Tsunami-induced local scouring at the landward toe of a coastal dike and the effectiveness of countermeasures to improve its tenacity are investigated using a numerical model that considers fluid-sediment-seabed interactions. From a comparison between experimental data and numerical results, the predictive capability of the model is demonstrated in terms of water surface elevations and final scour profiles. From the numerical results, it is found that armor blocks below the crown and berm of the landward slope of the dike receive large landward and upward force because of an increase in air pressure inside the dike and a decrease in water pressure induced by flow separation, suggesting that these blocks would be vulnerable against tsunami overflow. The numerical results also show that protection blocks covering the landward side of the dike move the scouring area landward and reduce its maximum depth, suggesting that they would be effective to extend the time before beginning the damage of the body of the dike.