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Monitoring concentration and grain size evolution of suspended sediments in rivers during floods using horizontal acoustic Doppler current profilers

Suivi en continu de la concentration et de l'évolution granulométrique des sédiments en suspension dans les rivières pendant les crues avec des profileurs de courants acoustiques à effet Doppler

Abstract : Horizontal acoustic Doppler current profilers (H-ADCPs) are increasingly used to monitor flow speeds in rivers. They can also be used to accurately measure the concentration of fine sediments in suspension when concentrations exceed 100 mg/L. This is done using the sediment attenuation that is calculated from the intensity profiles. Measurements are presented from a site on the Isère river (France) that is equipped with H-ADCPs operating at 300, 600 and 1200 kHz. At this location the river is 90 m wide and 4 m deep and the suspended sediments are silts and clays. Using data from a spring flood, a linear relationship is established between the sediment attenuation and the concentration measured with an optical turbidity meter. This relationship is used to interpret the attenuation data from a second flood in the same month. The optical and acoustical estimates of concentration agree within 30%. The multi-frequency attenuation data are used to estimate the mean grain size by assuming lognormal size distributions. The variation in grain size of the suspended sediment is measured throughout the flood and it is shown that there was a temporary coarsening of the suspended sediment on the rise of the flood. These measurements show that multi-frequency attenuation data from H-ADCPs can be used to monitor concentration and changes in grain size throughout floods. Nevertheless, the absolute values in mean grain size are still subject to large differences when compared to those obtained from in situ water samples. Laboratory experiments are needed to elucidate these discrepancies.