MATERIAL PLACEMENT IN THE NEARSHORE: LABORATORY AND NUMERICAL MODEL INVESTIGATION

Bradley D. Johnson, Ernest R. Smith

Abstract


Typical practice for a century has been to transport dredged sand to an offshore disposal site in deep water where the sediment is lost from the littoral system. The alternative of nearshore placement can retain the sand, but the fate of the material is poorly understood. A set of laboratory experiments were conducted, using tracer sand, with the intent of quantifying the migration of material with alternative dredged mound placements within the surf zone. Conventional depth-integrated tracer sand transport models can utilize a correction factor or a gradient diffusion mechanism to represent the effects of the depth variation. In the surf zone, however, an analytical correction factor is not available and a gradient diffusion coefficient is arbitrary with no physical basis. An alternative simple advective transport sand model is introduced herein that explicitly predicts both the advection associated with the return current and the wave-related onshore transport. With a simple
framework based on a suspended layer and a bedload layer of arbitrary transport directions, the Taylor dispersion of tracer sand is explicitly computed without any dependence on a diffusion mechanism. Both the modeled and measured results indicate transport directed offshore by the undertow, onshore by the wave asymmetry, and down-drift as forced by the longshore current.

Keywords


nearshore; wave breaking; dispersion; dredged material; surf zone

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