Experimental Study on Glacial Lake Outburst Floods Due to Waves Overtopping and Erosion of Moraine Dam

Abstract

氷河湖決壊洪水はその下流域において大規模な洪水を引き起こす恐れがあり，その80%は氷河や氷河湖(モレーンダム湖)内に浸水する氷河先端部の崩壊に起因する波によって生じる，統合型数値モデルの開発に向けて，(1)氷河の崩壊によって生じる波の予測，(2)越流およびモレーンダム侵食による流出ハイドログラフの予測，(3)洪水危険度評価のための下流域の洪水と堆砂過程の予測は不可欠であり，本研究では越流波と侵食によるモレーンダムの決壊に着目し室内実験を行った。室内実験では，氷河の崩壊規模，堤体形状と材料，ダム湖の水位を変化させ，越流波と侵食による流出ハイドログラフが複数のピークを有することを確認した。さらに，崩落規模とモレーンダム湖の水位が同様の条件のもと三角形形状と台形形状の堤体ではともに流出ハイドログラフの初期ピークは同様の傾向を示したが，その後のピークの規模と時刻は堤体形状と材料に応じて異なることを確認した。

Glacial Lake Outburst Floods (GLOFs) may cause floods of great magnitudes in the downstream river reaches. About 80% of GLOFs (based on 20 cases with known failure mechanisms) were initiated by displacement waves from ice avalanches that collapsed into the lakes from hanging or calving glaciers and rock avalanches. Therefore, an integrated model is essential for the i) prediction of waves generated by ice/rock avalanche, ii) prediction of outflow hydrograph due to waves overtopping and erosion of moraine dam and iii) flow and sediment routing in the downstream for flood risk assessment. Toward development of an integrated model, this study focuses on experimental study of moraine dam failure due to waves overtopping and erosion. Extensive laboratory experiments are carried out by varying size of ice/rock avalanche, shape of the dam, dam material and lake water level. The outflow hydrographs produced by waves overtopping and erosion consist multiple peaks. For the same falling block and lake water level, the initial peaks are similar for both triangular and trapezoidal dams. However, magnitude and timing of subsequent peaks are different according to dam shapes and dam materials.