Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

Abstract

Extensive measurements of ripple characteristics and dynamics along with associated suspended sediment fluxes and hydrodynamic conditions were made in the shoaling and surf zones of a macrotidal coarse grained beach at Sennen Cove, Cornwall, England (median grain diameter 0.69 mm). Suborbital vortex ripples were observed during the majority of the study period with height ∼5 cm and length ∼35 cm. The scale and shape of the ripples did not vary significantly as the bed shear stress increased during wave shoaling and breaking. However, ripple migration rates (onshore directed) were strongly dependent on their location relative to the breakpoint, increasing from ∼0.1 cm min−1 under shoaling waves to 2 cm min−1 in the outer surf zone during low‐energy conditions. Farther inside the surf zone, ripples persisted but migration rates slowed, probably owing to the presence of the offshore‐directed mean flow which impedes landward migration of the ripples. Under low‐wave conditions (during which measured sediment fluxes peaked around the outer surf zone and decreased through the saturated surf zone), bed form transport rates under shoaling waves were of the same magnitude as net suspended sediment fluxes but at least an order of magnitude smaller in the outer surf zone. Under high‐energy conditions (during which suspended sediment fluxes in the surf zone were offshore directed owing to the presence of the seaward directed mean flow), bed form transport rates were several orders of magnitude smaller than suspended fluxes.