# Study on the Control of Saltwater Intrusion Using Subsurface Dams

^{1}

^{2}

^{3}

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## Abstract

**:**

^{3}/min, the saline water wedge area was the smallest at 378 cm

^{2}, and the prevention effect of saltwater intrusion was the best. Building a dam too high, that was, the ratio of dam height to aquifer thickness exceeded 0.38, resulted in an increased saltwater wedge area and exacerbated aquifer pollution. When the dam was located at the minimum effective distance for preventing saltwater intrusion under a certain dam height and head difference between saltwater and freshwater boundary, that was, the ratio of the distance of the dam to the saltwater boundary to the total length of the aquifer was 0.063, the distance of the dam to the saltwater boundary was the minimum effective distance. Compared to other effective distances, when the dam was at the minimum effective distance, the freshwater discharge reached its maximum at 22.71 cm

^{3}/min, and the saltwater wedge area was the smallest at 378 cm

^{2}. These conclusions provide a theoretical reference for the impact of subsurface dam construction on the saltwater wedge. This study examines the impact of tides and waves on the water head of the saltwater boundary, and it is also necessary to verify these conclusions through actual field experiments. We will investigate this in future work.

## 1. Introduction

^{−8}m/s) could prevent the saltwater intrusion effectively. Although this study investigated the impact of subsurface dam permeability on saltwater intrusion, it did not investigate the effects of subsurface dam height, location, and the head difference between the saltwater and freshwater boundary on saltwater intrusion. Luyun et al. [39] studied the relationship between the height of subsurface dams and the thickness of the saltwater wedge using laboratory tests and SEAWAT. They found that, when the subsurface dams were higher than the thickness of the saltwater wedge, seawater intrusion could be prevented, and the saltwater trapped upstream could be flushed out. However, the effects of the construction location of the subsurface dams and the head difference were neglected. Vassilios K. Kaleris et al. [40] simulated the impact of different dam heights and locations on the saltwater wedge toe length. The above studies only use the saltwater wedge toe length as a criterion for evaluating saltwater intrusion. We used the fresh groundwater discharge to assess the environmental performance of the subsurface dam, as an increased fresh groundwater discharge is beneficial for carrying land-based pollutants and salt to the sea, which accumulate in traditional high subsurface barriers. In this study, a flow tank was used to compare saltwater intrusion without intervention with that following the construction of the subsurface dams. Based on the relevant flow tank parameters, a numerical model was built, corrected, and used to simulate the development of saltwater intrusion and freshwater discharge under different dam heights, locations, and hydraulic gradients in order to optimize the dam height and location under different scenarios.

## 2. Materials and Methods

#### 2.1. Experimental Methods

#### 2.1.1. Laboratory Materials

^{−2}cm/s, 0.52 cm, and 0.30, respectively.

#### 2.1.2. Experimental Setup

#### 2.2. Numerical Simulation

#### 2.2.1. Governing Equation

_{f}is the equivalent freshwater head [L]; ${\rho}_{f}$ is the density of freshwater [ML

^{−3}]; q

_{s}is the unit volume flow of the source (sink) [T

^{−1}]; $\theta $ is the effective porosity of the porous medium; S

_{f}is the unit water storage coefficient of equivalent freshwater [L

^{−1}]; K

_{f}is the permeability coefficient of equivalent freshwater [LT

^{−1}].

_{k}is the dissolved concentration of substance k [ML

^{−1}]; D

_{ij}is the hydrodynamic dispersion coefficient [L

^{2}T

^{−1}]; ${C}_{k}^{s}$ is the concentration of substance k in source or sink [ML

^{−1}]; R

_{n}is the reaction term for the utilized chemical substance.

#### 2.2.2. Numerical Model

#### 2.3. Evaluation Parameters

_{salt}(height of the saltwater wedge in the location of the subsurface dam); Q

_{0}(freshwater discharge at the freshwater boundary), ∆d (the difference between the freshwater head d

_{2}and the saline water head d

_{1}), $L/{L}_{Total}$ (distance $L$ from the location of the subsurface dam to the saltwater boundary divided by the total length of the aquifer, ${L}_{Total}$), L

_{Salt}(saltwater wedge toe length), and A (area of the saltwater wedge). The saltwater wedge was set at a 50% concentration, which was commonly used to describe saltwater wedges in aquifers with low dispersivity [44].

## 3. Results and Discussion

#### 3.1. Simulation Results and Calibration

#### 3.2. Analysis of the Control Effect of Subsurface Dams on Saltwater Intrusion

#### 3.2.1. Influence of Dam Height

_{min}, at which a dam can effectively prevent saltwater intrusion. At heights less than h

_{min}, increasing the dam height had no significant effect on reducing saltwater intrusion; however, at dam heights that were equal to or higher than h

_{min}, saltwater intrusion can be effectively intercepted.

