Impact of recent uneven channel evolution on hydrodynamic characteristics during flood season in the Pearl River Delta

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To explore the relationship between changes of riverbed topography and adjustments of flood dynamic characteristics of the Pearl River Delta (PRD) in 1999-2008, a one-dimensional river network mathematical model was established based on the surveyed topography in 1999. The 08·6 flood was first reproduced and model-data comparison was implemented; then, the impact of topography change of river network on hydrodynamic characteristics during flood season in the PRD was investigated. Results show that the flood diversion capacity of channels is increased due to the down-cutting of riverbed in the PRD. During the flood 08·6 (a recurrence period of 30 years), the water surface elevations are generally lower than before, with a maximum decrease about 0.9 m at the Makou station. The tidal range and tidal velocity are found to increase, making the tidal limit move to above Makou and Sanshui stations. The riverbed down-cutting in the Xijiang river networks is larger than that in the Beijiang river networks. Uneven riverbed down-cutting leads to changes of discharge ratio in river network nodes, during the flood season, flood diversion of the Xijiang river networks increases by 2%, and the peak discharge of the Makou and Tianhe stations increass by about 1 500 m3/s and 1 000 m3/s, respectively. The trend that the diversion ratio at Makou station decreases with the increase of upstream flood discharge does not change. Nevertheless, the uneven-riverbed-down-cutting caused diversion ratio changes increase. The riverbed uneven down-cutting in recent years is the major reason for the reduction of flood risk of the river networks in the PRD, especially in the central region of the PRD.