Characteristics of Wave-Induced Groundwater Dynamics Using Harmonic Analysis

Groundwater dynamics in coastal aquifers are important factors influencing various coastal engineering problems such as sediment transportation and beach stability. While tide-induced groundwater fluctuations were maturely understood by existing studies, characteristics of the wave-induced hydrodynamic processes, however, are not well-summarized. In the present study, wave-induced groundwater dynamics were investigated using harmonic analysis with respect to laboratory experiments to reveal the spatial and temporal features of amplitude attenuations and phase lags of the groundwater fluctuation, as well as the generation of high-frequency components. For the case that the uprush limit is located within the capillary truncated zone, wave-induced groundwater oscillation near the beach surface has the large amplitude and leads that near the bottom in the seaward generating zone, while the reverse trend occurs in the landward interior zone, which is consistent with the tide-induced groundwater hydrodynamics. However, capillary fringe behaviour leads to negligible phase lags, different from the tide scenarios. In addition, water surface elevation and seepage face dynamic have more pronounced effects on wave-induced hydrodynamics than that in tide scenarios, complicating the amplitude and phase features in the generating zone. Furthermore, the generation of higher frequency components is deemed to be related to the beach slope, seepage face, and capillary truncation. In the case that the uprush limit exceeds the capillary truncation zone, more complex groundwater dynamics appear due to the unsaturated groundwater flow resulting from the formation of the wetting front in the upper unsaturated zone.