Hydrological signatures of flood trends on German rivers: Flood frequencies, flood heights and specific stages

Climate change, land-use change and in-stream river engineering affect trends in river discharges and river stages, and distinguishing such overlapping contributions is a major challenge in hydrologic time-series analysis. In this study, a systematic investigation of river stages and discharges was carried out for 78 stream gauges of rivers in Germany. We analysed the available times series for trends in flood stages, flood discharges, flood frequency and in stage-discharge relationships over time. With respect to annual maximum discharges and flood frequencies, no significant trends could be identified consistently throughout the study area. Significant discharge trends were identified at a number of stations, however, and tended to be catchment-specific. In contrast, trends in flood stages tended to be gauge-dependent, as stages over time are influenced by changes in local rating curves and thus by local and reach-scale channel modifications. Specific gauge analysis is a suitable tool for analysing such changes. No significant trends in specific stages over time were identified at most of the investigated gauges, generally paralleling the trend-based stage results at the same sites. Nevertheless, we could identify several river gauges with significant decreasing specific-gauge trends (e.g., Danube at Ingolstadt, Elbe at Magdeburg, Weser at Intschede) and others with significant increasing trends (e.g., Elbe at Dresden, Ems at Greven, Fulda at Grebenau, Leine at Herrenhausen). The identified trends were small compared with trends identified on heavily engineered rivers in the US driven by local changes in the channel (e.g., incision or wing dike groyne construction) or changes on the floodplain (e.g., dike displacement or changing land use in the flood plain). The trends in discharges and stages documented here have contributed to past changes in flood frequency and intensity on German rivers.