Landslide triggering Budhi Gandaki River floods in Nepal unleashed energy equivalent to 3.5-magnitude quake: Report

A massive landslide in Nepal’s Gorkha district generated energy equivalent to a 3.5-magnitude earthquake, triggering repeated Budhi Gandaki River floods. The disaster left at least five people missing and damaged hydropower projects, roads and bridges, according to officials

Kathamandu: A massive landslide in the high Himalayas of Nepal’s Gorkha district on October 11, which triggered devastating floods in the Budhi Gandaki River, generated energy equivalent to that of a 3.5-magnitude earthquake, according to a preliminary assessment by the Department of Mines and Geology.

The landslide occurred in the upper reaches of the Ungdilung Khola, a tributary of the Budhi Gandaki River, triggering destructive flood surges after a large volume of debris blocked and subsequently released river flows, the technical report said. The report provides an initial explanation of how the high-altitude landslide and the repeated blockage and release of river flows contributed to the flooding downstream.

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The National Disaster Risk Reduction and Management Authority (NDRRMA) said in a social media post, citing the department’s report, that the Budhi Gandaki River experienced repeated flood surges in a single day following the massive landslide involving snow, water and glacial debris in the high Himalayas.

The latest disaster left at least five people missing and destroyed several hydropower projects, roads and bridges, according to the NDRRMA. The October 11 flooding was the second major flooding disaster, which occurred around one and a half months after devastating floods struck the Bhote Koshi and Trishuli rivers on August 26, which killed nearly 1,500 people and left more than 5,000 missing while devastating communities, houses, hydropower plants, roads and bridges along the affected rivers, the authority said.

Citing the department’s technical report, the NDRRMA said the largest of the repeated flood surges reached an estimated height of 20 to 25 metres. The incident occurred at around 4:20 a.m. in the upper reaches of the Ungdilung Khola, between Chamar Himal and Syerangi Himal, in Chumnubri Rural Municipality, which borders China.

According to the report, an avalanche originated at an altitude of approximately 6,000 metres in the Tibetan Tethys Himalaya, an area characterised by relatively weak rock formations, and a landslide occurred at around 5,500 metres.

“The massive landslide, comprising a mixture of snow, water and debris, flowed southward along the Ungdilung Khola, depositing a large volume of debris at the narrow confluence of the Ungdilung Khola and Syar Khola, another tributary of the Budhi Gandaki River, at an altitude of around 2,000 metres,” the authority said in its social media post.

The debris repeatedly blocked and released the flow of the Syar Khola, potentially generating successive flood surges. A similar process may also have occurred at the confluence of the Syar Khola and the Budhi Gandaki River, contributing to repeated flooding downstream, it said.

However, the report suggests that the enormous volume of debris carried by the Ungdilung Khola played a more significant role in generating the largest flood surge than the water accumulated behind the blockage of the Syar Khola. The first mud-laden flood reached the Salleri area at around 4:30 a.m., where a camp for the 341-MW Budhi Gandaki Hydroelectric Project was located. “The largest surge arrived at the hydropower camp in Salleri at around 6:30 a.m., reaching an estimated height of 20 to 25 metres before sweeping away the camp,” the report said. The powerful surge caused major damage downstream, particularly in the Jagat area along the Budhi Gandaki River, where several buildings were affected.

Permanent bridges over the Soti Khola and the Budhi Gandaki River were also among the damaged structures, the report said.

Field observations further indicated that the flow of the Syar Khola had stopped following the landslide approximately one hour before the largest flood surge arrived. “Water had accumulated over a stretch of around 1.5 kilometres in the upper reaches of the river. Smaller flood surges continued even after the largest surge had passed,” the report said.