Seawater Intrusion Processes Along the Tien River Mouth in the Period 2000–2020

Abstract

Seawater intrusion is a severe issue and is a main cause of serious freshwater scarcity during the dry season in the Mekong Delta. Insights into seawater intrusion processes and its controlling factors are needed to adaptation straterigies. In this paper, we use long-term salinity obvervation data and hydraulic modelling to simulate the salinization processes (spatial–temporal salinity distribution and concentration) in the context of changes in upstream discharge and tidal regimes over 20 years. The seawater intrusion model is based on DHI MIKE 11 software which is an one-dimensional equations of continuity, momentum conservation and transport in natural waters. In addition, longitudinal simulations along the Tien River mouth illustrate different hydrodynamic conditions of the river, help reproduce the observable salinity trends and contribute to a better understanding of this natural phenomenon. The Mike 11 model allows a quick assessment of saline intrusion in the Tien River mouth to predict a 4‰ salty border for agricultural production and a 0.3‰ salty border for domestic water exploitation for irrigation systems in Ben Tre to help ensure the plan to use water for production and daily life in the dry season 10 days in advance.

2.1 Introduction

Water resources are essential for the development of human society. This development, mainly in the industrial and agricultural sectors, increasingly affects other resources on multiple levels. Water shortage is considered as a limiting factor for the socio-economic development of an area. The main objective is to make companies to establish sustainable management policies and rules for managing water resources to ensure their permanence (Sam et al. 2006). The water source of Tien River mouth in the dry season is very unstable, causing random import along the river depends on each year and the amount of fresh water from Mekong River. The result of the process of river-sea interaction is that it provides low salinity, high salinity and affects the process of exploiting fresh water in the river mouth for agriculture and daily life. Typically, in the dry season of 2020, the salinity limit of 4 g/l in February 2020 on Ham Luong River is 75 km, Tieu Estuary and Dai Estuary rivers are 55 km, Co Chien is 68 km, Hau River is 66 km (Southern Institute of Irrigation Science). Also in February, the salinity measured at My Hoa on the Ham Luong River, 48 km from the river mouth, reached 14.5 g/l (February 14, 2020). Early and deep salinity intrusion has caused the largest freshwater reservoir in the West at Ba Tri-Ben Tre to be contaminated with 2 g/l of salinity. As a result of this drought, millions of people in Ben Tre lack fresh water for daily life and production.

Tien River mouth is a shallow estuary with many creeks flowing into the East Sea, including Tieu, Dai, Ba Lai, Ham Luong, Co Chien, Cung Hau estuaries, in which Ba Lai estuary has been degraded and prevented by saline sluices to remove salinity. The boundary of Tien River mouth is determined from My Thuan station 87 km from the sea (Legates and McCabe 1999; Perillo 1995). The salinity of the Tien River mouth varies from 0 to 31.5‰, along the length of the estuary. Salinity varies from year to year, depending on the freshwater flow in the main river. The typical tidal regime is an irregular semi-diurnal tide, with two peaks and two low tides in a day. Tidal amplitude ranges from 2 to 4 m. The low tide season can affect Tan Chau, which is 197 km from the sea.

The monitoring of salinity distribution along the main river in the Mekong estuary has been started since 1994. All salinity measuring stations are set up on the salinity evolution along the river according to their own installation criteria and not based on the calculation of the salinity dispersion along the river by hydrodynamics and river morphology. However, the monitoring process is in batches during the dry season, and there are limitations in terms of salinity measurement locations. In the current context that managers want to quickly estimate the evolution of salinity distribution along the estuary, one-dimensional mathematical models are suitable tools because they are easy to use and suitable for the management context. In addition, it is also a simple methodology to start approaching the approximate assessment of saline intrusion in the study area.

In this paper, we study the spatial and temporal variation of salinity distribution in the Tien estuary based on monitoring data of stations along the estuaries and detailed calculations according to cross-sections on the Mike 11 model calculation network. Firstly, simulation of the hydrodynamic regime of the river has been performed. Vital databases were used, including river morphology, friction factors, river flow and river level. These data were used to calibrate the hydrodynamic model, which was later linked to the AD modulus. The output of the AD modulus is the evolution of salts from upstream to downstream of the estuary under different hydrodynamic conditions. The model allows to estimate salinity changes even in areas where we do not need to measure in actual area. It also allows to plot isoline chart (areas of equal salinity) for a typical salinity.

