Search

我が国では梅雨や台風時に土砂災害が多発しており，土砂災害の中でも人的・物的被害が大きくなるものとして土石流がある。土石流の発生・発達域は一般的に急勾配であり，物理メカニズムからは不飽和土石流の存在が推定されるが，現地で観測した報告はほとんどない。しかし，土石流の発生・発達域における飽和・不飽和土石流の発生実態を明らかにしない限り，発生流下予測を適切に行うことはできない。そこで，本研究では土石流段波の波高と飽和帯の厚さの比を観測することで，飽和・不飽和土石流の流下形態を調べた。その結果，土石流段波の発生場所が上流であるほど不飽和土石流段波が発生しやすい傾向がみられた。, 本研究では科研費(18H02235，18K18917，17H02029)の助成を受けたものである。

大規模崩壊地から流下する土石流は，下流での直接的な被害を引き起こす場合があるため，流下規模の推定が防災や流域管理において重要だと考えられる。しかしながら，複雑な地形を有する大規模崩壊地での土石流の動態は，未だに理解が不十分である。そこで，大規模崩壊地（静岡県大谷崩）において，土石流と雨量の観測，土石流発生前後の地形測量を実施し，土石流の発達と地形変化の関係について検討した。その結果，土石流発生後には，土石流による侵食や堆積，支流や側岸部からの土砂供給によって複雑な地形変化が生じることが分かった。また，この地形変化は，その後の土石流の発達に影響を及ぼしていると考えられた。

It is essential to consider the fluidity of a debris-flow front when calculating its impact. Here, we flume-tested monogranular and bigranular debris flows, and compared the results to those of numerical simulations. We used sand particles with diameters of 0.29 and 0.14 cm at two mixing ratios, of 50% and 50% (5:5), and 30% and 70% (3:7), respectively. Particle segregation was recorded using a high-speed video camera. We evaluated the fronts of debris flows at 0.5-s intervals. We then numerically simulated one- dimensional debris flows under the same conditions, and we used the mean particle diameter when simulating mixed-diameter flows. For monogranular debris flows, the experimental and simulated results were in good agreement in terms of flow depth, front velocity, and flux, but the bigranular debris flows were not well-simulated; the simulated flow depth was less than that found experimentally, and the front velocity and flux were greater. The differences may be attributable to the fact that the dominant shear stress was caused by the concentration of smaller sediment particles in the lower flow layers; such inverse gradations were detected in the debris flow bodies. In this situation, most shear stress is supported by smaller particles in the lower layers; the debris-flow characteristics become similar to those of monogranular flows. Consequently, the calculated front velocities were underestimated; particle segregation at the front of bigranular debris flows did not affect fluidity either initially or over time.

Debris flows generally includes a wide range of grain sizes, in which fine sediment behaves as a fluid phase rather than as a solid phase and enlarges the pore fluid density. Although in existing models fine sediment constantly behave as fluid phase from initiation to deposition, previous researches have reported that behavior of fine sediment can vary through debris-flow propagations depending on the kinematic conditions (i.e., relation of turbulence and the settling velocity of the particles). To test the effects of this transitional behavior of fine sediment and compare with existing models, we conduct numerical simulations of debris flows with bidisperse granular materials, employing two models for the behavior of small particles: (i) all small particles constantly behave as a fluid phase (Model I); and (ii) the ratio of small particles behaving as a fluid phase varies depending on the kinematic conditions (Model II). In the simulations, we used an inclined channel with erodible bed at the upper stream end of the reach, where debris flows initiate by supplying water. Varying the inclination from 15�� to 20��, we measured the time series of discharges, flow depths, sediment concentrations and pore fluid densities at the downstream end. Hydrographs of the two models are significantly different at higher slopes, with a sharp peak at the front of debris flows in Model I and relatively moderate peak in Model II. These differences are caused by higher pore fluid densities from the front to the tail of debris flows in Model I, in contrast to lower pore fluid densities in Model II, where not all of small particles behave as a fluid phase. This infers that discharge rate of debris flows can be overestimated especially at higher slopes if the transitional behavior of fine sediment is not considered.

A modified moving particles simulation model (MPS-DF) to simulate inundation and sediment deposition of debris flows is presented. This model is based on the moving particles semi-implicit (MPS) method, which was originally used for incompressible viscous fluid flows with free surfaces. In the MPS-DF model, the constitutive equations of Egashira is introduced to the MPS method. In Egashira���s theory, debris flows are treated as a continuum and sand grains are expressed using sediment concentration. Thus, each particle has a variable sediment concentration value. In this study, we tested the applicability of the MPS-DF model for the formation process of alluvial fans. For this purpose, flume experiment was conducted. The experimental flume consisted of a straight channel 6.0m long and 0.1m wide, with an inclination of 15��, connected to an outflow plain. The inclination of the outflow plain decreased gradually from 12�� to 3��. At the straight channel, 5.0m long erodible bed with a thickness of 0.2m was present. Water was supplied from upper end for 60 s. at the rate of 3,000 cm3/s and debris flow was generated by entraining the erodible bed. Debris flow inundated and deposited sediment at the outflow plain and an alluvial fan was formed. Numerical simulations were also performed with the MPS-DF as well as a depth-integrated method based on the shallow water equations (2D simulation). 2D Simulation results of alluvial fan shape and flooding area were laterally spread and significantly different from those of experiment. The results of the MPS-DF were more similar to experimental results. Natural channels and lateral levees were formed as well as experiment. However, the alluvial fan shape of MPS-DF was slightly wider than that created during the experiment. This is thought to be due to the behavior of pore water of deposied layer, such as the seepage of water out of the deposited layer once the deposition process has been completed.

