<p>Seawater intrusion (SWI) is a continual challenge that threatens the sustainability of coastal groundwater resources. The behaviour of the seawater-freshwater mixing zone plays a critical role in controlling groundwater quality and solute transport; however, the combined impact of underground dams and sloping beach geometries on seawater-freshwater mixing-zone dynamics remains insufficiently understood. This study investigates the hydrodynamic response of SWI in sloping beach coastal aquifers using numerical simulations with beach inclination angles of 0°, 15°, 30°, and 45°, and under various underground dam heights (H_d). The outcomes indicate that the mixing zone initially expands during the transient stage before shrinking after the seawater intrusion wedge reaches the underground dam. Compared with no-dam conditions, underground dams reduced the final steady-state mixing-zone width and area, while only slightly affecting subsurface groundwater discharge and seawater intrusion length. Increasing dam height produced a narrower central mixing zone and lower subsurface groundwater discharge, whereas its effect on the bottom mixing-zone width was limited. In contrast, increasing the beach inclination angle significantly enhanced seawater intrusion and widened the mixing zone, particularly under steep slopes. Inclined beaches also delayed the time required for the seawater intrusion wedge to reach the dam because of the longer seawater intrusion pathway. The findings demonstrate that aquifer geometry and underground dam configuration strongly control the temporal evolution of seawater intrusion. Proper optimisation of underground dam height is therefore essential for reducing salinity expansion and improving groundwater protection in coastal aquifers. The study provides practical guidance for the design of physical barriers and sustainable groundwater management in vulnerable coastal regions.</p>