Dynamic mechanism of an extremely severe saltwater intrusion in the Changjiang estuary in February 2014

Abstract

Abstract. 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.

Salt Intrusion as a Function of Estuary Length in Periodically Weakly Stratified Estuaries

“…Examples are the Pearl River and Yangtze estuaries, China (Ding et al., 2021; Gong & Shen, 2011), the Mekong Delta (Vietnam, Thailand, and Cambodia, see Smajgl et al., 2015), the Tagus estuary, Portugal (Rodrigues et al., 2019), the Hudson River estuary, USA (Ralston & Geyer, 2019), and the Mary River estuary, Australia (Miloshis & Valentine, 2013). Catastrophic salt intrusion in the Yangtze estuary threatened drinking water supply to around 25 million people for 23 days in 2014 (Zhu et al., 2020). The Mekong Delta, a critical region for food production (rice, shrimp), recorded its worst salt intrusion in the 2019/2020 dry season that caused a 6‐month shortage of freshwater supply for the locals and threatened the global food security (IFRC, 2020).…”

Water and Salt Transports in the Hengsha Channel of Changjiang Estuary

“…The strong salinity front in the HC produces a strong southeastward baroclinic gradient force during middle tide after neap tide and during spring tide (Figure 12b,c); this force obviously enhances the water and salt transports during ebb tide and weakens them during flood tide (Figure 14). By contrast, during neap tide, the salinity gradient in the HC is weaker, and the water and salt transports during ebb tide are still distinctly larger than those during flood tide because the northerly wind is more influential during neap tide than during spring tide [38,39]. The residual salt transport distributions around the HC under the four tidal patterns in EX2 are shown in Figure 15.…”

“…Its water diversion ratio varies with the river discharge and tide from 65% to 75% [33][34][35]. The Qingcaosha Reservoir, the largest estuarine reservoir in the world, was built within the upper reaches of the NC in 2010 and supplies approximately 50% of the freshwater for Shanghai [36][37][38]. Saltwater intrusion in the NC originates from multiple sources.…”

“…First, saltwater intrudes from the open sea along the downstream NC; this intrusion is the weakest among the four outlets due to its higher water diversion ratio. Under extreme conditions, such as persistent strong northerly winds, the NC also experiences severe saltwater intrusion [38,39]. Second, saltwater spills over from the North Branch into the upstream South Branch during spring tide and is then transported downstream by runoff [40].…”

Changjiang Delta in the Anthropocene: Multi-scale hydro-morphodynamics and management challenges

“…Wind and waves can induce considerable irregular variations in the deltaic dynamics. At the daily time scale, strong wind plays a profound role in modulating saltwater intrusion (Li et al, 2012a;Zhu et al, 2020) and the effluent and plume dispersion off the delta (Chao, 1991;Chang and Isobe, 2003;Lee et al, 2017;Wu et al, 2018a). Large waves stimulate the formation of near-bottom hyper-concentrations and fluid mud in the mouth zone (Li and Zhang, 1998;Wan et al, 2014;Lin et al, 2021), which may enhance siltation in the navigational waterway along the North Passage (Wan et al, 2014).…”