Effect of Dikes on Saltwater Intrusion Under Various Wind Conditions in the Changjiang Estuary

Abstract

To improve navigation, the deep waterway project (DWP) was implemented in the North Passage of the Changjiang Estuary in 1998, which includes a deep channel and two dikes protecting it. By altering estuarine morphology, the DWP can affect saltwater intrusion and mixing, with implications for drinking water intake and supply. In this study, we employ a numerical model to study the influence of dikes of the DWP on saltwater intrusion in the estuary under the climatic and persistent strong northerly wind conditions that occurred in February 2014. The model results show that the dikes prevent the southward transport of relatively low‐salinity water at the mouth of the North Channel (NC) under climatic wind conditions, resulting in the weakening of saltwater intrusion and mixing in this channel. Under persistent strong northerly wind conditions, relatively high‐salinity water is transported southward to the mouth of NC and blocked by the dikes causing a water level rise at the mouth of the NC. As a result, a large amount of high‐salinity water advected into the NC and then out to the sea from the South Chanel, forming a counterclockwise horizontal circulation. The salinity increases abnormally, but mixing decreases in the NC for no more salinity variance input with the implementation of the DWP. Overall, the DWP favors water intake for the reservoir in NC under climatic wind conditions and is unfavorable to water intake under persistent strong northerly winds (>9 m/s), which can lead to extremely severe saltwater intrusion.

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

“…This underwater construction diminishes the freshwater partition in the South Channel while amplifies the onshore intrusion of saltwater in the South and North Passages, and alleviates the invasion of shelf waters in the North Channel by reducing/increasing the partition of riverine waters in the South/North Channel (Zhu et al, 2018a;Zhu et al, 2019). From the North to the South Channel, an anticlockwise circulation anomaly is thereby established (Li et al, 2020). The built underwater jetty spur (Figure 1B) has greatly increased the transport time by 50% (Wang et al, 2010).…”

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 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).…”

“…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].…”

Effects of Typhoon on Saltwater Intrusion in a High Discharge Estuary

“…Relatively strong intrusion events have been registered under strong northerly winds in winter when the landward Ekman transport of salt dominates over the steady shear dispersion in the North Channel (Li et al., 2012, 2020; Wu et al., 2010; Xue et al., 2009; Zhang et al., 2019; Zhu et al., 2020). Extreme events can also lead to conspicuous cases of saltwater intrusion.…”