砂防学会誌
Online ISSN : 2187-4654
Print ISSN : 0286-8385
ISSN-L : 0286-8385
62 巻, 3 号
選択された号の論文の14件中1~14を表示しています
口絵〈災害報告〉
口絵〈観測の現場を訪ねて-16〉
論説
論文
  • 清水 収
    2009 年 62 巻 3 号 p. 3-13
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    Wood debris (WD) transport processes accompanied with the formation and failure of WD dams were studied in a small fourth-order basin with steep bedrock low-order channels in Hokkaido, Japan. The basin experienced three floods between 1992 and 2006. WD dams are wood debris accumulations that completely block channels and retain large volumes of sediment upstream. They are common in forested mountain streams with small bankfull widths, and affect transport of both WD and sediment in mountainous watersheds. Mapping and dimensional measurement of the WD dams in the study basin have revealed that many dams are distributed throughout first- and second-order channels (<10 m in width) and that these dams trap a relatively small amount of sediment, whereas fewer dams occur in third- and fourth-order channels (15-45 m in width). A dam in the fourth-order channel in the study basin was found to retain a huge amount of sediment along a 500-m reach of riverbed upstream. Half of all the WD dams in low-order (first- and second-order) channels form at the place of WD recruitment by streamside landsliding, and the other half form after WD is transported in channels by debris flows. Repeated mapping of WD dams in the low-order channels before and after flooding in 2003 and 2006 revealed that 90% erosion of dams occurs during large floods and 40% erosion occurs during middle-scale floods. Three stages of WD transport processes were identified : the first stage includes processes from WD recruitment through WD dam formation in low-order channels ; the second stage comprises failure of WD dams in low-order channels, WD transport downstream, and WD deposition in high-order channels (dam formation in some cases) ; and the third stage includes failure of stable WD dams in high-order channels, WD release, and downcutting of elevated riverbeds. In the study basin, sediment storage behind WD dams was found to account for 40-70% of all sediment stored in low-order channels and 50-60% of that in high-order channels. Therefore, formation and failure of WD dams that function as major sediment reservoirs significantly affect sediment transport in mountainous terrain
  • 鈴木 拓郎, 堀田 紀文, 宮本 邦明
    2009 年 62 巻 3 号 p. 14-22
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    Mass conservation equations for a debris-flow body and bed sediment as well as an equation of motion constitute the governing equations for debris flow. The erosion rate equation is generally added to close the governing equation system. The entrainment coefficient of the existing erosion rate equation is derived on the basis of only the balance between the riverbed gradient and the sediment concentration or the balance between the sediment concentration and the riverbed gradient. This set of equations can produce good results under the condition where the riverbed gradient changes gradually. However, some problems occurred when we applied this equation set to highly unsteady conditions, such as those in the upstream area of the sabo dam and the riverbed evolution with the change in the channel width. Therefore, a non-entrainment erosion rate equation is proposed. Numerical simulations of debris flow are performed by using an existing model along with our model. The results for the condition where the riverbed gradient changes gradually show that both models indicate similar tendencies. Further, our model can yield clear results for the highly unsteady conditions, such as those in the formation of the convex upward deposition shape caused by the sudden change in the riverbed gradient from steep to low and those in the riverbed evolution with the change of the channel width. These results were experimentally confirmed in past studies. Further, in the upstream area of the sabo dam, the results given by our model indicate that many sediments are deposited as low as the scale of debris flow. These results seem to clearly explain the actual phenomena.
報文
  • 内田 太郎, 松岡 暁, 松本 直樹, 松田 如水, 秋山 浩一, 田村 圭司, 一戸 欣也
    2009 年 62 巻 3 号 p. 23-29
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    Landslide dams occasionally induce catastrophic outburst floods or debris flows mostly by overtopping. However, adequate information about changes of landslide dam features and grain size distribution due to overtopping are still lacking. The 2008 Iwate and Miyagi inland earthquake triggered many landslide dams. Here we report the results of prompt survey for landslide dam features and grain size distribution just after the overtopping at one of the landslide dams triggered by the Iwate and Miyagi inland earthquake. Erosion widths ranged from 19 to 32 m and longitudinal riverbed gradient after the overtopping ranged from 2.4 to 7.5 degree. Also, we found that amour coating occurred at the stream bed obviously. Based on the observed width and peak discharge, it can be estimated that the value of alpha in the regime theory was smaller than 1.9.
