1. INTRODUCTION

Beach erosion has been a serious problem in the past several decades. It has been especially significant in recent years as a consequence of reduction of sand supply from rivers following reservoir construction and dredging of submarine sand and minerals. Increasing needs for utilization of coastal zones and the construction of port facilities also block the littoral drift, resulting in severe erosion on the down-drift side and excessive accretion on the up-drift side. According to Bird [6], more than 70% of sandy coasts around the world are categorized as eroding beaches. The recent trend of sea level rise due to global warming may accelerate the erosion. In order to find effective countermeasures against beach erosion, we have to understand the mechanism of sediment transport in detail. However, our knowledge of sand transport mechanisms on natural beaches is still qualitative, which sometimes forces us to rely on experience.

In order to predict beach evolution quantitatively, we have to understand first the dynamics of waves and currents in nearshore environments. Waves in nature are, however, random and sediment transport mechanisms under random waves are complex. Laboratory experiments in wave flumes have been used as powerful techniques to investigate the mechanisms under well-controlled environments. A number of valuable data on the mechanics of the wave boundary layer, incipient motion of sand particles, and suspended sand concentration above sand ripples have been accumulated by laboratory measurements using monochromatic waves. However, they may not...