2.2 Data and Research Methods

2.2.1 Measuring Station Data

Tien river mouth area and measuring stations along the river mouth Fig. 2.1. Ben Tre province is located entirely in the Tien River mouth, so the fluctuation of salinity along the estuary directly affects the exploitation of water resources for domestic service, agriculture and industry of the province. Irrigation system is divided into two, North Ben Tre sub-region (Thuong Ba Lai, Giong Trom-Ba Tri Irrigation system, Ba Tri coastal area, Binh Dai coastal area, Binh Dai), South Ben Tre sub-region ( Irrigation Zone Tay Cho Lach, Cho Lach-Mo Cay, Mo Cay-Thanh Phu, coastal area of Thanh Phu). These irrigation systems ensure 75% of the cultivated area, 82% of domestic water for rural areas, and improve logistics for fishing, and effectively prevent natural disasters in coastal areas. The production model of these irrigation zones includes the model of aquaculture, giant tiger prawn, litopenaeus vannamei, wild shrimp, shutchi catfish, water rotation model of Shrimp-Rice (Binh Dai and Thanh Phu), Crayfish model on rice field (Thuong Binh Dai, Ba Tri, Thanh Phu in tidal and river dynamical transition zones), 3-Crop rice model (Winter-Spring I-IV, Summer-Autumn V-IX, Autumn-Winter IX-XII), two rice crops and one crop located in the freshwater area, raising shrimp and fish in garden ditches (Tay Cho Lach, Cho Lach-Mo Cay, Thuong Ba Lai, Thuong Mo Cay-Thanh Phu, river water predominates). However, freshwater systems have not been closed, so the years of deep and prolonged intrusion caused damage to production and people's lives.In the dry season 2015–2016, saltwater intrusion covered 162/164 communes and wards of Ben Tre province, causing damage to agricultural production about 1794 billion VND.

Fig. 2.1

Calculation diagram of Tien River mouth study area—Mekong Delta

The stations are made to measure the water level, salinity along the river. Measuring the water level 24 times a year all year round, measuring salinity in batches in the dry season from February to July, recently measured in January-VI, measuring level 12 times/day, 3–4 times/month, each 3 days. Manual method of measuring salinity was taken at 03 layers, the surface layer is 0.2 h, the middle layer is 0.5 h, the bottom layer is 0.8 h.

• The most recent model used for river systems in the Mekong Delta is MIKE 11 (DHI, 2007) which has been developed and updated over the decades by various organizations based on previous models and data. Organizations involved in model development, tributary updates and the cross-sections and model corrections include the Mekong River Commission (MRC), Southern Institute of Water Resources Science (SIWRR) and Thuyloi University (TLU). To simulate the problem of saline intrusion in the Mekong Delta, the model uses the MIKE 11 HD hydrodynamic module to calculate the water level and flow and the MIKE 11 AD convection diffusion module to calculate the salinity level in the river system.

• The MIKE 11 HD hydrodynamics module solves the Saint Venant equations including the equation of continuity (2.1) and the equation of momentum (2.2) (DHI, 2007):

  $$ \frac{\partial A}{{\partial t}} + \frac{\partial Q}{{\partial x}} = q $$

  (2.1)
  $$ \frac{\partial Q}{{\partial t}} + \frac{\partial }{\partial x}\left( {\varphi \frac{{Q^{2} }}{A}} \right) + gA\frac{\partial Z}{{\partial x}} + g\frac{Q\left| Q \right|}{{C^{2} AR}} = 0 $$

  (2.2)

where t is the time, x is the distance along the river, Q is the flow, Z is the water level, A is the cross-sectional area, R is the hydraulic radius, C is the Chezy roughness coefficient, q is the flow mass per unit length, g is the acceleration due to gravity, and α is the kinetic energy correction factor.

