{"id":38421,"date":"2021-08-02T09:38:18","date_gmt":"2021-08-02T09:38:18","guid":{"rendered":"https:\/\/blogs.agu.org\/landslideblog\/?p=38421"},"modified":"2021-08-02T09:38:18","modified_gmt":"2021-08-02T09:38:18","slug":"melamchi-satellite","status":"publish","type":"post","link":"https:\/\/blogs.agu.org\/landslideblog\/2021\/08\/02\/melamchi-satellite\/","title":{"rendered":"Melamchi – satellite images and helicopter videos start to explain the catastrophic floods in Nepal"},"content":{"rendered":"

Melamchi – satellite images and helicopter videos start to explain the catastrophic floods in Nepal<\/h4>\n

On 14 June 2021 a huge debris flow and flood destroyed Melamchi in Nepal<\/a>.\u00a0 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<\/a>.\u00a0 However, the event has been slightly mysterious, partly because satellite images have not been available of most of the area upstream of Melamchi.\u00a0 It is now clear that the event was more complex, and more concerning, than was initially obvious.<\/p>\n

Satellite imagery is slowly becoming available – this is slow as the area is still covered in monsoon cloud most of the time.\u00a0 To understand this event we have to go right up to the headwater area in the high mountains.\u00a0 This is the site:-<\/p>\n

\"Google

Google Earth image of the headwaters above Melamchi.<\/p><\/div>\n

.<\/p>\n

This is a very steep, very rugged terrain.\u00a0 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).<\/p>\n

Planet Labs captured an image of the same area on 29 July 2021.\u00a0 This is not the best image, but the change is startling:<\/p>\n

\"Satellite

Satellite image of the headwaters area above Melamchi, captured on 29 July 2021. Image copyright Planet Labs<\/a>, used with permission<\/p><\/div>\n

.<\/p>\n

Upstream of this area there are some landslides, most notably from the valley to the northwest.\u00a0 But there appears to be huge mobilisation of the sediment trapped behind the barrier.<\/p>\n

Further insight comes from a pair of Tweets<\/a> yesterday by Shiva Dahal (@dahalshivaji<\/a>), which included both photographs and a startling video of the site:-<\/p>\n

https:\/\/twitter.com\/dahalshivaji\/status\/1421754169904357376<\/p>\n

.<\/p>\n

The lower left image shows this area of sediment, clearly dramatically mobilised by the flood.\u00a0 The helicopter video captures this even more clearly:-<\/p>\n

\"The

The collapsing front edge of the sediment deposit upstream of Melamchi. Still from a helicopter video posted to Twitter<\/a>.<\/p><\/div>\n

.<\/p>\n

The view looking downstream is, if anything, even most dramatic:-<\/p>\n

\"The

The collapsing front edge of the sediment deposit upstream of Melamchi. Still from a helicopter video posted to Twitter<\/a>.<\/p><\/div>\n

.<\/p>\n

My interpretation is that this is effect the breach of an ancient landslide dam, which is now collapsing under successive rainstorm events.\u00a0 The sediment pile extends from about 2 km, and only a small proportion has mobilised to date.<\/p>\n

The sediment pile is collapsing rapidly – the video even captures small collapses occurring – with big slumps developing, as seen in the second video.\u00a0 Downstream, a huge amount of erosion has developed, and the walls of the channel have collapsed extensively:<\/p>\n

\"The

The deeply eroded channel upstream of Melamchi. Still from a helicopter video posted to Twitter<\/a>.<\/p><\/div>\n

.<\/p>\n

Further downstream, as I noted in a previous post<\/a>, there is a large landslide near to the village of Melamchigaon, but it appears that this was not the cause of the disaster:-<\/p>\n

\"Satellite

Satellite image of the landslide close to Melamchigaon. Image copyright Planet Labs<\/a>, used with permission<\/p><\/div>\n

.<\/p>\n

These images are grim in terms of the safety of Melamchi.\u00a0 There appears to be a huge, unstable mass of sediment, undergoing collapse, upstream of the town.\u00a0 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<\/a>.<\/p>\n

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.<\/strong><\/em><\/p>\n

There is a much larger consequence of this event though.\u00a0 Melamchi is source of a major drinking water project for the capital city of Kathmandu – the Melamchi Drinking Water Project (MDWP)<\/a>.\u00a0 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.\u00a0 The excellent Himalayan Times has a good article about the impacts<\/a>:<\/p>\n

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.<\/em><\/p>\n

The estimated costs to the project are 2 billion rupees (about \u00a312 million), but the full extent of the damage is unclear.<\/p>\n

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.\u00a0 Once again, this needs very urgent attention.<\/p>\n

.<\/p>\n

Reference<\/h4>\n

Planet Team (2021). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https:\/\/www.planet.com\/<\/a><\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

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