This paper presents an experimental evaluation method on the capture performance of debris flow by a steel open type Sabo dam (hereafter, steel open dam). First, the hydraulic tests for steel open dams are conducted to examine the outflow sediment, the capture height-time relation and the grain size distribution of outflow sediment by changing the riverbed slope and slit interval of dams. Herein, the capturing performance types by a steel open dam are proposed as the classification such as “complete blockage type”, “partial blockage type” and “control type”. It was found that the steel open dam with narrow slit interval showed “complete blockage type” which represents a good capture performance, regardless of riverbed slopes. It was also discovered that the capture height-time relation was a good measurement for the examination of capture process of debris flow. Second, the three different types of steel open dams in the most downstream are tested against three different scales of debris flow. It was confirmed that a new type steel open dam for the most downstream was proposed and showed good capture performance against different scales of debris flow.
Although the particle method is effective for high resolution simulation, it has a significantly large calculation time. Therefore, a coupling method of particle and grid-based simulations（PBS-GBS）for debris flows is developed in this study. In the connection part between the particle method（PBS）and the grid-based method, the flow rate, the transport concentration, and the water depth are converted. Numerical experiments were conducted under a constant gradient condition at 18 degrees and condition with a dam addition to it. A coupling method, PBS-GBS, and a standalone particle method, PBS, were used. Results of PBS-GBS show that sediment concentration and water depth in the connecting part are continuous. Moreover, even if the riverbed rises due to sediment deposition at the dam, the inflow wall particles increase their position accordingly, and the river bed position is also continuously connected. The temporal changes of flow rate and bulk concentration at the downstream end with PBS-GBS are almost equal to those with PBS. However, sediment discharge and transport concentration at the downstream end are lower when using the PBS method than those from the PBS-GBS method. This is because the grid-based method and the particle method have different evaluation methods of transport concentrations. In the particle method, the vertical distribution of the sediment concentration is calculated, therefore transport concentration is lower than bulk concentration. Result comparison of calculation time of PBS and PBS-GBS methods shows that the number of calculated particles decrease, and the calculation time is shortened by limiting the calculation region of the particle method. The calculation time is proportional to the number of particles. Therefore, PBS-GBS method is particularly effective in the case where the particle method is applied locally, such as evaluating the effect of the check dam.
It is important to understand the characteristics of sediment transport in gravel bed rivers for sediment disaster prevention. During a storm event due to Typhoon Lionrock (2016), large sediment discharge occurred in Pekerebetsu Creek, Hokkaido, Japan, where periglacial slopes were extensively observed. This study aims to find relationships between the sediment discharge and its control factors such as sediment supply and deposition, topographic features and bed materials. Digital aerial photographs were taken immediately after the event and were analyzed through comparison both with other aerial photographs taken of the same area and a 10 m digital terrain model simulated before the event. The supplied and deposited sediment volumes along the main and tributary channels were measured at the catchment scale. Small-scale debris flows were detected in the headwater, which developed into large scale flow due to the erosion of periglacial slope deposits with fine materials (<2 mm) at 35-45%. Sediment deposition and bank erosion along the main channel were caused by the debris or flood flows, and the river bed was 2-15 times wider than that before the event. Consequently, the supplied sediment volume from the river bed was dominant and accounted for 73.4% of the entire supplied sediment volume, in contrast to Ribira Creek where the supplied sediment volume from the river bed was 17.2%. Sediment discharge from the debris flow was estimated at 780,000 ㎥. Sediment estimated at 400,000 ㎥ was discharged at the outlet of the study catchment while sediment estimated at 630,000 ㎥ was trapped by the two erosion control dams. In the sediment sampling survey, fine materials (<2 mm) were selectively transported downstream, although a lot of coarse materials (≥2 mm) were deposited in the catchment.
Early morning on April 11, 2018, in the Yabakei area of Nakatsu city, Oita prefecture, an unexpected fatal landslide occurred without any usual causes such as rainfall and earthquake. According to the Oita prefecture's announcement, 4 houses were damaged, and 6 residents were killed. The rain observatory in the vicinity of the landslide slope recorded 4.5 mm and 1.5 mm rainfall on April 6 and 7, respectively. Rainfall had not been observed April 8 to April 11. With a simple in-situ survey, the landslide had a measured length of approximately 210 m, a height of about 120 m, a width of about 110 m, and a scarp of about 30 m in height. Currently, the major cause of the landslide was estimated to be weathered clay layer formed from tuff by mineral-groundwater reaction.