An analysis of longshore currents and associated sediment transport in the surf zone

Abstract

Two momentum based longshore current models and a preliminary longshore sediment transport model are derived, calibrated and tested in the present investigation. The Linear Longshore Current Model predicts the relatively small longshore current induced by monochromatic, two dimensional, gravity waves of finite height and near normal incidence breaking on a plane, impermeable, gently sloping bottom in the presence of a shorenormal jetty when the offshore wave height, wave period, wave angle and water depth are known, along with the beach slope and roughness. The Nonlinear Longshore Current Model predicts a longshore current using the same input as its linear counterpart, but the nonlinear model removes the assumptions of a relatively small current and near normal wave incidence and is valid only for uniform longshore conditions. The Linear Longshore Sediment Transport Model predicts the integrated, time averaged longshore sediment transport for a relatively small current and near normal wave incidence under uniform longshore conditions and also describes the initial response of a plane bed downstream of a shorenormal jetty. The longshore current models may be considered as a series of modifications of the original model of Longuet-Higgins (1970), while the Linear Longshore Sediment Transport Model is a surf zone application of the work of Madsen and Grant (1976a) on nonbreaking wave induced sediment transport. Calibration yields physically plausible behavior for the three model parameters while fixed bed, laboratory movable bed and field testing show a general longshore current model accuracy of about 20%, where the lat ter two data bases only test the Linear Longshore Current Model. The Linear Longshore Sediment Transport Model matches the laboratory data to an accuracy of about 20% but overpredicts the field data by a factor of 5; in view of the latter finding, the model should only be considered as an order of magnitude estimator of longshore sediment transport. To aid in model use, examples of the three models are presented in an appendix in the back of this report.