OBSERVATIONS OF RIPPLE GEOMETRY AND MIGRATION OVER TIDAL CYCLES ON A PLANAR MACROTIDAL BEACH

Ripple geometry and activity was investigated at mean sea level over seven tidal cycles on a planar macrotidal beach of quartz sand (D^^ = 7.5*\0~^m)'m the southwest of England. The affects of the oscillatory flows on the bed were compared between the flooding and ebbing tides in calm conditions within the shoaling zone (Hs< 0.3 m, Um~ 0.5 m/s) and in increasingly energetic conditions in the surf zone (Hs> 1.25 m, Um> 1.4 m/s). Ripple morphology was relatively stable (r[ ~ 0.05 m) and did not respond to the fluctuating forcing conditions over individual tidal cycles showing that ripples were in disequilibrium with the flow. However larger suborbital ripples (T| ~ 0.09 m,X~ 0.9 m) were observed in the surf zone under breaking waves. Ripple activity was shown in be directly influenced by the orbital velocity and significant wave height, combined with velocity asymmetry and cross-shore currents. Ripples were relatively inactive within the shoaling zone over high tide, becoming mobile as a result of tidal cycle oscillations. Ripple migration occurred when the mobility number >11.5. Tidal asymmetry in ripple morphology and activity was observed. Post-vortex ripples developed on the flooding tide forming large vortex ripples as the water depth increased. However on the ebbing tide, these larger vortex ripples were eroded as the tide turned, developing post-vortex ripples with increasing shear stresses. Peak shear stresses (M>200,0>1) occurred on the ebbing tide between 30 to 60 minutes after high tide, however the plane bed threshold was not reached. Ripple activity was typically onshore throughout the tidal observations. However ripples migrated offshore on the ebbing tide within the surf zone, during and after the period of peak shear stresses. Maximum bed load transport rates of 0.069 kg/m/s occurred as a result of stronger offshore wave asymmetry and bed return currents. This tidal asymmetry in ripple morphology and activity will have major implications when modelling cross-shore sediment transport on macrotidal beaches. Keywords: ripple migration, ripple geometry, cross-shore flow, wave asymmetryFaculty of Scienc