SIMULATING THE INFLUENCE OF GROUNDWATER SEEPAGE ON SEDIMENT TRANSPORTED BY THE SWEEP OF THE SWASH ZONE ACROSS MACRO-TIDAL BEACHES

A numerical model is developed to simulate the influence of groundwater seepage on swash zone sediment transport across macro-tidal beaches. The emergence of equilibrium profile morphology is examined in response to varying tide, sediment and initial profile characteristics. Due to the unresolved complexities of boundary layer flow and corresponding shear stresses in the swash zone, sediment transport across the beach face is modelled by incorporating an uprush phase described by the shallow water wave equations, within an heuristic model of equilibrium beach face slope. A new parameter, termed the ''M'' net transport parameter, is defined to parameterize the net flux of sediment per uprush-backrush cycle. The definition of differing equilibrium gradients across saturated versus unsaturated regions of the intertidal profile permits the saturation characteristics of the bed to be incorporated. The dynamics of the watertable exit point on the beach face is central to the scheme, as this defines the time-varying extent of saturated and unsaturated regions of the intertidal profile. The sensitivity of coastal seepage face development to tide, profile and sediment characteristics is highlighted through the derivation of a new seepage face parameter. The simulation model is found to successfully reproduce key aspects of observed profile adjustment and resulting equilibrium morphology. Complex morphodynamic feedback is observed between: the tidal stage, net transport per uprush-backrush cycle, profile slope, and the dynamics of the watertable outcrop. Net upper-profile steepening and lower-profile lowering is simulated on beaches composed of medium to coarse sand. This results in the emergence of a distinctive break in slope within the intertidal profile. Such morphology is commonly observed on macro-tidal coasts.