報告書番号

N09012

タイトル（和文）

ダム洪水吐の放流能力に及ぼす流木影響

タイトル（英文）

Effect of Driftwood on Discharge of Spillway

概要 （図表や脚注は「報告書全文」に掲載しております）

大規模出水や森林環境の劣化によって流木がダム貯水池に多量に流入し，その一部はダム洪水吐から流出しているが，場合によってはピアなどによって捕捉されてダム洪水吐を閉塞させる可能性がある．本研究では，クレストゲートを有するダム洪水吐を対象に，流木によるダム洪水吐の閉塞が放流能力に及ぼす影響を模型縮尺1/15相当の水理模型実験によって調べた．そして流木がクロスに絡み合わない仮想的な流木閉塞状態では，閉塞本数が多いほど水位は上昇するが，その水位上昇量には上限があることを確認した．また，同様の結果を，ゲート開操作に伴って閉塞状態となるダム貯水池一面に漂流する多量の流木群においても確認した．さらに，漂流状態と閉塞状態を判別する無次元パラメータ，ゲート直上流水面近傍流速，ゲート放流量算定式で構成される漂流・閉塞遷移水位の評価手法を提案するとともに，この評価手法を用いれば水位上昇量を安全側に評価できることを確認した．

概要 （英文）

This report deals with hydraulic effects of driftwoods flowing into dam reservoirs with heavy rain, because the driftwoods caught by the crest pier of the spillway could cause the water level rising. The maximum water level built by driftwoods stuck at the pier of the spillway is thoroughly studied using a 1/15-scaled hydraulic model. The relationship between number of the driftwoods captured by the pier and the water level rise is established with the experimental results, which indicates that the water level rises as the number of captured driftwoods increases, and that there are upper limits of water level in the two extreme cases. One is the case where driftwoods are captured with the same orientation parallel to each other without tangling, and another is the case where large amount of driftwoods are floating over a whole dam surface. In addition, an evaluation method is proposed to allow us to predict the maximum water level when the driftwood begins to be captured by the crest pier. This evaluation method is composed of a parameter to judge driftwood condition (floating or being captured), the velocity of the surface flow in front of the gate, and the equation of quantity of gate flow. The steadiness of the judgment parameter of transition was confirmed in various conditions of wood length, diameter and flow rate. Also the experimental results support the generality of the proposed evaluation method, since it turned out that the maximum water level realized with a lot of driftwoods captured is lower than that calculated by this evaluation method.

報告者

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