Sand Accumulation in Wave-Shelter Zone of Oharai Port and Change in Grain Size of Seabed Materials on Nearby Coast

Abstract

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.

Bacterial Community Structure and Diversity of Closely Located Coastal Areas

“…The higher abundance of the phylum Cyanobacteria of July's sampling agreed with the concept that the growth rate of Cyanobacteria is usually higher at high water temperature during the summer season [33]- [35]. The relative abundances of the class Betaproteobacteria were higher at the sea side station, may be because this station is influenced by riverine communities (and so terrestrial communities as well) of the Naka River [36] [37] (Figure 1).…”

Beach Changes Caused by Extension of Offshore Breakwater and Limitations of Artificial Reefs Constructed as a Measure Against Beach Erosion

“…Furthermore, the fact that the fine sand content on the Kokozura coast is higher than those on the Sekita and Suga coasts, where the medium-size sand content is high, suggests that fine sand was selectively transported south from the Sekita and Suga coasts and deposited on the Kokozura coast. Simultaneously, coarser sand was left on the Sekita and Suga coasts outside the wave-shelter zone of the offshore breakwater as in the case of Oharai Port (Matsu-ura et al, 2010).…”

“…When an offshore breakwater is constructed near the end of a pocket beach, sand is deposited in the wave-shelter zone of the offshore breakwater, and beach erosion occurs on the nearby coast. A typical example can be found in the vicinity of Oharai Port facing the Pacific Ocean (Matsu-ura et al, 2010), where a wave-shelter zone was formed following the construction of an offshore breakwater, resulting in the deposition of fine sand in the wave-shelter zone and beach erosion on the nearby coast. In this case, both the port authority responsible for port construction and the organization in charge of shore protection works were part of the Ibaraki Prefectural Government, and therefore, various adjustments were possible.…”