Tsunami simulation and boulder transport across contrasting coastal geomorphologies in Sanriku, Japan

Tsunami boulders serve as crucial indicators for reconstructing past tsunami events, yet their full transport process remains poorly understood. This study investigates tsunami boulder transport at two geomorphically distinct field sites in Japan affected by the 2011 tsunami, integrating field data with a validated tsunami simulation to track wave parameters at every transport stage. The simulation was validated against eyewitness accounts, local reports, and offshore GPS buoy data. Near the shoreline at the Settai site simulated flow velocities reached a maximum of 7.55 m/s, compared to 6.51 m/s at Hoshiyama beach. Inland at Settai, a maximum 15.56 m/s flow velocity and 17.49 m flow depth were recorded just inland of the partially destroyed barrier. Boulder transport information was compiled from previous studies alongside new field data to assess how tsunamis interact with coastal boulder deposits in different geomorphological settings. Comparison of boulder lithologies, shapes, and distributions revealed similarities in wave influence but differences in deposition patterns due to variations in slope. Commonly used initiation of motion equations were then tested against simulated data. For example, Boulder #1 at Hoshiyama beach had an A-axis of 7.35 m and a minimum calculated velocity for initiation of motion of 6.46 m/s, whilst the tsunami simulation estimated a maximum flow velocity of 6.51 m/s. Although these values are similar, the boulder's position on top of other large clasts suggests that greater energy would have been required for transport and emplacement. Overall, the comparisons showed that while recent refinements have significantly improved the accuracy of the equations, they may still overestimate the minimum flow velocity required for boulder transport. By combining detailed field observations with numerical modelling, this study provides a rare case study capturing parameters at all boulder transport stages, offering a critical reference for future tsunami boulder research and model development.