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Estuarine saltwater intrusions are mainly controlled by river discharge and tides. Unexpectedly, an extremely severe saltwater intrusion event occurred in February 2014 in the Changjiang estuary under normal river discharge conditions. This intrusion cut off the freshwater input for 23 d into the Qingcaosha reservoir, which is the largest estuarine reservoir in the world, creating a severe threat to water safety in Shanghai. No similar catastrophic saltwater intrusion has occurred since records of salinity in the estuary have been kept. During the event, a persistent and strong northerly wind existed, with a maximum speed of 17.6 m s -1 , lasting 9 d and coinciding with a distinct water level rise. Our study demonstrates that the extremely severe saltwater intrusion was caused by this northerly wind, which drove substantial landward net water transport to form a horizontal estuarine circulation that flowed into the northern channel and out of the southern channel. This landward net water transport overpowered the seaward-flowing river runoff and transported a large volume of highly saline water into the northern channel. The mechanisms of this severe saltwater intrusion event, including the northerly wind, residual water level rise, landward water transport and resulting horizontal circulation, etc., were systematically investigated. [ABSTRACT FROM AUTHOR]

Chen, Q.; Zhu, J.; Lyu, H., and Chen, S., 2019. Determining critical river discharge as a means to provide water supply security to the Changjiang River Estuary, China. Journal of Coastal Research, 35(5), 1087–1094. Coconut Creek (Florida), ISSN 0749-0208. Shanghai is one of the six cities worldwide experiencing severe water shortage in the 21st century. To solve the water supply shortage problem, a strategic water source transfer from the Huangpu River to the Changjiang Estuary has been completed; approximately 80% of the supply of fresh water in Shanghai is taken from the water sources in the estuary, but the estuary experiences saltwater intrusion during periods of low river discharge, e.g., the dry season. The reservoirs cannot take water if the salinity is higher than 0.45 psu at the water intakes. The longest water residence time of the Chenhang Reservoir (CHR) is 7 days; thus, the CHR will experience water supply problems if the duration of saltwater intrusion is longer than 7 days. In this study, saltwater intrusion was simulated, and the critical river discharge was ascertained under different river discharge conditions using the improved Estuarine, Coastal, and Ocean Model, semi-implicit. The saltwater spillover from the North Branch arrived in the middle reaches of the South Branch during the subsequent moderate tide and neap tide, threatening the water intake of the CHR. During a period of spring–neap tides, the longest periods for which the CHR could not take water from the Changjiang Estuary were 5.0, 6.5, and 8.2 days under river discharge conditions of 11,500, 10,000, and 9000 m3/s, respectively. The saltwater intrusion increased when the river discharge declined, and the availability of useable freshwater resources in the estuary became severe. By finely adjusting the river discharge in the model, the authors found that the critical river discharge was 9500 m3/s. If the amount of river discharge reaches 9500 m3/s, the Shanghai government can ask the Changjiang Water Resources Committee to discharge more water from the Three Gorges Reservoir to restrain the saltwater intrusion and ensure water security in the megalopolis of Shanghai. [ABSTRACT FROM AUTHOR]

The interactions between the extremely shallow, funnel-shaped topography and dynamic processes in the North Branch (NB) of the Changjiang Estuary produce a particular type of saltwater intrusion, saltwater spillover (SSO), from the NB into the South Branch (SB). This dominant type of saltwater intrusion threatens the winter water supplies of reservoirs located in the estuary. Simulated SSO was weaker than actual SSO in previous studies, and this problem has not been solved until now. The improved ECOM-si model with the advection scheme HSIMT-TVD was applied in this study. Logarithmic and Chézy-Manning formulas of the bottom drag coefficient (BDC) were established in the model to investigate the associated effect on saltwater intrusion in the NB. Modeled data and data collected at eight measurement stations located in the NB from February 19 to March 1, 2017, were compared, and three skill assessment indicators, the correlation coefficient (CC), root-mean-square error (RMSE), and skill score (SS), of water velocity and salinity were used to quantitatively validate the model. The results indicated that the water velocities modeled using the Chézy-Manning formula of BDC were slightly more accurate than those based on the logarithmic BDC formula, but the salinities produced by the latter formula were more accurate than those of the former. The results showed that the BDC increases when water depth decreases during ebb tide, and the results based on the Chézy-Manning formula were smaller than those based on the logarithmic formula. Additionally, the landward net water flux in the upper reaches of the NB during spring tide increases based on the Chézy-Manning formula, and saltwater intrusion in the NB was enhanced, especially in the upper reaches of the NB. At a transect in the upper reaches of the NB, the net transect water flux (NTWF) is upstream in spring tide and downstream in neap tide, and the values produced by the Chézy-Manning formula are much larger than those based on the logarithmic formula. Notably, SSO during spring tide was 1.8 times larger based on the Chézy-Manning formula than that based on the logarithmic formula. The model underestimated SSO and salinity at the hydrological stations in the SB based on the logarithmic BDC formula but successfully simulated SSO and the temporal variations in salinity in the SB using the Chézy-Manning formula of BDC. [ABSTRACT FROM AUTHOR]

Saltwater intrusion in estuaries is mainly controlled by tides and river discharge, as well as by topography and other factors. The Changjiang estuary has been seen a significant change in its topography from the data obtained in 2007 and 2017. In this study, a well-validated 3D numerical model was used to simulate and analyze the residual water and salt transport, water diversion ratio (WDR) in bifurcated channels and water resources in the Changjiang Estuary in 2007 and 2017. The comparisons of the model results showed that due to the North Branch becoming much shallower and narrower over the period from 2007 to 2017, the overall salinity in the North Branch decreased and the intensity of saltwater spillover (SSO) from the North Branch into the South Branch weakened. In the North Channel, the simulated residual or net transection water flux (NTWF) and WDR decreased during spring tides, resulting in increased saltwater intrusion. During neap tides, the saltwater intrusion was weakened despite the decreased NTWF and WDR because the water depth at the river mouth became shallower. The changes of topography during that period also resulted in changes of DWR, NTWF, salt transport across the tidal flats and dykes in the North Passage, South Passage and the South Channel, as well as overall dynamic mechanism. The results indicated that the salinity at the water intakes of the three reservoirs in the estuary slightly decreased, indicating that the time that reservoirs can take water from the estuary become longer in dry seasons. In the scenario of complete silt-up of the North Branch, the saltwater intrusion was weakened in the South Branch because of the disappearance of the SSO, which was favorable for the utilization of freshwater resources, but enhanced in the North Channel, North and South Passages. The overall influence from the topographic change over the period is that the saltwater intrusion is weakened in the North Branch, and enhanced during spring tides and weakened during neap tides in the North Channel, North and South Passages. Sediment accretion in the North Branch is favorable for utilization of freshwater resources. • Topography change in the Changjiang Estuary was remarkable in the recent decade. • Topography change had a great impact on saltwater intrusion in each channel. • Topography change was favorable for utilization of freshwater resources. [ABSTRACT FROM AUTHOR]