Hidroituango: the potential for extremely large landslides?

The latest news from Hidroituango is that the movement of the landslide continues without major change.  The Hidroituango twitter site posted a series of updates late on 5th June local time (early on 6th June UT) that indicate that:

1. In the last 24 hours, the behavior of the mountain on the right bank of the reservoir has been stable [I think this means stable sliding – i.e. no change in behaviour.  It is certainly not stable from a slope stability perspective]. According to the data of the interferometric radar  have presented some slides of surface material that reach Speeds less than 10 millimeters per hour, i.e. below the alarm threshold, which has allowed the continuous work of all the work fronts required in the project, except for the equipment that must advance work in the [plazoleta de compuertas = spillway?], due to the rain.

2. The two water leaks that have been detected in the dam wall are being kept under permanent observation. Both leaks, located in dimensions 375 and 375.3, do not record color or washing of fines and added its flow is less than 35 liters per second, so this Tuesday June 5 was progressed normally in the tasks programmed for its sealing by means of injections with bentonite, a clay that expands and improves the impermeability of the dam.

3. Water flows to the reservoir and downstream discharge are maintained at an average of 1200 m³/s. The reservoir level is 393.55 MASL, with an ascent rate of 0.4 meters per day

This information comes from the integrated information and alert Center, coordinated by EPM and operated 24 hours per day by specialized professionals, who adapt information obtained in real time and in a manner consistent with alerts notification protocols. For the PMU.

This situation was described yesterday as being “highly critical” by Jorge Londoño De la Cuesta, manager of EPM, yesterday.

The most interesting development though is a set of suggestions that the site could be affected by much larger landslides.  For example, El Colombiano has a report online that appears to imply that an American report – presumably the report by the US Army Corps of Engineers? – indicated that the maximum volume of a landslide at the site could be between 10 and 40 million cubic metres.  That would be an exceptionally large landslide – this is the realm of the high mobility (i.e. rapid) rock slide (although high mobility is not inevitable even at this volume), which could be very dangerous indeed.  It isn’t at all clear to me how the topography shown below can generate landslides on this scale, but it is far from impossible. It would be very interesting to see this report.

The slope at the Hidroituango dam, a part of which is unstable. Image via El Nuevo Siglo.

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Once again I am mindful of the extreme pressure on those individuals responsible for managing this crisis.  The current situation must be extremely difficult to address, with so few options to gain control in the short term.