Experts say Nepal's Trishuli hydropower rebuilds need new design rules

What happened

On 26 August 2026, a rock-ice avalanche upstream set off a fast, mud-heavy flood down the Bhotekoshi and Trishuli rivers. Clickmandu reported that eight operating plants and four projects under construction were hit, a total of 669 MW. The Independent Power Producers' Association, Nepal (IPPAN) said the flood moved at about 180 km/h and covered 22 km in seven minutes, too fast for early warning to help much. Many workers and residents were reported missing.

Energy expert Dr Mukesh Kafle told Clickmandu that the Trishuli riverbed had dropped by about three metres. In his view, the old design numbers no longer apply, and intakes, headworks and tunnel alignments need full redesign. Kulman Ghising, a former energy minister and former head of the Nepal Electricity Authority (NEA), said floodwater rose about 200 metres above the river. He said this went far beyond the 500-year and 1,000-year floods that designs had used.

Others urged caution before rebuilding. Thark Bahadur Thapa of NEA said reconstruction should not begin without a thorough glaciological study and full mapping, and that rebuilding at the same sites and heights would be dangerous unless scientists confirm a similar avalanche will not happen there. IPPAN President Mohan Kumar Dangi said project studies should cover the whole upper catchment, not only the area around each site.

The debate continued at IPPAN's Friday Forum, reported by New Spotlight on 13 September. Ghising asked the government to declare the Bhotekoshi and Trishuli corridors a special area for recovery. He proposed mandatory emergency exits in main access tunnels, remotely operated powerhouses like those in Austria and Switzerland, and at least one reservoir-based hydropower project in every river basin. None of these proposals had become a formal standard.

The engineering behind it

Hydropower structures are designed for a chosen flood. As general knowledge, engineers estimate a design flood from past river records and give it a return period, such as 1 in 1,000 years. This means a flood of that size has a 0.1 percent chance in any one year. The method assumes the future looks like the past, and that floods come from rain and snowmelt. It does not cover a sudden avalanche or a glacier lake burst unless engineers study those hazards separately.

Hydrometeorologist Dr Basantaraj Adhikari told the forum that this was a glacial outburst or high-altitude debris flood linked to climate change, not a normal monsoon flood. He warned that events once expected every 5,000 to 10,000 years now appear to recur much faster. If that is right, the statistics behind design floods need new inputs, such as glacier lake maps, slope hazard studies and satellite monitoring of the upper catchment.

The proposals also involve electronics. A remotely operated powerhouse lets staff control turbines, gates and switchgear from a safe place, through sensors, control systems and reliable communication links. NEA's Madan Raj Timilsina told the forum that he received word of the flood at 9:10 am, ordered the evacuation of the Upper Trishuli 3A underground powerhouse and the opening of dam gates, and water entered the powerhouse by 9:15 am. Five minutes is a very short time for people to leave an underground site.

Access tunnels are a specific weak point. Timilsina said the water entered the Trishuli 3A powerhouse through the tailrace and the main access tunnel. In general, an underground powerhouse is reached by a tunnel from the valley floor, and the tailrace returns water to the river at a low level. If the river rises above both openings, water can enter the cavern. This is why Ghising proposed raising access tunnels and adding emergency exits that lead to higher ground.

What it means in Nepal

The Trishuli corridor holds several important plants. Clickmandu names Rasuwagadhi, Sanjen, Chilime, and Trishuli 1, 3A and 3B in the affected catchment. NEA put its early estimate of losses at more than Rs 100 billion, and private developers were still counting theirs. The question for engineers is not only how to rebuild, but where, at what height, and with what warning time.

The insurance side is changing too. At the forum, Birendra Baidawar Chhetri of the Nepal Insurers' Association said insurers' losses on these corridors could reach Rs 15 to 23 billion. He said reinsurers increasingly see Nepal as high-risk, so cover may become harder to get and more expensive. A safer design can lower that risk, which is one reason the debate about standards matters to developers.

If new rules do require catchment-wide hazard studies, glacier mapping and remote control, the work will combine several fields. Hydrologists estimate floods. Geologists and remote sensing specialists map unstable slopes and lakes. Civil engineers place and protect intakes, tunnels and powerhouses. Electronics and communication engineers build the sensors, warning links and remote control systems. Each needs to understand enough of the others' work to design a safe project together.