It is apprehensive for the sea level rise due to global warming. The sea level rise increases vulnerability to inundation in coastal areas due to storm surges and high waves. In this study, it is investigated that a mobile storm surge barrier as one countermeasure against the storm surges and high waves. The mobile storm surge barrier is installed at a mouth of a harbor. Concretely, a flap gate is taken up. It can emergently close the mouth of the harbor in order to protect both of land areas and water areas from the storm surges and high waves without using obstacles to ship navigation. It has also a merit that is not to disturb seawater exchange because of it stored on a sea bed under normal circumstances.
　In this study, hydraulic model experiments are conducted to investigate the wave transmission behind the gate and wave forces acting on it. The experimental results shows availability of the wave forces evaluated by the Goda formula for the vertical gate and inclined gate toward the inside of harbor. The Coda formula is usually used for evaluation of the wave pressure acting on composite breakwaters. For the inclined gate toward the outside of harbor, the wave forces are about 1.2 - 1.4 times as large as the wave forces evaluated by the Goda formula. The wave transmission rate is lower for the inclined gate toward the outside of harbor than for the inclined gate toward the inside of harbor. The gate whose height is enough to reduce the wave transmission rate results in small difference of the waver transmission rate by directions of inclination of the gate.
　In order to examine the wave transmission and wave forces, the numerical model, CADMAS-SURF, is improved. The computational results are good agreement with the experimental results for the wave transmission rate and wave forces.
　Finally, the flap gate against a condition of the storm surge and high wave is structurally designed, especially for the vertical gate. Moreover, cost of the gate is estimated.