5 June 2018
Hidroituango – an increase in movement rate
In the last few hours, RCN Radio in Colombia has reported an increase in the movement rate in the landslide at the Hidroituango dam site, breaching the alarm threshold of 10 mm per hour. This has triggered evacuation of 45 workers at a part of the dam site. It is unclear as to whether this is merely the landslide showing typical stick-slip type behaviour, or a transition into a new phase of movement. Without a graph of the movement pattern this is impossible to ascertain. Meanwhile, the government is establishing a monitoring centre to collate the data, as recommended by the team from the United Nations and the US Army Corps of Engineers. This is a prudent move.
The same team has described a series of scenarios for the dam, as reported by BN Americas:
“They have established that we have a dam that is high risk, and that risk is associated with three main scenarios, some that are connected to the geotechnical risk of the mountainous mass or by landslide or damages to the powerhouse, which could trigger in the failure of the dam.
“The second scenario is that this unleashing would lead to a wave, as Minister Arce has just said, that will exceed the prey and also lead us to the failure of the dam.
“The third has to do with the dam itself: the top priority of level 385 to 410 and to 430, as has been established, can, by the way it was done to respond to the emergency, can generate some leaks and these filtrations could trigger in the failure of the dam, for which they recommend reinforcement of the priority filling, of the increase of the dam in these emergency conditions.
I think this means that they are concerned that: 1. Failure of the rockmass in the abutment (i.e. the landslide) could destroy the dam directly; 2. the landslide could trigger a wave that overtops the dam; and 3. the dam might fail because of seepage.
If so then this is a project is in deep, long term trouble. The lake level is currently rising at 70 cm per day, with the current level at 393 metres. The spillway is at 401 metres, so the lake level has to increase by another 8 metres yet. The groundwater will rise as the lake level comes up, with some lag, so the stability of the slopes will continue to decrease. Of course the lake level will increase beyond the spillway height in periods of heavy rainfall (there might be two metres depth of water flow through the spillway, unless the lake can be drawn down. If so then the lake level might top out about 10 m higher than at present, at some point.
Meanwhile, the two leaks at the foot of the dam are releasing 18 litres per second and 11 litres per second respectively. The authorities are trying to stem the leaks by sealing the site with bentonite. I remain unclear as to the source of water generating these flows.
Finally, a couple of weeks ago El Colombiano posted this video reportedly showing processes occurring within the tunnels at Hidroituango:
This was reportedly associated with the water flow in the machinery house. I have not seen anything like it – I am assuming that it was caused by cyclic pressure changes in the tunnel, driving condensation. I am not sure what this means, or why it was occurring. Does anyone have any insight?