2011年3月11日，東北太平洋沖地震が発生，地震に伴い発生した津波により，東北太平洋側地域に甚大な被害をもたらした。また，福島県白河市葉ノ木平地区等では，人的な被害を伴う土砂災害が発生した。被害は決して小さなものばかりではなかったが，全体的に見ると，地震の規模や記録された地震動の加速度の大きさにもかかわらず，山地部における斜面崩壊等の土砂災害は少数であり，規模も小さかった。また，人的被害を伴う土砂災害は，福島・栃木県に限定されていた。この原因について検討するために，地震発生時の土壌水分状態と地震の加速度を考慮した静的な斜面安定解析を行い，土壌水分状態が斜面安定性に与える影響を検討した。その結果，地震発生時期の先行降雨が少なかったことで，斜面安定性は大きく低下しなかったことが判明，仮に夏季の降雨イベント後に地震が発生していたら，実際よりも大きな被害がもたらされたと結論付けられた。, The pacific side of Tohoku region, Japan was suffered by a devastating earthquake on 11th March, 2011. Several hillside slopes were collapsed due to the earthquake in inland region, and a number of people were also killed by the sediment related disaster. Contrasting with the devastating earthquake which suffered wide area, the landslides were limited in its number, area, and size. To understand the reasons of this limited landslide occurrences, distribution of the maximum earthquake acceleration was investigated. The effects of the slope topographical properties such as slope gradient, direction, and bedrock geomorphology were also investigated by the field observation. Numerical experiments were also conducted to demonstrate the effects of the slope topographical properties and soil moisture conditions on the landslide occurrences. From the field investigations and numerical simulations, it is elucidated that the reasons of the limited landslide occurrences are 1) limited suffering area of strong acceleration by the earthquake and 2) relatively dry soil moisture condition of slopes at the earthquake occurrence.

インドネシアのメラピ火山は，2010年10月26日に噴火し，その後11月まで大小の噴火が継続した。火砕流や土石流による死者は約400人にのぼる大災害であった。生産土砂量は１億４千万m3と推算されており，それらは火砕流または火山灰として南斜面と南西斜面に流出した。著者らは噴火後2010年11月，12月，2011年1月および2012年1月に調査を行い，とくに，火砕流が流出した南斜面流域と火山灰が流出した南西斜面流域での土砂災害の違いに着目して，その原因，特性，対策について検討した。その結果，南斜面では，火砕流の流下・堆積による河川の上流部での地形変化により，降雨流出特性が顕著に変化し，これまで洪水の少なかった河川で洪水による橋梁の流失が生じたことや，火山灰が堆積した南西斜面では，火山灰堆積物のクラスト化によって降雨流出率が増加したことが原因で大洪水が発生し，土砂災害が生じたと考えられる事例があることがわかった。, Eruptions of Mt. Merapi, Yogyakarta, Indonesia produced a lot of pyroclastic flows in 2010. The pyroclastic flows directly killed more than three hundred people in south slope area and the topographical change due to the pyroclastic flows caused some severe floods in Opak River where little flood had occurred before the eruption. The widely deposited volcanic ash on south-west slope caused several large floods up to the end of rainy season in 2011. Particularly, the change in rainfall runoff process due to the deposition of volcanic ash seems to be one of the reasons why the large floods occurred frequently after the eruptions. As a result a lot of infrastructures such as roads, bridges and sabo structures were severely damaged in the tributaries of Progo River and Opak River.

This study examined whether accurately simulating vertical air temperature (AT) distribution within forest canopies is essential for predicting vertical photosynthesis and transpiration distribution using multilayer canopy models. Inspecting earlier observational studies that reported vertical AT distribution within forest canopies, we showed that the common vertical AT difference within forest canopies was lower than 3.0℃. We showed, using a leaf-scale transpiration-photosynthesis model, that a 3.0℃ AT difference caused smaller differences in leaf- scale photosynthesis and transpiration rates than a common vertical difference in photosynthetic active radiation (PAR) intensity within forest canopies when AT was higher than ca. 15℃. While, the AT difference caused larger differences in leaf-scale photosynthetic and transpiration rates than the PAR difference when AT was lower than ca. 10℃. However, the ranges in the rates with changing AT by 3.0℃ were comparable with predictability of a leaf-scale transpiration-photosynthesis model. Thus, we conclude that accurately simulating AT distribution is not essential at this stage for calculating vertical photosynthesis and transpiration distribution using multilayer canopy models., 本研究では、林内気温鉛直分布を正確に再現することが、林内光合成・蒸散鉛直分布を多層モデルで推定するのに不可欠かどうかを調べた。筆者らは既存文献を踏査して、林内で計測される鉛直方向の気温差が通常3.0℃以下であることを示した。つづいて、単葉スケールの蒸散・光合成モデルによる計算によってつぎのことを示した。気温が約15℃以上のとき、3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、林内で通常計測される光合成有効放射量の鉛直方向の差が引き起こす光合成・蒸散量の違いよりも小さい。一方、気温が約10℃以下のとき、3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、林内で通常計測される光合成有効放射量の鉛直方向の差が引き起こす光合成・蒸散量の違いよりも大きいが3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、単葉スケールの蒸散・光合成モデルの予測精度と同程度である。したがって、現時点において、林内気温鉛直分布を正確に再現することは、林内光合成・蒸散鉛直分布を多層モデルで推定するのに不可欠ではないと結論した。

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.