WATER-TABLE OUTCROPPING ON MACRO-TIDAL BEACHES - A SIMULATION-MODEL

During the fall of the tide the coastal water table within a beach may become decoupled from the ocean, resulting in the formation of a seepage face while water outcrops on the inter-tidal profile. A simulation model (called SEEP) is developed to simulate the point of decoupling, and the subsequent motion of the seepage face as it continues to fall across the inter-tidal profile. The model is based entirely on the dynamics of an isolated water particle on the seepage face, i.e. the pressure distribution within the beach is ignored. Sensitivity analysis of the SEEP model to beach face slope and permeability characteristics suggests that small variations to these parameters can result in significantly differing rates of exit point motion and hence seepage face extent. To test the predictive capability of the SEEP Model, results are presented from a field investigation of exit point dynamics on a macro-tidal beach, North Queensland Australia. The simulation model is found to accurately reproduce the observed seepage face motion at this field site. In addition, a re-analysis of seepage face extent reported in Nielsen (1990) from a protected. micro-tidal environment, shows similarly strong agreement between the reported and modelled exit point elevation.