Morphological characteristics and conditions of drainage basins contributing to the formation of debris flow fans: an examination of regions with different rock strength using decision tree analysis

Abstract

Abstract. Debris flows cause severe disasters that can result in human casualties and the collapse of houses. The establishment of early warning systems in basins with high debris flow risks is needed to reduce the negative impacts of debris flow disasters. Because debris flows often form debris flow fans near the mouths of valleys, debris flow fans are regarded as important topographical elements that indicate the occurrence of debris flows. The presence or absence of a debris flow fan makes it possible to clarify the morphological conditions of the contributing area that has generated debris flows. These morphological conditions may depend on rock strength, which controls the weathering activity and grain size of sediments. In this study, we investigated the morphological conditions of a drainage basin that contribute to the formation of debris flow fans using decision tree analysis. The analysis was conducted at two sites with clear differences in rock strength due to geological processes: Neogene sedimentary rock and Paleogene accretionary complex sites. As a result of decision tree analysis using data sets containing a total of 158 basins, the thresholds of morphological parameters needed for forming debris flow fans differed depending on the geological features. When the relief ratio was less than 0.29 at the Paleogene accretionary complex site, coarse-grained sediments were less likely to pass out of the valley, resulting in the absence of debris flow fans. On the other hand, at Neogene sedimentary rock sites, short basins were determined to form debris flow fans, even if the relief ratio was less than 0.36, because the sediments were fine-grained and tended to flow downstream. In contrast, morphological factors that influence the presence or absence of debris flow fans were common at both sites. The first, second, and third most important morphological factors were the relief ratio, most frequent slope gradient, and basin length, respectively. Therefore, these morphological factors are considered important in evaluating debris flow risks. This study demonstrates that the decision tree analysis is an effective tool for determining the hierarchy and threshold of morphological factors that classify the presence or absence of debris flows that reach valley mouths.