5 May 2015

The Langtang ice and rock avalanche in the Gorkha Earthquake

Posted by dr-dave

The Langtang ice and rock avalanche

Over the weekend the picture is slowly becoming clearer in terms of landslides associated with the Gorkha Earthquake in Nepal.  It is now apparent that the level of landsliding in the Hill Districts at least is rather lower than we might have feared, although it is still far from insignificant.  Understanding the situation in the higher mountains remains very challenging due to the amount of cloud cover that is still preventing satellite images from being available.  We have to hope that a window opens up before the monsoon.

So far the stand our landslide event appears to be the ice and rock avalanche at Langtang.  This appears to have caused substantial loss of life.  Not for the faint-hearted, this BBC report from the scene leaves little to the imagination in terms of the likely losses.

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NASA have managed to collect some imagery of the site.  This is a post-event Landsat 8 image that clearly shows the avalanche:

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This is the Google Earth image of the village of Langtang before the ice and rock avalanche:

Langtang

Google Earth

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The Digital Globe satellite imagery of the site captures the avalanche track and deposit pretty well:

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This is a slightly odd event because of the lack of a clear large deposit.  My initial (and very provisonal) interpretation would be that a chunk of ice and rock broke off from the slope above the village – a comparison of the Google Earth image from January with the Digital Globe image suggests this strongly:

Before:-

Langtang

Google Earth

After:

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I suspect that this mass then went through freefall / rapid sliding into the valley below (removing rock from the surface en route – the planed off surface is clearly apparent in the image above). Google Earth suggests that the fall was about 700 or 800 m vertical distance – and that the mass then fragmented on impact to create a massive blast of air, ice and rock.  This is similar to the Seti River rock avalanche that I featured three years ago, although on a smaller scale.  This mechanism would account for the huge damage that occurred in Langtang.