One dimensional bed deformation model is very useful in obtaining long-term and long-reach bed deformation characteristics. However, sediment discharge will be underestimated by assuming a rectangular cross-section. Furthermore, when cohesive material is exposed at scoured areas in rivers, erosion of bed tends to be suppressed. Sediment transport rate is also suppressed on bed composed of cohesive material. In the present study, in order to improve the estimation of the sediment transport rate, the model that we suggest here does not assume cross-sections as rectangular shapes ; the bed elevation change is considered in the transverse direction but the water surface elevation is constant in the transverse direction. Furthermore, sediment transport and bed deformation characteristics on bed composed of cohesive material are discussed with attention focused on sediment sorting, based on the results obtained from numerical model of bed deformation and the model is applied to the Tonle Sap River in Cambodia. The results show that the change in sediment discharge with bed configuration is calculated automatically using the suggested model. The sediment discharge obtained by the suggested model is greater than that using a rectangular cross-section model. Furthermore, the transverse change in sediment size has been reproduced by considering the sediment transport width change. The temporal change of sediment size distribution on bed with cohesive material is faster than that on bed with non-cohesive material, because sediment is transported to downstream area without bed erosion process.
The need for low-cost spur roads with a simple structure is rapidly increasing. Cracks and collapse are caused by the insufficient compaction of embankments on spur roads. A simple method is required for inspecting embankments on low-cost spur roads. Therefore, an inexpensive portable dynamic penetrometer known as the Tottori FK-type penetrometer was developed. Penetration tests were carried out at the center and shoulder of eight spur roads geologically characterized by granite. The results are summarized as follows. 1) When a weight is dropped five times, the penetration depth exceeds 25 cm at the site where a crack has already occurred. 2) The cracks most likely occur when the penetration depth at the shoulder is more than three times greater than that at the center. 3) In the case of 2), the crack occurs at a position near the shoulder. 4) When the penetration depth exceeds 25 cm at both the center and the shoulder, the crack occurs at a position near the center. On the basis of the results stated above, we proposed an inspection method that divides the compaction condition of embankments into four categories. The collapse of the shoulder actually occurred in the part of the shoulder that was categorized as weak according to the proposed inspection method. We confirmed that the proposed method can be adopted to evaluate the structural strength of embankments on spur roads.
Shutter sabo dams let sediment flow past on typical days ; at other times, they retain sediment that may cause trouble downstream. This paper describes the conditions under which shutter sabo dams are applicable. These dams can be used in the bedload transport reaches of most torrents. Fixed-open-type sabo dams are appropriate in torrents where debris flows are infrequent, e. g., once every few decades or longer. In torrents where the ordinary sediment discharge causes problems downstream, closed-type sabo dams should be installed and must be excavated regularly. The operation of the shutters is also discussed. The shutter should be set or closed when new landslides occur or when natural landslide dams form upstream and create sediment discharge that may cause sedimentation problems downstream.
The 2011 off the Pacific coast of Tohoku Earthquake (M 9.0) occurred at 14 : 36, March 11, 2011. This main shock and series of aftershock gave destructive damage to large area from Aomori Pref. to Shizuoka Pref. in Japan. The most sensational damage was given by Tsunami at the coastal area from Aomori Pref. to Chiba Pref. while many landslides occurred and gave damages at Fukushima, Tochigi, and Ibaraki Pref.. This paper reports the results of field survey of landslides occurred at Shirakawa, Fukushima Pref. and Nasukarasuyama, Tochigi Pref.. In this area, the surface of hillside is covered with pyroclastic deposits from Mt.Nasu, so that many of the landslides occurred at the layer of pyroclastic deposits. Slip surfaces of Landslides observed at this survey were located within the pumice or loam layer. These landslides were characterized by long distance of landslide mass movement. The long distance movement of landslide mass is supposed to be due to generation of excess pore water pressure at the slip surface, although rainfall amount at January to March is relatively small. The mechanism of the generation of excess pore water should be examined.
In this paper, we introduced landslides induced by the northern Nagano Prefecture earthquake on March 12, 2011. The landslides occurred in an area where covered by 2-3 m depth of snow. Due to this situation, the landslides mass were displaced long distance mixing with large amount of snow. Moreover, in the case of the earthquake was triggered by the Mianohara fault, landslides distribution analysis revealed that in the study area, most of landslides occurred on the hanging wall of the source fault, within a distance of about 14.5 km from the fault.