Erosion and Sedimentation Caused by Glacial Lake Outburst Floods in the Nepal and Bhutan Himalayas

摘要

Various erosion and sedimentation disasters have been induced by glacial lake outburst floods (GLOFs) occurring along various segments of rivers in the Nepal and Bhutan Himalayas. This paper discusses such phenomena from the point of view of GLOF hazard management. In a U-shaped valley formed by past glacia-tion, a GLOF occurs as a debris flow from breached moraines. GLOFs may cause the collapse of banks along hills and terraces of glacial and fluvioglacial origin by toe erosion, and the vegetation may not recover from the damage caused by the GLOFs for a long time and may act as a sediment source for the downstream river. The breaching of temporary dams by the sediment-loaded flow of a GLOF or during associated landslides could aggravate the secondary damage along narrow river courses in V-shaped valleys. In the Lesser and Greater Himalayas, a dense distribution of old deep-seated landslides and rock creep indicates the re-activation of landslides on slopes of rivers undercut by GLOFs. Woody debris may be produced from riparian forests in V-shaped valleys. In the case of gently sloping rivers in the basins of such valleys, bank erosion and the river blockage caused by woody debris from those valleys often pose serious risks, particularly, in terraces along the lower reaches of the rivers. Sand deposition in rivers and irrigation canals may adversely affect the downstream alluvial plain areas. In the case of topographical units such as river terraces that are prone to bank erosion, planning should be done for constructing bridges with appropriate dimensions. Geomorphological classification maps of the regions along rivers and DEM maps of the basins and alluvial plains are essential for planning for GLOF hazard management.