Gabriel Stecher , Mathew Herrnegger

Dams and hydropower reservoirs constructed in the headwaters of river basins alter the hydrological characteristics as well as other physical and biological conditions of rivers downstream. In this study the impact of reservoirs on floods is systematically assessed for 8 heavily modified river basins in Austria. Since discharge data prior to the construction is not available the natural unaffected annual peak discharges downstream of the reservoirs were estimated by transposing the peak runoff of an unaffected reference catchment by considering the spatial proximity, the catchment area and the precipitation conditions in both catchments. The potential impact is then calculated by comparing the estimated and the observed discharge peaks. The results show significant flood peak reductions not only at gauges directly affected by the reservoir but also further downstream at receiving rivers. The propagation of the flood peak reduction downstream is also related to the ratios (i) between the reservoir catchment and the total catchment area and (ii) between the storage capacity and the catchment area. This indicates the decrease of possible flood control further downstream. Considering the return periods, results indicated that flood peak reductions are most significantly for events with higher return periods. Extremer events with return periods larger than 30 years show a considerable flood peak reduction of over 33 % on average. These findings show that reservoirs foremost constructed for hydropower generation also contribute significantly to flood control and flood risk mitigation.