Interesting times at the Imja lake in 2016. The Nepali Army are lowering the water level by 3m. The thought behind this is that is a lot of water hemmed in behind an ice cored moraine, and that this poses a flood threat as (i) an ice cored dam is fundamentally not very reliable and is expected to be less so under continued regional climate change and (ii) ongoing glacier retreat causes the lake to keep expanding. A nice clear summary is given on the From a Glaciers Perspective blog written by Mauro Pelto.
This photo of the lake, looking upglacier is by Jeff Kargel. I took it from an article on www.scitechdaily.com so unfortunately I don’t know the exact date of it, but as its from the air it gives the best view! Note the bright blue of some of the lakes that are not turbid as the water is still within them and the sediment is settling to the bottom of the waterbody.
According to what I’ve read in the news the army has airlifted a lot of heavy equipment up to do this engineering work at just over 5000m. Quite a feat already.
From the Kathmandu Post: “The DHM in technical and financial support from the United Nations Development Programme and the Global Environment Facility is implementing the $7.2 million Community Based Flood and Glacial Lake Outburst Risk Reduction Project (CFGORRP) that aims to reduce possible loss of human lives and infrastructure from a glacial lake outburst flooding in Solukhumbu and the downstream Tarai and Churia districts of Mahottari, Siraha, Saptari and Udaypur.“
Army Airlifts equipment to drain Imja water by 3 metres [Kathmandu Post 03.06.2016]
Nepal lake: Work begins to drain rising waters near Everest [BBC news 02.06.2016]
Dave Rounce of the University of Texas contributed a great blog about the Imja lake to the EGU website: http://blogs.egu.eu/divisions/cr/2016/07/28/fieldwork-at-5000-meters-in-altitude/, which includes some rare footage of floodwaters exiting the nearby Lhotse glacier and flowing towards Chukung on 12 June 2016, by Elisabeth Byers.
The risk assessment of glacier lakes such as the Imja is a tricky business (e.g. Emmer and Vilimek, 2013). They are considered more dangerous if (a) the moraine dam is narrow, and if (b) ice or rock avalanches into the lake are likely, as these trigger waves that can overtop the moraine dam and in the process of doing so instigate a rapid erosion of the dam which could ultimately cause an outburst to occur. The Imja lake was previously being drained by a narrow spillway that meandered across 100s of metres of moraine dam to enter the downvalley river flow, deeming it relatively stable. Furthermore, the Glaciersonline site about the Imja lake notes that geophysical data collected by Reynolds International shows that much of the moraine dam is not ice cored and therefore is much more stable than some moraine lake dams. However, this lake, which is more than 1.5km long, 600m wide and 90m deep in places, is currently the fastest growing lake in Nepal, expanding at over 40m/year, and a series of lakes are expanding around this spillway. If these small lakes within the moraine dam continue to expand, the effective width of the moraine dam will be reduced over time. At the same time, as the lake expands upglacier, eating away at the ice which is no longer replenished by sufficient from accumulation from above the upglacier expansion of the main lake might eventually mean that the lakewaters will be overlooked by steep slopes from which avalanches are likely.
Thus clip from the forthcoming movie Outburst illustrates some of the main risk criteria that are causes for concern regarding glacier lake outburst floods, using the example of an un-named lake in the Hongu valley:
The Imja lake, and the threat it may or may not present has been a cause of tension between the local community and researchers and media outlets that emphasize, and are alleged to exaggerate, the catastrophic likelihood of a flood. For example, it was sad for me to hear stories of how residents of Namche, which lies 100s of metres above the river in the valley below, were so panicked about the threat of a flood from the Imja lake occurring during the earthquake of April 2015, that some people were fleeing uphill, even though even if all of the water from the Imja lake were to escape, Namche itself would certainly not be affected directly. This seems to indicate a failure to communicate the threat accurately and effectively, despite the ongoing efforts of groups such as ICIMOD and The Mountain Institute and the HiMAP project. Hopefully their continued efforts, and wider community collaboration will change this over time.
Imja lake GLOF risk has in the past been assessed as moderate, and on this basis I guess (not very scientific I know, but I have not had time to really dig into the limited number of high quality hazard assessments made on some of these lakes!) there might be glacier lakes in the Himalaya that are more likely to be the sources of outburst floods in the near future. However, as the fastest growing glacier lake in Nepal, the government recently identified it as one of the 6 most dangerous lakes in the country, and, as this region is so important for tourism in Nepal, and downstream communities are concerned about potential flooding, the decision to act has been taken. Given the size of the lake and how it is developing it may be a timely intervention, although of course the catch with averted disasters is that its never clear if there would have been a disaster without the intervention.
To my knowledge, the most comprehensive survey of glacier lakes in the Himalaya is that carried out by ICIMOD, which is freely available to download here, and is well worth a read for its balanced view on our current knowledge, risk assessment and communicating risk.
Here is the flowchart used to identify the lakes posing a critical risk:
Emmer, A. and Vilimek, V (2013) Review Article: Lake and breach hazard assessment for moraine-dammed lakes: an example from the Cordillera Blanca (Peru). Nat. Hazards Earth Syst. Sci., 13, 1551–1565
ICIMOD (2011) Glacier lakes and glacial lake outburst floods in Nepal. Kathmandu: ICIMOD.