Impact of beach face slope variation on saltwater intrusion dynamics in unconfined aquifer under tidal boundary condition

Density-dependent saltwater flow in coastal aquifers varies with the seaside tidal and beach slope conditions. This paper presents a set of laboratory-scale saltwater intrusion experiments under different beach slopes (15 degrees, 20 degrees, 25 degrees, and 30 degrees) for unconfined aquifer conditions. Experimental porewater pressure measurements were utilized for quantitative analysis. A new G channel-based image analysis technique is used for experimental image analysis and freshwater-saltwater interface identification. Experimental results were numerically vali-dated using the two-dimensional Finite Element subsurface FLOW (FEFLOW) model. The time-varying analysis revealed that saltwater intrusion occurs rapidly on flatter beach slopes. In the case of a steeper beach slope, saltwater intruded less. Steeper slopes take more time to reach a quasi-steady condition. Tidal oscillations alter hydraulic gradient changes across the beach slope. This gradient change generated clockwise circulating saltwater flow within porous media in the inter-tidal zone. This circulating flow resulted in the formation of the upper saline plume (USP). The USP expanded with time and moved in a downward direction. Finally, a deformed elliptic-shaped USP was observed under quasi-steady-state conditions. Submarine groundwater discharge (SGD) pathways move the saltwater region through the intermediate zone of a saltwater wedge and USP on varying beach slopes. It is evident that SGD particles move along the saltwater-freshwater interface (SWI) zone and rise upward (up to the intersection point). The tracer experiments were started after attaining the quasi-steady state condition and continued till the tracer reached the intersection point of saltwater level and sloping beach face. The experimental data sets can be used as benchmark test cases.