Beach nourishment using 87,000 m3 of gravel with grain size between 2.5 and 13 mm was experimentally carried out until April 2008 at the Jinkoji coast, which faces the Pacific Ocean and is surrounded by artificial headlands. Monitoring surveys, a boring test and core sampling on the beach were carried out to investigate the mechanism of the formation of gravel layers and their effect on protecting the foot of the seawall. It was found that the nourishment gravel was deposited with a slope of 1/8 at the foot of the seawall, thus providing some protection to the seawall, and that the nourishment gravel was stably deposited without offshore discharge.
Beach changes around Oharai Port facing the Pacific Ocean were investigated using bathymetric survey data collected over 25 years between 1979 and 2004. Between the south and offshore breakwaters of Oharai Port, 1.50×106 m3 of fine sand was deposited in this period, i.e., at an annual rate of 6.0×104 m3/yr, which was originally supplied from the Naka River 3 km north of the port. Also in the wave-shelter zone of the offshore breakwater south of the port, 7.0×106 m3 of fine sand was deposited between 1979 and 2004 at an annual rate of 2.8×105 m3/yr, which was transported by the northward longshore sand transport induced from outside to inside the wave-shelter zone of the offshore breakwater, resulting in severe beach erosion on the south coast outside the wave-shelter zone. Taking into consideration the fact that sand transport to the Kashimanada coast is now completely obstructed by the port breakwaters, sand bypassing and sand back passing to the coast from the sand deposition zone of Oharai Port are required.
A strong low-pressure system passed along the coast of Pacific Ocean side of the northern part of Japan in October 2006. High tide level was recorded, many beaches and dunes were eroded and coastal defense systems were damaged on the Ibaraki coast by the low-pressure system. This high tide level due to the low-pressure system continued for over 24 hours with big wind waves and met astronomical high tide two or three times. The results of the numerical simulation in this paper show that Ekman transport was the main physical force to generate this high tide level. Unusual time and spatial distribution of winds around the low-pressure system affected the force considerably.
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