Progress in natural flood retention at the Bavarian Danube

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The public generally expects restoring natural flood retention as an effective flood mitigation measure, given that river regulation or losses of natural retention storage space through human intervention of nature are often perceived as the cause of recent disastrous floodings. In order to investigate the above mentioned hypotheses and to investigate the retention potential of a large natural river before the river was modified by man, we developed a 2D-hydrodynamic model for a 270 km section of the Bavarian Danube under its historical and present states. In other words, we modeled the flooding of the river prior to the implementation of any river regulation measure, i.e., a state at around 1,800, and compared the results with the present state. Surprisingly, even though the retention volume during the historical state was much larger compared to the present (approximately 350 million m3 of additional retention space during a 100-year flood event), only minimal or even negative effects on the damping of flood peaks could be detected for some cross sections. The delay of the flood peak in the historic scenario, however, conforms with general expectations. Two main hydraulic effects could be observed: Firstly, overtopping of the accompanying floodplains is nowadays delayed by dikes, so that the smaller retention volume is activated very efficiently during times of flood peak passage. Secondly, the timely interaction of the Danube flood wave propagation with flood waves from tributaries and with bypass-overland-flows parallel to the main channel is important. We furthermore performed steady-state simulations to show the process of overtopping, the activation of retention volume and the flow velocities on the floodplains for both states and quantify the loss of retention volume over the last 200 years. This consideration explains the natural retention effects in more detail.