Debris flow is composed of mixed-size gravel, and the grain size distribution and concentration of sediment strongly affects the flow behavior. To predict more accurate debris flows, we need a numerical model that considers mixed-size gravel and can describe sediment sorting. We propose a more accurate method for predicting mixed-size debris flow behavior, from torrents to alluvial fans. We combined a debris flow model that can describe sediment sorting with an integration model, and proposed an integrated model for mixed-size gravel. Applying this model, we simulated debris flow on a virtual watershed under different conditions of inflow sediment distribution. Our results showed that an integrated model for mixed-size gravel provide a more rational description of debris flow in torrents and alluvial fans than models of uniform-sized gravel.
In this study, we tried to assess the factors that could affect the landslides occurrence (slope angle, forest practice and precipitation) in the eight sub basins of catchment of Miyagawa Dam, Mie prefecture, Japan. Our findings are summarized as follows : 1) Collapsed area ratio is proportional to mean slope angle. 2) In each sub basins, the collapsed area ratio is in proportion to collapsing possibility area ratio index (APhr (t)) that was derived from a prediction model of slope stability considering forest practice. 3) Collapsed area ratio has significant correlation with both daily maximum and hourly maximum precipitation in each observation period. 4) Based on these findings, a new model was proposed considering slope angle, forest practice conditions and precipitation. The model was successfully applied to predict the collapsed areas in the study basins for the last forty years.
The quality and quantity of sediment transport in watersheds must be evaluated for comprehensive sediment management in sediment transport systems. Field observations are thus important. Indirect methods, such as the use of turbidity sensors, to observe suspended and wash loads and the use of hydrophones, to observe bed load, have been applied recently in several catchments in Japan. We used these methods to collect data at four points in the Ashiaraidani watershed, in the district of the city of Takayama, Gifu Prefecture. This paper reports the results of analysis and three main observations. First, sediment discharge was recognized in each blanch and was influenced by sediment supply from riverbed and side banks. Second, we effectively estimated sediment transport routes during rainfall events by comparing the turbidity curves and changes in hydrophone pulses at two different observation points. Third, the sediment source area of suspended and wash loads was estimated by classifying the relationship between turbidity and water level. Thus, these indirect methods were successfully adapted to monitor sediment discharge in mountain streams.
To examine how to promote the evacuation of inhabitants, we investigated the evacuation of inhabitants in 5 major disasters in 2009-2010 using disaster reports provided by the Ministry of Land, Infrastructure, Transport and Tourism. We collected the information on evacuation, including the area, designation of the sediment-related disaster (special) warning area, time of issue of evacuation call, type of evacuation call (“evacuation order” or “evacuation instruction”), the total target households and inhabitants, and the total actual evacuees. The results suggested that the evacuation ratio (i. e., the ratio of the total actual evacuees to the total target inhabitants) in the individual areas decreases with increasing target households (especially > 50), that the type of evacuation call and the time of issue do not affect the evacuation ratio, and that designation of the sediment-related disaster (special) warning area can improve the evacuation ratio when the target households are ≤ 50. Hence the high evacuation ratio can be expected when (1) the sedimentrelated disaster (special) warning area is designated or (2) the evacuation call is issued for each “Chiban” or “Koaza” (small administrative block in Japan generally containing households ≤ 50).
Many analytical approaches for capture mechanism of woody debris were proposed by using Distinct Element Method with spherical assembled element, as well as cylindrical element or cylindrical assembled element. This paper presents an analytical approach of influence of surface shapes on the analysis for capture mechanism of woody debris. When the woody debris was modeled by spherical assembled element, larger amount of woody debris were captured than that of experimental results. It was caused that the debris was caught each other by the uneven surface made by connection of sphere elements. On the other hand, when the woody debris was modeled by cylindrical element or cylindrical assembled element, the experimental result of capture mechanism and capture probability were simulated well. It is pointed out that the smooth surface of cylindrical element can express the frictional effect between woody debris and structural member in the capture process very well.
On the Kii Peninsula, a number of large-scale collapses of landslide dams occurred in 1889 due to a typhoon that brought rainfall of over 1000 mm between August 19 and 20. More than 33 landslide dams were formed and most of them collapsed, causing serious damage. In 2011, many large-scale collapses occurred again due to Typhoon No.12 that lasted from August 30 to September 6, causing more than 17 river blockages and landslide dams. In this report, the authors examined the distribution characteristics of the 1889 and 2011 disasters, with the focus on the Totsukawa area in southern Nara Prefecture. It has been reported that 28 landslide dams were formed in this area in 1889 (total sediment volume : 200 million m3). In 2011, 13 river blockages (mostly partial) occurred in the same area (total sediment volume : 35 million m3), forming 4 landslide dams over 20 m in height.
Many catastrophic landslides occurred in the Kii Peninsula, following typhoon No. 12, 2011. Some of them occurred in areas had been labeled as landslides in the landslide map published by National Research Institute for Earth Science and Disaster Prevention (NIED). A comparison of 20 catastrophic landslides with areas greater than 0.01 km2 and the landslide map for the Kii Peninsula, showed that 12 of these landslides had occurred in areas or parts of areas labeled as landslides. Moreover, 7 out of 8 catastrophic landslides with areas greater than 0.1 km2 coincided with areas labeled as landslide in the landslide map. This research showed that the potential area of catastrophic landslides could be determined by analyzing geographical features using tools such as the landslide maps published by the NIED.