The MIKE 11 AD convection diffusion module solves the conservation of mass Eq. (2.3) to estimate the salt concentration (DHI, 2007)

$$ \frac{\partial AC}{{\partial t}} + \frac{\partial QC}{{\partial x}} - \frac{\partial }{\partial x}\left( {{\text{AD}}\frac{\partial C}{{\partial x}}} \right) = - AKC + C_{{\text{s}}} q $$

(2.3)

where C is the salt concentration, C_S is the source/sink amount of the salt concentration, K is the zero possible linear decay coefficient for salinity, and D is the diffusion coefficient.

Calculation diagram for simulation of salinity in the Tien estuary model calibration and verification are as follows: The upstream boundary is calculated from My Thuan station in Vinh Long province, the downstream boundary uses saline and water level stations to serve simulations of Vam Kenh, Loc Thuan, An Thuan, and Ben Trai. The stations used to calibrate and inspect models Son Doc, Hoa Binh, My Tho, Dong Tam, My Hoa, Giong Trom, Huong My, Khanh Thanh Tan, Vung Liem.

The calibration parameter is the roughness of the Manning coefficient. Initial values of the Manning roughness varied according to the approach studied, with mean values of 0.02–0.027. In the calibration procedure, the Manning coefficient is modified to the same extent along the study range because we assume that the sources of error associated with its evaluation are the same for all grids. Calibration and validation were performed using water level data at Cho Lach station, My Hoa, My Tho Hoa Binh. The time period selected for calibration is from January 1, 2016 to June 30, 2016. Figures 2.2 and 2.3 show the results of the calibration. This figure shows a good correspondence between the calculated water level and the observed water level at the station.

Fig. 2.2

Test results of water level between calculation and actual measurement in which a Cho Lach, b Hoa Binh, c My Tho, d My Hoa

Fig. 2.3

Salinity simulation results and actual measurements in which a Vam Kenh, b Binh Dai, c Son Doc, d Hoa Binh

In addition, the model performance was checked by statistical indicators: root mean square error (RMSE), Nash–Sutcliffe model efficiency index (NSC) and correlation coefficient (R2). The equations used to determine these indices are available in Sanchez (1992). RMSE indicates a perfect combination between observed and predicted values when it is zero. NSC ranges from − ∞ to 1. It indicates a perfect combination between observed and predicted values when it equals 1. Values between 0 and 1 are generally considered to be an acceptable level of performance (Zhang et al. 2015).

2.3 Results and Discussion

• (a) Maximum Salinity in Dry Season

Figure 2.4 shows the maximum salinity at the measuring stations in the period 1994–2020 according to the distance from the sea estuary to the river estuary. The lowest salinity is in 2014 and the highest is in 2020. The station area which is 4–7.8 km from the sea, the salinity is always high from 20 to 32‰, the stations along the river which are 10–28 km from the sea, the salinity fluctuates greatly from 3 to 27‰. The area which is 45–63 km from the sea, the level ranges from 1 to 14‰. The change of salinity peaks in areas along the main river ranges from 10 to 28 km each year depending on the freshwater flow in the main river. However, according to the salinity data of stations along the river, we divide the Tien River mouth by region based on salinity according to the Venice system (Ippen and Harleman 1961; Venice System 1958) as follows: the station area which is less than 10 km-distance-from-the-sea is the saltwater tidal area, stations which are 10–20 km from the sea is brackish tide area (tidal dynamics always prevail), distance from 20 to 28 km is brackish tide, 28–37 km is brackish tidal area (tidal and river dynamics transition zone, tidal predominates), 37–45 km is brackish tidal zone (transition zone between tidal dynamics and river flow but river flow predominates), 45–63 km is freshwater tide (dominant river dynamics region).

Fig. 2.4

Highest salinity value at measuring stations along the river in the period 1994–2020

