Mechanisms of hyperconcentrated flood propagation in a dynamic channel-floodplain system

•A downstream increasing discharge peak occurs in hyperconcentrated floods, for which the floodplain effects remain poorly known.•Numerical experiments are done to unravel how a channel-floodplain system reacts to a hyperconcentrated flood process.•A 2-D depth-averaged fully coupled model of non-capacity sediment transport is deployed.•The typical feature of channel erosion and floodplain deposition can be reproduced.•The interaction between channel and floodplain deformation and their impacts on the flood hydrographs are thoroughly discussed. The downstream peak discharge increase during hyperconcentrated floods in the Yellow River has been attributed to bed erosion, roughness reduction and floodplain effects. While great improvements have been made on the understandings of the roles of bed erosion and roughness reduction, the effects of floodplain remain poorly understood. Here, as a first step to reveal the floodplain effects, we present a numerical experimental study on how the channel-floodplain system reacts to a hyperconcentrated flood process. For this purpose, schematized channel-floodplain systems are designed and the classical 1992 flood record data is prescribed at the upstream boundary. By applying a fully coupled morphodynamic model, numerical experiments are conducted for a comprehensive analysis on the effects of bed erodibility, floodplain width, bed roughness variation, symmetry and longitudinal variability of geomorphology. Our results show two distinct trends for the response of channel- floodplain system depending on bed erodibility. For a small bed erodibility, both channel and floodplain experience erosion. For a moderate/large bed erodibility, only the channel experiences erosion whereas deposition occurs on the floodplain. The variation of the floodplain width does not affect these erosion-deposition behaviors while changing the magnitude and patterns of floodplain deposition. The longitudinally discontinuous channel-floodplain divided by either water storage areas or housing/farming banks diminishes the floodplain deposition at the discontinuous locations. The present numerical experiments do not show an obvious peak discharge increase, nonetheless, the recognized erosion-deposition characteristics would help further study of the floodplain effects on the peak of hyperconcentrated floods.