ON THE TOPOGRAPHIC CHANGE OF THE BEACH OF TOKAI VILLAGE ON THE COAST OF KANTO DISTRICT, JAPAN

Abstract

As the results of surveys on the topographic change of the beach of Tokai Village, the author concluded as follows; (1) In the map of the coast line exaggerating its scale only in the perpendicular direction to its general trend, it was found that the shore line shows a composite wave-like form, of which the wave lengths are 2, 000, 300 and 30 meters. The loops of the largest waves of the shore line with the wave length of 2, 000 meters were situated in front of long continued submarine bars, and those of the medium waves with the wave length of 300 meters were formed superimposing the largest waves and were situated in front of individual submarine bars. The wave lengths of the medium waves were superimposed by the minor waves that were cusps with the wave length of 30 meters, and coincided with the unit lengths of the periodic change of the sizes of cusps. (2) The undulating shore line with the medium wave length of 300 meters was formed by the action of swash that is more powerful at the front of the central parts of the submarine bars than the terminal parts of the bars and by that of rip currents that flow out through the channels among the bars. (3) The undulation of the shore line shifted southwards in accordance with the movement of the bars during the period from December 1956 to August 1957. At the same time the bars shifted by about 70 meters toward the shore, though the shifting of the shore line in the perpendicular direction to it was almost negligible. (4) The change of the shore line in the short period of about twenty days was rather conspicuous. When winds blew strongly, the shore line retreated in front of the bars and advanced in front of channels among the bars. On the other hand, when winds weakened, the shore line advanced in front of the bars and retreated in front of, the channels. (5) Considering the relation between the changes of the shore line and cusps in this period, cusps disappeared where the shore line retreated and occurred where the shore line advanced. Where cusps existed throughout this period during which surveys were repeated three times, the author found the tendency that cusps were enlarged where the shore line retreated and became smaller where the shore line advanced. It is inferred, therefore, that cusps are not altered on stable shores and the intercusp space becomes irregular where shore lines repeat advancement and retreat. (6) During the period from December 1956 to August 1957 when the bars shifted toward the shore, the bars became smaller and the declivities of their outer slopes became less without exception. At the same time, the inner slopes of the bars decreased their declivities in the northern part of long continued bars and incressed in the southern part. Accordingly, the bars in the southern part became to show very asymmetric features in their transversed profiles. On the other hand, the southern part of the shore retreated remarkably and the grain size of the beach materials became coarser, while the northern part of the shore advanced conspicuously and the beach materials became finer. (7) Off the shore, the flat submarine bottom 5 to 20 meters deep in the northern part became deeper and the depth of the bottoms decreased around the flat bottom in the northern part and on the submarine bottoms in the southern part during this period. On the other hand the bottom sediments became coarser where the depth increased and finer where the depth decreased.