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Sediment transported from debris‐flow initiation zones is typically stored in a topographic feature called a debris‐flow fan, the formation process of which governs secondary sediment transport further downstream. Although sediment transport from debris‐flow fans can impact sediment regimes and change landforms, the determinants of progressive fan formation are not well constrained. To identify such determinants, this study monitored debris flows and performed topographic surveys of debris‐flow fans in the Ichino‐sawa torrent (Japan) during 2016–2017. In this period, eight debris flows occurred, two of which eroded the existing fan and formed a new channel with a short recurrence interval (~40 days). Consequently, these two cases induced substantial sediment transport further downstream from the fan. The examined rainfall indices did not provide a threshold for diagnosing the occurrence of such sediment transport. Debris flows with a large flow depth and a long duration led to changes in the runout direction and subsequently formed new channels. Before these processes, the existing channel was backfilled and plugged by previous debris flows, forming a steep fan surface around the fan apex. The results suggest that increasing the magnitude and the duration of debris flows potentially triggers sediment transport from fans coupled with channel plugging. The annual sediment transport from the fan exceeded almost all sediment yields of the world rivers and was found comparable with that linked with volcanic eruptions and their aftermath. Thus, the fan‐formation process can induce substantial sediment transport, independent of volcanic perturbations and extreme climatic events, and is dependent on the sediment supply from repeated occurrence of debris flows in the initiation zones. [ABSTRACT FROM AUTHOR]

The dynamics of debris flow depend on internal stress components, such as particle–particle stress, the stress exerted by pore water, and interactions between particles and pore water. Although dominant internal stress components depend on the grain size composition, the effects of grain size on the dynamics of debris flow are not fully understood. To investigate the effects of grain size on the dynamics of debris flows, pore fluid pressures were measured in an open channel experiment. In the experiment, monodisperse debris flows were triggered for five different grain sizes: 0.2, 0.8, 1.3, 2.2, and 2.9 mm. The pore fluid pressures in debris flows of 0.2 mm grains had larger excess pressures over the hydrostatic pressure, and were close to the total normal stress, while those of other grain sizes had smaller excess pressures and were relatively close to the hydrostatic pressure. Comparing the measured friction factors and theoretical ones for stony debris flows, particle–particle stress dominated in debris flows, except for 0.2 mm grains, and the measured excess pore pressures could be explained by the Reynolds stress of pore fluid due to shear by particles in laminar motion. By contrast, particle–particle stress did not dominate in debris flows of grain size 0.2 mm, and a large portion of the particles was in suspension affected by turbulence. These differences in flow dynamics may correspond to the flow transition from laminar to turbulent flow described by the threshold of relative flow depth, which is the ratio of the flow depth to grain size.

The long‐term behaviour of radiocaesium (137Cs) activity concentrations in forest ecosystems and their downstream impacts remain important issues in the deciduous broadleaf forests of Fukushima, Japan following the Fukushima Daiichi Nuclear Power Plant accident. To predict 137Cs cycling and discharge in the forest ecosystem, it is important to understand the spatial dynamics of the 137Cs inventory and transport along hillslopes. Therefore, we observed the spatial distribution of the 137Cs inventory and 137Cs transport via sediment and litter of a deciduous forest hillslope in Fukushima, Japan in 2016 and 2017 and examined how the spatial distribution of 137Cs inventory was formed using a mass balance model. In 2017, the 137Cs activity concentration was significantly greater in the downslope riparian area (455 kBq/m2) than in the upslope ridge area (179 kBq/m2). Annual 137Cs transport within litter and sediment contributed <0.5% to the current 137Cs inventory and cannot explain the current spatial variation of 137Cs inventory on the hillslope. The mass balance model results showed that if the initial 137Cs deposition was distributed uniformly in 2011, the spatial distribution of the hillslope 137Cs inventory was influenced mainly by the movement of leaf litter with a high 137Cs activity concentration. [ABSTRACT FROM AUTHOR]