Still chasing the Melamchi mirage

Two months after a massive debris flow on the Melamchi River knocked out Nepal’s most expensive water project, geologists investigating the disaster say the danger will remain for decades to come.

Torrents of boulders and mud paste buried the headwork of the Melamchi Water Supply Project under 20m of debris on 15 and 30 July. Waves of mud washed away the highway and bridges connecting the site, submerged settlements, and killed at least 30 people.

The much-delayed $800 million project had started test distribution of water from the Langtang National Park through a 26.5km tunnel to Kathmandu residents. Fortunately, the tunnel had been sealed for inspection a few hours before the flood hit that day, and was saved.

Geologists and experts are now trying to study the upper reaches of the Melamchi watershed to determine what caused the disastrous debris flow, and to see if anything can be done to mitigate future floods. What they found does not bode well.

Project engineers say that the two big floods in July caused damage worth more than Rs2 billion, including a 22km link road, 6 bridges, warehouses and offices. Six project staff from Nepal, India and China were killed.

“Melamchi is now uncertain, we do not know when we can restart the project,” says senior divisional engineer Rajendra Prasad Pant. “We need to make a much more extensive study of the watershed upstream to determine if we can control future floods, and if so how long it will take.”

Indeed, aerial inspection and satellite imagery of the Melamchi Valley upstream shows a massive sediment field that is constantly collapsing and being washed down by heavy monsoon downpours.

Engineers say they are trying to mobilise heavy earthmovers to clear the debris and send water into the tunnel again through a temporary channel by the end of the year to resume water supply to Kathmandu. Just this part of the job will add another Rs1 billion to the cost of the project.

But there remains the enormous challenge of dealing with the 2.5km long crumbling sediment field upriver, which they say will pose a danger for decades to come. No one has an estimate of how much any mitigation measures will cost, or if it is even possible.

But besides immediate repairs at the headworks, the disaster has also put on hold the second and third phase of the project, which was to extend the tunnel to Yangri and Larke, two parallel valleys to the east to augment the tunnel capacity by another 340 million litres per day.

Although engineers had studied the upper reaches of the Melamchi to scope out any glacial lake or unstable slopes, they had never expected the sediment field to be so fragile and unstable.

“So far, we think the disaster was caused by record rainfall in the catchment area which had been destabilised by the 2015 earthquakes,” says Shiva Baskota of the Department of Mines and Geology, who recently flew over Bhemthang region, and said he saw extensive slope failure on the Melamchi and its tributary stream, the Pemdang.

Aerial photo of the confluence of the Melamchi and Pemdang Rivers taken after the 30 July flood.

Baskota estimates that some 150 cubic metres of fluvial, glacial or lacustrine deposits still sit on the river upstream at an elevation of 3,500m, and this is constantly being washed down by the river because of continuing heavy rains. On 11 July, a weather station in nearby Sermathang recorded 110mm of rain in 24 hours.

Some scientists blame the extreme weather events on climate change, and say that heavy downpours are now falling on higher altitudes where there used to be snowfall. Other researchers believe that in addition to heavy rainfall, a glacial lake higher up could have burst, adding material to the debris flow.

“It looks like there has not been an adequate disaster risk assessment of the catchment area,” Baskota says. “This means we have to study the glaciers above Yangri and Larke before starting the next phase of the Melamchi water project.”

There is a way to reduce the danger from future flooding along the Melamchi by building check dams along the river to slow the flow of the debris. But engineers admit it is a huge and expensive undertaking which has to start as soon as the monsoon ends to be prepared for next year’s rains.

Climate scientist Raju Pandit Chhetri says that the impact of global warming in the Himalaya is only going to increase, so current and future infrastructure along the mountains have to take the risk into account.

“Bridges, highways, water supply all have to be designed to withstand extreme events,” says Chhetri. “It may drive up the cost of the project, but it will prevent greater loss. Planning for a disaster like this would not have affected the Melamchi project as badly as it did.”

Some water experts say that an expensive project like Melamchi should never have been built in the first place, when cheaper alternatives to supply water to Kathmandu through reservoirs on the Valley rim to store monsoon water was possible.

“Even back then, some of us had warned that Melamchi was not suitable,” says water scientist Ajaya Dixit. “We had not foreseen a disaster like the one this year, but we knew that Sindhupalchok’s rugged terrain made it a risky place to invest in expensive infrastructure.”

Government officials, however, say that although the Norwegian consultants of the Melamchi project had assessed seismic and environmental risk and designed it for a lifespan of 100 years, nothing like the flood in July was ever expected.

“What was perhaps not factored in was the new risk added by accelerated global warming,” says Pant.

A British consultant had conducted a study in 1988 of rivers in and around Kathmandu Valley to select the most suitable river for a project to augment the Valley’s water supply, and Melamchi was chosen as the most appropriate on the basis of flow, natural gradient and proximity.

The Melamchi project was thus launched with support from the Asian Development Bank (ADB), but it was delayed due to political interference, local opposition, earthquakes, blockades, pandemic and other upheavals.

The project therefore took three times longer than it should have and was inaugurated only on 4 April 202`1 by President Bidya Devi Bhandari. Households and businesses in Kathmandu felt a great sense of relief that the water had finally arrived, but within three months the flood damaged the project and taps went dry again.

In the Ministry of Water Supply, there is already talk of finding alternative sources of water for the Valley even if Melamchi is restored – because of the continued flood risk on the river, and the fact that the project will not be able to satisfy the thirst of an expanding capital.

At present the water utility for Kathmandu supplies between 90-170 million litres to consumers every day from existing reservoirs, streams and groundwater. Melamchi had doubled that supply, but it still did not meet demand. Pumping groundwater has also lowered Kathmandu’s water table, and that supply is dwindling.

Adapted from the Nepali original in the September-October edition of Himal Khabarpatrika.