Abstract Rainfall patterns that trigger debris flows may vary depending on the amount of sediment storage in their initiation zones. However, the current literature did not fully explore the relationship between the rainfall patterns triggering debris flows and changes in sediment storage. In this study, we investigated the variation in rainfall patterns for debris-flow occurrences via field-based, four-year monitoring in a steep mountainous torrent (the Ichino-sawa torrent) in Japan. In this catchment, the sediment storage changes seasonally owing to the sediment discharge by debris flow from summer to fall and sediment recharge due to freeze–thaw cycles during winter. We found that the readily mobilized sediment was locally accumulated in response to rainfall above the 5-mm 10-min−1 threshold. Subsequent debris-flow surges caused the entrainment of the accumulated sediment, which led to their development and propagation. Accordingly, the rainfall patterns that triggered debris flow tended to include longer consecutive periods above the rainfall threshold compared with the rainfall patterns that did not trigger debris flow. However, the decrease in sediment storage due to previous debris-flow events reduced the period above the threshold required to trigger debris flows. Hence, the decrease in sediment storage during summer to fall seasons reduced the rainfall threshold for debris-flow occurrences. Conversely, after sediment recharge during the subsequent winter, a relatively long period above the rainfall threshold was again necessary for debris-flow occurrences. Therefore, the rainfall pattern that triggers debris flow varies in response to the cycle of sediment discharge and recharge.

中文翻译：

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日本大屋山体滑坡 Ichino-sawa 洪流起始带引发泥石流的降雨模式变化

摘要 引发泥石流的降雨模式可能因泥石流起始区的沉积物储存量而异。然而，目前的文献并没有充分探讨引发泥石流的降雨模式与沉积物储存变化之间的关系。在这项研究中，我们通过对日本陡峭山地洪流（Ichino-sawa 洪流）进行为期四年的实地监测，调查了泥石流发生的降雨模式的变化。在该流域，由于夏季到秋季泥石流的排沙和冬季冻融循环造成的沉积物补给，沉积物储存量随季节变化。我们发现容易移动的沉积物在降雨量超过 5 毫米 10 分钟-1 阈值时会局部积聚。随后的泥石流涌动导致堆积的沉积物被夹带，从而导致它们的发展和传播。因此，与未触发泥石流的降雨模式相比，触发泥石流的降雨模式往往包括更长的连续高于降雨阈值的时期。然而，由于先前的泥石流事件导致沉积物储存量的减少，使触发泥石流所需的时间缩短了。因此，夏季到秋季沉积物储存量的减少降低了泥石流发生的降雨阈值。相反，在随后的冬季沉积物补给后，泥石流发生再次需要高于降雨阈值的相对较长时间。所以，