Measuring the River Ice Floe Sizes Broken by 2011 Tohoku Pacific-Coast Earthquake Tsunami

The tsunami of 2011 Tohoku Pacific-Coast Earthquake broke river ice and transported thousands of meter-long ice floes in rivers of Hokkaido, Northern Japan. The objectives of this study are to investigate sizes and areas of ice floes from photographs obtained during the field survey conducted two and three days after the tsunami and, on that basis, to estimate the impact forces of ice floes transported by tsunami on river structures. This study mainly focused on the Mu River, where the largest number of ice floes was found. Ice floe sizes were measured using composite photographs of field survey that were superimposed both on 10 m grid data and river planforms. The river mouth of the Mu River is about 500 kilometers away from the epicenter of the earthquake, but the first wave with an amplitude of only 0.4 m above the initial river water level seems to have caused breaking of river ice and its jamming at 2.6 km away from the river mouth. Downstream from the ice jam, ice floe deposits were found only around sluice ways and on floodplains with relatively small sizes. Likewise, the tsunami propagated 2.4 km upstream from the jam causing the river ice cover to crack. In the upstream regions, relatively large ice floes were floating around bridge piers with the sizes ranging from several meters up to about 40 m. From the results of image analysis of survey photographs, the authors have also estimated the impulsive force due to tsunami-induced collision of ice floes and river structures, based on a formula for measuring impact forces by Ikeno et al. (2003). This report provides valuable information about the criterion for the design of river structures assuming winter-time tsunami disasters.