A STUDY ON THE AEOLIAN BED-FORMS IN ENSHUNADA BEACH, SHIZUOKA PREFECTURE, CENTRAL JAPAN-THEIR CONFIGURATIONS AND CRITERIA OF FORMATION

Abstract

Enshunada beach located in Shizuoka Prefecture, Central Japan, is divided into two similar arcs by the advancement of the delta of River Tenryu (Fig. 1). In this region, the weather is so fine that the surface of beaches is dry in winter. Sand drift occurs by strong westerly to northwesterly wind which is prevailing in winter (Fig. 2). Various aeolian bed-forms are formed on the backshores in this beach (Fig. 3). These aeolian bed-forms are classified into sand ripples (Photo. 1(a)), granule ripples (Photo. 1 (b)), and dunes (Photo. 1 (c) and (d)) according to the wave-length. Regardless of size of the wave-length, aeolian bed-forms have two characteristics. One is that they have transverse ridges against the wind direction, and the other is that they have a gentle slope in the windward and a steeper slope in the leeward (Fig. 4). In dunes, two kinds of the bed-forms are observed; one has the smooth curve all over a dune from windward slope to leeward slope (Photo. 1 (c)), and the other has the straight and steeper slope, which is called “slip face” in the leeward (Photo. 1 (d)). The former is named “wavy dune”, and the latter is named “dune with slip face” by the author. In addition to ripples and dunes, two plane forms are observed. They are “flat bed” which involve large amount of sand drift, and “plane bed” which involve the least amount of sand drift. Bed-fours observed in this region are classified into following six types by their wave-lengths and the characteristics of configulations: “plane bed”, “sand ripples”, “granule ripples”, “wavy dunes”, “dunes with slip face”, and “flat bed”. Sand ripples are generally formed everywhere in these beaches and granule ripples are observed on backshores of Nakatajima where bed materials contain granule and coarse sand (Table 2). In many cases, dunes are formed on the backshores of eastern districts in both arcs. For the formation of the dunes, a long fetch of wind and a supply of sand are neccessary. In the eastern districts, the direction of prevailling wind is parallel with the strike of beach lines (Table 1). Because the mean diameter of bed materials varies from place to place, there are some differences in wind velocity (V1.0), which corresponds to the formation of sand ripples. As the mean diameter is larger, V1.0 is slightly larger in Nakatajima than in any other places. The transformation of the dune profiles were measured by leveling on Dec. 6, 8, and 14, 1974 in Chihama (Fig. 6 and Table 3). The wave-length change is generally small. The wave-height change is clearly recongnized and it is classified into following three types: Type 1 There is no distinct change of wave-height, Type 2 Wave-heights increase markedly, and Type 3 Wave-heights decrease prominently. There is a good correlation among these three types of changing dune profile, the wind velocity (V1, 0), and the condition of formation or disappearance of ripples (Table 4). When the wind velocity is small, the dune profile change becomes the type 1 and the ripples are formed clearly. When the wind velocity is medium, the dune profile change becomes the type 2 and the ripples disappeare. When the wind velocity is large, the dune profile change becomes type 3 and the ripples completely disappeare. Thus it seems that the types 1, 2, and 3 correspond to the cases of ripple formation, dune formation, and flat bed formation respectively. Aeolian bed-forms are the results of processes occuring at the interface between the sand bed and air flow.