ABSTRACT
Sedimentation and the management of hydropower reservoirs is one of the big global issues concerning future hydropower use. Exemplarily for Asia a reduction of 80 % of the potential storage volume is predicted by 2035. For Europe those thresholds in sedimentation are predicted by 2080. Related consequences − beside a reduction of the storage volume and the restriction in hydropower production − are effects on aquatic ecology linked to artificial flushing of deposited fines. Impacts on aquatic ecology, however, can be manifold, especially in a change in habitat quality with possible clogging of the gravel matrix and an increase of stress due to an increased turbidity.
The presented case study is dealing with a lowering of the water surface level of the reservoir Gepatsch (Tyrol) beyond the operational level. This was a controlled drawdown of the reservoir, which has to be distinguishes from reservoir flushing. However, also from the controlled drawdown an increased load of suspended sediments was expected. Based on the awareness of possible negative ecological consequences a complex set of measures and an integrative monitoring design was been developed. Monitoring is based on detailed event based quantification of eroded sediments. High resolution turbidity data are available for the entire Inn river in Tyrol. Moreover, we analysed the biological quality elements macrozoobenthos and fish at selected stretches. In addition, freeze-core samples were taken before and after the lowering of the reservoir volume, in combination with cocooning of brown trout during the spawning period. This case study brought up an extensive data set, however for future monitoring activities in similar projects we suggest to concentrate on a limited number of monitoring sites, including a reference station as well as a station to assess the emission. Downstream effects could be assessed numerically, but it has to be considered that additional stations provide the possibility for a detailed process study, i.e. the analyses of processes that are causing a natural increase of suspended sediments.
