In the 2011 off the Pacific coast of Tohoku Earthquake, at many places, coastal dykes fully collapsed by deep over-topping tsunami. In this study, the resistance of coastal dyke against deep over-topping tsunami was examined by conducting a series of model experiments. In the model dykes, inclina-tion of upstream and downstream slopes, covering system of the dyke such as the thickness of surface gravel layers or the opening size between the slope protection panels, way of reinforcing by geogrids such as length of the geogrids inside of the dykes were changed. In the model experiments, the height of the model dykes was 10cm, width of the flume was 20cm. Flow rates applied in the experiment were in-creased step by step. The over-flow depth against dykes increased with increasing unit water flow rate. It was found that GR coastal dykes having the following structures can exhibit high resistance against ero-sion of the backfill caused by deep over-topping tsunami: the inclination of the upstream slope is gentle while it of the downstream slopes is steep; the cover system should include gravel and panels; and the geogrid layers are arranged over the full width of the embankment and the reinforcement layers are con-nected to the panels.
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