Shallow western end of Tsho Rolpa
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Shallow western end of Tsho Rolpa GLOFs: the past predicts the futureNotes from Bridges-PRTD 1999 |
To appreciate the need for attention to such hazards as loom in the upper Rolwaling valley, as well as to understand the nature of the potential event, it is useful to recite some of the details of unmitigated disasters.
The following paragraphs are excerpted from Ives' summary of the ICIMOD report (1986):
On 4 August 1985, a glacial lake [Dig Tsho, below Langmoche Glacier in Khumbu] drained suddenly and sent a 10 to 15 metre high surge of water and debris down the Bhote Koshi and Dudh Koshi rivers, for more thsan 90 km. An estimated 1 mill. m3 of water was released, creating an initial peak discharge of 2,000 m3/sec; two to four times the magnitude of maximum floods due to heavy monsoon rains. This spectacular natural event destroyed the nearly completed Namche Small Hydel Project [being built by the Austrian "donor agency" Oeko-Himal], at a cost of about NRs 40 million. It eliminated all the bridges, for 42 km downstream between Thamo and Jubing; four or five people lost their lives. In addition, more than 30 houses were destroyed, as well as considerable cultivable land, livestock, and forest, together with long stretches of the Lukla-Namche Bazar main trail. If the flood had occurred two months later, during the trekking searon, the death toll could have been as high as 100 to 200 persons.
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Other GLOFs described by Ives include a larger one which occurred on a tributary to the Boqu River (Sun Koshi) in China in 1981; with a peak discharge estimated at 16,000 km3/sec, it destroyed a large section of the China-Nepal road as well as the Friendship Bridge, and impacted 30 km of valleys in Nepal. A 1977 GLOF near Ama Dablam in Khumbu destroyed downstream bridges for 35 km.
The apparent cause of the Dig Tsho outburst was an avalanche that cascaded onto Langmoche Glacier and splashed high waves across the lake, which was already near the top of the moraine bank. This occurred in August, after a long period of unusually warm and clear weather led to increased melting on the rockwall above Langmoche Glacier. Obviously, other causes have led to a similar result at another point. It is worth pointing out, however, that all of the relevant factors at Dig Tsho (a moraine-dammed lake, full to the brim, exposed to huge avalanches) also pertain to Tsho Rolpa.