Bedload Transport in the Surf Zone: Fixing Energetics Models

Calantoni, J., and Drake, T.G., 2000, Bedload transport in the surf zone: fixing energetics models: EOS Trans. AGU, 79 (45), Fall Meeting Suppl., F416 (Special Session: Nearshore Processes)
Energetics models for sediment transport in the surf zone predict storm-generated, offshore bar migration at Duck, NC, but fail to predict subsequent post-storm, onshore bar migration. Fluid motion during offshore bar migration is typically dominated by pervasive offshore-directed currents that are relatively steady and unidirectional, thus approximating the conditions under which Bagnold originally derived energetics-based sediment transport models. On the other hand, onshore bar migration takes place during relatively quiescent conditions, when unsteady oscillatory fluid motions dominate steady, unidirectional ones. A key assumption in the energetics models is the quadratic dependence of bed shear stress on flow velocity. We question the validity of this assumption under highly unsteady flow conditions that occur commonly in the surf zone, and assert that under such conditions the oscillatory terms in the energetics models for bedload transport must be replaced. Discrete-particle simulations of bedload motion under surf zone bores suggest steep, horizontal fluid-pressure gradients generated by the passage of breaking waves produce impulsive forces that are directly related to bedload transport. We find that the bedload transport is directly proportional to the impulse for impulses greater than a critical value. We will present a derivation of the bedload-impulse relationship and a suitable algorithm for calculation of the impulse from field data. The constant of proportionality has been calibrated using the simulation data. A new bedload transport formula eliminates the oscillatory term in the Bailard bedload formula, which is proportional to the cube of the velocity, and replaces it with the impulse term.
Supported by the Coastal Dynamics Program of the Office of Naval Research.