  • 山田 孝, 森 健司, 沼本 晋也, 林 拙郎
    2009 年 62 巻 3 号 p. 30-38
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    Little is known about the actual situation of torrent bed scouring-induced debris flow generation, despite developing theoretical and experimental research. Herein we report subsurface flows discharged and spouted from cross-section of torrent bed material as well as overland flow generation by debris flow occurrence using an automated interval camera installed at the No.6 Sabo Dam construction site in the Nishinogaito torrent, Mount Fujiwara , Inabe-shi,Mie prefecture, Japan.
    Subsurface flows spouted from cross-section of torrent bed material during heavy rainfall, and their volumes increased as the amount of rainfall increased. In addition, a lot of sediment spouted from several holes created at cross-section of torrent bed materia with a subsurface flow prior to debris flow. Serial camera-footage analysis revealed a time difference between overland flow and debris flow. Subsurface flows may travel in several pipes developed in debris flow deposits, and water can concentrate in pipes during heavy rainfall, resulting debris flow generation.
  • 執印 康裕, 鈴木 雅一, 堀田 紀文, 田坂 聡明, 小川 紀一朗
    2009 年 62 巻 3 号 p. 39-46
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    Here we indicate a result of the effect of rainfall characteristics and soil strength on the potential for shallow landslides over large areas using a simplified physically based model. The model was based on 50-m grid digital elevation model of the Miyagawa upper river basin at Odai-cho, (a former Miyagawa village) in Mie prefecture. The model calculates the distribution of the shallow-infiltrated water level by Darcy's law and a safety factor estimated using infinite slope stability analysis for each 50-m grid cell. Response analysis of this model with a steady rainfall intensity showed that the area of the potential shallow landslide ratio increased with rainfall intensity, stabilizing at rainfall intensities of over 50 mm/hr. The spatial distribution of the potential shallow landslide area during steady rainfall was initially highest in the vicinity of the channel heads and hollows, expanding as one moved upstream. Response analysis using actual five heavy rainfall events data, revealed the potential for shallow landslide development decreased dramatically when apparent soil cohesion ranged between 3 to 9 kPa, indicating that the time rate change of the shallow landslide potential was specified by both temporal rainfall patterns and the value of apparent soil cohesion.
研究ノート
  • 小嶋 伸一, 小山内 信智, 西本 晴男, 小川 紀一朗, 松田 昌之
    2009 年 62 巻 3 号 p. 47-54
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    It is important to warn local inhabitants of sediment disasters and inform them of their types and sizes. It is necessary and important to develop a worldwide standard index to represent the types and sizes of sediment disasters. The authors carried out a questionnaire survey of prefectural office staffs, in order to understand their images of disasters. Based on the results of analysis of the questionnaire survey, the authors proposed a disaster damage index to represent the level of the sediment disasters using several damage factors concerning human societies such as numbers of dead and missing persons, injured persons, completely collapsed houses, half collapsed houses (including partially damaged).
技術ノート
  • 宮瀬 将之, 菊井 稔宏
    2009 年 62 巻 3 号 p. 55-61
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    It is rare case that local residents take evacuation, before sediment-related disaster occurred. It is pointed out the reason why people didn't take evacuation was because they had not experience of a sediment disaster, and they thought that their home was safer, it is called normalcy bias. Therefore, we analyzed the reason about the past disaster that it took evacuation, or no evacuation. As a result, it has been understood that it is important that the resident maintain consideration to the danger of sediment-related disaster. These supplement factor are the sediment-related disaster and disaster tradition, etc. We showed the only factor that did not evacuate is that local residents didn't recognize the danger of the sediment-related disaster. On the basis of analysis, we showed it about “the factor that local residents takes evacuation succeed". And we suggested that “the making of the check list to sense sediment-related disaster" and “the necessity of upbringing of the leader disaster prevention" were important.
災害報告
  • 古川 浩平, 海堀 正博, 久保田 哲也, 地頭薗 隆, 権田 豊, 杉原 成満, 林 真一郎, 池田 暁彦, 荒木 義則, 柏原 佳明
    2009 年 62 巻 3 号 p. 62-73
    発行日: 2009/09/15
    公開日: 2014/02/01
    ジャーナル オープンアクセス
    The deluge on July 21 2009 in Yamaguchi, with the area's exceptionally heavy rainfall of 297 mm, maximum intensity 72.5 mm/hr and countless traces of debris flow, resulted in tremendous damage and high number of causalities in the area. Hence, urgent field investigation and reconnaissance were conducted by the society to overview this sediment related disaster. Here, the various results and the information obtained through this investigation were reported, mentioning the damages, the meteorological condition featuring precipitation, geologic-geomorphologic features and hydraulic characteristics of the debris flows, vegetation effects, and the efficiency of Sabo facilities against the sediment runoff in this disaster.
現場情報コーナー
観測の現場を訪ねて-16
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