Japan has a long stretch of coastal dykes along its shoreline to protect against storm surges, tsunamis and high wind waves. The 2011 Tohoku Earthquake and Tsunami caused serious damage to these coastal dykes. To improve the coastal dykes along the damaged coast and also to reconsider tsunami risks in other parts of the Japanese coast, it is important to understand the effect that a tsunami can have on a coastal dyke. The present paper thus aims to analyze a tsunami flow around a coastal dyke using a LES numerical model. To simulate both an impact phase and an overtopping phase of a tsunami flow, which were observed in the 2011 tsunami, a dam break flow and a pump flow were used to generate a tsunami-like flow in the numerical model. Based on the numerical results, the pressure and the velocity field around a coastal dyke were ascertained. These effects were considered to have a big influence on the dike failure in the 2011 tsunami, and thus it is recommended that the design of countermeasures should include a calculation of both of these parameters.