1. Introduction
The estuary is where the seawater and freshwater meet. Different forms of estuaries, different tides and runoff intensities will have an important impact on the mixing type of saltwater and freshwater, whereas different mixing types of saltwater and freshwater will have a significant impact on saltwater transport. Regarding large-scale saltwater intrusion, previous researchers have done a large number of research on the length of saltwater intrusion. Earlier studies of saltwater intrusion length assumed that runoff and tidal transport of saltwater were balanced [
1,
2], and on this basis, many researchers proposed estuarine salinity transport balance equations. Festa and Hansen [
3] used a two-dimensional mathematical model to discover that the length of salinity intrusion would change with the estuary water depth, vertical diffusion coefficient and runoff. Chatwin [
4] further provided the salinity intrusion length, including estuary depth, runoff, vertical diffusion coefficient, estuary salinity, etc.
With the development of computer technology, more and more researchers begin to use mathematical models to study the saltwater intrusion. Park and Kuo [
5] used a two-dimensional mathematical model to derive the response process of the length of saltwater intrusion to the periodic variation of large and small tides. Banas et al. [
6] pointed out that the estuary’s response to external changes depends on the estuary response time scale and the external time scale. Nebiyu [
7] used a three-dimensional numerical model to study the process of saltwater intrusion in the estuary under climate change. Meselhe and Noshi [
8] used the finite difference method to construct a three-dimensional numerical model at the Calcasieu-Sabine estuary, and simulated the hydrodynamic and salinity distribution of the estuary. Kurup et al. [
9] applies a two-dimensional numerical model to the Swan estuary, and analyzed and calculated the impact of different sea inflows on the location and extent of saltwater wedges in different seasons in the area. Acertsl [
10] and Nguyen [
11,
12] et al. used this model to analyze and study the saltwater intrusion in the Gorai and Mekong estuaries, respectively. Tiruneh, N.D. et al. [
7] used a three-dimensional numerical model of the estuary to simulate and analyze the effects of changes in water volume and sea level on estuarine saltwater intrusion. Bhuiyan and Dutta [
13] reported a 1 m sea level rise (SLR) could produce an increase of 1.5 psu in the salinity in the Gorai river network, Bangladesh. A salinity model study due to SLR in the James River (USA) showed that salinity could intrude about 10 km farther upstream for a 1 m SLR [
14]. Grabemann et al. [
15] reported a 2 km upstream advance of the brackish water zone in the approximately 80 km long Weser Estuary in Germany for a 0.55 m SLR scenario. In the Chesapeake Bay, the mean salinity, salinity intrusion distance and stratification would increase with rising sea level [
16].
Saltwater intrusion occasionally occurs in the Yangtze River Delta and the Pearl River Delta during the dry season in China. At the same time, the estuary is usually one of the most densely populated and economically developed regions. With the development of economy, the demand for freshwater for industry, agriculture and urban living is increasing. The estuary is the most direct and important source of freshwater, and the intrusion of saltwater in the estuary is the main obstacle affecting the utilization of freshwater resources in this area [
17]. The saltwater intrusion not only has a negative impact on the population of the estuary, but also affects the hydrological environment. The saltwater intrusion causes huge losses to various countries every year [
18,
19,
20].
As the largest estuary in China, the Yangtze River estuary has always been the focus of saltwater intrusion research. The studies began in the 1980s. In 1980, Shen Huanting [
21] conducted a preliminary study on saltwater intrusion in the Yangtze River estuary, provided the distribution characteristics of saltwater in the Yangtze River estuary and mentioned the important phenomenon of spilling from the North Branch (NB) of the Yangtze River estuary. The Yangtze River estuary belongs to the middle tidal estuary, which is dominated by half-day tides, and its upstream runoff is abundant. The interaction between tide and runoff has become the emphasis of many scholars’ research, including Mao Zhichang et al. [
22], Xiao Chengtai et al. [
23], Song Zhirao et al. [
24], Jianrong Zhu et al. [
25] and Yazhen Kong et al. [
26]. With the development of computer technology, scholars have studied the Yangtze River estuary saltwater intrusion through numerical simulation quantization. Wu Hui [
27] used the ECOM-si model to study the quantitative relationship between saltwater spillover, runoff and tidal range in the NB of the Yangtze River estuary. Chen Li [
28] used the statistical law to establish a saltwater intrusion prediction model for Chenkeng reservoir in the Yangtze River estuary. Xu Zhi et al. [
18] used the MIKE21 model to study the relationship between river discharge of the Yangtze River estuary and saltwater intrusion, and initially established a saltwater intrusion function. Li Lu [
29] pointed out that Stokes transportation was the main reason for NB spillover, and the tidal pump was the main reason for NB saltwater spillover. Qiu Chen et al. [
30] did quantitative research on the changes of saltwater intrusion characteristics in the Yangtze River estuary after sea level rises in the future.
