GLOF and Flood Damage to Nepal's Hydropower Plants

Nepal's electricity system is overwhelmingly hydropower-based: at the end of fiscal year 2081/82 BS (2024/25 AD), installed capacity on the national system was about 3,591 megawatts (MW), more than 90 percent of it hydropower, according to the Nepal Electricity Authority (NEA). Almost all plants are run-of-river schemes built directly on steep Himalayan rivers that drain glaciers and glacial lakes in Nepal and the Tibet Autonomous Region of China.

That geography creates a specific hazard: the glacial lake outburst flood, or GLOF. As glaciers melt under a warming climate, meltwater ponds behind unstable moraine dams or on the glacier surface itself; when such a lake breaches, it releases a sudden wall of water and debris far larger than any normal monsoon flood. The International Centre for Integrated Mountain Development (ICIMOD) records 26 GLOF events affecting Nepal since 1977, of which 14 originated inside Nepal and the rest across the border, mostly in Tibet.

For hydropower, the damage pattern is consistent: headworks and gates are smashed or buried in sediment, canals and desanders fill with debris, riverside transmission towers are undercut, and powerhouses are flooded. Because Nepal's most energy-rich rivers — the Trishuli, Bhote Koshi, Dudh Koshi and Tamakoshi — all head in glaciated terrain, GLOF and extreme-flood risk is now a central question in Nepali hydropower planning, insurance and financing.

At around 3:15 am on 8 July 2025 (24 Asar 2082 BS), a flood surged down the Lhende (Lende) Khola into the Bhote Koshi–Trishuli river at Rasuwagadhi, the main Nepal–China border crossing in Rasuwa district. Nepal's Department of Hydrology and Meteorology (DHM) and satellite analysts including ICIMOD attributed it to the sudden drainage of a supraglacial lake just north of the border in Tibet, which had grown from a small meltwater pond in December 2023 to roughly 638,000 square metres by 7 July 2025 and shrank to about 435,000 square metres after the outburst — a climate-driven GLOF, with no heavy rainfall recorded that night.

The flood destroyed the Nepal–China Miteri (Friendship) Bridge, swept away containers at the Rasuwagadhi customs yard and under-construction dry port, and killed at least nine people, with around 19 reported missing in initial tallies. For the power sector it was the most damaging single event in years: NEA reported about 240 MW of generation halted, while the Independent Power Producers' Association of Nepal (IPPAN) counted projects totalling more than 600 MW affected across Rasuwa and Nuwakot once under-construction schemes were included.

The worst-hit plant was the 111 MW Rasuwagadhi Hydropower Project, barely 200 metres from the border: the flood tore away its dam gates and destroyed the headworks, forcing a complete shutdown with damage later estimated at around NPR 3.51 billion. Recovery took roughly a year — one 37 MW unit resumed on 7 December 2025, two units (about 68 MW) were online by early 2026, and after a final reconstruction shutdown from 5 to 27 June 2026 the plant returned to full three-unit capacity at the end of June 2026.

Note on names: the Bhote Koshi in Rasuwa (the upper Trishuli) is a different river from the Bhote Koshi in Sindhupalchok hit by the 2016 GLOF described below; both drain from Tibet and both corridors are lined with hydropower plants.

On the evening of 5 July 2016 (21 Asar 2073 BS), the moraine-dammed Gongbatongshacuo lake in Tibet's Zhangzangbo valley — a tributary of the Poiqu, which becomes the Bhote Koshi in Sindhupalchok — burst about 24 kilometres upstream of the 45 MW Upper Bhote Koshi Hydroelectric Project. A debris-laden surge swept down the river at around 9 pm, choking the plant's spillway, overtopping the intake and burying the reach upstream of the dam in sediment and boulders reported to be up to 14 metres across. The flood also swept away houses, a school building and part of the Tatopani customs area, and damaged the Arniko Highway to China.

It was the third disaster to strike the plant in three years. The Jure landslide of 2 August 2014, about 16 kilometres downstream on the Sunkoshi, killed at least 156 people, submerged the powerhouse of the small Sanima (Sunkoshi) hydropower plant and toppled three of Upper Bhote Koshi's transmission towers, cutting the plant off for around six months. The April 2015 Gorkha earthquake then caused damage estimated at about NPR 3 billion, and the 2016 flood destroyed the dam, headworks and electro-mechanical equipment.

Rebuilding the dam and damaged tunnel sections and replacing transformers cost roughly NPR 7 billion. The plant — Nepal's first large private hydropower project, generating since 2001 — began test transmission only in late 2019 and returned to commercial operation in early 2020. Upper Bhote Koshi is also notable as one of the first Nepali plants to operate a sensor-based GLOF early-warning system on its river.

