浅水域での軽石群の漂流過程に関する実験的検討

抄録

Pumice from large volcanic eruptions can float for months and spread across coasts and ports, disrupting navigation, water intakes, aquaculture, and shoreline work. In shallow water, however, we still lack clear knowledge of how pumice travels and how it starts moving again after it settles, because its properties vary and long drift times are hard to reproduce in the lab. We addressed this gap with controlled experiments in a large wind-tunnel wave flume. We examined two topics: (1) how much pumice passes through wave-driven flow around coastal structures (the structure-crossing discharge, or throughput), and (2) how deposits that build up near those structures are re-mobilized. Across a range of wind speeds, wave conditions, grain sizes, and representative structure shapes, two clear controls emerged. First, both the speed at which pumice moves and the amount that crosses structures increase strongly with wind speed. This shows that, in surf and harbor zones, wind is the main driver of pumice transport. Second, whether deposits are re-mobilized, and how strongly, depends on structure geometry and wind characteristics. In practice, local design and prevailing winds set the threshold for exporting pumice from pockets such as gaps and flanks. We also turn these findings into simple tools. Air-dry pumice density provides a practical estimator for how long pumice will stay afloat and the resulting exposure window. In addition, a residual-fraction predictor combines wind statistics with grain-size information to estimate what portion of deposits are likely to remain after a re-mobilization event. Together, these experiments provide a reproducible data set and an evaluation framework that improve forecasts of nearshore pumice transport and accumulation, supporting intake protection, port operations, and cleanup planning after future eruptions.