Utilizing instrumentation to measure acoustic reflectance as a surrogate for suspended-sediment concentrations along a Piedmont River in Rockdale County, Georgia
Stephens, Daniel P.
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Erosion and sedimentation is a prevalent water-quality issue in many Piedmont streams in Georgia (Landers and others, 2002). Typically, concentrations of suspended sediment are determined by collecting water samples and making physical measurements of sediment mass per unit volume in a laboratory setting. Particle-size distributions of individual samples also are typically per-formed in a laboratory. The U.S. Geological Survey is con-ducting an ongoing research project in Rockdale County, Georgia, and is using state-of-the-art instrumentation in an attempt to develop surrogate in situ, real-time measure-ments of particle-size and suspended-sediment concentra-tions in streamwater. The ultimate goal is to use in situ, real-time particle-size- and suspended-sediment concentra-tion measurements for computing continuous sediment loads. Instrumentation installed in the Yellow River near Mil-stead, Georgia, includes the Sequoia LISST™-100 (Laser In-Situ Scattering and Transmissiometery) size distribution sensor (Fig. 1) and the SonTek® Argonaut-Shallow Water (SW) velocity meter1 (Fig. 2). The LISST™-100 is designed to report real-time in situ measurements of particle-size distribution (Gartner and others, 2001). The Argonaut-SW is a bottom-mounted, acoustic Doppler meter that is designed to report water velocity utilizing two vertically oriented beams and water-surface elevation using a third beam. The instrumented site is characterized by gradually-sloping, nonvegetated granite bedrock that controls flow of the river from baseflow to flood conditions. The grad-ual slope of the granite bedrock presented design and in-stallation obstacles to locating the instruments at the proper depth and outside of an area of recirculating back-flow. Site installation was designed to protect the instru-mentation from potentially-damaging flow-transported de-bris, premature wear, and vandalism, and to allow removal of the sensor for periodic maintenance during low to high baseflow conditions (Wagner and others, 2000). To meet these requirements, a polyvinyl chloride (PVC) pipe “de-ployment” tube was assembled and attached with steel wedge anchors bolted into the granite bedrock. ________ Figure 1. Sequoia LISST™-100 before deployment at study site. The deployment tube was raised approximately 10 cen-timeters off the streambed to ensure that the LISST™-100 would only measure entrained particles in the primary flow and not be skewed by larger sediments transported within the variable flow zone resulting from the bedrock channel. The LISST™-100 sensing tip was exposed ap-proximately 10 centimeters beyond the end of the de-ployment tube such that the sensor would not be fouled by the collection of fine sediment within the interior of the deployment tube. To prevent excessive strain on the LISST™-100 data cable, a light-gauge wire tether was connected to the instrument casing and anchored to the bedrock streambed. The degree of exposure of the sensor from the deployment tube end could be adjusted by chang-ing the length of the wire tether. The Argonaut-SW was designed to use Doppler tech-nology to measure stream velocity. The instrument func-tions by measuring the Doppler shift in reflected acoustic energy from entrained sediment particles within the water column. In order to use the Argonaut-SW to produce a surrogate for sediment concentrations, data compiled from the reflectivity function within the velocity computation are compared to actual sediment concentrations (Gray and others, 2003) measured from traditionally collected sam-ples. In theory, a larger measured reflectance will corre-spond to a greater sediment concentration.