Abstract
Abstract. The severity of floods is shaped not only by event- and catchment-specific characteristics but also depends on the river network configuration. At the confluence of relevant tributaries with the main river, flood event characteristics may change depending on the magnitude and temporal match of flood waves. This superposition of flood waves may potentially increase the flood severity downstream in the main river. However, this aspect has not been analysed for a large set of river confluences to date. To fill this gap, the role of flood wave superposition in the flood severity at downstream gauges is investigated in four large river basins in Germany and Austria (the Elbe, the Danube, the Rhine and the Weser). A novel methodological approach to analyse flood wave superposition is presented and applied to mean daily discharge data from 37 triple points. A triple point consists of three gauges: one in the tributary as well as one upstream and downstream of the confluence with the main river respectively. At the triple points, differences and similarities in flood wave characteristics between the main river and the tributary are analysed in terms of the temporal match and the magnitudes of flood peaks. At many of the confluences analysed, the tributary peaks consistently arrive earlier than the main river peaks, although high variability in the time lag is generally detected. No large differences in temporal matching are detected for floods of different magnitudes. In the majority of cases, the largest floods at the downstream gauge do not occur due to perfect temporal match between the tributary and the main river. In terms of spatial variability, the impact of flood wave superposition is site-specific. Characteristic patterns of flood wave superposition are detected for flood peaks in the Danube River, where peak discharges largely increase due to inflow from alpine tributaries. Overall, we conclude that the superposition of flood waves is not the driving factor behind flood peak severity at the major confluences in Germany; however, a few confluences show the potential for strong flood magnifications if a temporal shift in flood waves was to occur.
Highlights
Floods result from an interplay of several factors along a cascade of processes beginning with precipitation and runoff generation in a catchment down to river routing
The aim of this study is to investigate the role of flood wave superposition in flood severity downstream of relevant confluences in the main rivers in Germany, including Austrian tributaries
A perfect match of flood waves is detected for the small catchment of Zschopau, where flood wave superposition enhances the majority of flood events
Summary
Floods result from an interplay of several factors along a cascade of processes beginning with precipitation and runoff generation in a catchment down to river routing. Eventspecific characteristics such as the intensity and spatial patterns of precipitation exert an impact on river discharge. Flood patterns are characteristic for each catchment due to the specific physiogeographic conditions, i.e. the elevation and slope or geological formation, that result in site-specific runoff generation processes. Floods are impacted by the river network configuration and the related geomorphological catchment characteristics. Several studies have indicated the impact of drainage density (or hillslope lengths), which is related to the network configuration, on the runoff coefficients The river network configuration can lead to a higher/lower probability of flood wave superposition
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