Tsunami-proof Breakwater Designed to Rise Up from Sea Bed
Obayashi Corporation (OBYCF), TOA Corporation, & Mitsubishi Heavy Industries Bridge & Steel Structures Engineering Co., Ltd. (MBE) a wholly owned subsidiary of Mitsubishi Heavy Industries, Ltd. (MHI),will begin construction of a Vertical Telescopic Breakwater (VTB), a buoyancy-driven vertical piling breakwater, in the harbor at Shimotsu in Wakayama Prefecture. Construction is scheduled to begin in early October 2012.
The breakwater system is designed to swiftly rise from the sea bottom in the event of tsunami, and contribute to prevention or mitigation of tsunami impact to harbors and coastal areas. When completed, it will be the first such movable steel pipe breakwater system in the world.
The three companies will undertake the construction, which will also serve to verify the system's viability, across a 10-meter span near the navigation channel as a part of a larger breakwater system that is slated for completion by the spring of 2020. The three companies plan to complete their portion by February 2013.
The VTB mainly consists of multiple sets of larger- and smaller-diameter steel pipes, set in a row at specified intervals in the sea bottom to form a fence-like wall when activated. In each set of steel pipes, the larger-diameter "lower pipe" set into the sea bottom houses a smaller-diameter "upper pipe" that emerges from the lower pipe and rises to the surface to function as a breakwater in the event of a tsunami or other emergency. The upper pipe is made to rise by buoyancy that is generated by blowing air into it. When the exhaust valve at the top of the upper pipe is opened, the air is expelled and the upper pipe sinks to the sea bottom of its own weight.
As the breakwater system remains in the seabed at ordinary times, it enables breakwaters to be constructed at port entrances and the mouths of rivers, something that is impossible to do with conventional breakwaters because they would block sea traffic. In addition, the new system has minimal impact on tidal currents and ambient scenery. The system's seabed mounting also makes it relatively resistant to earthquakes and corrosion, resulting in significant reductions in maintenance and management efforts. As the raising and lowering of the upper pipes are executed by simple air blowing and venting, reliable operation is feasible even in times of emergency. The time required for the upper pipes to rise to the surface and form a breakwater can be as short as several minutes.
The breakwater construction work is a project of the Kinki Regional Development Bureau of the Ministry of Labor, Infrastructure, Transport and Tourism, which plans to erect movable breakwaters, 230 meters in total length, in the navigation channel at the harbor entrance as part of a major initiative to cope with the threat of tsunamis in the Kainan district of the port city of Shimotsu in Wakayama Prefecture.
Due to the physical characteristics of the Kainan district at Shimotsu, which is located at a deep indent in the coastline facing the Kii Channel, this area has suffered serious damage from tsunamis caused by the Nankai Earthquake of 1946 and Chile's cataclysmic temblor of 1960.
In the event of another earthquake in either the Nankai or Tonankai zones – an event expected to occur in the foreseeable future with a high degree of probability – a tsunami exceeding the height of the existing breakwater is projected. As the projected tsunami inundation zone includes not only residential areas but also administrative offices, disaster prevention centers, and numerous commercial facilities, measures to cope with tsunamis are urgently sought.
The construction work on order, to be carried out by a JV formed by Obayashi, TOA and MBE, calls for the creation of a 10m long VTB near the channel, incorporating piles capable of rising from the 13.5m deep seabed to form a 7.5m high breakwater above the sea surface within 10 minutes. Nippon Steel Engineering is currently manufacturing the upper and lower pipes-the core components of the system.