As Nepal grapples with the aftermath of the devastating flood that killed hundreds, it now faces a fresh flooding threat following the possible breach of “a significant barrier lake” that formed on the Tibetan side.
Chinese state media reported that this lake had formed after the likely glacial collapse — which triggered Wednesday’s Nepal disaster — caused an ice-and-rock debris blockage at the confluence of the Chhochen and Purepu Tsangpo rivers.
On Friday, a post by Nepal Police in Nepali warned that a “water barrier/dam” on the Tibetan side had breached.
Both Nepal and China had issued warnings earlier about the possibility of a breach that could cause a second flood downstream. The barrier lake had been overflowing as of Thursday morning, with its volume estimated at 2 million cubic metres, according to state-run China Central Television. Another 3 million cubic metres of water — the equivalent of about 1,200 Olympic swimming pools — was expected to flow into the lake over the next three days, posing a high risk of a breach, the report said.
What, however, are barrier lakes, how devastating can they be, and what do Chinese authorities say they are doing?
What is a barrier lake?
While Chinese authorities are calling this formation a barrier lake, it is more commonly known as a landslide dam or lake. According to the US Geological Survey, it is created in a river valley when landslide debris completely blocks a stream or river.
The most common triggers for such landslides are earthquakes and excessive rain.
These temporary lakes can accumulate huge quantities of water with no outlet. If water continues to accumulate, it can overflow or breach the dam — as the recent one is reported to have done — triggering a potentially devastating disaster known as an LLOF, or landslide lake outburst flood. This is similar in effect to a GLOF, or a glacial lake outburst flood, which occurs when the water dammed by a glacier or a moraine is released suddenly.
The timescale of a breach can vary from days to even decades after the formation of a dam. The worst LLOF event, which we talk about later in this article, took just 10 days. In contrast, a dam formed by a rock-and-debris fall over the Tagermarch River in Kyrgyzstan took 131 years to fail (1835-1966).
Incidentally, a GLOF was initially speculated to be the cause of the Nepal flooding, but the most likely possibility now appears to be a glacier break-off — which has now created the landslide lake.
According to a 2017 study published in Current Science, landslide dams commonly form in places with high, steep mountains and narrow river valleys that witness activity such as rock, soil and mud avalanches, slides, or flows — a description that would fit the Himalayas. Indeed, a National Disaster Response Force document says that LLOFs are common in the Himalayan river basins. One way to mitigate their effect is to create outlets — something China now appears to be exploring.
How devastating can a breach be?
Very. A failure of the dam can potentially release millions of cubic metres of water in a short period, causing catastrophic flooding downstream. This flooding can carry debris and also create secondary landslides downstream, compounding its effect. In Nepal, the worry is all the more greater as it has just experienced devastating flooding that has already swallowed settlements, infrastructure and lives.
The world’s worst recorded LLOF occurred in 1786, after an earthquake in China’s Sichuan Province triggered a huge landslide and dammed the Dadu River. After 10 days, the water breached the landslide dam — travelling as far as 1,400 km downstream and killing about 1,00,000 people.
These unpredictable natural disasters also have cross-border effects.
In April 2000, the giant Yigong landslide created a 60 metre high dam on the Yigong Tsangpo River. The dammed lake collapsed two months later, producing a catastrophic flood that surged downstream through 500 km of Arunachal Pradesh to reach the floodplain of the Brahmaputra River in Assam, says a 2020 study in Scientific Reports. It destroyed 20 bridges and left 50,000 people homeless.
According to a 2008 study published in the journal Natural Hazards, LLOFs in 2000 and 2005 caused extensive damage to infrastructure along the Satluj and Spiti rivers in Himachal Pradesh. At least 156 people were killed in the 2000 LLOF though the toll was possibly much higher, says the study. Both were cross-border disasters; the landslide lakes formed in Tibet.
In 2008, landslides and rock avalanches triggered by the Wenchuan earthquake produced 257 landslide dams, mainly situated along the eastern boundary of the Qinghai-Tibet Plateau. The largest of these dams blocked the Tongkou River (a tributary of the Fujiang River) at Tangjiashan.
The lake threatened to unleash a massive flood on downstream communities, putting up to 1.3 million people at risk. Authorities built a spillway to drain the lake in a controlled manner and evacuated downstream residents.
In August 2014, heavy precipitation induced a huge landslide in Nepal that created a landslide lake across the river Sunkoshi. The lake was emptied using geo-engineering methods after 45 days.
What is happening now?
Due to the risks from the barrier lake, search and rescue operations had to be suspended near the Nepal-Tibet border, according to Chinese state media reports.
The X post by Nepal Police also asks rescue personnel to move away.
“All security personnel, rescue workers and members of the public involved in rescue and relief operations are requested to remain alert immediately and move to, or inform others to move to, a safe location,” the post said.
Chinese authorities had mobilised an eight-member engineering team to conduct aerial surveys and 3D modelling of the barrier lake, according to state broadcaster CCTV on Friday.
One of the engineers, Chen Hanxiao, told China’s state broadcaster CCTV that the general approach to reducing the lake’s threat is to excavate a drainage channel at a lower point of the dam. The width and depth of that channel needed calculations based on how fast the water will flow and how quickly the lake will fill.
Data from their survey found that the lake was about 150 metres long and 40-50 metres deep with a reservoir of 1.5-2 million cubic metres of water.
The lake has formed in a location upstream from the site of Wednesday's flash flood. (Photo by China's CCTV) 
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