Variation in rainfall patterns triggering debris flow in the initiation zone of the Ichino-sawa torrent, Ohya landslide, Japan

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.