The Hidroituango crisis: new landslides threaten the site

Over the last few days the Hidroituango crisis in Colombia has deepened considerably.  El Pais reports (in Spanish that) the Colombian President, Juan Manuel Santos, stated yesterday that professional advice is that the level of risk has increased in recent days.  The major problem is once again that of landslides, with new slope instabilities becoming evident as the level of the lake has risen.  As noted previously, higher lake levels are generally associated with increased instability, and that appears to be the case here.  The major issue appears to be a new landslide that has developed on the slopes above the dam.  There are some good images of this problem on Twitter – for example, Mabel Gasca has tweeted this image of some of the tension cracks that have developed:-

The Hidroituango crisis: tension cracks on the slope above the dam. Image via Mabel Gasca on Twitter.

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Meanwhile, Diario Siglo XXI has tweeted this image of the site:

The Hidroituango crisis: tension cracks delineating the new landslide. Image from Diario Siglo XXI via twitter.

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Whilst it might be hoped that this is a comparatively shallow slope failure, images from within the tunnels demonstrate that the cracks extend into the rock mass, unfortunately:-

The Hidroituango crisis: tension cracks within the tunnels in the hillslope. Image posted online by Semana.

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Thus, the mass that is mobile is reported to have a volume of about 130,000 cubic metres.  Whilst this is not a hazard on the scale of Vajont, there are two obvious threats.  The first is that the landslide might induce a wave that overtops the dam, with potentially catastrophic consequences.  The second is that the landslide blocks the spillway and intakes, meaning that the water levels can no longer be managed.

There is a little information about the specifics in this case, though news reports suggest that the authorities are getting advice from the UN and the US Army Corps of Engineers, which is good.  The usual way to respond to this crisis would be to draw down the lake level (slowly) to increase stability in the slope, allowing time to develop a mitigation plan.  However, in this case that may not be possible because the tunnels are blocked and unstable and, even worse, El Pais reports that rainy weather is on its way.  This is a translation from the article:

On the other hand, the President said that starting next Wednesday there will be a tropical phenomenon in the area that will produce rain and possible landslides, but warned that they have nothing to do with the dam.  For his part, the Minister of Mines and Energy, Germán Arce, said that the studies reached the conclusion that if the landslides continue, a wave could be generated that would put the area around the project at risk.    “If a landslide occurs in a significant volume of material, this slip could generate a wave that exceeds the dam. That is, it returns us to that initial risk condition, “said Arce.  A few hours ago Empresas Públicas de Medellín (EPM) had said that by next June 7, it was expected to reach the level of 415 meters above sea level in the construction of the dam so that the emergency risk could be reduced.

If the lake levels are going to rise, and the slopes are going to be wetted by rain in coming days, then the stability will likely decrease.  This does not mean that a slope failure is inevitable, but it it is right to monitor closely and to evacuate those at risk.  The Bogota Post reported yesterday that 25,000 people have been evacuated from downstream of the dam.

Acknowledgement

Many thanks to the various people who have sent info to me about the Hidroituango crisis, and in particular to Lance Brown, who has been tireless in his monitoring of this situation.  You can see his various comments, with key links, at my earlier post.  Apologies for my slow update – I was on leave in Singapore last week.