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TYPES OF SAND RIPPLES AND THEIR ARRANGEMENT OBSERVED ON THE TATADO BEACH, SHIMODA CITY, IZU PENINSULA

1972; Association of Japanese Geographers; Volume: 45; Issue: 3 Linguagem: Inglês

10.4157/grj.45.216

2185-1735

Tadashi Machida, Masao Inokuchi, Eiji Matsumoto, Takayuki Ishii, Hiroshi Ikeda,

Earthquake and Disaster Impact Studies

Sand ripples formed by oscillatory flow under waves have interested many researchers in hydraulics, and the hydraulic conditions for their formation havee been examined by using small scale experimental wave tanks. The results of these studies, however, are insufficient to explain the hydraulic conditions for the formation of many kinds of the ripple types under natural conditions, which have been described by physical geographers and sedimentologists, because the experimental studies have been concerned almost exclusively in the most simple ripple type, that is, a parallel type of ripples. The authors tried to examine some hydraulic conditions for the formation of various types of ripples by the observations and measurements in a real field. A series of observations was made from 19th to 22nd of July, 1969, on the shallow sea bottom of the Tatado Beach, Shimoda City, Shizuoka Prefecture, Central Japan. This coast is a sand beach with a length of 500 m and faces southwards directly to the Pacific Ocean. An observation range, along which observations and measurements of ripples, waves, and bed materials were made, was established perpendicularly to the shore line through the middle part of the beach (Fig. 1). The bed materials sampled along the range were medium sands (0.24mm in median diameter) with high content of shell fragments and did not show any certain tendency of size distribution and sorting along the range (Figs. 2 and 3). Keeping pace with the measurements of wave and tide conditions at an observational tower, observations of ripple forms were carried on by skin divers from shore to the point about 6 m in depth along the range, at intervals of one to two hours. The ripples here were classified into following ripple types based on the patterns of their crests and troughs and also on the relations between the directions of ripple crests and of surface wave ridges. 1) Parallel type (Type P). This type of ripples shows regular alternation of ripple crest and trough lines parallel to those of surface waves. This type may be subdivided into long crest parallel type (Type Pl) and short, sinuous crest parallel type (Type Ps). 2) Diagonal type (Type D). This type includes various patterns of ripples which have the crest lines diagonal to the wave ridge lines. Usually, this type of ripples consists of a set of ripple crest lines which cross each other, and are also subdivided generally into two types, long crest diagonal type (Type Dl) which reveals a ladder-like pattern, and short crest diagonal type (Type Ds). 3) Lunate type (Type L). This type shows lunate-shaped depressions with an offshore-faced convex arch. They are arranged discrepantly each other on a flat sea floor. Cross profiles in the off-and onshore direction of the depressions show a distinct asymmetrical form, that is, gentle offshore-faced slope and steep onshore-faced one. Three-dimensional forms of some typical ripple patterns were measured by the “comb method” which uses an apparatus shown in Photo 1, and contour maps of the ripple forms were shown in Fig. 4. By the repeated observations of the ripples at regular time intervals, it was made clear that these ripple types were always arranged towards the shore in the following regular order: parallel type, diagonal type and lunate type, and that their zone of occurrence fluctuated in such a general trend as they retreated offshore when waves increased in height or tide level dropped, and vice versa (cf. Fig. 5 and Table 1). This might indicate that each type of the ripples is formed closely in response to the water movements on the sea bottoms under the surface waves. Some hydraulic conditions for the formation of each ripple type were examined. During the observation period on the Tatado Beach, it was seen that the wave form in the offshore zone showed a transformation by shoaling.