Model for Predicting Topographic Changes on Coast Composed of Sand of Mixed Grain Size and Its Applications

Abstract

In beach nourishment, predicting the changes in the longitudinal profile is important for estimating the effect of nourishment. When material composed of not only coarse sand but also fine sand is used in beach nourishment, a considerable amount of fine sand may be lost owing to offshore sand transport by waves, depending on the conditions of the beach slope. On the basis of the contour-line-change model proposed by Serizawa et al. (2003), which was derived using the concept of the equilibrium slope (Bakker, 1968), Uda et al. (2004) developed a model for predicting the change in the longitudinal profile as well as the change in the grain size distribution. Then, the model was applied to the results of the movable bed experiment using a large wave tank, in which the model beach was composed of sand of mixed grain size, to evaluate the model applicability (Fukuhama et al., 2007). The experimental results showed that sand of mixed grain size was sorted into fine and coarse sand, and that coarse sand was transported shoreward, forming a berm, whereas fine sand sunk to a zone deeper than the depth of closure. Although the experimental and predicted results were in good agreement in this application, the applicability of the model to the coasts was still inadequate. Therefore, the model was applied to the following two model cases: beach changes associated with the beach nourishment using fine and coarse materials on the Chigasaki coast in Kanagawa Prefecture, Japan (Yoshioka et al., 2008), and the formation of a ‘conveyer belt’ composed of gravel and fine sand observed on the Shimizu coast (Nishitani et al., 2008). In this paper, these applications of the model are summarized and the effectiveness of the model is shown.