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.

Many large-scale disasters have occurred in mountainous rivers due to the transport of woody debris. A retention facility installed within the river channel that retains such woody debris is expected to be a useful method for preventing these disasters. A numerical model that accurately calculates woody debris behavior is necessary for the efficient design of a debris retention facility. The writers have developed a new 3-D numerical analysis model to calculate the movement of wood pieces in rivers. This paper introduces two verifications conducted to improve the accuracy of this numerical model.

This paper reviews the proposed models and previous experimental results focusing on elucidating the mechanism of particle size segregation within the collective sediment-transport flows such as a debris flow and a two-layer flow composed of water and collective transport sediment. These are the foundations of addressing the urgent disaster prevention issues of countermeasures toward a simultaneous inundation by sediment and flood in Japan. The constitutive laws governing the flows composed of mixed particle sizes and the mechanisms of particle size segregation are often considered as independent factors mutually influencing by changing of particle size distribution. Many proposed theories on particle size segregation are based on reverse grading, but there is no decision on whether its formation is caused by the upward movement of coarser particles and the downward movement of finer particles in the flow’s interior (or both). Based on the proposed models and previous experimental results, we suggested that the dimensionless shear stress, τ_＊, which can simply represent the flow characteristics, serves as an indicator for evaluating particle size segregation in a debris flow. Using τ_＊ as an indicator for the flow composed of mixed particle sizes may also contribute to understand the sediment-transport mechanisms at the connection area between the steep stream where collective sediment-transport flows are formed and the river where individual sediment-transport flows are formed.