• (b) Evaluation of Salinity Along the River in the Dry Season

Evaluation of salinity changes over time from measuring stations along the river showed that there were years of high salinity going deep into the estuary, and low salinity for the transitional area between the tide and the headspring flow. For general purposes, we choose 03 years for analysis 2005, 2016, 2017. The year 2005 is the highest salinity year in the period when there have not been much changes according to the freshwater flow from the Mekong River to the Mekong Delta., in 2016 the highest salinity in the period (2006–2016), 2017 in low salinity represents significant changes in hydropower dams upstream of Mekong coming into operation. The process of salinity evolution in the dry season from January to June is presented in Fig. 2.5a–c. In 2005, 2016–2017 in the dominant tidal area, the salinity did not change, the dominant tidal dynamic transition zone had a great change in salinity, specifically at Son station, the highest salinity in 2005 was 24.0‰, 2016 is 27.4‰, in 2017 it is 13.7‰, Hoa Binh 18‰, 15‰, 9‰, Tra Vinh 10.8‰, 14‰, 9.6‰. The area of fresh tide and river flow dominates at My Hoa station 10.9‰, 11.8‰, 1.6‰, My Tho 4.6‰, 3.4‰, 0‰, Khanh Thanh Tan 8.7‰, 2.6‰. The salinity variation at the stations changes due to the interaction between river currents and tides. In terms of flow amplitude upstream of Tien station My Thuan estuary, Fig. 2.6a, b show the flow amplitude tends to increase. In three years, choose to analyze and arrange in ascending order 2005, 2016, 2017, however, in terms of average day there is a change in the order of 2016, 2005, 2017 which explains why in 2016 salinity is larger than in 2005 and 2017.

Fig. 2.5

a Salinity evolution along the Tien River estuary in 2005 in which (a) Cua Tieu, (b) Ham Luong, (c) Co Chien. b Salinity evolution along the Tien River in 2016 in which (a) Co Chien, (b) Ham Luong, (c) Cua Tieu. c Salinity evolution along the Tien River estuary in 2017 in which (a) Ham Luong, (b) Co Chien, (c) Cua Tieu

Fig. 2.6

a Flow volume in dry season at My Thuan in dry season 2005, 2016, 2017, b average daily flow in dry season of My Thuan station

The flow with a positive value indicates the upstream current flowing towards the sea at low tide and vice versa at high tide. Figure 2.6a show the flow amplitude gradually increases for the time it shows signs of tidal action which tends to increase in the Tien River estuary. Therefore, according to the salinity monitoring data of stations along the river mouth, Ham Luong estuary has a stronger interaction than other estuaries based on the salinity of My Hoa and Son Doc stations, in 2016 the highest salinity at My Hoa station was 11.8‰, Son Doc 27.4‰, in 2017 My Hoa decreased by 86–1.6‰, Son Doc decreased by 50–13.7‰. Stations at Co Chien and Estuary Tieu decrease by 30–70% depending on the location away from the sea. Thus, for Ham Luong estuary, when the headspring water is reduced in salinity, it will be pushed up higher than that of other estuaries, causing difficulties in water resource management in the dry season for the year when the salinity is high.

• (c) Salinity Along Hydrologic Depths

Saline intrusion is generally divided into three categories according to the degree of stratification (Nguyen Sinh Huy 2010; Cameron and Pritchard 1963) including saline wedge, partial disturbance and total disturbance. Saline wedges form when the amount of fresh water flowing into the estuary is greater than the tidal flow. Partial turbulence is a strong freshwater current that lies above the salty tidal current. When the high tide forces the upper freshwater current, the salinity layer increases with the water flow to the sea. Estuary total disturbance occurs when the flow in the river is small, tidal flow plays a major role in the water cycle in the estuary. The result of this process is uniform salinity at all depths in the river. This classification is determined through the classification of Ne (Kettab 2014). Considering the salinity monitoring data according to the depth along the Tien River estuary, the results of salinity layers measurement data collected are very few, mainly collected at Ham Luong estuary, the other estuaries are only available in 2015. Therefore, 2015 was used to analyze and evaluate the distribution of salinity according to the depth of the Tien River mouth. Figure 2.7a–c show the salinity along the depth of the measuring stations Tieu, Ham Luong and Co Chien estuaries, symbols D: bottom salinity 0.8 h, G: salinity depth 0.5 h, M: salinity salinity on the water surface at a depth of 0.2 h.

Fig. 2.7

a Distribution of salinity by depth of stations along Estuary Tieu River 2015 in which (a) Vam Kenh, (b) Hoa Binh, (c) My Tho. b Salinity evolution along Ham Luong River in 2015 in which (a) An Thuan, (b) Son Doc, (c) My Hoa. c Salinity evolution by depth along the Co Chien River in 2015 in which (a) Ben Trai, (b) Huong My, (c) Khanh Thanh Tan

Tieu Estuary has the uniform disturbance in the area 4–18 km from the sea, My Tho has a difference in salinity between layers, but it is not significant, ranging from 0.2 to 0.6‰.

