2 August 2021
Melamchi – satellite images and helicopter videos start to explain the catastrophic floods in Nepal
On 14 June 2021 a huge debris flow and flood destroyed Melamchi in Nepal. The event was triggered by heavy rainfall, and as I noted a few days after the event, there is a very large landslide upstream of the village that might explain the surge that came down the valley. However, the event has been slightly mysterious, partly because satellite images have not been available of most of the area upstream of Melamchi. It is now clear that the event was more complex, and more concerning, than was initially obvious.
Satellite imagery is slowly becoming available – this is slow as the area is still covered in monsoon cloud most of the time. To understand this event we have to go right up to the headwater area in the high mountains. This is the site:-
This is a very steep, very rugged terrain. Note in the centre of the image the large volume of sediment, partially vegetated, in the valley floor, trapped against some sort of barrier (very possibly an old landslide deposit).
Planet Labs captured an image of the same area on 29 July 2021. This is not the best image, but the change is startling:
Upstream of this area there are some landslides, most notably from the valley to the northwest. But there appears to be huge mobilisation of the sediment trapped behind the barrier.
भीमथाङको यो विशाल पहिराेले मेलम्ची क्षेत्रमा लामाे समय दु:ख दिन्छ जस्तो छ ( भिडियो बाबू लामा) pic.twitter.com/0oEqWFnTu1
— ShIvA DaHaL (@dahalshivaji) August 1, 2021
The lower left image shows this area of sediment, clearly dramatically mobilised by the flood. The helicopter video captures this even more clearly:-
The view looking downstream is, if anything, even most dramatic:-
My interpretation is that this is effect the breach of an ancient landslide dam, which is now collapsing under successive rainstorm events. The sediment pile extends from about 2 km, and only a small proportion has mobilised to date.
The sediment pile is collapsing rapidly – the video even captures small collapses occurring – with big slumps developing, as seen in the second video. Downstream, a huge amount of erosion has developed, and the walls of the channel have collapsed extensively:
Further downstream, as I noted in a previous post, there is a large landslide near to the village of Melamchigaon, but it appears that this was not the cause of the disaster:-
These images are grim in terms of the safety of Melamchi. There appears to be a huge, unstable mass of sediment, undergoing collapse, upstream of the town. Heavy rainfall is likely to bring further major floods and debris flows, and indeed on Saturday a further dozen houses were lost to floods in the area.
My analysis is not definitive – indeed it is only provisional – a much more detailed examination of this site is needed (and is hopefully underway), led by local teams.
There is a much larger consequence of this event though. Melamchi is source of a major drinking water project for the capital city of Kathmandu – the Melamchi Drinking Water Project (MDWP). The works, which started to supply water to the city on 28 March 2021 after 23 years of construction, were substantially damaged in the June event. The excellent Himalayan Times has a good article about the impacts:
The head works of MDWP has been covered 10 metres high with flood debris according to officials. Worst of all, the flood has swept away around 10 kilometres road from Melamchi Bazaar up to the headworks, including two major concrete bridges.
The estimated costs to the project are 2 billion rupees (about £12 million), but the full extent of the damage is unclear.
However, there is a real but unquantified risk that the problems are much more serious, with continued large-scale mobilisation of sediment in future rainstorms. Once again, this needs very urgent attention.
Planet Team (2021). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/