A glacier came off a ridge on the Nepal-China border and travelled 100 km down multiple rivers, the longest part of the run being in the Trishuli.
Before ice, rock, mud and water hurtled down one river after another, partly in Tibet (China) and mostly in Nepal, a seismic event occurred where the two countries’ borders meet, at an altitude of more than 5,000 metres in the Himalayas. That seismic event is at the centre of multiple theories since, ranging from what caused it in the immediate to what really fell.
The Shaking: What and Where Exactly?
The disaster began at 8:37 am Nepal time (8:22 am IST) on August 26 with a tremor recorded by monitoring networks across the region as a magnitude 4.4 earthquake, at first. Three days on, scientists remained divided over what that signal represented. The United States Geological Survey (USGS) later revised its interpretation of that event, saying that the seismic waves were generated by a landslide involving a glacier in Langtang National Park in Nepalese territory.
The energy released by that landslide (‘slope failure’) was the equivalent of a magnitude 5.2 earthquake, it said.
India’s remote sensing centre offered a different preliminary reading, saying that a magnitude-4.9 earthquake triggered the collapse of a glacier; and that the sequence began on the Tibetan side. The International Centre for Integrated Mountain Development says the trigger remains unconfirmed.
What Fell, And Travelled
Geological reconstructions of the event, based on satellite images, reported that a part of the glacier slid down the Nepal side of a mountain; debris blocked a river on the Tibetan side; and the blocked water tore through, destroying a border port nearby before the catastrophe entered Nepal.
The US agency simply said it was “likely triggered by rapid slope failure involving a glacier in Langtang National Park, Nepal, near the border with China”. The USGS has not settled on how the landslide occurred, saying, “It is unclear if the initial slope failure was a landslide incorporating part of a glacier or a glacial collapse.”
Whether an earthquake set off the avalanche, or the avalanche triggered an earthquake-like shaking, the sequence that followed is not in dispute. The mass surged nearly 100 kilometres through the rivers Lhende Khola, Bhote Koshi and Trishuli, destroying areas in Rasuwa, Nuwakot and Dhading in Nepal. The country’s department of hydrology recorded the Trishuli rising nine metres in 30 minutes.
That’s an inland tsunami by all measures.
Having claimed hundreds of lives, the surge calmed somewhat, and the dirt-brown water crossed into India through the Narayani river, reaching the Gandak barrage in Bihar by night. Upstream, the danger has not passed. China’s Ministry of Water Resources has warned that a new barrier lake – formed likely due to another landslide or leftovers of this one – near the border is already overflowing and at high risk of breaching.
But let’s rewind.
Where it went
Before the deadly surge passed, it left heartbreaking devastation in villages and towns of the small Himalayan nation.
Once the landslide/glacier debris tore through the river Lhende Khola, it first hit the nearby Gyirong Port, a Chinese inland port at the border, 13 km from the landslide-cum-avalanche site. That site is now flattened; no sign of the grand building.
This port serves as a major crossing for Hindu pilgrims travelling to the Hindu sacred site of Kailash Mansarovar. Cameras recorded mudflow engulfing the building.
Having entered Nepal, the surge ran down the Bhote Koshi into the Trishuli, striking Timure and Syapru Besi in Rasuwa next.
The USGS extent map traced the continuous run of close to 100 km for the primary bulk of the debris.
The wave covered its first 22 km at an average velocity of 193 kmph, experts analysed. More than 10 hydropower projects went offline in the region, taking more than 12% of Nepal’s electricity capacity.
How It Calmed, Finally
Past Dhading, the Trishuli river leaves the mountainous gorge and meets the Kali Gandaki at Devghat to become the Narayani, and crosses into India at Triveni, above the Gandak barrage at Valmikinagar in West Champaran.
Bihar’s water resources department recorded 86,000 cusecs at the barrage at noon on August 26, and rising at the time. All 36 gates were opened, discharge peaked at 1.50 lakh cusecs late that night, and had fallen to 1.04 lakh cusecs by Thursday morning, the department said in a statement. It was a scare, but the barrage has seen far more water before during the rains.
