Abstract
The results of a near shore survey conducted along the Ninomiya Coast during four periods in 1962, clarified the following facts regarding sediment circulation between inshore and offshore areas.
(1) Submarine Erosion
Submarine erosion during the summer is dependent on long wave action. Submarine erosion takes place only in restricted areas such as the seaward extension of giant cusps by convergence of wave energy by wave refraction. Such areas had been filled with fine sediments and are slightly elevated. Bottom currents transport fine sediments (less than 2φ median diameter) more readily than coarse materials (greater than 2φ median diameter). Fine sand particles behave similarly to medium and coarse sand particles in the threshold mean flow velocity, but the settling velocity is smaller for fine sand particles. Because the fine particles have been transported, only the coarse materials remain.
(2) Long Shore Current
Long shore currents with velocities of 40 cm/sec flow both eastward and westward along the Ninomiya Coast. The course of flow was near the shoreline near giant cusps and a considerable distance from the shore near bays. Such a meandering course illustrates the existence of normal onshore and rip currents as well as currents parallel to the shore. The normal currents transport suspended materials both onshore and offshore through giant cusps.
(3) Submarine Sedimentation
Due to turbulance caused by typhoons in late summer, sedimentation occurs in the near shore areas in contrast to the eroded beaches. Deposition of finer sediments (less than 2φ median diameter) takes place at the seaward extention of the giant cusps. The giant cusps tended to migrate and alter their form during the periods of survey. The deposits of fine sediments correspondingly migrated with the shifting of the giant cusps. Fine sediments that were deposited disappeared as inshore discharge of particles ceased. As beaches erode, submarine sedimentation takes place by means of rip currents in front of giant cusps.
