Inundated area and the time of the debris flows triggered by large-scale shallow landslides in Izu Oshima Island on 16 October, 2013

Abstract

This article is intended to clarify the inundated area and the time of the debris flows triggered by large-scale shallow landslides in Izu Oshima Island on 16 October, 2013. A large number of shallow landslides occurred on slopes consisting of the volcanic sediment, and debris flows which contain fine particles such as volcanic ash flooded down wide area. In addition, it is characterized that debris flows traveled over a small ridge and widened the damaged area because of the undeveloped topography. It is important to analyze the situation of debris flow disaster peculiar to a volcanic area for the countermeasures in other similar areas. First, we conducted field investigation to designate the inundation area in detail, such as measuring the height of flood mark on a wall of houses. Next, interviews to municipal officials, fire authorities and inhabitants were carried out to clarify when the debris flow damage occurred in each districts. As a result, the inundation area of the debris flows was widespread from upstream to the shore, and the sedimentation more than one meter depth was mostly around the vicinity of Okanesawa stream. In the other area nearer the sea, sedimentation was equal or less than 50 cm depth. As a result of interviews, damages occurred 2 : 20-2 : 30 in Kandatsu district that is most upper area and it occurred about 2 : 30-3 : 00 in the middle stream area. It is suggested that four times of surge flow hit the area around Motomachi Bridge in the downstream around 3 : 00 and 3 : 30. We concluded that the damaged area widened from Kandatsu district where the first event occurred to the downstream area sequentially during one to two hours.