ETS - Influence of turbidity currents on reservoir sedimentation venting through a bottom outlet as an alternative

Sedimentation is an increasing problem in Alpine reservoirs. It leads to a reduction of storage capacities of reservoirs (economical impacts) on one hand and might endanger the operating equipment like bottom outlets as well as power conduit inlets and gates on the other hand. During flood events it occurs that the inflows into a reservoir plunge after a certain distance in the stagnant water into deeper layers. This procedure results from the difference in concentration and/or temperature between the inflows and the water in the reservoir. These density- or turbidity currents might flow along the Thalweg up to the dam where the solids settle due to the reduction of the velocity of the turbidity currents. A possibility to avoid these problems would be to vent turbidity currents through the bottom outlet during a flood for retarding the settling of sediment in the reservoir for the most part. In this project it will be analysed whether a turbidity current, which flows along the Thalweg up to the dam during a flood, may be vented through the bottom outlet in order to prevent therewith the deposition of fine material. Whether a reservoir is appropriate to perform these venting campaigns or not depends on several parameters like the shape of the reservoir, the inlet structure of the bottom outlet as well as hydrological and sedimentological characteristics. Knowledge about flow conditions, sediment concentrations and temperature distributions in the reservoir are additional and crucial basics. For operating the bottom outlet the time of flow from the plunge point to the bottom outlet has to be known as well. The measurements are the determination of the velocity distribution (ADCP), turbidity, temperature and conductivity in two locations in the reservoir Großsölk in Austria. The velocity measurements have been carried out with two down looking SONTEK Argonaut XR. The other probes have been measured at the same locations. Besides the measurements in the reservoir additional probes are installed upstream and downstream of the reservoir. First measurements show that a venting of turbidity currents might be possible during higher floods when the bottom outlet will be opened.