報告書番号

N08085

タイトル（和文）

津波による地形変化予測数値モデルの適用性―流砂量式の特性と矩形港湾実験の再現―

タイトル（英文）

Applicability of numerical model for seabed topography changes by tsunami flow - Analysis of formulae for sediment transport and simulations in a rectagular harbor -

概要 （図表や脚注は「報告書全文」に掲載しております）

数値計算モデルの掃流砂量式と巻き上げ量式の地形変化に及ぼす影響を検討し，以下のことが明らかになった．掃流砂による浸食・堆積効果は水面勾配とフルード数で規定される．津波波形前面では，掃流砂はFr<6/7で堆積に，Fr>6/7で浸食に働く．浮遊砂となる巻き上げ量が掃流砂より地形変化への影響が大きく，その効果は10倍以上である．
当所の提案した流砂量モデルによる数値計算モデルを0.08mm粒径の砂を用いた矩形港湾模型実験に適用したところ，港内中心部の堆積量と港口の浸食量について，高橋ら(1999)のモデルよりも高精度に再現した．
次に，実機スケールを想定して，模型の大きさを100倍に，海岸の一般的な砂の粒径0.2mmの矩形港湾を想定し，数値計算モデルを適用した．上限濃度1％と5％のケースを比較したところ，上限濃度が大きいほど浸食深さや堆積高さが大きく，実機スケールに本数値計算モデルを適用した場合に，上限濃度の値が地形変化に大きな影響を与える可能性があることが明らかになった．

概要 （英文）

Characteristics of formulae for bed-load transport and pick-up rate in suspended transport are investigated in order to clarify the impact on seabed topography changes by tsunami flow. The impact by bed-load transport was depended on Froude number and water surface slope. Bed-load transport causes deposition under Fr < 6/7 and erosion under Fr > 6/7 at face front of tsunami wave. Pick-up rate has more predominant influences for seabed topography changes than that of one brought by bed-load transport.
2-D Numerical simulations with formulae by Ikeno et.al were carried out to simulate topography changes around harbor by tsunami flow in the flume. The result indicated that the numerical model is more applicable than a numerical model with previous formulae for estimation of deposit and erosion by topography changes. It is for this reason that the formula of pick-up rate is adaptable for wide-range diameter of sand, from 0.08mm to 0.2mm.
Upper limit of suspended sediment concentration is needed to set due to avoid overlarge concentration in the numerical model. Comparison between numerical results in a real scale with 1% and 5 % upper limits clearly shows topography changes have a deep relevance with the upper limit value. The upper limit value is one of dominant factors for evaluating seabed topography changes by the 2-D Numerical simulations with the formulae by Ikeno et.al in a real scale.

報告者

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