HIGH WATER

Scientists trace the origin of the damming of the Arun River in April to a bursting glacial lake near Mt Makalu

At 4:00 p.m. on 20 April a flood came down the Barun River that was blocked by debris at the Arun River, forming a lake up to 3 km long and 500 m wide above the village of Barun Bazar.

The rising lake displaced 10 families and threatened 80 more households as well as downstream riverside communities in Bhojpur and Dhankuta districts. The government swiftly deployed the Nepal Army and Nepal Police to help, and unblock the river. Fortunately, the lake drained spontaneously the next day.

Attempts to identify the source of the flood by the Nepal Army were thwarted when bad weather prevented helicopter reconnaissance of the upper Barun region.

Speculation ranged from heavy rains, flooded tributaries of the Barun, to a glacial lake outburst flood (GLOF) from the Lower Barun glacial lake, one of Nepal’s most potentially dangerous lakes.

On 2 May Dhananjay Regmi, Daene McKinney, and I flew from Dingboche over the AmphuLaptsa and West Col passes into the Barun Valley. We discovered that at least part of the flood’s source was the rather small Langmale glacial lake that had obviously undergone a rapid and recent drainage. The flood was most likely triggered by some event that created a surge wave, breaking through the lake’s terminal moraine and causing a GLOF downstream.

As a result, the beautiful Barun Valley was transformed from a lush, green Shangri La into a barren wasteland of treeless flood plains, debris, mud, and boulders. No one was killed, but three buildings in Yangle Kharka were lost as was valuable grazing land. The flood was dramatically captured in a video filmed by a visiting German climbing team. Six local bridges  were washed away, and villagers were concerned about the flood’s impact on the tourist, pilgrimage, and yarsubumba seasons.

Three weeks later, I returned to the Barun Valley and spent the next three weeks studying the cause and impact of the Langmale outburst flood. Tashi Sherpa of the Makalu Basecamp Yak Hotel and Lodge in Langmale pointed to what was most likely the primary trigger: a massive rock wall that broke off from the southwest face of Saldim Peak (6,388 m) on the day of the outburst flood. This slope failure was possibly linked to the destabilising effects of the April 2015 earthquake, which had caused at least one other GLOF in the Everest region. The rock mass plummeted into the Langmale glacier below, in turn triggering an avalanche of ice, boulders, and debris that fell further into the Langmale lake.

Dendi Sherpa, a climbing guide, was camped 200m above Langmale lake with a client and says there were two rockfall events, the first at 3 pm and the second at 4:45 pm. Fog prevented him from seeing the avalanche and flood as they occurred, but Sherpa could hear it and surveyed the damage the next morning. We were thus able to reconstruct what was most likely the series of events that led to the Langmale GLOF in April.

The first slope failure was small and only caused a minor rise in the Barun’s water level, largely ignored by villagers living downstream in YangleKharka. The second slope failure, however, consisted of over 1 million m3 of solid rock that plunged 300m down to the Langmale glacier, creating a massive blast upon impact that hurled house size boulders and icebergs up to 1 km in all directions.

A huge cloud of whitish dust settled over everything—shrubs, boulders, lodges, mani walls--over a 12 sq km area. A debris flow of mud, sand, and rocks washed up and over the right and left lateral moraines and into adjacent basins to the east and west. Because the total estimated flood volume was far larger than that contained in the pre-flood Langmale lake, the flood was most likely composed of lake water, water created by friction during the rockfall, and/or water released from caves and conduits within the Langmale glacier.

The debris-filled floodwater cascaded over a 200 m rock wall and into the Barun river below, creating a huge torrent that picked up more material and debris as it barrelled down the Barun River. Massive new, canyon-like river channels and flood plains were created that destroyed hundreds of hectares of grazing and forest land, killed at least 24 yaks and dzo. The flood attenuated at the wide and flat valley of Yangle Kharka, but nevertheless continued its destructive journey downstream to dam at the Arun river confluence with the debris-choked floodwater.

When I left the valley to begin the 8-day trek back to Tumlingtar on 3 June, villagers were desperately seeking funds to rebuild the bridges in the Barun valley.

We estimated the total flood volume was 7.6 million m3 of water + 3 million m3. This is nearly twice the estimated flood volume of the famous Langmoche (Dig Tsho) GLOF of 1985 in the Khumbu, which destroyed a nearly completed hydropower station, all bridges for 80km downstream, and killed at least five people. According to Dorje Sherpa, a lodgeowner and eyewitness of the flood at YangleKharka, the Langmale GLOF was short lived (owing to its finite supply of water), with the second major flood surge reaching Yangle Kharka around 5:30 p.m. and lowering back to its normal water level within the hour.

Although Nepal has most likely entered an era of increased GLOF and glacier flood risk, these are natural events and a normal part of Himalayan geomorphology. The Langmale GLOF was largely unexpected, but also represents an excellent example of how GLOFs are not only the result of climate change processes but also that of the dramatic geologic setting of the Himalaya.

Every glacial lake and glacial lake outburst flood in Nepal is different, complex, and defies oversimplification. As earthquakes continue and more glacial lakes form with each passing year, scientists need to continue their work to better understand these hazards, their formative processes, and mitigation techniques.

Downstream infrastructure, such as hydropower projects, need to recognise and plan for these events. Risk awareness and disaster management training will also be of critical importance to people living in villages, and even cities like Pokhara, located downstream of high mountain glaciated landscapes.

Pictures by Alton C Byers

Online package by Sonia Awale