STUDY ON IMPACT FORCE OF RIVER ICE FLOES CAUSED BY PROPAGATING TSUNAMIS IN A FROZEN RIVER

The tsunami of 2011 Tohoku Pacific-Coast Earthquake broke river ice and transported thousands of meter-long ice floes in the Mu River, Hokkaido, Japan. The objectives of this study are to investigate sizes and areas of ice floes from photographs obtained during the field survey after the tsunami and, on that basis, to estimate the impact forces of ice floes transported by tsunami on river structures. Ice floe sizes were measured using composite photographs of field survey that were superimposed both on 10-m grid data and river planforms. 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. The impact forces were approximately 22 to 55 kN on average and 100 to 300 kN as a maximum when ice floes collide with a structure. This paper provides valuable information about the criterion for the design of river structures assuming winter-time tsunami disasters