Many port structures were severely damaged by the tsunami of the Great East Japan Earthquake. Meanwhile, most of the pontoons were only slightly damaged in Miyagi Prefecture and remained usable even after the earthquake. Damage survey has shown that the pontoon contributed to decrease the damage to the ship moored to the pontoon. However, there is little knowledge and design examination of the pontoon on the occurrence of tsunami, and design technique for such case is not established yet. In this study, various failure situations of pontoon in Miyagi Prefecture were carefully investigated. In order to clarify the characteristics of pontoon at the time of tsunami, hydraulic model experiments were conducted with pontoon of the mooring pile system. Mooring reaction force and dynamic behavior under the tsunami have been clarified through the experiments. Furthermore, the situation of the tsunami of the Great East Japan Earthquake was analyzed using numerical computation to verify the failure mechanism closely and to establish the countermeasures and design techniques against tsunami. Major conclusions out of the experiments are as follows;
(1)Horizontal mooring reaction force corresponds to the dynamic pressure of the flow. Resistance coefficient is estimated about 1.2.
(2)Vertical mooring reaction force occurs when the pontoon goes up by tsunami, and the mooring roller and the access bridge come into contact with mooring pile head. When pontoon is totally below the water surface, vertical mooring reaction force is almost at the same level with the surplus buoyancy of the pontoon.
(3)When the surplus buoyancy acts on the mooring pile head, pull-out force, which hardly occurs in the normal situation, is caused and acts to the mooring pile. Mooring pile is pulled off the ground resulting in the flow-away of pontoon to make the damage extensive.
(4)When the horizontal speed of the flow is about 2.5～3m/s, the pontoon shows the behavior of overturn due to the partial submersion. However, when pontoon is buried in water totally, pontoon is not overturned.
(5)It is recommendable that the distance between mooring piles is amply wide, and mooring roller should be installed at the lower part of pontoon, which eventually suppresses the lurch of pontoon by the speed of the flow.