26 January 2011

Before and after: high resolution images of the Brazil landslides

Posted by Dave Petley

Thanks to AAM for pointing this out as a comment in yesterday’s Brazil landslide post.  The private Geoeye satellite has obtained a very high resolution image of the an area affected by the recent landslides in Brazil.  The image, which was collected on 20th January, can be viewed and downloaded here. 

The complete image is shown below (obviously compressed here):

Satellite image by GeoEye

Helpfully, Google Earth also has a Geoeye image of the same area collected in 2008:

However, what has happened here becomes much clearer when the image above is draped over the (somewhat rudimentary but still useful) Google Earth terrain model to create a 3D view:

There are several interesting elements to this, of which I will highlight a few.  I invite others to use the comments to make their own observations. 

1.  The area has a mixture of forest and cleared land – and of course we always think that forestry (or lack thereof) plays a key role in determining landslide occurrence in heavy rainfall.  However, I don’t see much evidence from the above that the woodland has played a key role in determining the landslide locations.  Some cleared areas have failed, but so have some wooded areas as well, and vice-versa. 

2. The key factor in determining landslide location seems to be slope gradient, with the landslides notably picking out the steeper slope areas, and in those areas the landslides extend to the ridges. 

3. In several cases the failures the landslides appear to extend down to bedrock – and indeed the areas with thin soil cover before the landslides are visible in the vegetation patterns.  In some cases it appears that the landslides have preferentially picked out these areas.  

4. The settlements are very poorly located in terms of risk from these landslides.  Note that these are not urban areas, but rural communities.  In particular, the area of houses in the centre of the image above has been decimated by the slides that have occurred in the bowl-shaped topography above them.  The horror of being in such a location during these landslides is unimaginable.

5. In the bowl-shaped area above much of the debris appears to have been removed, with two different watercourses being visible (one heading to the east, one to the south, then southwest).  It is unsurprising that the watercourses have had so many debris flows.

Other thoughts and comments?


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