^{2}, respectively. At dam heights of 20, 21, and 27 cm, saltwater wedge heights of 22, 22.3, and 25.8 cm were observed, respectively, corresponding to areas of 389, 406, and 429 cm

^{2}, respectively. It can be seen that, at dam heights greater than 27 cm, the saltwater wedge height at the dam exceeds the saltwater wedge height achieved without intervention, with both the saltwater wedge height and area increasing alongside the dam height. Therefore, dams that are too high lead to seawater pollution in the aquifer, and dams that are only slightly higher than hmin are associated with the best prevention and saltwater intrusion control.

^{3}/min, which is higher than that observed without intervention (21.24 cm

^{3}/min). However, the saltwater wedge area was the smallest (Figure 4c). These results suggest that the change in freshwater discharge is related to the size of the saltwater wedge area under the influence of the dam, and that, the smaller the saltwater wedge area, the greater the freshwater discharge.

#### 3.2.2. Influence of the Distance between Dam and Saltwater Boundary

_{Tota}

_{l}ratio of <0.063 (L = 10 cm), the saltwater wedge toe length slowly decreased from 100.92 cm to 99.47 cm as L/L

_{Tota}

_{l}increased. At L/L

_{Tota}

_{l}= 0.063, the saltwater wedge toe length decreased rapidly to 10 cm, and the dam effectively prevented saltwater intrusion. As L/L

_{Tota}

_{l}increased, the saltwater wedge toe length gradually increased until it reached the saltwater wedge toe length observed without intervention. Therefore, a minimum effective distance (L

_{min}) of L/L

_{Tota}

_{l}= 0.063 was obtained for the dam to saltwater when the head difference was 13 mm and the dam height was set at 20 cm.

_{min}required to block the saltwater, the saltwater wedge area was close to that observed without intervention. If the dam was at L

_{min}, then the saltwater wedge area increased with the distance between the dam and the saltwater boundary until it reached that observed without intervention. As the distance between the dam and the saltwater boundary increased, the saltwater wedge height at the location of the dam gradually decreased (Figure 7c), meaning that the effective dam height gradually decreased. Therefore, the dam’s effective height gradually decreased. It is recommended to build a subsurface dam at a distance from the saltwater boundary as this reduces engineering costs but increases the saltwater wedge area.

_{Total}increased from 0.038 to 0.094, and freshwater discharge (Q

_{0}) decreased from 24.17 cm

^{3}/min to 22.72 cm

^{3}/min, which was higher than without intervention (21.24 cm

^{3}/min). This is because the saltwater wedge area gradually increases and the freshwater discharge gradually decreases.

#### 3.2.3. Influence of the Hydraulic Gradient

_{1},b

_{1},c

_{1})), both with and without the presence of a dam (Figure 7(a

_{2},b

_{2},c

_{2})). Thus, increasing the hydraulic gradient improves the prevention and control of saltwater intrusion.

^{3}/min to 27.23 cm

^{3}/min, while the freshwater discharge increased from 12.93 cm

^{3}/min to 26.3 cm

^{3}/min without intervention. The freshwater discharge gradually decreased both without intervention and in the presence of a dam when the groundwater head difference increased.

## 4. Comparisons with Previous Studies

^{−8}m/s. Luyun et al. [39] studied the relationship between the height of subsurface dams and the thickness of the saltwater wedge using laboratory tests and SEAWAT. They found that, when the subsurface dams were higher than the thickness of the saltwater wedge, seawater intrusion could be prevented, and the saltwater trapped upstream could be flushed out. Luyun et al. [29] presented laboratory-scale investigations on how the effectiveness of cutoff walls varies with their depth and distance from the coast. The investigation showed that, when a cutoff wall is located in an area of saltwater intrusion, its protective effect increases with decreasing distance from the coast and increasing penetration depth. Antoifi Abdoulhalik et al. [28] completed numerical and laboratory experiments in a laboratory-scale aquifer in which the effectiveness of cutoff walls was assessed in three different configurations. The results show that the cutoff wall was effective in reducing the saltwater wedge in all the investigated cases of layered aquifers with a toe length reduction of up to 43%. The differences between previous research and this study are as follows:

- (1)
- Difference in whether the impact of subsurface dam location and height on the prevention and control of saltwater intrusion has been considered simultaneously.

- (2)
- Difference in whether freshwater discharge is used as a criterion for evaluating the effectiveness of subsurface dams in preventing saltwater intrusion.

