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

Abstract. Debris flows cause severe disasters, such as human damage and the collapse of houses. Establishment of the early warning systems in the basins with high debris flow risks is needed to reduce 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 the rock strength, which controls 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 clearly differences in rock strength due to geological sedimentation processes: Neogene sedimentary rocks and Paleogene accretionary prism sites. As a result of decision tree analysis using data sets of total 158 basins, the thresholds of morphological parameters needed for forming debris flow fans differed depending on the geological features. In the Paleogene accretionary prism site, when the relief ratio was less than 0.29, coarse-grained sediments were less likely to pass out from the valley, resulting in the absence of debris flow fans. On the other hand, in Neogene sedimentary rocks 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 demonstrated that the decision tree analysis is an effective tool to obtain hierarchy and threshold of morphological factors that classifies the presence and absence of debris flows reaching valley mouths.