Author(s): Jessica Laible; Benoit Camenen; Jerome Le Coz; Guillaume Dramais; Gilles Pierrefeu; Francois Lauters

Linked Author(s): Jessica Laible

Keywords: Hydroacoustics; HADCP; Suspended sediment; Suspended sand sampling; Engineered alpine rivers

Abstract: The knowledge of the flux, grain size distribution and temporal and spatial distribution of suspended sand is of major importance for the sediment management of engineered alpine rivers. In these rivers, sediment is trapped in hydropower reservoirs, requiring regular dam flushes to maintain dam storage capacity. Measuring suspended sand concentration continuously remains an issue, since it is characterized by vertical and lateral gradients requiring frequent, spatially-distributed sampling. The classical approach, the solid sediment gauging based on water samples distributed on several verticals and depths throughout the water column, offers limited spatial and temporal resolution. The aim of study is to establish time series of sand flux and concentration by improving temporal resolution applying an acoustic multi-frequency method using acoustic attenuation and backscatter to measure indirectly the suspension. Based on the experience of Moore 2012 and the USGS , the use of two frequencies allows the separation of the fine sediment fraction dominating the acoustic attenuation and the sand fraction dominating the backscatter. Correcting the signal intensity for transmission losses for each of the frequencies, the concentration and grain size of suspended sediment can be quantified by signal inversion. Compared to the Colorado where the first USGS stations have been operated, the sediment suspension in the Isère River in France is characterized by a bimodal distribution and a broader sand grain size distribution. Applying the bi-frequency method to this alpine River will certainly need an adapted method. Two monostatic Horizontal Acoustic Doppler Current Profilers (HADCPs) of 400 and 1000 kHz were installed at the long-term hydro-sedimentary station of Grenoble Campus. Frequent isokinetic samples made with US P-72 and US P-6 samplers within the measuring volumes of the HADCPs are used to establish a relation between the acoustic backscatter signal and the suspended sediment concentration. Simultaneous solid sediment gaugings throughout the river cross-section were achieved using a Delft bottle and ADCP measurements to determine the total sediment flux. These results, together with the samples in the HADCP range, are used to establish the relation between the suspended sediment concentration in the HADCP measurement zone (or index concentration) and the cross-sectional average concentration. Conducting frequent sampling campaigns, a time series of suspended sediment concentration and flux at Grenoble Campus is determined. First results link suspended concentration of fine sediments and intensity of the acoustic signal. High concentrations of fine sediments during floods (up to 4 g/l), measured by turbidimetry, led to significant attenuation of the acoustic signal and a measurement range reduced to less than 5 m for the 1 MHz sensor. Observed heterogeneity along the acoustic beams relates to variation in grain size and concentration and requires cell-by-cell analysis based on local concentrations instead of a supposedly homogeneous average.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022899

Year: 2022