Cross-shore exchange on natural beaches
Brown, Jenna A.
MacMahan, Jamie H.
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The cross-shore exchange of material is examined on beaches of varying morphology and hydrodynamics. On a dissipative, rip-channeled beach in Monterey, California, field measurements of the rip current vertical structure reveal depth-uniform flows inside the surf zone, and surface-dominated flows with rotation over depth outside the surf zone. Aperiodic, very low frequency pulsations in the near-surface velocity were found to control the mean rip current flow. Observations of Lagrangian surface drifters released on the inner shelf resulted in a new theory of material transport on a rip-channeled beach, where rip current pulses episodically expel material offshore through rip channels, and momentum-driven surfzone circulations pull the material back onshore over shoals to satisfy a mass balance, with no material being completely removed from the nearshore system. Advection and diffusion processes on a steep, reflective beach at Carmel River State Beach, California, are also investigated using measurements of the temporal and spatial evolution of dye released in the surf zone. A net offshore transport did exist, and the mixing and transport processes inside and outside the surf zone were independent. Overall, this work provides new insights into the fate of material transported between the surf zone and inner shelf on natural beaches.
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