Seawater intrusion (SWI) is one of the real threats to groundwater resources around the globe, specifically in highly populated coastal areas as groundwater is heavily exploited. Gaining a better understanding of SWI in coastal aquifers is very important for the efficient and sustainable management of fresh groundwater resources. Saline water dynamic is affected by many hydrological and hydrogeological parameters that include: hydraulic conductivity of the aquifer, recharge rate, abstraction rate, aquifer geometry, geological settings, and density of seawater among others. Many studies have been conducted to address different aspects of SWI using both physical experiments and mathematical (analytical and numerical) modeling. This includes various initial and boundary conditions, and other hydrological settings under different scales. There is no study investigated the dynamics of saline water under managed aquifer recharge (MAR) practice using a sand tank experiment similar to the one presented in this thesis. The main objective of this thesis is to investigate the dynamics of saline water in a coastal unconfined aquifer under MAR practices. The effectiveness of MAR in mitigating SWI was explored under different hydrological conditions that include various values of hydraulic conductivities, hydraulic gradients, injection rates, location of injection wells from the coastline, and depth of injection (well screen). This was achieved using both sand tank experiment and numerical modeling. For the numerical modeling part, a density-dependent flow and transport code, SEAWAT, was selected. Investigating the effect of hydraulic conductivity and salinity concentration using sand tank experiments is challenging and requires tremendous efforts. Hence, numerical modeling is an excellent tool to study seawater dynamics under various conditions because it is cheaper and less time demanding compared with the sand tank physical experiment. A conceptual model that mimics the designed sand tank was developed and calibrated. Sensitivity and grid resolution analyses were performed to ensure the reliability of the developed model when simulating observed sand tank runs and other different scenarios. Results of the sand tank experiments showed that injecting water close to the toe of the saline water interface was more effective to reduce the SWI by 32% to 61.4% compared with the situation when MAR was practiced away from it depending on the injected volume (0.5 L to 1.4 L). Injection of water within the dispersion zone (behind the interface) retarded the re-intrusion of saline water as it built a hydrological barrier against intrusion. Injection of water near the sea boundary resulted in large immediate loss of the injected water through the coastal boundary. The results revealed that injection at shallow depth was more effective in reducing the salinity in the intruded zone of the aquifer compared with the scenario when the injection was performed deeper (near the bottom of the aquifer). For example, injecting 1.06 L of freshwater near the toe of saline water interface at shallow depth (22.5 cm) reduced more salinized volume by 25.5% compared to when the same injection was at deeper depth (42 cm). With injecting more volume, the saline water interface receded further seaward direction (increasing the injected volume by 32%, the retreated distance increased by 43%). However, it is very important to monitor the height of the water table mound under injection as a high water mound may result in a steeper hydraulic gradient. As a consequence, the rapid discharge of the injected water occurred through the constant head boundaries (in this case acted as a discharge boundary) and hence less vii impact in controlling SWI. In a real situation, abstraction wells can be the discharge (or drain) facility and so injection and abstraction rates must be balanced to increase the effectiveness of injection in mitigation of SWI problem. As that, optimization of MAR (in terms of the location of injection, rate and volume of injection, depth of injection, duration of injection, along with maintaining water balance for the aquifer) is necessary to avoid ineffectiveness or failure of MAR to mitigate SWI problem. The calibrated numerical simulation (Root-Mean-Square Error (RMSE) is 0.3) showed that the efficiency of MAR is affected by aquifer's conductivity. Our numerical simulation showed that applying MAR to highly conductive porous media drain out quickly the injected water resulting in a smaller water table mound and hence small impact in controlling the SWI. Moreover, the rate of SWI was high under higher hydraulic conductivity. Therefore, the permeability and the injection rate and volume need to be well studied for any site. The results also revealed that the denser is the intruding saline water, the more challenging is the effect of MAR in controlling the SWI. When the density of the intruding saline water increases by 30%, more of the injected volume required (increased by 100%, from 1.06 L to 2.12 L) in order for the saline water interface to recede seaward direction by more than 20%. In general, the SWI intensity and its response to any mitigation solution is a site-specific and depends on a combination of factors. The MAR system to control SWI must be well planned and assessed before implementation to ensure its effectiveness and success. The study illustrated the importance of conducting laboratory scale experiments that indeed helps a lot to gain a better understanding and enriching our knowledge about real-life challenges like the SWI. Sand tank experiments allow change, mimic, and manipulate the different possible conditions/variables that you may face when dealing with reality. Misunderstanding or oversimplification of reality would result in the failure of efforts to solve real problems in fields. Definitely, the field condition is more complicated than the one simulated in our sand tank experiments, but obtaining insights about the effect of each factor and parameter is important for comprehensive understanding and hence more successful implementations in reality. The work presented in this thesis to be advanced further to study the seawater dynamics in different layered heterogeneities and hydrological conditions (e.g., abstraction rate, and tidal fluctuation at the sea boundary) under MAR practices.