Watertable outcropping on macro-tidal beaches and its influence on the morphology of the intertidal profile

As the tide rises and falls across the intertidal profile of macro-tidal beaches the beach face is transgressed through every tidal cycle by the contrasting hydrodynamic regimes of: wave shoaling and breaking, surf zone bore propagation and beach face runup-backwash. The relative occurrence of shoaling, surf and swash processes is controlled by the tide, but tidalregulation is not the sole determinant of equilibrium profile morphology. The tidally-driven rise and fall of the watertable within a beach results in an upper region of the intertidal zone. draining through the tide cycle, in contrast to a lower region that remains in a permanently saturated state. This is of particular significance to sediment transport mechanisms in the swash zone. Runup infiltration on an unsaturated beach face promotes net onshore transport and profile steepening. In contrast, beach groundwater seepage across a saturated beach face enhances offshore transport and profile lowering. The rate of rise and fall of the watertable outcrop determines the relative extent of saturated and unsaturated regions of the intertidal profile. A model (called 'SEEPQ was developed to simulate the dynamics of the watertable exit point (the upper limit of the watertable outcrop) through the tide cycle. The assumption that the exit point represents a unique point on the free surface-whose motion is independent of the pressure distribution within the beach -permits the derivation of a simple criterion to determine both the instant of watertable-tide decoupling and the subsequent rate of continued exit point fall. The model is found to be highly sensitive to beach face sediment and slope characteristics. Comparison of observed and simulated exit point motion from three central Queensland macro-tidal beaches demonstrates the considerable success of the model across a range of morphologies and neap to spring tides. A runup correction factor is incorporated to correct for runup infiltration above the still water level. A field survey of the central Queensland macro-tidal coast reveals that a (typically distinct) slope break in the intertidal profile was a characteristic of all beaches. Empirical analysis shows that the elevation of this feature within the profile strongly correlates with the drainage characteristics of the beach. An increase in extent of the steeper, upper-intertidal region is associated with beaches composed of increasingly permeable sand. It is proposed that this feature corresponds to the modal elevation at which the rising tide over-tops the falling watertable outcrop. The extremely low gradient, featureless and dissipative lower-intertidal profile is consistent with this region remaining in a perpetually saturated state. Statistical analysis (Principal Components Analysis or Empirical Orthogonal Function Analysis) of five years of monthly profiles from a subset of these beaches confirms that an intertidal break in slope is indeed a characteristic and persistent feature. Higher order eigenftmctions reveal that the upper-intertidal profile regularly undergoes significant adjustment in response to cyclonic storm events. Erosion and overall lowering of the profile gradient results, followed by a rapid recovery period of profile accretion and steepening. In contrast, the morphology of the lower-intertidal profile exhibits low variability, at all times held by groundwater outflow in a low gradient, dissipative and stable state. A numerical model (called 'OUTCROP') was developed to simulate the influence on intertidal profile morphology of the sweep of the swash zone across macro-tidal beaches. A physical description of the hydrodynamics of uprush is combined within an heuristic parametrisation of net sediment transport per swash cycle. The elevation of the watertable exit point (and hence extent of the watertable outcrop) is central to this dynamic-equilibrium model, defining regions of enhanced onshore or offshore transport. Simulation results incorporating a range of sediment, tide and slope characteristics are presented. Qualitative comparison with the prior field survey suggests that the model is capable of reproducing key aspects of profile adjustment and equilibrium profile morphology. It is concluded that the influence of an outcropping watertable to intertidal sediment transport mechanisms will only be quantified following further investigation of modified shear stresses due to infiltration and seepage in the swash zone.