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Institut für Wasser- und Umweltsystemmodellierung - IWS



"Analysis of Hydraulic, Morphological and Ecological Effects of Reservoir Flushing downstream of a reservoir: Case Study Bodendorf, River Mur, Austria"

The loss of storage volume of reservoirs is caused by Sediment deposition. Sedimentation occurs where river-transported material cannot be carried by the flows. When the reservoirs are filled, the volume available for water storage decreases and they do not function as was originally thought. Several measures exist that could help to minimize this problem, whereof sediment flushing (hydraulic flushing) is one, to deal with sedimentation problems. Sediment flushing means that the reservoir water is released through bottom gates in the dam with the intention to erode deposited material from the reservoir bed and carrying these to the river downstream from the dam. Sediment flushing is used to remove sediments up to and including sands and gravels. It is the sands and gravels which determine the ultimate morphology of a river downstream of a reservoir. With the new knowledge, the management of the reservoirs could be improved and by that also the efficiency of flushing operations. It is not until recently, though, that the downstream effects have received any particular attention. Some negative effects have been known for long time, but practically no thorough investigations have been carried out. The main aim of this thesis is to analyze the possible effects of flushing in the downstream reaches of reservoir Bodendorf using mainly DHI softwares (MIKE 11 and MIKE 21C) and also the supplementary habitat model, CASiMiR (IWS), and to evaluate the performance of these softwares (especially MIKE 21 C) in representing the effects of such high flow situations downstream of a reservoir. The results of hydraulic and morphological, and hence hydraulic, changes from MIKE 21C because of sedimentation and erosion occurred during flushing and the flood events are worth representative. A very interesting trend of sedimentation and erosion which is approved by the observed sedimentation and erosion trends has been seen. In general, it can be concluded that erosion dominated processes are taken place during high flow events, flushing and flood, in the downstream of reservoir Bodendorf in River Mur though sedimentation also occurs in the processes. As a 2D curvilinear model, the impor! tance of modeling with MIKE 21C has also been observed by its morphological results which have sedimentation in the inner parts of the river bends of the two case study sites, Campingplatz and Gestütsof, which has been also seen in the after flushing and flood event measured bathymetry. Applying the habitat model CASiMiR (developed by Institute of Hydraulic Engineering, Stuttgart University), a general improvement of the habitat for spawning graylings has also been observed after flushings.