Forensic Profiling Analogue Approach for the Investigation of Natural Hazards – A Case Study from Onokoba Elementary School, Unzen Volcano, Japan

Internal temperature variations of pyroclastic flows and their deposits are arguably the most challenging data to acquire. As a preliminary study of the temperature variation inside pyroclastic flows, the remains of Onokoba Elementary School (Shimabara, Japan) were investigated. The elementary school is located in the close vicinity of Unzen volcano and was hit by one of the largest pyroclastic flows during the latest active period of the volcano on 15th of September 1991. This present preliminary study aims to determine the temperature exposure of various portion of the school building using field-forensic and urban geology. Natural hazard methods applied to the damaged materials exposed to high temperature have generated a temperature fingerprint the maximum temperature distribution. Charred wooden parts and plastic gutters installed on the schoolyard-side faced of the building turns out to be the most useful temperature indicators. The various deformation and alterations of the studied materials show significant differences in the temperature exposed to. Such differences on the second-floor section (between 75-110°C and 120-150°C) and on the first-floor section (above 435-557°C) of the building do not simply imply significant temperature heterogeneity in short distance (some ten to ≤100 m) inside the pyroclastic flow, but also points toward the possible effects of the building architecture on some key dynamic parameter of the pyroclastic flow. Such information may be important for planning future hazard mitigation actions

Laboratory investigation of the effects of grain size on the dynamics of debris flows: Measurement of pore fluid pressure in an open channel

The dynamics of debris flow depend on internal stress components, such as particle–particle stress, the stress exerted by pore water, and interactions between particles and pore water. Although dominant internal stress components depend on the grain size composition, the effects of grain size on the dynamics of debris flow are not fully understood. To investigate the effects of grain size on the dynamics of debris flows, pore fluid pressures were measured in an open channel experiment. In the experiment, monodisperse debris flows were triggered for five different grain sizes: 0.2, 0.8, 1.3, 2.2, and 2.9 mm. The pore fluid pressures in debris flows of 0.2 mm grains had larger excess pressures over the hydrostatic pressure, and were close to the total normal stress, while those of other grain sizes had smaller excess pressures and were relatively close to the hydrostatic pressure. Comparing the measured friction factors and theoretical ones for stony debris flows, particle–particle stress dominated in debris flows, except for 0.2 mm grains, and the measured excess pore pressures could be explained by the Reynolds stress of pore fluid due to shear by particles in laminar motion. By contrast, particle–particle stress did not dominate in debris flows of grain size 0.2 mm, and a large portion of the particles was in suspension affected by turbulence. These differences in flow dynamics may correspond to the flow transition from laminar to turbulent flow described by the threshold of relative flow depth, which is the ratio of the flow depth to grain size

Spatial Distribution of Drifted-wood Hazard following the July 2017 Sediment-hazards in the Akatani river, Fukuoka Prefecture, Japan

In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had increasing impacts on communities in Japan, because of climate change and structural immobilism in a changing and ageing society. Decreasing rural population lowers the human vulnerability in mountains, but hazards can still leave the mountain to the plains and sea, potentially carrying drifted-wood. The aim of the paper is to measure the distribution of wood-debris deposits created by the 2017 Asakura disaster and to rethink the distribution and spatial extension of associated disaster-risk zoning. For this purpose, the authors: (1) digitized and measured the distribution of drifted-wood, (2) statistically analyzed its distribution and (3) calculated the potential impact force of individual drifted timber as a minimal value. The results have shown that there is a shortening of the wood debris as they travel downstream and that the geomorphology has an important control over deposition zones. The result of momentum calculation for different stems’ length show spatially differentiated hazard-zones, which limit different disaster-risk potentials. From the present finding, we can state that we (1) need to develop separate strategies for sediments and wood debris (2) and for wood hazards, zonations can be generated depending on the location and the size of the deposited trees that differs spatially in a watershed

