The effect of beach slope variation on saltwater intrusion dynamics in the unconfined coastal aquifer (experimental and numerical)

Over-exploitation of groundwater resources often causes seawater to intrude into coastal aquifers. This study aims to evaluate how different beach slopes (90°, 75°, 60°, and 45°) affect the extent and behavior of seawater intrusion in unconfined coastal aquifers under transient conditions. A three-dimensional laboratory model was constructed to simulate seawater intrusion under varying beach slopes. Experimental data were analyzed using image processing techniques, and results were validated using the SEAWAT numerical model. Key parameters—including wedge toe length, height, and area—were measured over time to assess the transient response of the saltwater wedge. The results showed that under static conditions, flatter slopes produced larger saltwater wedges. During transient conditions following a groundwater-level decline, the wedge toe advanced approximately 57% further in the vertical slope than in the 45° slope, while the final wedge size remained smaller on the steeper beach. The wedge height stabilized earlier than the toe length and area during intrusion, whereas in the recession stage, all three indices reached equilibrium almost simultaneously. The geometry of the beach slope has a significant effect on both the extent and temporal behavior of seawater intrusion. The toe length index showed a strong relationship with wedge area and can serve as a reliable indicator of intrusion volume under both steady and transient conditions. These findings emphasize the importance of considering beach slope in the design and management of coastal aquifer systems. Understanding how slope geometry influences the evolution of the saltwater wedge can improve the prediction and control of seawater intrusion in response to groundwater-level fluctuations.