A sheet flow model to estimate the sediment transport rate for beaches backed by seawalls

摘要

To consider the effect of reflective structures on coastal sediment transport and beach morphology, experiments have been performed at laboratory model scale on a partially reflective seawall located in the surf zone. The main objectives of these experiments were to measure beach profile evolution on an open beach with two different sizes of sand and to compare the results with those on a beach fronting a partially reflective seawall. The results obtained from the profile evolution measurements in front of reflective seawalls indicate that the extent to which a seawall affects the processes on the fronting beaches largely depends on its location relative to the active shore face. A seawall located well landward of the active shore face will behave in much the same way as a natural beach whereas seawalls located in the active shore face will modify the near-shore beach profile because of the effects of reflected waves. A simple sheet flow model, based on the measured probability density functions of near-bed horizontal velocities, is developed to predict the short-term response of a partially reflective structure to random wave attack. The main conceptual innovation of the model is taking the probability density functions of the velocities into account and integrating them to calculate the sediment displacements across the profile using a threshold criterion for initiation of sediment motion. The results obtained from the model and comparison with the experimental results are promising and encouraging for further developments of the preliminary model.