Regarding the study of saltwater spillover in the NB of the Yangtze River estuary, it is difficult to observe the path of NB spillover visually and determine the salinity distribution of the NB spillover in some areas (Qingcaosha, Beigang and Nangang), due to the mutual mixing and doping of the saltwater intrusion from the South Branch (SB) and the spillover saltwater from the NB. In the past, researchers mainly depended on the relationship between the peak and the valley values of the salinity and the tide pattern, the phase relationship between the salinity process and the flow velocity, and the vertical distribution of the salinity to determine the main source of the saltwater [
31]. Some scholars have qualitatively divided the estuary area of the NB spillover and the conventional offshore on the basis of the changes in salinity values [
32]. There is a lack of clear and accurate methods to reflect the time varying process of the NB spillover under various conditions, and the specific distribution of spillover saltwater in the SB. In addition, the topography of the NB has changed significantly in recent years, and there is still no relevant research on its influence on the saltwater spillover on the NB of the Yangtze River estuary. The advantages of the MIKE21 model include the following: (1) an unstructured mesh with a high resolution was applied, which better fit the shoreline areas; (2) the influences of Hangzhou Bay were considered and (3) the use of spatially varying bottom roughness was included in the experimental process. This paper combines the measured data, builds a MIKE21 model, which simulates the movement of spillover saltwater group and salinity variation, under the influence of NB of Yangtze River estuary alone, and discusses the effects of factors, such as runoff, tidal current and wind on the activity and concentration of the saltwater group.
4. Discussion
4.1. Tidal Distributions
Tide-induced mixing mainly includes two aspects. On the one hand is the turbulent mixing caused by the frictional force of the river channel on the water during the period when the tide flows in the river channel. The second aspect is the interaction of tidal waves and rivers, which produces large-scale flows, including discrete shear flows, and the “pumping” and “blocking” effects of tidal waves. Under different tidal conditions, the fluctuation range of the saltwater group core within a tidal cycle increased with the tidal dynamics. With the increase of tide dynamics, the average downward velocity of the tide of the saltwater group core did not change much within the entrance, but it slowed down significantly near the entrance.
The model boundary hydrodynamic condition flow rate was 15,000 m
3/s. We increased the tidal amplitude of the hydrodynamic boundary of the open sea by a certain proportion coefficient (see
Table 6), kept the other conditions unchanged and performed simulation calculations to study the effects of different tidal intensities on the saltwater group. In this article, only path b1 was taken as an example. The position of the saltwater group changed under different tidal current conditions, as shown in
Figure 14.
During the third to sixth day (during the period of the NB spillover tide), the core of the saltwater group oscillated near the original point, and there was no obvious downward movement. The stronger the tide is, the greater the amplitude of the oscillation. With the weakening of the NB spillover, the saltwater group started to move downward on the sixth day. When the tide was weak, there was almost no fluctuation during the downward movement of the saltwater group. Under the advection of the runoff, the core of the saltwater group moved basically at a uniform speed and moved downstream. As the tide power increased, the core of the saltwater group fluctuated significantly with the flood and ebb during a tide cycle. The stronger the tidal power, the larger the range of fluctuation.
4.2. Wind Distributions
Wind mainly drives the flow of water by friction on the surface of the water, which has an impact on the material transport of the water. Wind often acts as the main source of energy for large lakes, oceans and certain coastal areas, but in estuary areas, the role of wind may not be significant. For long and narrow estuaries, the tide and runoff generally have a major impact on the flow structure and material transport of the water. If the estuary is wider, the influence of wind power will increase relatively.
Due to the lack of a large amount of measured data, the effect of wind on the flow structure and material transport of the Yangtze River estuary is less studied. Wu Hui et al. [
42] proved that northerly wind produced land-ward Ekman transport at the Yangtze River estuary with numerical simulations, and confirmed that this transport was helpful for spillover at the NB of the Yangtze River estuary. According to the measured data from Chongxi Hydrological Station, Li Lu [
43] found that under the conditions of northerly winds, the salinity would increase abnormally during the low tide and mid tide. This further confirmed that wind has an important effect on the saltwater spillover at the Yangtze River Estuary, and based on this, the influence of wind on different transport was analyzed through the study of numerical simulation.
