A NUMERICAL MODEL OF BEACH CHANGE DUE TO SHEET-FLOW

Akira Watanabe, Kazuhiko Shiba, Masahiko Isobe

Abstract


First a discussion is made on the relation between the time-average of a Boussinesq-type equation and the mean flow equation. Then nonlinear wave transformation on a sloping bed is computed by a set of Boussinesq equations including a breaker-induced energy dissipation term. Comparisons are made between the computations and laboratory measurements for cross-shore distributions of the wave height and mean water elevation, and for time-histories of the near-bottom velocity near and after breaking. Undertow current velocity in the nearshore zone is calculated by a semi-empirical formula and is also compared with measurement data. A beach profile change model is set up by combining the Boussinesq-type equations, a sediment transport rate formula for the sheet-flow proposed by Dibajnia and Watanabe, which incorporates the asymmetric orbital velocity due to wave nonlinearity as well as the undertow current, and a sediment mass conservation equation proposed by Watanabe et ah, which includes the effect of local bottom slope. The validity of the model is examined through the comparisons of the computed transport rate distributions and beach profiles with the laboratory data obtained in large wave flume experiments.

Keywords


numerical model; beach change; sheetflow

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