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Innovative concept for the management of sediments in alpine torrents

Rindler, Rolf, Holzapfel, Patrick, Hauer, Christoph, Jury, Gerhild, Moser, Markus, Fischer, Andreas, Gumpinger, Clemens, Habersack, Helmut
Österreichische Wasser- und Abfallwirtschaft 2019 v.71 no.3-4 pp. 179-190
bedload, dams (hydrology), ecology, gravel, hydraulic engineering, limestone, protective effect, rivers, sediment transport, spawning, stream channels, transponders, Alps region, Austria
Sediment balance and dynamics of running waters are crucial for its ecological and morphological state. Alpine rivers provide the link between sediment production in the alpine region and the need for sediment of downstream rivers. Nevertheless, protective hydraulic engineering measures like e. g. bedload retention dams may cause a total interruption in sediment continuity or at least retard sediment availability. In further consequence, downstream sediment deficit reduces the potential for morphodynamic processes, leads to deepening of the river bed, and decreases the availability of spawning gravel. However, sediment continuity is becoming increasingly important in many water management and hydraulic engineering projects. In the seventies, two slit dams were constructed at the Strobler Weißenbach, an Alpine river in the Northern Limestone Alps of Austria, because of hazard mitigation issues. Frequent clogging of the structure by woody debris leads to annual retention of huge amounts of limestone gravel Besides the downstream sediment deficit, this implies high costs in restoring the protective effect of the structure (e. g. costs for excavation, hauling or landfilling fees).The goal of the presented study is to identify possible measures concerning the emptying of the structure considering downstream river ecology and morphology. Therefore, an extensive pre- and post-monitoring concept was implemented at the study site focusing on water level, sediment transport (PIT tag tracer, suspended sediment probes), changes in biotic data (fishing data, mapping of spawning grounds), and stream morphology (bathymetry, weighted usable area). The findings of this work should lead to the development of a novel integrative sediment management concept for alpine headwaters in Austria.