Ham Luong estuary has a difference in salinity between layers, but it is not significant, ranging from 0.3 to 1‰, in the area 20–37 km from the sea, there is almost no difference in salinity between layers, My Hoa station which is far from the sea 45km has the difference between layers 0.5‰.

The difference between layers of Co Chien estuary is 0.4‰ at Ben Trai, Huong My, and Khanh Thanh Tan stations at 0.2‰.

Because the salinity difference between layers according to the depth from 0.2 to 1‰ is small, if only based on the salinity measured value according to the depth, it is possible to classify the Tien River mouth in the estuary area with uniform disturbance and dominant tidal flow. However, the distribution of salinity in the estuary depends on the hydrodynamic regime of the estuary, where the estuary morphology plays a decisive role. Therefore, to evaluate more accurately the 2D-3D model because the 1D model cannot simulate the stratified salinity by degrees.

2.4 Evolution of Water Level and Salinity

To study the variation of salinity over a 24-h period, March 28, 2016 was selected as the date of the highest salinity at measuring stations along Tieu estuary and Ham Luong estuary. Measuring mode 12 op/day Fig. 2.8. The blue line represents the water fluctuation at stations along the river, the orange line the salinity variation along the river as determined by the measuring stations. Salinity varies with the rhythm of the sinusoidal tidal water level. Salinity fluctuates in Ben Trai with the highest 24‰, the smallest 10.7‰ falling at highest tide and lowest tide.

Fig. 2.8

Salinity evolution along Tien River mouths on March 28, 2016 in which a Co Chien, b Ham Luong, c Cua Tieu

Considering the fluctuation of water level at stations along the river, the Tieu estuary branch, the water level and salinity at the measuring stations show that the higher the upstream, the higher the peak and the low tide are compared to the downstream stations due to the participation of currents in the river. River. Salinity changes at stations along the river are opposite, salinity decreases sharply when moving upstream. Thus, the water level of measuring stations along the estuary which is influenced by tide is directive by astronomical factors, while the distribution of salinity along the main river upstream is determined by the flow of freshwater and velocity. Evaluation of salinity change on the day when the salinity peaked in the dry season in 2016 showed that: Co Chien estuary used salinity at Ben Trai as a landmark; at Hung My station 12 km away, the salinity decreased by 34; and Huong My at 20 km away, the salinity decreased by 69%; Tra Vinh 23 km away decreased 71%; Khanh Thanh Tan 34.5 km away, salinity decreased 91.2%. Ham Luong branch takes An Thuan station as a landmark, 5.29 km from the sea, the highest salinity of the day is 31.2‰, Son Doc station 15 km away, the salinity decreases by 16%, Giong Trom station 31.7 km away, the salinity decreases by 49%, the My Hoa station 42 km away salinity decreased by 62%. Tieu Estuary and Vam Kenh branches 4.1 km from the sea, the highest salinity of the day is 25.9‰, at Hoa Binh station 17.8 km away salinity reduced by 47%, My Tho station 50 km away salinity decreased by 87%. Based on the salinity reduction process along the main river, it can be seen that the flow determines the salinity along the river. Co Chien receives more fresh water, 2‰ is the highest salty border of the day, located 40 km from the sea, Tieu estuary receives the smallest 3.4‰ at 55 k away. Particularly in Ham Luong, the amount of fresh water accounting for 16% is larger than that of Tieu estuary, but in My Hoa, 47 km from the sea, the maximum salinity reaches 11.8‰, 3.47 times higher than My Tho station, 5.9 times higher than Khanh Thanh Tan station. On the other hand, at the inlet of An Thuan, the salinity is 31.2‰, higher than the other two estuaries 5.9–7.6‰, so the salinity is pushed higher and deeper than these. Thus, the saline tide at Ham Luong River is stronger than the other one due to the different morphology of this estuary. Therefore, the Ham Luong estuary is very sensitive to the amount of fresh water flowing from upstream. In years with smaller amount of fresh water, the higher the salinity, the deeper the saltwater intrusion. In years with larger amount of fresh water, the salinity decrease more sharply than other rivers belong to Tien River mouth.

