Impacts of topography change on saltwater intrusion over the past decade in the Changjiang Estuary

Abstract

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.

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

“…The increased discharge rate in the dry season, due to impoundment of Three Gorges Dam could be favorable for ensuring the freshwater supplement (Cai et al, 2019) and has potentials to suppress the injection of saltwater from the North Branch to the South Branch. The incremental anthropogenic impacts, such as the narrowing and shallowing of the North Branch, also progressively reduced the saltwater intrusions from the North Branch to the South Branch, and the topographic changes in the North Channel from 2007 to 2017 strengthened/weakened the intrusion of shelf waters during spring/neap tides (Chen et al, 2019). However, the role of Three Gorges Dam in changing the estuarine and coastal circulations is considered to be secondary referring to the underwater infrastructure in the North Passage (Wu et al, 2018c).…”

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

“…The prevailing monsoon climate results in a strong northerly wind of 5.5 m s −1 during winter and a southeasterly wind of 5.0 m s −1 during summer (Zhu et al, 2015). Saltwater intrusion in the Changjiang estuary is also mainly determined by river discharge and tides (Song and Mao, 2002;Gu et al, 2003;Shen et al, 2003;Luo and Chen, 2005;Qiu et al, 2012;Chen et al, 2019a) but is also influenced by wind (Xue et al, 2009;Li et al, 2012;Ding et al, 2017;Zhang et al, 2019) and topography (Li et al, 2014;Chen et al, 2019b). The impact of wind on saltwater intrusion has been studied but only with a climatic wind (Xue et al, 2009;Qiu et al, 2012;Chen et al, 2019a) and a strong northerly wind induced by ordinary cold fronts in winter lasting 1-2 d, which could cause a change in the observed salinity (Li et al, 2012).…”

Beyond Human Interventions on Complex Bays: Effects on Water and Wave Dynamics (Study Case Cádiz Bay, Spain)

“…This flushing time is analyzed in both sections of Figure 3a in order to understand how the ocean-outer bay and outer bay-PC water renewal rates are modified as a consequence of the interventions. Chen et al [23] analyzed the water and salt transport from another perspective, defining the residual water unit width flux (R w ) and the residual unit width salt flux (R s ) as:…”

“…The main drawback of the latter is that these simulations require extensive calibration and validation which is not easy to perform [5]. However, there are still some aspects of the dynamics of these systems that have not been analyzed in depth, despite the fact that a significant number of the world's main bays and estuaries are made up of interconnected basins, including the San Francisco Bay [19,20], the Chesapeake Bay [21], the Yangtze River estuary [22,23] or the Harvey River estuary [24].…”