報告書番号

N05045

タイトル（和文）

大陸棚上における津波のソリトン分裂と砕波に関する研究

タイトル（英文）

Study on tsunami soliton fission and its breaking on continental shelf

概要 （図表や脚注は「報告書全文」に掲載しております）

津波のソリトン分裂現象と分裂波の砕波限界を把握するために，長さ200mの大型造波水路を用いて大陸棚上を伝播する津波の実験を行った．大陸棚は長さ約100mで，地形勾配は1/100，1/150，1/200と3つの条件でそれぞれ実施し，砕波近傍で波形を詳細に計測した．また，水位の時系列データから，砕波限界の指標である水面の空間波形や水表面流速波速比を算定する方法を提案し，実験データを適用した．その結果，砕波限界の水表面流速波速比は0.5～1.2と従来用いられていた水平床上の砕波限界値より大きいことが明らかになった．さらに，実験の成果を反映して，数値計算モデルの改良を行った．従来モデルでは，ソリトン分裂波に，分裂波と同程度の振幅の引波が先行する場合に，砕波水深と砕波波高を正しく評価できなかった．そこで，ソリトン分裂波発生を検知するために水位波形の曲率に着目したパラメターを提案し，次に砕波限界の水表面流速波速比を0.85に設定して，拡散型の砕波モデルの改良を行い，津波の浅水変形，ソリトン分裂，その砕波現象を精度良く再現できる数値計算モデルを得た．特に，ソリトン分裂波の砕波地点及び砕波波高の再現性が向上した．

概要 （英文）

To study tsunami soliton fission and split wave breaking, undistorted experiment was carried out for tsunami shoaling on a continental shelf in the LARGE WAVE FLUME, which is 205m long. Three models of the continental shelf were set up, which have 100m lengths, and 1/100, 1/150, and 1/200 slope respectively. Water surface elevations were measured across the flume, and ten or eleven wave gages were thickly installed around the point of wave breaking. We proposed a new method for calculating wave breaking criterion expressed by time series data of water surface elevation, and applied to the experiment. As a result, at the point of wave breaking, the ratio of surface water particle horizontal velocity to wave velocity is from 0.5 to 1.2. This range for breaker limit parameter is larger than one by the former study.Next we upgraded wave breaking model in numerical simulation based on nonlinear dispersion long wave theory. New parameter, which is consists of water thickness and curvature of water surface, was picked up to search for occurrence of soliton fission. It means intrinsically subtracted amount from ratio of water surface horizontal velocity to wave velocity by ratio of depth averaged horizontal velocity to water velocity. Criterion of wave breaking is the ratio of water surface horizontal velocity to wave velocity, which calculated by new equation expressed by water surface level and water depth. This modification can enhance reproducibility of tsunami propagation with breaking near coast even if large descent wave lead before adjacent soliton fission.

報告者

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