The longest continuous days unsuitable for water intake of the lower biandan shoal in the changjiang river estuary

The Chenhang Reservoir, Qingcaosha Reservoir and Dongfengxisha Reservoir have been built in the Changjiang River Estuary, which supply about 80% freshwater for Shanghai. With social and economic development, there is still insufficiency of water supply in Shanghai. The lower Biandan Shoal is located at the upstream of bifurcation between the North Chanel and South Chanel, which reveals wet sand during ebb in spring tide. We applied the 3 dimension numerical model of saltwater intrusion in the Changjiang River Estuary which is developed and applied by the authors' research group for long term, and calculated the longest continuous not suitable take water times in the case of the extremely low river discharge in 1978-1979 to know whether the lower Biandan Shoal can be as a reserve water resource. The model was validated with the observed data at the 8 sites in the North Branch from February 19 to March 1, 2017. The results show that the mean correlation coefficients, root-mean-square error and skill scores for comparison of modelled and observed surface and bottom salinity are 0.85, 1.82 and 0.82, respectively, indicating that the calculated salinity agrees well with the observed data, and the model can preferably simulate the saltwater intrusion in the Changjiang River Estuary. The model calculation shows that the longest continuous not suitable take water times at the model output site in the lower Biandan Shoal is 13.79 d, the source of saltwater intrusion is mainly from the upstream saltwater spilling over from the North Branch into the South Branch during the later spring tide and middle tide after spring tide, and the downstream saltwater intruding directly from the sea during the middle tide after neap tide, and the former influence is greater than the latter one. The duration in which the freshwater can be taken just occurs during the middle tide after neap tide, which is advected by ebb current along the southern side of the South Branch. The longest continuous not suitable take water times at the lower Biandan Shoal is greatly shorter than the ones at the Qingcaosha Reservoir and Dongfengxisha Reservoir, meaning that the freshwater resource at the lower Biandan Shoal is more sufficient than the ones at the upstream and downstream of the South Branch, and is an excellent reserve water resource.