Based on model B, based on the upstream boundary flow of 1500 m3/s, the salinity changes of the saltwater group on the path b1 under the wind parameters of northerly 5 m/s, southerly 5 m/s, easterly 5 m/s and westerly 5 m/s were studied.
It can be seen from
Figure 15 that different wind conditions had a certain influence on the activities and concentrations of NB spillover and spillover saltwater group, but its significance was smaller than that of runoff and current. Among them, the northerly and easterly could promote the NB spillover to a certain extent, and westerly could inhibit the NB spillover. The wind circulation from the north port to the south port formed by the north wind reduced the downward velocity of the saltwater group passing through the north port, while the downward velocity of the saltwater group passing through the south port increased.
4.3. Topography of the Area
Since the 21st century, the topography of the Yangtze River estuary has been changing rapidly under the influence of natural and artificial effects. In particular, the influence of the topographical changes in the NB on the spillover of the NB of the Yangtze River estuary has been the focus of attention. Based on the shoreline planned for the NB Reduction Project in 2004, Song Zekun [
44] et al. used a mathematical model to study the changes of the tidal current characteristics of the NB, and the sediment and riverbed erosion and deposition of the NB below the planned shoreline. Wu Hui [
42] studied the tide load, water level velocity and salinity changes in the NB, and analyzed the changes in the salinity of the SB water source point after the project.
During 2000–2008, the annual topographic change significantly weakened the inverted irrigation in NB, and the phenomenon of inverted irrigation will be further weakened after the implementation of the mid-narrowing scheme. The increase or decrease of the amount of back-irrigated saltwater caused by topographic change will make the salinity of the back-irrigated saltwater mass active in the southern branch increase or decrease significantly, but it has little effect on the activity law of the back-irrigated saltwater mass in SB [
45].
5. Conclusions
In this paper, the transport of the saltwater group from the NB was studied. The author adopted the nesting model to better study the influence of spillover saltwater in the NB on the SB, selected different river discharge values to simulate salinity changes caused by different river flows. The main findings of this study are summarized as follows:
Taking b1 as an example, it can be seen from the moving position of the saltwater group core on path b1 under different runoff conditions that before the sixth day, due to the relatively stable spillover, no significant movement occurred. After the sixth day, the downward movement of the initial saltwater group became slowly. With the sharp decrease of the upstream spillover saltwater, the upstream saltwater supply was cut off by the drained runoff, and the movement of the saltwater group core accelerated gradually. At the end of the downward movement, the larger the runoff value, the further the downward movement distance reached by the saltwater group core, and converged to about 90 km gradually.
At different flow rates, the relationship between the average position and time of tide cycle of the saltwater group core of each water channel was consistent with the Gompertz model, and its parameters had a non-linear relationship with the flow rate. The fitting results of the model demonstrated that when the flow Q < 10,000 m3/s, the saltwater group core in each water channel could not move down to the entrance within half a month, and the continuous accumulation of the saltwater group in the entrance would cause serious saltwater pollution. As the flow increased, it took longer for the saltwater group core to reach the entrance, around 3–8 days. The average relative salinity decay rate of the saltwater group core in the entrance shows a S-shaped change that increased first and then decreased and then increased again along with the increase of the flow rate. This is closely related to the residence time of the saltwater group core in the entrance changing with the flow.
The semimonthly average tide salinity of the three major reservoirs along the Yangtze River Estuary appeared symmetrical hump. Under the same flow rate, the closer to upstream water intake, the bigger the hump curvature of the salinity change line; the closer to downstream water intake, the smaller the hump curvature of the salinity change line, which is related to the lower vertical gradient of the saltwater group. The hump of the average salinity process line at different tide levels decreased gradually with the increasing flow rate and the position of the maximum value advanced gradually with the increasing flow rate.
The fluctuation range of the saltwater group core increased with the tide current within a tidal cycle. With the increase of the tide dynamics, the average downward velocity of the tide of the saltwater group core did not change much within the entrance, but it slowed down significantly near the entrance. Different wind conditions had a certain influence on the activity and concentration of the NB spillover, but relatively less significance. The increase and decrease of the amount of the spillover saltwater caused by topographic changes will lead to marked increase or decrease of the salinity of spillover saltwater group in SB, whereas the impact on the activity rule of the saltwater group in SB will be little.