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.
Debris flow hydrographs at the outlet of a basin were calculated by numerical simulations. In order to discuss the differences among locations of debris flow generation, we set the locations at upper reaches, middle reaches and lower reaches of the basin. In addition to the settings, we set small landslide dams. Debris flow hydrographs generated in the middle reaches showed the sharpest increase of sediment concentration at the end of steep slope in the lower reaches but at the outlet of the basin hydrographs showed moderate increase. Results showed debris flow hydrographs change drastically by locations and their generated locations.
This study investigated storm event which triggered debris flows in Nagiso-Town, Nagano Prefecture, on July 9, 2014, by using antecedent precipitation indices (APIs) with various half-life times (HLTs). The storm was characterized by a large amount of precipitation during anteceding 2 days as well as a large rainfall intensity after a cumulative rainfall of more than 100 mm. At the occurrences of debris flows, combinations of APIs with HLTs of 13-173 h and APIs with HLTs of 0.1-2.0 h exceeded their past maximal values, and the storm was proved to be the biggest event for recent 30 years. We proposed a method to compute the minimal hourly rainfall which is required to cause the excesses of APIs over their past maxima. The value was estimated to be 34 mm at 40 minutes before the debris flow occurrences. Thus, the method was expected to provide useful information for early warning and evacuation against debris flow disasters.
In this article, we proposed an indirect method of measuring seepage of natural dam and considered the effect of seepage on prediction of water level in natural dam. It is one of the most important subjects to predict overtopping time of natural dam. It has not been obvious whether pondage depends on seepage in natural dam reservoir, because monitoring of inflow and seepage is difficult in natural river bed. First, we estimated seepage volume of natural dam with hydrological data. Second, we back-calculated inflow volume. The data was observed around a natural dam in Nagatono Valley in Kii Peninsula, from Sep. 2012 to March 2014. In our proposed method, it was assumed that seepage was followed Darcy's law. We compared observed inflow with back-calculated inflow by the saturated hydraulic conductivity, and obtained the same results. Using the back-calculated inflow, it clarified that 30-90% of inflow volume leaked from a natural dam, and 30-70% of seepage is observed as spring water from lower of the natural dam during rainfall period and 24 hours after stopping rainfall. The existence of substantial seepage in this natural dam shows necessity of consider in prediction of water level in natural dam.
On 9 July 2014, due to intensive rainfall of Typhoon Neoguri (T1408), slope failure and debris flow occurred at Nagiso town in Nagano prefecture. Especially at Nashizawa River, debris flow caused devastating damage on houses, railway tracks and Route 19. Tragically one person was killed and three injured. After the disaster, research team was immediately organized by Japan Society of Erosion Control Engineering with the aim of investigating the processes of generation, motion and deposition of debris flows, estimating extent and severity of damage from debris flow and evaluating the effect of, and damage on sabo-related facilities.