Nepal flash flood from Tibetan side washes away Rasuwa villages and hydropower sites

A flash flood that officials trace to the Tibetan side of the border tore through northern Nepal on Wednesday morning, washing away villages, market streets, bridges and hydropower sites along the Bhote Koshi and Trishuli rivers. District administrator Narendra Pariyar told Reuters that Syapru Besi and Timure in Rasuwa district were badly hit. “We have reports of villages and infrastructure project sites being swept away,” he said. “There could be heavy casualties or loss of property. But I cannot say for sure now.”

The surge struck the Bhote Koshi around 9:00 a.m. and moved downstream into the Trishuli toward Nuwakot. Early local reports said the Timure market, the Timure customs post and parts of Syabrubesi took the first blow. Nine Armed Police Force personnel stationed at a border outpost in Timure were unaccounted for by mid-morning. Nepal News later reported that about 28 security personnel were out of contact and that around 60 houses and two bridges had been swept away.

Prime Minister Balendra Shah dispatched the army, police and relief teams and told people in low-lying areas to move to higher ground. The Flood Forecasting Division said the water had reached Galchhi in Dhading by late morning and was expected to pass Mugling within hours. High-risk notices covered riverbanks in Rasuwa, Nuwakot, Dhading, Gorkha and Chitwan. Officials asked travellers to stay off the Prithvi Highway and other riverside roads. Public transport from Kathmandu toward seven districts was halted.

Not a rain flood

Senior meteorologist Meen Kumar Aryal of the Department of Hydrology and Meteorology said rainfall was not the cause. In the previous 24 hours Rasuwa had seen only light rain, no more than seven millimetres. Hydrologists suspect a glacial lake outburst on the Tibetan plateau. The department said it was working with Chinese counterparts and looking at satellite images to locate the source. A deadly flash flood on the same river last year was later tied to the draining of a glacier lake.

The Bhote Koshi rises in Tibet and joins the Trishuli, which becomes the Gandak after it crosses into India. A pulse of water that starts above the border therefore travels through a chain of hydropower plants, customs yards, highway bridges and farm towns before it reaches the plains. The water resources ministry said hydroelectric projects were among the damaged sites. Power in the area was cut. For a country that sells electricity and depends on mountain roads for trade with China, a single outburst can close a border post and a generating station in the same hour.

Rasuwa’s affected valleys hold about 50,000 people. Many of the houses closest to the river are shops, inns and customs warehouses that grew up around the Kerung-Rasuwa trading route. When the river jumps its banks, it does not only flood fields. It removes the buildings that make the crossing work.

A familiar Himalayan pattern, arriving faster

Glacial lake outburst floods have a known sequence. A lake held by ice or loose moraine drains in minutes to hours. The water picks up debris. By the time it reaches a town 20 or 30 kilometres downstream it is a slurry that can move houses. Warning time depends on whether anyone saw the lake fail and whether a sensor or a phone call beat the water. On Wednesday morning the first public alerts came after the surge had already hit Timure.

Nepal has invested in hydropower along these rivers because the drop from the Tibetan plateau is steep and the flow is usually reliable. That same drop turns a lake failure into a weapon against the plants built to harvest it. Last year’s flood on the Bhote Koshi was a rehearsal that the country is now living through again, with the source again on the other side of a border that complicates inspection and early warning.

Shah convened an emergency session of the disaster council. Search teams were still moving into Rasuwa as the flood pulse continued south. Casualty figures were not firm by early afternoon. The official instruction to people on the banks remained the same: leave the river and stay off the roads that run beside it. The water that left Tibet at dawn had not finished its route through Nepal by the time those instructions were issued.