Tidal water table fluctuations in a sandy ocean beach

Tidal water table fluctuations observed for 27 days in a gently sloped ocea n beach are predicted well by numerical models based on the Boussinesq equa tion driven with the observed 10 min-averaged shoreline (ocean-beach inters ection) motion. Diurnal and semidiurnal water table fluctuations are almost completely damped 100 m landward of the mean shoreline location on this fi ne-grained sand beach, but fluctuations at spring-neap periods (approximate to 14 days) are attenuated less. Comparison of the observations with the p redictions suggests that the asymmetries in the water table level time seri es measured in this study result from nonlinearity owing to the large (rela tive to the wavelength) horizontal shoreline excursions, rather than from n onlinearity owing to finite-amplitude water table fluctuations. Cross-shore variations of the aquifer depth are predicted to have a small effect on th e landward decay rate of the water table fluctuations. The seepage face wid th is predicted accurately and depends on the nonplanar beach profile. In g eneral, the development of a seepage face is predicted to have little effec t on the water table level landward of the intertidal region.