Glacial Lake Outburst Events in the Bhutan Himalayas

摘要

Maintaining proper preparedness for GLOF hazards and drawing attention to the task of mitigation are important goals, necessitating exploration of past cases of outbursts. The frequency of GLOF occurrences is still unknown, because major outbursts which cause significant damage downstream are rare and records have been kept on them for only the past several decades. We have to collect as much data on past outburst events as possible, including unpublished and previously unknown incidents. As is obvious in the traces of known GLOFs, an outburst event leaves typical topographical and sedimentological features, i.e., 1) V-shaped trenches, 2) huge debris fan depositions and 3) subsequent devastated river beds. Hence, these features can be used as proof of past outburst events. We describe the 2009 Tshojo flood as the most recent case of a GLOF in the Bhutan Himalayas. The flood, which was initiated by dam leakage and water splashing on the surface from en-/sub-glacial passage, was a potentially dangerous hazard. Attention to such outburst events from invisible sources will be required in the future. As for evaluating the frequency of GLOF incidents, besides the six cases reported in previous studies, we revealed a total of 15 other outburst cases in the Bhutan Himalayas using field survey data; Corona KH-4A, Hexagon KH9-9, Landsat7/ETM+, and ALOS/PRISM satellite data; and images from Google Earth. These 21 cases were found on the Tibetan branch of the Kuri Chu, and the Chamkhar Chu, Pho Chu, Mo Chu and Soe Chu rivers. Lake outbursts at the foot of cliffs with hanging glaciers were the most frequent cases, accounting for ten of the GLOFs. Seventeen of the 21 cases occurred before the 1970s, while four cases were counted during the period from the 1970s to 2010. Hence, the current frequency of outburst occurrences does not seem to have increased. Further research is urged, covering minor outburst events, and the scope has to be broadened at least to the Sikkim and Nepal Himalayas.