Groundwater Waves and Water Exchange in Beaches

Abstract

The generation of groundwater movement in beaches at wave and surf-beat frequencies is not a trivial process. Measurements of beachface porewater pressures beneath the swash zone show a clear spectral peak that coincides with the spectrum for the horizontal movement of the watertable exit point across the beachface, rather than the “usual suspects” of surf zone water-level or wave runup. The generation of beach groundwater waves at these frequencies is determined by a combination of the movement of the swash front and the characteristics of meniscus drainage within the upper layer of the beachface. Based on experimental data, recommendations are provided for describing the propagation of watertable waves which are influenced by the capillary fringe above the watertable. With respect to small amplitude waves, this is easily done by incorporating experimentally based effective porosities ne in the dispersion relations. The effective porosity corresponding to time-periodic waves is complex because of the phase lag of the total moisture relative to the watertable. To a great degree of accuracy ne has been found to depend on a single dimensionless variable: ne=ne(omegaHc/K) where omega is the angular frequency, Hc is the capillary height and K the hydraulic conductivity. Incorporating the experimental ne-information into non-linear wave equations is challenging: By analogy with the hysteresis free model of Parlange & Brutsaert, the experimental ne-values suggest the use of fractional (2/3) order derivatives.