Tidal water table fluctuations in a sandy ocean beach

Tidal water table fluctuations observed for 27 days in a gently sloped ocean beach are predicted well by numerical models based on the Boussinesq equation driven with the observed 10 min-averaged shoreline (ocean-beach intersection) motion. Diurnal and semidiurnal water table fluctuations are almost completely damped 100 m landward of the mean shoreline location on this fine-grained sand beach, but fluctuations at spring-neap periods (approximate to 14 days) are attenuated less. Comparison of the observations with the predictions suggests that the asymmetries in the water table level time series measured in this study result from nonlinearity owing to the large (relative to the wavelength) horizontal shoreline excursions, rather than from nonlinearity owing to finite-amplitude water table fluctuations. Cross-shore variations of the aquifer depth are predicted to have a small effect on the landward decay rate of the water table fluctuations. The seepage face width is predicted accurately and depends on the nonplanar beach profile. In general, the development of a seepage face is predicted to have little effect on the water table level landward of the intertidal region.