ヨウレイリン ノ ジュカンシャダンリョウ ソクテイ ノ タメ ノ オオガタ ウリョウケイ ノ ケンテイ

幼齢林における樹冠遮断量の測定上の問題点を解決できるような大型雨量計のデザインを提示し,実際に作製した大型雨量計を用いてその実用性を検討した。雨量計の重要な要素である受水面積は,我が国の一般的な幼齢林の植栽密度,植物への影響,メンテナンス性,既往の大型雨量計を用いた樹冠遮断研究における受水面積の決定基準などを参考にして,約5m^2が適当であると決めた。大型雨量計の実用性は,次の各実験により確認した。まず,降雨強度の大きい降雨イベントにおいても雨量計からの排水水量を正確に測定できるようするために,本研究で用いた500mLの転倒マス型量水計を対象に大流量を含めた流量検定を行い,流量と1転倒に要する水量との関係を求めた。次に,大型雨量計の初期損失量を求めたところ0.2mm以下であり,大型雨量計の排水性は良好であった。さらに,大型雨量計の受水面積を厳密に求めるために,自然降雨を対象にした大型雨量計と貯留型雨量計の比較観測を行った。転倒マス型量水計の検定結果を考慮して補正した大型雨量計からの排水水量と,貯留型雨量計が示した雨量の関係は,良好な直線関係を示しており,その直線の傾きから大型雨量計の厳密な受水面積を決めた。また,その直線関係は,通常の降雨イベントだけでなく強度の大きい降雨イベントにおいても成り立っていたため,本研究で提示した大型雨量計は降雨強度の大きいイベントにも耐えうる実用性の高い雨量計であることがわかった。We proposed a design of a large plastic-sheet net-rainfall gauge for measurements of interception loss at young forests in Japan, and calibrated the proposed gauge with a standard rain gauge. The appropriate orifice area of the proposed gauge was suggested to be approximately 5m^2, considering the prevalent stand density of young forests in Japan, negative influences of water-logging on nearby plants, and the standard method to determine orifice areas in previous studies. The calibration of the proposed gauge was made by the following three steps. First, in order to examine robustness of the proposed gauge under strong rain conditions, we made a dynamic calibration of an attached tipping bucket flow meter under a variety of inflow water fluxes. This calibration provided a correction function revealing accurate water volume which is necessary to register a tip of the flow meter, as a function of the inflow water flux. Second, we found that initial loss of water, being retained on the proposed gauge, was less than 0.2 mm, indicating good drainage of the proposed gauge. Third, to calculate the accurate orifice areas of the proposed gauges, we made comparative measurements of rainfall using three proposed gauges and a standard gauge. Good linear relationships were found between total drainage water volumes from the proposed gauges, which were corrected by the above correction function, and rainfall observed by the standard gauge. By making use of the slopes of lines, we determined the accurate orifice areas of the proposed gauges. The line successfully fitted to a rainfall event with strong rainfall intensity, indicating that the proposed gauge was applicable to monitor rainfall interception even in strong rainfall conditions

Deposits’ Morphology of the 2018 Hokkaido Iburi-Tobu Earthquake Mass Movements from LiDAR & Aerial Photographs

On 6 September at 03:08 a.m. local time, a 33 km deep earthquake underneath the Iburi mountains triggered more than 7000 co-seismic mass movements within 25 km of the epicenter. Most of the mass movements occurred in complex terrain and became coalescent. However, a total of 59 mass movements occurred as discrete events and stopped on the semi-horizontal valley floor. Using this case study, the authors aimed to define planar and vertical parameters to (1) compare the geometrical parameters with rain-triggered mass movements and (2) to extend existing datasets used for hazards and disaster risk purposes. To reach these objectives, the methodology relies on LiDAR data flown in the aftermath of the earthquake as well as aerial photographs. Using a Geographical Information System (GIS), planform and vertical parameters were extracted from the DEM in order to calculate the relationship between areas and volume, between the Fahrböschung and the volume of the deposits, and to discuss the relationship between the deposit slope surface and the effective stress of the deposit. Results have shown that the relation S=k[Vd]2/3 (where S is the surface area of a deposit and Vd the volume, and k a scalar that is function of S) is k = 2.1842ln(S) − 10.167 with a R2 of 0.52, with less variability in deposits left by valley-confined processes compared to open-slope processes. The Fahrböschung for events that started as valley-confined mass-movements was Fc = −0.043ln(D) + 0.7082, with a R2 of 0.5, while for open-slope mass-movements, the Fo = −0.046ln(D) + 0.7088 with a R2 of 0.52. The “T-values”, as defined by Takahashi (2014), are displaying values as high as nine times that of the values for experimental rainfall debris-flow, signifying that the effective stress is higher than in rain-triggered counterparts, which have an increased pore pressure due to the need for further water in the material to be moving. For co-seismic debris-flows and other co-seismic mass movements it is the ground acceleration that “fluidizes” the material. The maxima found in this study are as high as 3.75

Laboratory investigation of the effects of grain size on the dynamics of debris flows: Measurement of pore fluid pressure in an open channel

The dynamics of debris flow depend on internal stress components, such as particle–particle stress, the stress exerted by pore water, and interactions between particles and pore water. Although dominant internal stress components depend on the grain size composition, the effects of grain size on the dynamics of debris flow are not fully understood. To investigate the effects of grain size on the dynamics of debris flows, pore fluid pressures were measured in an open channel experiment. In the experiment, monodisperse debris flows were triggered for five different grain sizes: 0.2, 0.8, 1.3, 2.2, and 2.9 mm. The pore fluid pressures in debris flows of 0.2 mm grains had larger excess pressures over the hydrostatic pressure, and were close to the total normal stress, while those of other grain sizes had smaller excess pressures and were relatively close to the hydrostatic pressure. Comparing the measured friction factors and theoretical ones for stony debris flows, particle–particle stress dominated in debris flows, except for 0.2 mm grains, and the measured excess pore pressures could be explained by the Reynolds stress of pore fluid due to shear by particles in laminar motion. By contrast, particle–particle stress did not dominate in debris flows of grain size 0.2 mm, and a large portion of the particles was in suspension affected by turbulence. These differences in flow dynamics may correspond to the flow transition from laminar to turbulent flow described by the threshold of relative flow depth, which is the ratio of the flow depth to grain size