On 15 June 2021 (1 Asar 2078 BS), the first day of the monsoon month of Asar, a devastating debris flood came down the Melamchi river in Sindhupalchok and struck Melamchi Bazaar, killing five people, leaving around 20 missing and damaging more than 500 households in Helambu and Melamchi municipalities. ICIMOD's post-disaster assessment described it as a cascading multi-hazard event rather than a simple rainstorm flood: intense rain high in the catchment combined with snowmelt, the breach of glacial meltwater ponds and the collapse of an old landslide dam at Bremthang, mobilising an estimated 13 million cubic metres of sediment.

The flood is best known for crippling the Melamchi Water Supply Project — the nearly three-decade national project built to divert 170 million litres of water a day to Kathmandu — barely a year after it first delivered water. Its headworks were buried under some six metres (20 feet) of debris, and a World Bank-supported drone survey quantified the damage and residual risk. Since then the intake has been operated seasonally, closing each monsoon while a more flood-resilient permanent headworks is designed.

Hydropower along the Melamchi and Indrawati rivers was hit as well. The 7.5 MW Indrawati III plant lost four transmission towers to the Melamchi flood and had its powerhouse damaged by the swollen Indrawati, with losses of roughly NPR 25 million before it resumed generation the following month. The event became a case study in planning Himalayan infrastructure against chains of hazards acting together rather than any single design flood.

GLOFs are the most dramatic threat, but ordinary — and increasingly extraordinary — monsoon floods now damage Nepali hydropower almost every year. On 27–28 September 2024 (11–12 Asoj 2081 BS), record-breaking late-monsoon rainfall triggered floods and landslides across central and eastern Nepal: IPPAN tallied 13 operating projects of about 626 MW forced to halt generation and around 15 more projects totalling roughly 1,010 MW, many under construction, with major damage.

Landslides struck the NEA-built 456 MW Upper Tamakoshi project, Nepal's largest plant, where four workers went missing; the powerhouse of the 22 MW Bagmati Small Hydropower Project was submerged; and the 86 MW Solu Dudhkoshi project suffered repeated flood damage. The National Disaster Risk Reduction and Management Authority (NDRRMA) estimated hydropower losses of over NPR 3 billion from that single weekend, and the NEA imported roughly 300 MW of extra electricity from India to cover the shortfall.

A month earlier, on 16 August 2024, a glacial pond above Thame village in Solukhumbu burst, destroying houses, a school and a health post — a reminder that even small supraglacial ponds, like the one that later burst above Rasuwagadhi, can flood destructively without warning.

GLOF damage to Nepali hydropower is not new. On 11 July 1981, an outburst from the Zhangzangbo glacial lake in Tibet — the same valley that produced the 2016 flood — roared down the Sun Koshi/Bhote Koshi, destroying the Nepal–China Friendship Bridge, sections of the Arniko Highway and the diversion weir of the Sunkoshi hydropower station.

Four years later, on 4 August 1985, an ice avalanche into the Dig Tsho glacial lake in the Khumbu sent a 10–15 metre wall of water and debris down the Dudh Koshi, destroying the nearly completed Namche Small Hydropower Plant along with bridges, houses and farmland. The disaster put GLOF risk firmly on the national agenda, leading eventually to the lowering of Tsho Rolpa lake in 2000 and today's ICIMOD glacial-lake inventories.

A 2020 inventory by ICIMOD and the United Nations Development Programme (UNDP), prepared with Nepal's Department of Hydrology and Meteorology, mapped 3,624 glacial lakes in the Koshi, Gandaki and Karnali basins and ranked 47 as potentially dangerous: 42 in the Koshi basin, 3 in the Gandaki and 2 in the Karnali. Critically for hydropower planners, 25 of the 47 lie in the Tibet Autonomous Region of China and only 21 inside Nepal (one is in India) — meaning most worst-case floods, like those of 1981, 2016 and 2025, would originate beyond Nepal's borders and monitoring network. The 2025 Rasuwa event added a further warning: it came from a fast-growing supraglacial pond that was not on the dangerous-lakes list at all.

The exposure is concentrated exactly where Nepal's generation is clustered. The Trishuli corridor alone carries more than 600 MW of operating and under-construction capacity below the Lhende catchment, while the Sindhupalchok Bhote Koshi, Dudh Koshi and Tamakoshi — the last below Tsho Rolpa, Nepal's largest dangerous lake — each combine glaciated headwaters with cascades of run-of-river plants. Studies after the 2016 and 2021 events have repeatedly warned that design floods based on past river records understate what a GLOF or cascading debris flow can deliver.

Risk reduction is possible, though costly. Measures adopted or recommended after these disasters include GLOF-aware siting and higher design floods, gated spillways that can pass debris, powerhouses set back or built underground, upstream sensor networks linked to gate operation, transboundary data-sharing with China, disaster insurance, and community early-warning systems of the kind ICIMOD recommended after Melamchi. Lake-lowering works — as done at Tsho Rolpa in 2000 and Imja in 2016 — remain the most direct, if expensive, defence.