Using a hydroacoustic method to establish continuous time series of suspended sand concentration and grain size in the Isère River, France

High frequency measurements of the concentration and grain size of suspended sand in rivers remain a scientific challenge due to the strong spatio-temporal variability. Applying a hydroacoustic multi-frequency method can improve temporal resolution compared to the classical approach by solid gauging (water sampling) and provides an interesting surrogate for suspended sediment concentration and grain size in rivers characterized by a bimodal suspension. The aim of this study is to establish time series of concentration and grain size of suspended sand in the Isère River (France) using a hydroacoustic method. Measurements with 400 and 1000 kHz Horizontal Acoustic Doppler Current Profilers (HADCP) are used to determine the acoustic attenuation and backscatter. Using frequent isokinetic water samples obtained with a US P-06 sampler close to the ensonified volume, a relation between the acoustic signal and the sediment concentration and grain size can be determined. In a next step, regular solid gaugings help to establish a relation between the concentration and grain size in the ensonified volume and on average in the river cross-section. Finally, time series of concentration and grain size of suspended sand may be established based on this relation. Results show a good correlation between the concentration of fine-grained sediments and acoustic attenuation as well as between the sand concentration and backscatter. While the acoustic signature of fine sediments is mostly driven by concentration changes, the acoustic signature of the sand fraction is impacted by changes not only in concentration but also in grain size distribution (the median diameter  varying between 150 and 400 µm). The homogeneity of concentration and grain size along the acoustic beam seems to be a main factor for successfully establishing concentration time series based on a cell-by-cell analysis.