Measurements of sediment flux in rivers with a multi-frequency echosounder

At present, SSC fluxes in rivers are typically estimated by multiplying the river discharge with the
average suspended sediment concentration (SSC). The latter is typically obtained from optical turbidity
measurements in one single point of the river cross‐section. The optical turbidity is converted in
average SSC based on a relation that is derived from the laboratory analysis of regular SSC samples.
This method has the disadvantages that it is based on a one‐point measurement and that it is
expensive.

The SSC distribution in an entire profile – vertical or horizontal – can also be derived from the
backscatter of single‐frequency echosounders. The disadvantage of this method is that the particle size
of the suspended sediment needs to be known in order to convert the profile of backscatter into a
profile of SSC.

Here we present a hydro‐acoustic method based on multi‐frequency echosounding. Operating on
multiple acoustic frequencies allows estimating the mean particle size directly from the backscatter at
the different frequencies. The method based on multi‐frequency echosounding is illustrated with
measurements on the Rhône River just upstream of Lake Geneva in Switzerland. The results are
compared to measurements based on optical turbidity measurements and to measurements based on
single‐frequency echosounding.