After the 2011 off the Pacific coast of Tohoku Earthquake Tsunami, coastal dikes in Japan are required to be improved their tenacity. It is reported that scours of coastal dikes caused by tsunami overflow mainly occurred at the toe of the landward slope. In this study, fixed- bed hydraulic physical experiments and numerical simulations were conducted to understand the distribution of local pressure on a dike under tsunami overflow. As a result of the experiments the negative pressure at the top of the slope was observed and it can be significantly reduced by the suggested small weir type countermeasures.
The tsunami generated by the 2011 off the Pacific coast of Tohoku earthquake caused catastrophic damage to coastal dikes in Japan. In order to prevent the lives and property from tsunami, the tenacity of coastal dikes s hould be improved to secure sufficient time as long as possible, at least long enough for the evacuation of residence near dikes. Kato et al. (2012) investigated the failed pattern of damaged dikes due to tsunami and reported that scours of coastal dikes caused by tsunami overflow mainly occurred at the toe of the landward slope of a dike. Yoshimori et al. (2015) conducted the hydraulic physical model tests and showed the scour of coastal dikes was reduced by sheet piles installed at the toe of the slope. Kato et al. (2013) proposed countermeasures to improve the tenacity of coastal dikes such as covering the back toe, making the slope milder, increasing the weight of armoring blocks and placing steel bars inside the parapet. It was also pointed out that the armoring blocks at the top of the landward slope could be lifted up and damaged by the uplift force induced by the pressure reduction at the top of the slope due to the high-speed flow. Kotake and Isobe (2012) also showed the armoring blocks at the top of the slope could be lifted up easier when the landward slope was steeper. According to the knowledge obtained by these researches, a reduction of negative pressure at the top of the slope by decreasing the speed of the flow can lower the scouring rate that eventually leads to improve the tenacity of coastal dikes.
