The caissons of many breakwaters either slid or overturned due to overflow pressure of the tsunami triggered by the 2011 Tohoku earthquake. For example, the offshore tsunami breakwater of Kamaishi Port, constructed to protect Kamaishi City, was destroyed by the huge tsunami overflowing the breakwater. Up until the 2011 disaster, tsunami overflow had not been considered in breakwater design; rather, it had been estimated by Tanimoto’s tsunami force formula. However, Tanimoto’s formula cannot express the force of tsunami overflow. Since the widespread damage engendered by the Tohoku tsunami overflow, the pressure formula under tsunami overflow has been studied using hydraulic experiments, and, in 2015, the experimental results were introduced in the Tsunami-Resistant Design Guideline for Breakwater. In the guideline, the new tsunami pressure formula is expressed by the quasi-hydrostatic pressure due to the tsunami water level. The front and the rear pressure are expressed as modified hydrostatic pressures. The modification factor is 1.05 at the front and 0.9 at the rear.
Many breakwaters in Japan have been inspected and redesigned using the new design formula. However, some design problems remain. Because the new design formula is intended for simple rectangular caisson-type breakwaters, the uplift and overburden pressure are not considered; instead, only the buoyancy force acting on the caisson is considered in the guideline. In the hydrostatic condition, the buoyancy force is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid. The buoyancy force is equivalent to the pressure difference between the bottom and the top of the immersed object. However, under tsunami overflow, the pressure difference between the bottom and the top of the breakwater caisson, especially a caisson having a large parapet, can be extremely larger than the buoyancy force.
In order to examine this buoyancy force, a series of hydraulic experiments were conducted. The experiments were conducted in an experimental wave flume in which a large pump was installed to produce tsunami overflow. Pressure gauges, water level gauges, and velocimeters were installed at the top and bottom of the caisson model. The caisson model (32cm high, 26.4cm wide) was built on a scale of 1/36. Tsunami height in the experiment ranged from 4 to 14 cm.
The experimental results clarified the following matters: 1) The uplift pressure distribution is a triangular shape whose front (rear) end pressure is almost the same as the static water pressure corresponding to the front (rear) water level. 2) The front end overburden pressure is smaller than the static water pressure corresponding to the water level. This small pressure is caused by the eddy at the front edge of the caisson. 3) The large uplift pressure and the small overburden pressure cause the upward force to be larger than the buoyancy force. This upward force degrades the stability of the caisson, especially a caisson having a large parapet.