**1. Introduction**

On a coast subject to strong wind action, foreshore sand may be transported to the hinterland, causing damage to the houses and coastal roads along the coastline. When a detached breakwater or an artificial reef (submerged breakwater) is constructed as a measure against beach erosion on such a coast, sand accumulates in the lee of the structure owing to its wave-sheltering effect, forming a salient [1]. In Japan, detached breakwaters have been widely used as a shore protection measure against beach erosion. Moreover, artificial reefs have often been constructed as a measure instead of detached breakwaters in recent years because they are submerged and thus do not block the ocean view. An artificial reef has the similar wave-dissipating effect as a detached breakwater; thus, sand deposition occurs in the lee of the constructed structure. For example, on Kimigahama Beach in Chiba Prefecture, Japan, salients were formed after the construction of two artificial reefs owing to the wave-sheltering

effect of the reefs. Then, a significant amount of fine sand was transported inland from the foreshore by wind.

Thus, in this area, topographic changes due to the actions of waves and wind simultaneously occur on coast after artificial reefs were constructed, and therefore, the prediction of the beach changes is required to consider the effective measures to maintain the shoreline and reduce windblown sand to the hinterland. In the previous studies regarding the sand transport due to waves, a number of models predicting the beach changes have been proposed [2–11]. Moreover, regarding windblown sand, many studies have been carried out for more than half a century, and not only many formulae of windblown sand transport but also models for predicting topographic changes have been proposed [12–20]. However, there are few studies to predict topographic changes while taking the combined effects of waves and wind into account. Yokota et al. [21] developed a model for predicting these beach changes, in which the BG model (a model for predicting 3-D beach changes due to waves based on Bagnold's concept) [22–24] is combined with a cellular automaton method [20]. In this study, this model was tried to apply to the prediction of beach changes on Kimigahama Beach, which is of importance in practical coastal engineering.

In Yokota et al. [21], the BG model was employed, which is a model based on the concept of the equilibrium slope and is derived by an energetics approach [25, 26], and there are eight types [22]. Among them, Type 8 is a model taking the effects of both wave action and nearshore currents into account ([22], Fujiwara et al. [27]). However, In this study, the simplest model of Type 1 BG model was used, which uses a simple sediment transport equation expressed by the wave energy flux at the breaking point, and does not calculate the nearshore currents field, so the calculation load is small [22]. Beach changes caused by wave action in the lee of a detached breakwater have already been successfully predicted when the Type 1 BG model is employed [22].

In general, when artificial reefs are installed near the shoreline, not only shoreward current but also rip current generated by forced wave breaking on the reef can strongly affect the deformation of the beach. In this case, it is necessary to include this effect in the prediction [22, 27]. However, the artificial reef on Kimigahama Beach has a large offshore distance of 300 m or more, so this effect is considered to be small, and the effect of the artificial reef on the beach can be regarded as similar to that of the detached breakwater. Based on this, in this study, the coefficient of wave height transmission of the artificial reef is given and calculated as a breakwater.

The model of topographic changes due to windblown sand used by Yokota et al. [21] is based on the cellular automaton method (Katsuki et al. [20]). This model has a relatively small calculation load compared with the other models and can perform calculations. Here, this model is used for predicting beach changes when artificial reefs are constructed on a coast subject to both waves and wind on Kimigahama Beach. Reproduction calculation was first carried out on the basis of the field data. Then, beach changes were predicted after the artificial reefs were removed to avoid excess sand accumulation behind the artificial reefs and erosion on nearby beaches. Moreover, the effect of beach nourishment around the artificial reefs was investigated. It was concluded that landward sand transport by wind is accelerated when wave-sheltering structures such as an artificial reef are constructed on a coast composed of fine sand.
