Coastal watertable processes are of importance in resolving environmental engineering problems involving salt water wedge migration, beach erosion and accretion processes, and the seaward movement of pollutants from coastal waste disposal sites. Some knowledge of these processes is required for the construction of coastal structures such as foundations for buildings and invert levels for canals. Field observations of the coastal watertable and its tidal response reveal dynamic characteristics which have not previously been identified, and which cannot be explained using available theories.
Field observations of the coastal watertable have been supplemented with laboratory experiments using a vertical Hele-Shaw cell to study the tidal dynamics of unconfined aquifers. A number of analytical models are proposed and tested against the Hele-Shaw and field data. It is shown that while some time-averaged characteristics can be explained using one-dimensional analytical models, dynamic characteristics require more sophisticated models. Some success is obtained using an extension of the one-dimensional Boussinesq equation to incorporate vertical flow effects. The interaction between the aquifer and the capillary zone above the watertable is also seen to be a significant process in beach watertable dynamics. An analytical model which links the two zones and compares favourably with field data is presented.
Some consideration is also given to the decoupled boundary condition which occurs during the falling tide and manifests itself as a seepage face. Observations in the Hele-Shaw cell challenge classical theories on seepage face formation, and field data indicate that the decoupled boundary condition may be a significant factor in beach watertable processes.