Combining Optical and Acoustic Backscatter Measurements for Monitoring of Fine Suspended-Sediment Concentration Under Changes in Particle Size and Density

Optical and acoustic backscatter measurements are routinely utilized to monitor suspended-sediment concentration (M); however, both measurements are affected by changes in particle size and density. In this study, optical and acoustic backscatter measurements are combined to a single parameter, the sediment composition index (SCI), to quantify M, mean particle radius by number (a(o)), the fraction of sediment <20 mu m by diameter (f20 ), and particle bulk apparent density (rho(bulk)). Data are analyzed from Chesapeake Bay and five rivers of Queensland, Australia. SCI is utilized to predict the ratio of M to acoustic backscatter under changes in a(o) and rho(bulk) (R-2 ranged from 0.6 to 0.98 across all data sets) and combined with acoustic backscatter to predict estimates of M that are independent of changes in a(o) and rho(bulk). SCI is proportional to log10(a(o)) and f20 for SCI from acoustic backscatter measured at 6 MHz (R-2 = 0.8 and 0.74, respectively, p-value < 0.001, n = 133), while SCI(log10(a(o))) and SCI(f20 ) from acoustic backscatter measured at 2 MHz or lower are sensitive to changes in floc fractal dimension. Estimates of rho(bulk) from SCI are biased by changes in particle size (R-2 is 0.1-0.5 across all datasets). This study builds upon recent work that derived SCI to quantify composition of sand and mud in suspension and demonstrates the utility of the approach in systems transporting flocculated silt and clay. Future research directions are discussed. Plain Language Summary Accurate measurement of suspended-sediment load, the product of discharge and suspended-sediment concentration, is needed to inform numerous management efforts that aim to either mitigate or enhance the supply of sediment to waterways. Furthermore, given sediment-related impairments on aquatic ecosystems and the importance of sediment supply to shoreline stability, suspended-sediment load measurements delineated by particle size across the freshwater to marine domain are of increasing interest. Optical and acoustic backscatter measurements are routinely utilized to monitor suspended-sediment concentration needed to compute suspended-sediment load; however, both measurements are affected by changes in particle size and density. In this study we evaluate the use of combining acoustic and optical backscatter measurements to improve the estimation of suspended-sediment concentration and particle size in suspensions dominated by muddy flocculated sediment. We demonstrate from theory and field observations that combining optical and acoustic backscatter measurements provide estimates of suspended-sediment concentration that are not biased by changes in particle size and density.