Author(s): Patrick Holzapfel; Rolf Rindler; Helmut Habersack; Michael Tritthart; Christoph Hauer
Keywords: No Keywords
Abstract: Sediment balance and dynamics of running waters are crucial for its ecological and morphological state. Nevertheless, protective hydraulic engineering measures like e. g. bedload retention dams act as an interruption in sediment continuity. 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. Due to hazard mitigation issues, two slit dams were constructed at the Strobler Weissenbach, an Alpine mountain stream located in the Northern Limestone Alps of Austria. Frequent clogging of the structures by woody debris leads to annual retention of huge amounts of limestone gravel. To re-establish state of the art requires the structure completely empty. However, increased sediment release from the structure can result in significant ecological damage and/or geomorphological alterations. Hence, the goal of the presented study is to identify possible measures concerning the emptying of the structure by simultaneously taking into account downstream ecological consequences. In a first step, an extensive monitoring concept was implemented at the study site focusing on sediment transport (PIT tag tracer, suspended sediment probes), changes in biotic data (fishing data, mapping of spawning grounds) and ecohydraulic analyses of alterations in stream morphology (bathymetry, habitat modelling). After more than one year of premonitoring, as a first measure, clogging of woody debris was disentangled. Special focus was given on the assessment of responses of fish to increased suspended sediment concentrations (SSC) and changes in habitat quality in general. Hence, an impact assessment model according to Newcombe (2003) was implemented in a software application and applied to recorded SSC time series. “Severity of ill effect” (SEV) values which may range from 0 (no effect) to 14 (up to100% mortality) were modelled before and after sediment release from the sediment retention dam showing high accordance with in-situ observations and data from electro fishing. Furthermore, survey of selected river stretches showed a filling up of pool habitats in consequence of increased sediment release. For this reason, a decline in weighted usable area for adult brown trout (Salmo trutta) and an impairment of suitable spawning grounds could be modelled and documented. Finally, the findings of this work should contribute to the development of an integrative novel sediment management concept for alpine headwaters in Austria.