8 November 2015

Deciphering the Samarco tailings dam disaster in Bento Rodriguez

Posted by Dave Petley

The Samarco tailings dam disaster in Bento Rodriguez

Latest reports from Minais Gerais state is Brazil suggest that there are now two known fatalities from the Samarco tailings dam disaster that devastated Bento Rodriguez, but that a further 28 people remain missing.  News reports suggest that about 60 million cubic metres of sludge were released when two tailings dams collapsed. Samarco is now coming in for some criticism for their response to the disaster, and it will be very interesting to see how they handle the inevitable investigation into the dam itself.  The fact is that tailings dams should not fail like this – although they do so alarmingly often.

I have seen few images of the location of the source accident itself.  The reports consistently suggest that two dams failed.  Based on the configuration of the tailings dams, this is most likely to have been the set of dams shown in the Google Earth image below, in which two structures lie upstream of a smaller downstream dam:


Samarco dam configuration via Google Earth


So presumably one of the major structures upstream ruptured, triggering a collapse that then over-ran the smaller downstream dam.  The best image I have seen that indicates what might have happened in this one (via Estadao):


Samarco dam failures via Estadao



This image shows the two upstream tailings dams.  The left hand dam (the one to the west in the Google Earth image) above is clearly intact, whilst the one to the west appears to have failed.  So, based upon this, the problem seems to lie with this dam:

Samarco dam viaSamarco Google Earth

Samarco dam via Google Earth


It is hard to know what might have happened beyond this, but the surge that came down the valley was enormous, based on the erosion in the valley below.  Note the height of the trimline in the vicinity of the second dam.  And of course there is now a vast amount of tailings that are not being retained by any structure.  Dealing with this debris must be a priority.

Interestingly there is some information on the internet that there has been recent work to increase the height of at least one of the dams.  There is nothing to suggest at this stage that these works had any role in the accident, but it will be a factor in the investigation