Lü Ruren, Li Deji
The Tangbulang Gully located in Gongbujiangda, Tibet has a drainage area of 75.67km2 almost over 4800 m a.s.l., where mountains are high, gullies deep, slopes precipitous arid frostweathering and snow erosion intensive. There have been several glaciations above 4600 m a.s.l. since Quaternary Period, leaving behind 2-3 lateral and terminal moraine banks. The watershed 9.40 km2 in area, is occupied by existing glaciers and snow-covers. There are 7 major glacial end moraine lakes there. The Damenla-hai Ice Lake ranks first in size. It is located at the source of the left branch on the upper reaches of the main gully. At about 11 p.m.,on Sept. 26, 1964, about 5 million m3 material at the front of the Damenlahai Glacier plunged into the lake in the slumping mode, banking up the water level and stirring up a wave more than 10 m high. The wave overrode the terminal moraine bank and caused the collapse of the original overflow outlet. The peak discharge after the collapse reached 2010 m3/sec. The flood dashed through all obstacles along its path, mixed and carried quantities of loose material. When it reached the mouth of the gully, a viscous debris flow with specific gravity of 2.0 t/m3 and discharge of 2812.0 m3/sec was produced. It destroyed the forest and pasture in the gully, submerged a 2.2 km long section of the Sichuan-Xizang Highway, 100 mu of farmland, 12 houses, and killed one person below the gully mouth. Boulders were everywhere, and the volume of residual deposits was 1.63 million m3. The Niyang River was blocked up for 10 hours, leaving backwater as long as 3 km. The actual measurement shows that the maximum depth of water when the Damenlahai Ice-lake dam burst was 17 m and the total discharge reached 3,698 million m3.After exploration and analysis, we think that it is impossible to occur ice lake burst debris flow as in 1964. Only Dajia Glacier may advance, collapse or slide into the lake, thus Dajia Ice Lake will burst and may induce small flood or debris flow in the middle and lower reaches of Tang-bulang Gully.As to the glacial debris flow of Xizang, the Damenlahai ice lake burst debris flow occurred in the late autumn of 1964 was not an accidental phenomenon, but relative to climatic fluctation of the 1960s. The large glacial accumulation was the principal cause of its advance. Climatic fluctuation changing from relative cold and moist to dry and warm and intensive glacial thawing caused the immediate collapse or slide of glacial front. To all ice lakes having possibility of such danger proper drainage of water from the ice lake and lowering its level beforehand can eliminate the happening of ice lake burst catastrophe.
