultrasonic acoustic technology to measure the concentration of clay-sized sediments transported by streams has the potential to greatly increase the temporal and spatial resolution of sediment measurements while reducing the need for personnel to be present at gauging stations during storm events. In order to improve measurement capabilities for fine sediments in stream channels, The National Center for Physical Acoustics at The University of Mississippi has developed a remote, autonomous acoustic attenuation system for deployment in streams. The system was tested on the Middle Rio Grande near San Acacia, New Mexico, and in Goodwin Creek in Panola County, Mississippi. The acoustic instruments were compared to sediment concentrations from physical samples in both deployments. Daily patterns were found in the acoustic signals from the Middle Rio Grande, and a follow-up experiment at The University of Mississippi Biological Field Station was used to investigate the potential effects of algal movements on acoustic attenuation measurements. The results showed daily patterns in attenuation that were correlated with similar patterns in light, temperature, and dissolved oxygen. These results, along with previous related efforts from the literature, led us to hypothesize that daily movement of algal colonies in the water column may interfere with high-frequency acoustic measurements in natural environments and that acoustic methods have the potential to allow ecological researchers to detect mass algal movements in the field. Results from Goodwin Creek demonstrate that, in the absence of daily cycles in attenuation, the acoustic system was able to provide measurements of sediment concentration with high temporal resolution that tracked well with expected sediment transport patterns in response to discharge hydrographs.