受流域洪水归槽造成水位抬升和区域河床下切引起水位下降影响，珠江三角洲泄洪情势发生了显著变化。 为探明这种影响和变化，利用洪水、地形实测资料，分析当前洪水归槽和地形下切态势，基于河网二维水动力学模 型，分别量化洪水归槽和地形下切对西、北江三角洲泄洪能力的作用大小，并在此基础上评估现状主干河道过流 能力。结果表明：洪水归槽导致珠江三角洲防洪压力陡增，相对 20 世纪 90 年代，防洪控制断面思贤滘 50 年一遇 设计洪峰流量增幅达 11.7%，西江、北江相应洪水位最大抬升幅度分别为 0.67、0.56?m；河道大规模下切增加了河 道泄洪能力，1999—2016 年西、北江主干河道平均下切幅度分别为 2.54、1.21?m，相应的洪水位普遍降低，50 年一 遇重现期洪水位最大降幅分别西江为 0.76?m、北江为 0.67?m；除三角洲上部局部河段外，洪水归槽对三角洲洪水 位抬升作用显著大于河道下切的水位降低作用。现状三角洲主干河道 50 年一遇洪水位有近 900?km 河段超堤防 设计水位，超出河段主要集中在天河、南华、三善滘附近的三角洲腹部。研究成果可为后续珠江三角洲防洪体系 整体安全评估和提升提供依据。

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.