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Characteristics of Debris Flow Caused by Outburst of Glacial Lakes on the Boqu River in Xizang, China

Xu Daoming ( Lanzhou Institute of Gluciology and Geocryology, Chinese Academy of Sciences)  
An outburst catstrophe originated from a moraine dammed lake at the head of a tributary of the Boqu River on the southern slope of the Tibetan Himalayas in 1981. The outburst flood with a peak discharge of 15920m3/s at the outburst breach and 2316m3/s at Bharabise 50 km down stream away from the breach, 16 times higher than the average annual maximum discharge of the river, caused a large scale sediment movement-debris flow.Along the over 50 km long section of valley, the debris flow involved a total solid materials of about 4 mil. m3 during the process. The debris flow valley can be divided into three sections; the section of debris flow formation, the section of flow passage and solid materials replenishment and the section of debris flow deposition. Half of the total solid materials came from the vertical erosion in the first section and the other half from the lateral erosion in the latter two sections. This debris flow is considered as the sediment-laminated movement under the condition of an extraordinary flood; the moving layer of sediments is estimated at a range of 4—10m thick. 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 levees ( side deposits ) , boulder piles (a flow surge deposits ) and residual terrace ( flow remainders). The sedimen-tology of these deposits is characterized by the grain-coarsed and grain-mixed conposition, lack of bedding and sorting and presence of inverse grading and long axial parallel and imbrication. All these features show that the mechanism of debris flow movement is accordant with that of "grain flow" developed by R.A.Bagnold. A brief discussion about the survey and control of dangerous glacial lakes is presented at the end of this paper.
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