Measuring suspended sediment concentrations in a reservoir based on laser diffraction

The sediment balance in a natural river may change when water enters a reservoir, resulting in a natural trapping effect of the sediment particles. These sediment depositions reduce the capacity of hydropower reservoirs in an amount of about 1 % per year worldwide. This study focuses on the suspended sediment transport in the Angostura reservoir in Costa Rica. To get better knowledge of the transport processes in the reservoir during dry season, measurements of suspended sediment concentrations were conducted by using a Laser In-Situ Scattering and Transmissometry instrument (LISST). The instrument is based on a laser-diffraction method and measures sediment concentrations as well as the grain size distributions (32 grain sizes in the range between 1.25 and 250 μm) with a frequency of 1 Hertz. The measurements were applied at two locations of the reservoir (in the middle part and in the downstream part), in order to assess the spatial distribution of the suspended sediment concentrations. All measurements showed an increase of the suspended sediment concentrations over the depth. In the upstream profile, the increase was about 48 % and in the downstream profile about 75 %. However, the measurements also showed that the increase is not linearly distributed over the depth. A subdivision into the sediment groups, clay, silt and sand showed that the transported sediments contain a high amount of silt (>60 % for all measurements and locations). The results also showed that the percentage of sand increased over the depth and the percentage of clay decreased. The LISST instrument was able to measure suspended sediment concentrations in this case where the concentrations were low, compared to the wet season, and where the transported sediments contained mainly fine material. Additional conducted measurements with an acoustic Doppler current profiler (ADCP) showed very low flow velocities (in average between 0.1 m s-1 and 0.15 m s-1) in the reservoir and proved that there was no influence of the flow field on the sediment concentrations in a slight bend. In general, from the results of the LISST measurements, it could be seen that the suspended sediment concentrations and therefore the suspended sediment transport is low during dry season and has no significant influence on the depositions along the reservoir. However, a flood event that happened during the measurement campaign showed that the suspended sediment concentrations may increase rapidly and that the sediment balance can also be influenced also during dry season.