## 5. Conclusions

- (1)
- When the dam cannot effectively intercept saltwater, increasing the height can still delay saltwater intrusion. For a dam to have a preventive effect, it must reach the minimum effective dam height; increasing the height of a dam below this limit had no significant impact on reducing the saltwater wedge area, whereas the dam can effectively intercept saltwater intrusion if a dam is equal to or higher than the minimum effective height. However, dams that were far above this height were associated with an increase in the saltwater wedge area, exacerbating saltwater pollution. When the dam was slightly higher than the minimum effective height, the prevention and control effect of saltwater intrusion was the best. The change in freshwater discharge was related to the size of the saltwater wedge area under the influence of the dam. The smaller the saltwater wedge area, the greater the amount of freshwater discharge was;
- (2)
- Under a certain dam height and head difference between the saltwater and freshwater boundary, there was also a minimum effective distance for a dam to prevent saltwater intrusion. If the minimum effective distance was not achieved, the saltwater wedge area was close to the area under the natural state. If the minimum effective distance was achieved, the saltwater wedge area increased with the distance until it reached the natural state. The freshwater discharge gradually decreased as the distance between the dam and the saltwater boundary increased, as did the minimum effective height of the dam, reducing engineering costs but increasing the saltwater wedge area;
- (3)
- The greater the hydraulic gradient, the shorter and lower the saltwater wedge, both in the presence and absence of a dam. Without intervention, the saltwater wedge toe length decreased as the hydraulic gradient increased in an approximately linear fashion. The freshwater discharge increased gradually as the hydraulic gradient increased, and the freshwater discharge gradually decreased as the head difference increased, both with and without intervention. Therefore, a large head difference can play a positive role in the prevention and control of saltwater intrusion, thereby reducing the construction costs of the dam.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 2.**Distribution of saltwater wedge obtained through a (

**a**) laboratory experiment and (

**b**) numerical simulation, and (

**c**) the observed and calculated values of the saltwater wedge toe length over time.

**Figure 3.**Distribution of saltwater wedge after subsurface dam was set through a (

**a**) laboratory experiment. (

**b**) Numerical simulation. (

**c**) Observed and calculated values of saltwater wedge toe length over time.

**Figure 4.**Relationship between dam height (h/H) and (

**a**) saltwater wedge toe length (L

_{Salt}), (

**b**) saltwater wedge height at the location of the dam (h

_{Salt}), (

**c**) saltwater wedge area (A), and (

**d**) freshwater discharge (Q

_{0}).

**Figure 5.**Saltwater wedge distribution obtained with (

**a**) 6 cm, (

**b**) 8 cm, (

**c**) 10 cm, (

**d**) 13 cm, (

**e**) 15 cm, and (

**f**) 25 cm between dam and saltwater boundary.

**Figure 6.**Relationship between dam position (L/L

_{Total}) and (

**a**) saltwater wedge toe length (L

_{Salt}), (

**b**) saltwater wedge height at dam (h

_{Salt}), (

**c**) saltwater wedge area (A), and (

**d**) freshwater discharge (Q

_{0}).

**Figure 7.**Saltwater wedge distribution. Hydraulic gradients of (

**a**) 12 cm without intervention; (

_{1}**a**) 12 cm with a dam; (

_{2}**b**) 13 cm without intervention; (

_{1}**b**) 13 cm with a dam; (

_{2}**c**) 14 cm without intervention; (

_{1}**c**) 14 cm with a dam.

_{2}**Figure 8.**Relationship between hydraulic gradient (Δd) and (

**a**) saltwater wedge toe length (L

_{Salt}), (

**b**) saltwater wedge area (A), (

**c**) saltwater wedge height on dam location (h

_{Salt}), and (

**d**) freshwater discharge (Q

_{0}).

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**MDPI and ACS Style**

Chang, Y.; Chen, X.; Liu, D.; Tian, C.; Xu, D.; Wang, L.
Study on the Control of Saltwater Intrusion Using Subsurface Dams. *Water* **2023**, *15*, 3938.
https://doi.org/10.3390/w15223938

**AMA Style**

Chang Y, Chen X, Liu D, Tian C, Xu D, Wang L.
Study on the Control of Saltwater Intrusion Using Subsurface Dams. *Water*. 2023; 15(22):3938.
https://doi.org/10.3390/w15223938

**Chicago/Turabian Style**

Chang, Yawen, Xuequn Chen, Dan Liu, Chanjuan Tian, Dandan Xu, and Luyao Wang.
2023. "Study on the Control of Saltwater Intrusion Using Subsurface Dams" *Water* 15, no. 22: 3938.
https://doi.org/10.3390/w15223938