SENSITIVITY ANALYSIS OF SEDIMENT FLUXES DERIVED BY USING ACOUSTIC BACKSCATTER

摘要

The evaluation of suspended sediment concentrations and sediment fluxes is an important task in river engineering. Such an evaluation is nowadays more often conducted by using the intensity of backscattered sound from acoustic measurement devices. Advantages of the analysis of the acoustic backscatter signal are a high spatial resolution of the suspended sediment concentrations and accurate calculations of the sediment fluxes. In the presented study the evaluation of the sediment fluxes is based on the measured acoustic backscatter signal from a single frequency ADCP in moving operation mode and suspended sediment concentration measurements with a device based on laser diffraction. In a first step the number of suspended sediment samples is varied along a chosen transect to analyze the sensitivity of the developed correlation function regarding the spatial distribution of the samples. The developed correlation function is used in a further step for the sediment flux estimations. The results show the importance of a balanced sampling along the transect so that a broad range of backscatter intensities is covered. In a next step the sediment fluxes within the near bed blanking zones, where no data from the ADCP measurements is available, are evaluated with respect to different gradients for the conducted extrapolation. From the results, it can be seen that the sediment fluxes in the near to bed blanking zone, calculated by an extrapolation, may reach in absolute terms even higher values as the measured ones. It can also be seen that the gradient used for the extrapolation may influence the total amount of calculated sediment fluxes significantly.