Numerical modeling of salinity changes in the desalination stage of a coastal reservoir

In recent years, population growth and rapid economic development have exacerbated the problem of water shortage, especially in coastal areas, to the point that meeting freshwater demand has become a serious challenge for coastal communities (Herrera-Leon et al., 2018; Phan et al., 2018). This situation is further complicated by the irregular spatial and temporal distribution of freshwater resources in these areas. A coastal reservoir is defined as a water storage structure constructed at river estuary or other coastal area to store fresh water and control water resources. One of the obvious advantages of coastal reservoirs is providing additional fresh water storage capacity for water supply networks (Xu, 2001). In areas under water stress, coastal reservoirs, which are often the basis of local economic development, can help reduce water shortage (Li and Chen, 2005). Many coastal reservoirs have been constructed in China, South Korea, Hong Kong and Singapore (Yuan et al., 2007).Despite the importance of coastal reservoirs, there is little research on this issue in the literature and no studies have been conducted in this regard in Iran. In addition, there are many issues about the performance of these reservoirs that have attracted widespread attention worldwide. One of the most important issues is salinity changes in the coastal reservoir, which is the main focus of the present study. Accordingly, in this study, numerical simulation of flow and salinity transfer in a coastal reservoir along the Caspian Sea is developed as a case study.

Methodology:

Tajan river is one of the most important rivers in the Caspian Sea watershed, which originates from 3251 meters above the northern slope of the Alborz mountain range in the south of Sari city in the north of Iran. The flow in this river is influenced by hydraulic structures built at upstream of river, such as Shahid Rajaei Dam. In March 2016, due to heavy rains in the upstream basins, a large flood occurred in this river. Measurements showed that the peak discharge of flood was 880 m3/s and the maximum volume of flood was 3112560 m3. The return period of this flood was more than 1000 years.The modeling region in the present study is located between the estuary of Tajan river to the Caspian Sea and Neka river. The dimensions of the coastal reservoir in this study include 1 Km wide and 9 Km long (along the coastline) and the water supply to it is provided through a flood channel from the Tajan river. In the present study, MIKE3 which is a 3D numerical model was used. Two different models were developed and the results of each were studied. Model No. 1 where desalination of the coastal reservoir was considered by average monthly discharge of the Tajan river (Inflow boundary condition) and buttom outlets (Outflow boundary condition). The simulation period in this model was determined as 1 year. On the other hand, in Model No. 2 desalination of the coastal reservoir was considered by a 1000 year return period flood in Tajan river (Inflow) and an Ogee spillway (Outflow). Finally, similar to the water quality of the Caspian Sea, the initial salinity in the reservior was considered as 12 PSU.

Results and discussion:

  In this part, the results obtained from the both models No. 1 and No. 2 are presented and analyzed. The results of different models are also compared. Results in Model No. 1 showed that changes in water level and current speed were negligible with the maximum current speed of about 0.08 m/s. In addition, after 1000 hours from the start of the simulation, the salinity in the reservoir was about 8 PSU, and after 3000 hours it was about 3.5 PSU and after 8760 hours it was reached a maximum value of about 2 PSU. On the other hand, results in Model No. 2 showed that the current speed in the flood channel was about 7 m/s. However, the current speed inside the reservoir was low with a maximum value of about 0.2 m/s. This is about 10 times more than the current speed in Model No.1. Furthermore, result showed that at time step of the flood peak entry, significant decrease in salinity of the reservoir happened. Actually, the salinity of nearly half of the reservoir was less than 3 PSU in this time step. Finally, at the end of the simulation, the salinity of the reservoir was less than 1 PSU.

Conclusion:

A numerical study was carried out on the dynamic of salinity transfer and diffusion in a coastal reservoir under desalination condition. Two numerical models were developed. In Model No. 1, flow and salinity changes during one year simulation period with average monthly discharge of Tajan river were studied. In Model No. 2, changes in flow and salinity of the reservoir under a historical flood flow condition with peak discharge of nearly 200 times the average monthly discharge were studied. Salinity profiles in the depth of the reservoir and at different time steps showed that desalination occurred in the depth of the reservoir. In addition, the comparison of the two models showed that the salinity stratification in model No. 2 was more intense due to the rapid changes in the hydrograph flow. In both models, the salinity difference at the surface and depth of reservoir decreased over time from the beginning of modeling.