Unravelling increasing flood hazard and influential factors in a tidal river

Flood disasters are destructive especially in prosperous and urbanized estuarine regions, where the flood regime is much more complex due to multiple fluvial–estuarine impacts. The Shenzhen River (SZR), located in one of the most prosperous regions of southern China, is vulnerable to increasing flood risk. Unravelling the influential factors is of particular significance to flood hazards prevention and urban safety for the SZR. Based on the field-measured floods on June 13, 2008 (“2008.06”) and August 29, 2018 (“2018.08”) with roughly equal magnitudes of rainfall and tide, the changing flood risk in the SZR basin was assessed. Considering the substantial development of tidal flat plants in the past two decades, a physical model of the SZR was built to quantify the impacts of changing river regime on the flood stage. The model covers the whole mainstream of tidal reach and half of the Shenzhen Bay (SZB), which was well calibrated and validated by in situ flow process. Several situations with different ranges of riverine vegetation and estuarine mangrove, including all vegetation (actual situation), half vegetation, no vegetation, mangrove in 2002 and 2018, were modelled to explore flood stage variations in 2-yr and 50-yr return period. The results found that the “2018.08” flood stage was about 1.4 m higher than “2008.06” flood. Moreover, the rainfall–runoff duration in “2018.08” was significantly decreased by 1 h less than that of “2008.06” flood, indicating increased flood risk in the SZR. The flood stage in the middle reach increases by more than 0.6 m driven by the riverine vegetation during the 50-yr return period flood, while the flood stage rises less than 0.1 m for the flood with 2-yr return period. Moreover, the extended estuarine mangrove forest resulted in about 0.2 m flood stage increment in the lower reach. The effects of sea level rise and sediment deposition after channel dredging on the flood risk in the SZR were further discussed. The effects of sea level rise and sediment deposition after channel dredging on flood risk were further discussed in the SZR. Channel infilling probably causes a flood stage increase of approximately 0.5 m for all reaches, while the influence of sea level is relatively slight but cumulative. Sufficient river management and planning, such as seasonal removal of riverine vegetation, mangrove management and regular topography surveys, should be taken into consideration in the near future.