Response of flood level to flood returning to main channel and channel deepning in the West and North River Delta
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Abstract:
In recent years, the flood situation in the Pearl River Delta has changed significantly due to the change of water and sediment conditions from the upstream and the impact of human activities. On the one hand, the construction of the upstream embankment reduces the flood detention volume and increases the energy of flood movement in the main river channel, resulting in an increase in the peak flow of the downstream section, a rise in water level and a steep increase in flood control pressure. On the other hand, since the 1980s, due to the impact of human activities such as large-scale and high-intensity artificial sand mining and channel excavation, the riverbed of the Pearl River Delta has been generally cut down, and the river channel has developed into a narrow and deep type. Under the condition that the incoming flow has little change and the incoming sediment volume has dropped significantly, the water level at the same flow has dropped significantly, and the flood discharge capacity of the river channel has improved. The design flood in the river network area has changed greatly due to the influence of water level rise caused by the basin flood returning to the channel and water level drop caused by the regional riverbed undercutting. In this case, a new flood control system must be rapidly reconstructed under the changed conditions to find a new "balance" point.The current situation of flood diversion and terrain downcutting is analyzed to explore the impact and change using flood and terrain measurement data. Based on a two-dimensional hydrodynamic model of the river network, the effects of flood diversion and terrain downcutting on the flood discharge capacity of the West and North River Delta are quantified, and the current flow capacity of the main river channel is evaluated.The results shows that: ① The flood control pressure of the Pearl River Delta increases sharply as the result of the flood returning to main channel. Compared with the 1990s, the design peak flow of the flood control section Sixianjiao with a 50 year return period has increased by 11.7%, and the maximum rise of the corresponding flood levels in the West and North River is 0.67 m and 0.56 m respectively.② The large-scale downcutting of the river channel increases the flood discharge capacity. From 1999 to 2016, the average downcut amplitude of the main channel of the West River and the North River is 2.54 m and 1.21 m respectively, and the corresponding flood water level generally decreases. The maximum drop of flood level with 50 year return period frequency is 0.76 m in West River and 0.67 m in North River.③ Except for the upper reaches of the delta, the effect of flood returning to main channel on the rise of flood level in the Pearl River Delta is significantly greater than that of channel deepening. At present, the design water level of the main river course in the delta is mostly 50 year return period. The water level of nearly 900 km reaches exceeds the design water level of the embankment, which is mainly concentrated in the hinterland of the delta near Tianhe, Nanhua and Sanshanjiao.The main conclusions are: ① The return of flood to the channel will cause significant rise of flood level; ② The undercutting of river bed will cause the flood level to decrease generally; ③ Considering the influence of flood return channel and river channel topographic undercutting, flood return channel has basically greater effect on water level rise than river bed undercutting; ④ Under the combined influence of flood channeling and uneven topographic undercutting, abnormal backwater of water level in the abdomen still exists. The rapid contraction of the overflow section along the river and the formation of the bottleneck section are the main reasons for the abnormal backwater of the water level in the abdomen since 1999. The research results can provide basis and technical support for the subsequent overall safety assessment and improvement of the Pearl River Delta flood control system.
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