Swash-zone sediment transport and foreshore evolution: field experiments and mathematical modeling

Abstract

Data from field experiments carried out in the swash zone on two Pacific beaches in Japan were analyzed to determine net sediment transport rates and the resulting foreshore response. The experiments involved artificial modification of the foreshore slope with a bulldozer, either making the slope steeper or milder, after which a wide range of measurements were performed as the slope evolved back towards its equilibrium value under the action of the incident waves. A sediment transport formula was developed to predict the net transport rate over many swash cycles and compared with the derived transport rates from the surveys of the foreshore. This formula was then combined with the conservation equation for sediment volume to yield a mathematical model for simulating the foreshore evolution under varying waves and water level. Predictions by the model were compared to measurements yielding satisfactory agreement, although the optimum values on the main calibration coefficient displayed some variation between experiments. Further analysis of a limited number of cases indicated that this coefficient may be dependent upon the ratio between the incident wave period and the swash period, which characterizes the interaction between individual waves in the swash zone. A simple analytical solution was also derived for a schematized foreshore shape to estimate the typical time scale of the morphological response.