Characteristics of debris flow caused by outburst of glacial lake in Boqu river, Xizang, China, 1981

Abstract

A catastrophic outburst of a moraine dammed lake at the head of a tributary of Boqu river on the S-flank of the Tibetan Himalayas took place in 1981. The flood with a peak discharge of 15920 m³/s at the breach and 2316 m³/s at Bharabise, more than 50 km downstream, was 16 times larger than the average annual flood of the river, and caused a large scale sediment morement. Spreading over 50 km or more along Boqu river, the debris flow involved a total of about 4 mio. m³ of solid material. The debris flow valley may be divided into three sections according to erosion and deposition: the section of vertical erosion, the section of lateral erosion-flow passage, and the section of lateral erosion-deposition. Half of the total solid materials was derived from the vertical erosion in the first section and the other half from the lateral erosion in the latter two sections. This debris flow was a sediment-laminated movement under the conditions of an extraordinary flood. The moving layer of sediment may be estimated as being 4 to 10 m in thickness.

Debris flow deposits with well developed morphologies are chiefly scattered along the last section of the debris flow valley. The most significant morphologies include the levee (a leteral deposit), the stone pile (a flow surge deposit) and the residual terrace (the residue of the flow). The sedimentology of these deposits is characteristicly coarse grain and of mixed composition with a lack of bedding and sorting, the presence of inverse grading, parallelism of long axes and imbrication. All these features imply an accordance with the “grain flow” concept developed by R. A. Bagnold in the mechanics of sediment movement.