2.5 Simulation Results from Model Number 1D

The year 2016 was chosen to simulate in mike 11 because this is a year of high salinity, the flow at the beginning of the dry season has decreased sharply due to the operation of the upstream reservoirs of the Mekong. Figure 2.9 shows the results of the highest salinity intrusion in the dry season 2016. The salinity limit for agriculture is 4‰ (Legates and McCabe 1999) from Ham Luong estuary to Cho Lach. Co Chien estuary climb 4‰ salty border beyond Vung Liem-Vinh Long, and Tieu estuary climb about 50 km from the sea. The saline boundary exploits domestic water when the salinity in rivers and canals is less than 0.3‰, beyond Cho Lach, Cuu Tieu River, Quoi An river, Co Chien River. 2016 is the year causing great damage to water supply for the winter-spring crop of people in the Tien river mouth because the winter-spring crop lasts from January to April. Model of Dong Xuan Rice, Shrimp Rice belong to Mo Cay-Thanh Phu, Giong Trom-Ba Tri, Binh Dai irrigation zones in the transition zone between tidal dynamics and river flow. When tidal currents dominate, high saline water at the time of sowing will not be enough for agricultural production because these irrigation systems have not been closed. Therefore, when the salinity in the main river increases, it will go into the canal system of Ben Tre province, causing damage to agricultural production or daily life. Irrigation systems in brackish and fresh brackish water in the Ham Luong estuary are at higher risk of saline intrusion than in Giong Trom-Ba Tri, the downstream of Thuong Ba Lai, Mo Cay-Thanh Phu. Currently, the ability to predict the salinity of the model is done in a short time of 10 days. Actively storing water or operating irrigation works in agricultural production areas can be done soon, but the water demand for agriculture is great, so when there is saline intrusion in the river during the prolonged time cannot avoid damage. Freshwater areas such as Thuong Ba Lai, Nam Cho Lach, Cho Lach will be affected by saline intrusion when the salinity in the main river increases and penetrates deep upstream. Predicting the salinity in advance in the river is a necessity to prepare water sources for production and daily life. Therefore, the fastest way to know changes in salinity in the main river in the dry season is to use a one-dimensional model like Mike 11.

Fig. 2.9

Simulation results of saline intrusion along Tien estuary in dry season 2016

2.6 Conclusion

The purpose of this study is to learn about the mechanism of saltwater intrusion in the Tien estuary in space and time, its dependence on the hydrodynamic regime of the estuary using the Mike 11 model and actual measured data along the estuary stations of the Tien River. Measuring stations are carried out for many years during the dry season from January to June every year. However, the salinity measurement in the dry season is not continuous for a month, only 3–4 times at high tide and low tide. Through the analysis, it was found that the tides and the amount of fresh water in the river dominated the distribution of salinity in the main river. The monitoring values of salinity combined with the basic data of the model such as river cross-section, estuary morphology, water level boundary conditions, salinity, flow are used to simulate in one-dimensional model.

This model includes hydrodynamic module combined with new diffusive transport module. Mike 11 model has been calibrated with actual values of water level and salinity measurements at main river stations with values calculated from the model is suitable. Model can be used to forecast the evolution of salinity distribution along the estuary during the dry season 10 days in advance, in order to ensure production or daily activities with salty border 4‰, 0.3‰. This forecast is the most effective and practical for the transitional area between tidal and river flow dynamics because this is a sensitive area of the 4‰ salty border pushing upstream deeply when the tidal current is dominant and 0.3‰ salty border pushing down very close to the mouth of the sea when river currents prevail. For the Tien River mouth, these sensitive areas are the Ba Tri-Giong Trom, Binh Dai, Mo Cay-Thanh Phu and Binh Dai irrigation zones.

Finally, based on the actual salinity values measured at stations along the river, we analyze the salinity reduction along the main river, distinguishing the typical areas of the Tien River mouth. Temporarily assessing the distribution of salinity by depth. However, the one-dimensional model does not evaluate the salinity stratification according to the cross-sectional depth, so the study has not concluded the salinity stratification in the Tien estuary. The actual measured values are to verify the calculated results of the one-dimensional model.