SIMULTANEOUS OBSERVATIONS OF CLIFF-TOP GROUND MOTION RESPONSE TO EXTREME WAVES OVER VARYING BEACH AND BATHYMETRIC FRONTAGES

Observations of cliff top ground motion and nearshore hydrodynamics under highly energetic storm conditions are notoriously difficult to obtain. The delivery of wave energy to cliffs and the subsequent cliff top ground motion response has been found to be greatest in the infragravity (< 0.05 Hz) and sea swell (0.1 – 0.05 Hz) frequencies, during periods of high tide and/or large waves. Cliffs fronted by steeply sloping beaches and bathymetry demonstrate higher cliff top ground motion response under storm conditions compared to more dissipative beaches. Customarily, nearshore wave climate is inferred from offshore wave measurements or modelling and/or cliff-toe observations of waves and water levels. The nuances of how varying beach slopes control the delivery of this wave energy and wave transformation across the foreshore are often overlooked. Coupling the hydrodynamics across the intertidal zone with the cliff top ground motion is not typically carried out in such studies. Here we consider the influence of the offshore (approx. 17 km offshore, 60 m depth), inshore (approx. 2 km offshore, 15-20 m depth) and intertidal (2 – 100 m from cliff face) wave climate on coastal cliff top ground motions at two sites, simultaneously, with different beach and bathymetric slopes under both calm and stormy conditions. It is found that cliff top ground motions are 1 -2 times greater at the site fronted by a steeper (reflective) beach and bathymetric slope compared to a cliff fronted by a dissipative beach, in both the infragravity and sea swell frequencies during not only calm but energetic wave conditions (>2.5 m H_s). Modelled wave data showed an over prediction of significant wave height (~ 0.3 m) for the steeper bathymetric slope than that of the flatter bathymetric profile, leading to the perhaps incorrect assumption that the reflective beach experiences a more energetic wave climate consistently, regardless of the offshore conditions. Most surprisingly, however, cross shore measurements of waves and water levels in the intertidal zone indicate that the cliff top ground motion response is linked to the wave climate not only at the toe of the cliff, but also deeper into the intertidal zone. This indicates that a more detailed understanding of the full propagation of waves at not only the incident swell frequencies but the infragravity frequencies, including the transformation and dissipation of wave energy from deep water to breaking and/or the point of cliff-impact is required to further our understanding of how cliffs respond to nearshore hydrodynamics.