Beach Cycle of the Ninomiya Coast, Kanagawa Prefecture, Kantô District

Abstract

A series of coastal and near-shore survey was carried out in 1962 by members of the Japanse Hydrographic Office on the beach of Ninomiya, Kanagwa Prefecture. The author, leader of the survey, made an investigation into the seasonal change of the beach and the sea bottom near shore from the results of the survey conducted several times within the one-year period.
Situated at the northwest corner of Sagami Bay, the Ninomiya coast has a narrow beach of 60 to 70 meters in width, running east-west on the south side of the Ôiso Hills. The sea bottom near the shore of Ninomiya coast is flattened by both wave abrasion and sedimentation down to 20 meters in depth and continues to a steep slope of deeper than 20 meters which several gullies incise. Some beach developments by small waves were observed during summer season and erosion during late summer and late spring by stormy waves was also recognized. While, the sea bottom near shore was eroded during summer season by long. period waves and deposited during late summer by stormy typhoon waves.
Major records of the beach cycle observed are as follows: The berm was cut back and extinguished by stormy waves during late spring and the beach fore-shore became more gently sloping. In the calm season of small waves, sands started piling up on the beach to form a new berm, which later showed a growth. At the typhoon season of late summer, again the berm was cut back and the beach fore-shore became gently sloping. Beach cusps occurred in accordance with the formation of berm during summer season, but they were sharpened upward with berm development. Both berms and beach cusps disappeared because of the typhoon. From measurement of the inner structure of beach sediments, it may be said that the beach was rapidly deposited with sands of about 1 meter in thickness after it was eroded by stormy waves. Then, gravels were deposited on the seaward edge of the berm as cusp materials no sooner than the formation of berms and cusps started. Distribution of gravels extended on the beach with the devel opment of berms and cusps during calm season.
The sea bottom near shore showed a gentle profile with minor steps in front of large cusps in late spring. During summer season it was eroded and the profile was further smoothed away because of distinguishable erosion of the edge of the steps. Eroded areas were formed in front of the large cusps and elongated in north-south direction. Sea bottom in front of the cusps was filled with finer sediments, and was slightly elevated against other areas. The author thinks that the large cusps were on the route of the near-shore circulation of sediments, since the cusps adjacent to the eroded areas of the sea bottom received much sedimentation during summer season.
At the typhoon season of late summer, the sea bottom near shore received considerable sedimentation in contrast to the beach where erosion took place simultaneously. Deposited areas were found on the seaward extention of the large cusps, coincident with the eroded areas formed during calm season. In such areas, the sea bottom again showed a profile with minor steps due primarily to the sheet flow of sediments by rip curreat. At the same time, offshore bars occurred temporarily by stormy waves, but within a month after the storm those offshore bars disappeared.