The seaward boundary of an unconfined coastal aquifer is typically assumed to be under a static condition given by the mean sea level for practical applications. However, in reality the aquifer is exposed to the influence of oceanic oscillations due to tides and waves. Previous studies have shown that tidal oscillations acting on a sloping beach induce relatively rapid seawater circulation in the intertidal zone below the beach surface, which has been identified as a major contributor to the overall submarine groundwater discharge (SGD). Salt transport associated with this seawater circulation leads to the formation of an upper saline plume (USP) within the intertidal zone. The presence of the USP alters the salt distribution in the near-shore aquifer with fresh groundwater discharging in a zone between the upper saline plume and the saltwater wedge (SW). However, how the tidally induced circulation interacts with the inland hydraulic condition and density-gradients in controlling the extent of the SW remains an important question to be addressed.
The study presented in this thesis aimed to investigate the mechanisms that control the extent of aquifer salinisation, aquifer-ocean exchange, and salt transport and mixing in the near-shore aquifer under both oceanic and terrestrial/inland forces. Laboratory experiments and numerical simulations were conducted to examine the tidal influence on the extent of aquifer salinisation (saltwater intrusion), solute transport and mixing between the recirculating seawater zones (SW and USP) and freshwater discharge zone (FDZ), and water exchange between the aquifer and ocean.
In the investigation on the extent of saltwater wedge under the tidal influence, the experiments showed that the presence of the upper saline plume shifted the fresh groundwater discharge zone seaward to the low tide mark and restricted the intrusion of the saltwater wedge. The overall seawater intrusion extent, as indicated by the wedge toe location, was reduced significantly compared with the non-tidal (static) case. The Glover solution was modified to include the tide-induced circulation flow in estimating the salt-freshwater interface (SFI) under the tidal condition. The predictions of the modified Glover solution matched the experimental data and numerical simulation results for both the laboratory-scale and field-scale models. The findings highlight the significant impact of the tide in modulating the groundwater behavior and salt-freshwater dynamics, not only within but also landward of the intertidal zone.
Similar to the investigation of the tidal effect on seawater intrusion, both laboratory experiments and numerical simulations were carried out to study the combined influences of tides and seasonal inland flux oscillations on the ocean-aquifer exchange in an unconfined coastal aquifer. While the seasonal inland flux oscillations drive a relatively large volume of seawater exchange across the aquifer-ocean interface as the saltwater wedge moves landward and seaward over the seasonal cycle, the physical experiments demonstrated that this driving mechanism was restricted under the tidal influence, compared with the non-tidal case. Both the experiments and numerical simulations showed that the rate of water exchange across the seabed induced by seasonal fluctuations of inland groundwater flux decreased significantly as a result of the tidal influence.
The investigation on the tide-induced mixing was conducted through laboratory experiments and simulations based on a particle tracking model. The result showed that the solute transport and salt-freshwater mixing in different zones (USP, SW and the corresponding salt-freshwater mixing zones and FDZ) exhibited distinctive characteristics under the tidal influence. In the experiments, the residence times of solute travelled in the USP were much shorter compared with those associated with the FDZ and SW. The results indicated that the USP represented a more active zone with the highest replenishment rate of groundwater and degree of spread for the solute transport. The transport pathway of the injected tracer in the FDZ was altered when it reached the SFI of the USP due to the buoyancy effect. In contrast, no tracer solution was found to move across the SFI from the saltwater zone into the fresh groundwater zone due to relatively strong advection along the corresponding SFIs and buoyancy effect.