Delayed Survey of the 2011 Tohoku Tsunami in the Former Exclusion Zone in Minami-Soma, Fukushima Prefecture

Abstract

Post-tsunami field surveys in the Minami-Soma exclusion zone in the Fukushima Prefecture were delayed for 15 months after the 2011 Tohoku tsunami. The area was subject to access restrictions until June 2012 due to high radiation levels caused by the meltdown at the Fukushima Dai-ichi Nuclear Power Plant. The distribution of the measured tsunami heights is presented in combination with observed infrastructure damage. The enhanced tsunami heights in the areas along the shoreline are attributed to wave reflection, funneling and splash-up at cliffs and seawalls, as well as the increased flow resistance as the tsunami plowed through coastal pine-tree forests. Consequently, large tsunami heights exceeding 10 m were limited to areas within 500 m from the shoreline. Onshore profiles of the maximum inundation levels were dependent on inland topography: tsunami heights increased inland in steep V-shaped valleys, while decaying with inundation distance along flat coastal plains. Tsunami flood levels in the coastal plains are affected by the extent of seawall damage: coastal flood levels are higher behind completely destroyed seawalls than behind partially damaged coastal defenses. Remnant seawalls provided valuable lessons to be implemented in future designs of tenacious structures based on the Japanese concept of ‘nebari’ representing resiliency to endure tsunami overflow as the original design height is exceeded.