Pipe hydrophone is an acoustic sensor that is widely used to measure sediment discharge in mountainous rivers in Japan. Monitoring data has been dramatically increased in the last decade. However, because the sound pressure of small colliding grain is smaller than that of the noise due to water flow, there is a lower limit of detectable grain diameter. Also because some of sediment grains do not collide against the pipe, the sediment discharge may be underestimated. Here we carried out the flume experiments on the same installation condition with general field measurements and measured the sound pressure of the noise due to water flow and that of the colliding grain. Then we analyzed a lower limit of detectable grain diameter and estimated the collision ratio by the same method Uchida et al. (2015) proposed. The results of this study are as follows. (1) On the experimental flume because the magnitude of the noise due to water flow was dependent on the noise near the water surface, the noise due to water flow became larger with the water depth became more shallow and the turbulence of the water surface became larger. (2) Based on the result, we estimated that on individual collision in case the diameter was 2 mm detecting probability was around 40% and in case the diameter was 3 mm detecting probability was more than 90%. On collective collision in case the diameter was 1 mm detecting probability was around 30% and in case the diameter was 2 mm detecting probability was almost 100% (3) In case dimensionless tractive force took a value within the range of 0.1≦τ ＊＜0.3 and the collision ratio ranged between 0.3 to 0.7.
A debris flow disaster can cause considerable damage in the stricken area, which can affect the people dwelling there. We investigated changes in communities each year after a disaster. We also proposed a method for calculating the monetary cost of living as a refugee, which causes long-lasting mental trauma. The results were as follows : (1) after a debris flow disaster, the community does not recover to its original situation, and people are forced to out-migrate to neighboring regions or public housing, as per the examples of Harihara, in Kagoshima Prefecture, and Minamata, in Kumamoto Prefecture, Japan. It became apparent that it is difficult for single elderly persons to rebuild residential housing ; (2) the recovery period from a debris flow disaster in the Minamata area was estimated to be around 10 years ; (3) when the method was applied to a sediment-related disaster hazard area in Aso, in Kumamoto Prefecture, the total cost of mental damage was estimated to be 221.2 million yen ; and (4) when the method was applied to the entire basin, which contains 68 streams that pose a danger of mud and debris flows, the monetary cost of living as a refugee, and the cost-benefit ratio of the long-lasting mental trauma, increased by 0.1.
Microtopographic examination was conducted to identify geomorphologic factors related to an onset of a deep-seated landslide at Kitagawa, Kochi where several deep-seated landslides occurred in 2011 and 2014 by heavy rainfalls. ELSA Map (Kokusai Kogyo, Co., Ltd.) with 1 m mesh was produced from LiDAR Data for the examination. The geology of studied area is made of alternative layers of mudstone and sandstone with faults and fractured zone in accretionary prism. Three kinds of microtopographic characteristics are identified for the slope with the possibility of the onset of deep-seated landslides. The first one is a gentle slope on the ridge with many steps. Landslides are prone to occur below the gentle slope on the ridge. The combination of the gentle slope on the ridge and existing landslides below it is also included in this case. The second one is the combination of small gentle slope at the middle of small ridge and steep slope below it. Several small gentle slopes can be found at almost same altitude in the middle of several ridges along a stream in this case. The third one is that a toe of a slope with landslide configuration is collapsed. The first case is dominant in studied area.
We developed a method for measuring detail channel morphology using a low elevation photographic scanning. This study was conducted in a 36-m step-pool channel segment in a headwater stream of the Ooborasawa monitoring watersheds. A plastic case that equipped with two digital cameras has been installed at a 2.2 m pole ahead. We took photos from 1.8 m above ground surface every 5 seconds and then obtained 3006 photos for 4 hours. Eleven ground control points were installed for measuring relative coordinates of the segment. In addition, we measured the number, height, and length of steps for examining the accuracy of data. Particle size distribution of channel substrate was obtained with 100 random sampling. PhotoScan Professional edition 1.0.0 was used to make a 3 D model, an orthophoto, and a digital surface model. A 5-cm contour was then developed using ArcGIS. Resolution of an obtained topographic model was 0.6 mm per pixel. Because minimum particle size measured by an orthophoto was 1 cm, approximately 88% (greater than 1 cm) of channel substrate was able to be estimated using this method. All of the step structures in the channel were identified. Estimated values of step heights by the topographic model were comparable to the measured values. Our method permits us developing a high-resolution topographical model in a headwater channel, which is important for understanding the dynamics of sediment movement.
Detection of volcanic phenomena associated with the eruption of Mt. Merapi in 2010 was examined through several analytical methods using ALOS images. The eruption triggered various phenomena as follows: pre- and post-volcanic deformation, widespread tephra fall, pyroclastic and debris flows, and induced sediment disasters around the volcano. The areas affected by each phenomenon could be extracted using ALOS images, and adequate analytical methods for each phenomenon were determined. In some cases, it was difficult to distinguish a particular area affected by multiple phenomena using only ALOS images. When various phenomena occurred for short periods after eruption, it was necessary to use not only ALOS images but also information on volcanic-eruption processes to extract the location of affected areas. These results suggest that satellite images provide useful information about phenomena associated with eruption, practically independent from weather conditions and magnitude of volcanic eruptions.
The landslides on April 16th 2016 in Kumamoto prefecture, especially in the Aso volcanic range, with intense earthquake of M 7.3 (maximum acceleration=1,791 gal) yielded countless instances of landslide and debris flow that induced tremendous damages and causalities in the area. Hence, field investigation and reconnaissance were conducted by the Japan Society of Erosion Control Engineering to delve into this sediment-related disasters. The various results and the information containing phenomena of slope movements, obtained through this investigation were reported as the primary report, mentioning damage, geologic-geomorphologic features and geo-technical characteristics of the landslides, vegetation effects on the slope instability phenomena, possibility of occurrence of secondary disasters, urgent measures for mitigating secondary disasters, and the efficiency of Sabo facilities in this disaster.