Blog
By David Coltart
11th February 2014
A top South African based specialist dam engineer has studied the photographs of Tokwe Mukorsi and has made these remarks. They are very technical but do convey the seriousness of the situation:
“Tokwe-Mukorsi Dam is a concrete faced rockfill, or “CFRDâ€. This dam type comprises a rockfill embankment, with a concrete slab on the upstream face to create the impermeable barrier. The concrete face slab is generally constructed after the rockfill embankment is complete, or is at least well in advance, in order to ensure as much settlement as possible occurs before the concrete slab is in place.
From the recent photographs of Tokwe-Mukorsi dam, it is clear that the construction river diversion capacity has been exceeded, either as a result of a design for an unusually high risk, which has some precedence in Zimbabwe, or due to the extreme intensity of the flood that recently entered the dam.
As a consequence of the upstream concrete face slab not yet having been constructed, the rising reservoir simply flows through the permeable rockfill embankment. Although the rockfill embankment is normally structured to allow flow-through without damage, the expected flow is seepage, rather than a torrent. Furthermore, a subjective observation suggests that the downstream shell rockfill contains excessive fine materials, and consequently might be erodible, while a distinct pattern of the water emerging on horizontal surfaces is a typical sign that some of the fill might be less permeable than it should be. As long as the apparent situation persists, progressive erosion will continue and a real risk exists that the unraveling process could finally lead to the failure of the dam.”
In separate comments the same engineer has advised that even if the “progressive erosion” is stopped and the dam is saved the wall will have to be rehabilitated which is going to be a very expensive exercise.
What astounds me is how the Herald in particular has studiously ignored this story as if there is not a major crisis. All the engineers I have spoken to have said that whilst it is unlikely that the wall will break, it is seriously damaged now and, as the above statement concludes, if the progressive erosion is not stopped the dam could still fail.
What the Zimbabwe public needs are regular reports from Government based on the factual developments at the dam as they evolve. There is far too much at stake for this issue to simply be wished away.