Highly active sediment transport induces morphodynamic changes exacerbating flash flooding

Rainfall-induced flash floods often trigger active sediment transport and substantial morphological changes, which in turn affect flood dynamics. Previous numerical studies have mostly overlooked the strong interactions between flow, sediment transport, and morphological evolution, and inevitably their impacts on flash flooding have remained poorly understood. Here a two-dimensional (2D) coupled shallow water hydro-sediment-morphodynamic (SHSM) model is applied to reconstruct an extreme flash flood in the Jiuyuan Gully catchment, Beijing, China, in July 2023. The impacts of sediment transport and morphological change on the flash flood are evaluated by comparing the results of a shallow water hydrodynamic (HD) model and the SHSM model. The SHSM model is shown to outperform the HD model in reproducing the observed maximum water levels, as sediment transport and morphological changes are explicitly taken into account. Bed load sediment prevails, while its transport rate may amount up to 30 kg/(m·s) in the flood conveyance channel and deviate from the transport capacity determined in line with local flows, as sediment entrainment is hindered by the concrete-lined bed. Aggradation in the flood conveyance channel is significant, reaching 3.0 m locally. Accordingly, the water level increases, leading to overbank flooding and extended inundation over the floodplains and adjacent areas. Consequently, flash flooding is exacerbated. The current findings highlight the critical role of morphological change induced by active sediment transport in modifying flash flooding, underscoring the significance of morphological change for practical flood warning and development of effective risk management strategies.