A flap-gate breakwater is a coastal defense structure that ordinarily lies down on the seabed, and rises due to its buoyancy to form a continuous seawall before a tsunami attack. Because the 2011 Tohoku earthquake tsunami seriously damaged many coastal defense structures, the resilience of these structures must be considered so they do not immediately collapse against tsunamis exceeding design levels. In this study, we examined resistance properties of the flap-gate breakwater against tsunamis through a series of hydraulic experiments using a flap-gate model that could detect reaction forces from a rubble mound.

In addition to this study, we conducted a field experiment to prove the flap-gate breakwater's basic performance. The paper also describes the results of this experiment.


A flap-gate breakwater (hereafter called a flap-gate) for tsunami and storm surge protection was previously developed, and is expected to be used as a movable flap-type tsunami barrier. Figure l shows the flap-gate's mechanism. The 2011 Tohoku earthquake tsunami severely damaged many coastal defense structures, which are required to be resilient enough to not immediately collapse when subjected to tsunamis that exceed design levels. The flap-gate was initially developed through hydraulic model experiments (Kimura et al., 2009; Kiyomiya et al., 2006; Kiyomiya et al., 2007), and a field experiment was later conducted to consider its function, applicability, and effectiveness (Kimura et al., 2012).

The purpose of this study is to determine the flap-gate's resistance properties against tsunamis exceeding over the design level through an experiment that subjects a hydraulic model of the flap-gate to excessive external force. Moreover, we considered how a seepage flow formed in a rubble mound due to sea level differences between a port inside the flap-gate and the ocean outside would affect the bearing force of soil.

Figure 1 illustrates the flap-gate behavior during a tsunami or storm surge. The flap-gate, which usually lies on the seabed, rises above the sea surface due to its buoyancy, and then closes of the port or canal entrance. The flap-gate stands up to its prescribed angle (90° in Fig. 1 (c)) due to the water elevation outside of the flap-gate. Upper and lower tension rods support the forces of water pressure acting on the gates. Resistance plates between the lower tension rods, shown in Fig. 1 (c), reduce the speed at which the gate rises and reduce impacts on the tension rods when the gate is upright. A substructure, shown in Fig. 2 (a), holds gates, tension rods, and other movable equipment, and the weight of the substructure supports all forces caused by water elevation.

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