Impact of hydropower reservoirs on floods: Evidence from large river basins in Austria

Abstract

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.

Toward a Catchment‐Scale Assessment of Flood Peak Attenuation by Multiple Reservoirs

“…A few methods are available in the literature to derive simplified indices that allow for a rapid, although approximated, determination of the impact of a system of reservoirs on flood (see, e.g., Del Giudice, Gargano, et al., 2014; Del Giudice, Rasulo, et al., 2014; Miotto et al., 2007; Wang et al., 2017). In many cases, the approaches are data‐driven (see, e.g., Stecher & Herrnegger, 2022), that is, derived from observing the behavior of a system under meteorological forcing. Two popular indices are the Flood Attenuation by Reservoirs and Lakes (FARL) (Scarrott et al., 1999) and the Reservoir Index (RI) (López & Francés, 2013).…”