Saltwater flushing by freshwater in a laboratory beach

Experiments were conducted to investigate the flushing of saltwater out of a laboratory aquifer (or beach) by freshwater propagating seaward. After a steady state distribution was achieved with a seaward hydraulic gradient, freshwater was introduced while keeping the total head constant at each boundary. This caused the propagation of freshwater seaward. Two initial uniform concentrations were used: Case 1: 2.0 g/L (low salinity case) and Case 2: 34.0 g/L (high salinity case). The observed salinity and pressure data were closely reproduced using the MARUN (Boufadel et al. 1999a) numerical code. The results indicated that buoyancy plays an important role for Case 2 but is negligible for Case 1. The results also indicated that the flow in the offshore beach aquifer (submerged portion of beach) was negligible especially for Case 2. For this case, the pressure increased with time until reaching a peak and then decreased (i.e., humps were formed). This was not observed in the low salinity case. Investigations revealed that the increase in pressure is due to a combination of remnant high salinity and a rise in the water table at that location. Numerical investigations revealed that for the same difference in total head, the seaward flow of freshwater increases with a decrease in the seawater salinity. The increase, however, was nonlinear as a function of seawater density. For example, the discharge in the high salinity case was 20% lower than that in the low salinity case.