Outburst floods caused by landslide dams have brought devastating damage to various parts of the world. In July 2012, a landslide dam subsequent to the large-scale collapse of a mountain edifice occurred on Ambon Island in the eastern part of Indonesia. A height of the dam was approximately 140 m and a volume of dammed water was approximately 25 million m3. The dam failed about a year after its formation in July 2013, and a large-scale flood swept away two third of houses and buildings in a downstream village with a population of more than five thousand. Authors have conducted field reconnaissance survey several times before and after the outbreak of the landslide dam and installed interval recording cameras. We fortunately recorded images of the process of the outburst flood by one of the installed cameras, and video images which had been taken by the Ministry of Public Works of Indonesia and villagers were obtained. In this study, we derived events leading the progress of the outburst flood from obtained images and clarified the process of landslide dam outburst flood through the analysis of chronology of those events. Some of characteristic events, such as abrupt expansion of the width of water channel formed on the slope of the landslide dam and large-scale collapse of side bank of water channel was observed by the monitored images. The process of destruction of partially-built structures on the landslide dam, changes of longitudinal profile through the outburst flood and some other processes of the flood are discussed, and results of this paper and previous studies based on numerical models and channel experiments are compared.
If sediment disaster risk has increased because of such processes as natural damming of a river, the authority responsible for disaster prevention measures must judge, as promptly and accurately as possible within the framework of limited human resources, whether it is necessary to issue an evacuation order or advisory or not. That requires providing training so that the organization has efficient, uniformly trained personnel. Generally speaking, efficiency can be improved by increasing the ratio of finished work and decreasing the ratio of rework necessitated by calculation or other errors included in the obtained results. If the degree of effectiveness of such training is to be evaluated quantitatively, it is necessary to analyze the ratio of finished work and the ratio of rework in view of the time required for obtaining accurate results. In this study, a mathematical model consisting of a one-dimensional advection-diffusion equation that approximates Kolmogorov's forward equation was proposed as a means of estimating the time required for disaster response activities. The proposed mathematical model was applied to the training to be provided for emergency surveys conducted by the Ministry of Land, Infrastructure, Transport and Tourism to estimate the ratio of finished work (μ) and the ratio of rework (λ) in the cases where obtained results contain some kind of error. The calculation results thus obtained were μ=0.5505 and λ=0.4495 for work activities related to the estimation of the area flooded because of natural damming and μ=0.5519 and λ=0.4481 for work activities related to the estimation of the time of occurrence of a natural dam break.
Recently, it is reported that debris flows with woody debris cause damage. For example, jamming at bridges causing high water levels and flooding, or leading sudden high flow depth surges at downstream when blockaded woody debris break away from temporarily building structures and standing trees. This study is based on a flume experiment focus on how the woody debris causes or enlarges above mentioned phenomena. To investigate the woody debris factor, such as the shape and provenance of the wood, debris volumes and water flow, authors used hydraulic models that reproduced actual landforms. The main findings are as follows. (1) Debris woods accumulated when logs carried along by debris flows passes from forest area, then debris woods move in frontal part of the debris flow, and the flow with water and sediment follows. This causes temporary blockages on bridges in mountainous rivers. (2) The peak discharge that occurs after the blockages collapse is around 1.2 times higher than debris flow without blockage collapse.