Mandavu Dam Wall

     If I had made any New Year's resolutions (I didn't) I might well have resolved to post to this blog on a regular basis and I would already be close to breaking my resolution. That's entirely normal of course but... I would have an excuse (and a real one at that!) For some time after we got back from our Christmas break, we were without a reliable internet connection. Eventually we discovered that the problem was not some arcane bit of IT or satellite programming that had gone wrong but something we could solve fairly easily - the mounting for the satellite dish was coming away from the wall and putting the dish out of alignment. I fixed it back up and it seems to be OK now, so here goes......

   Mandavu Dam.

    Fourteen kilometers from Sinamatella is the biggest body of open water in Hwange National Park - Mandavu Dam. It's a hugely productive dam, thanks to the nutrients introduced by the hippos and the hundreds of buffalo, elephants and others that drink there in the dry season so it is popular with anglers who are allowed to fish there at weekends. It's also a vital part of Sinamatella's game water supplies, and an important breeding and feeding place for numerous birds.

 

 We pass the dam regularly and often spend time there counting and listing birds but until recently we had never given a thought to the maintenance of the actual dam wall. I'm certain we wouldn't have thought of it now if Colin Gillies from Wildlife and Environment Zimbabwe hadn't e-mailed me with some concerns over the state of the wall. I knew about as much about dam wall maintenance as I do about computers (i.e. next to nothing) so I asked my good friend Professor Google for help. He, of course, knows more or less everything, and as I read through various websites that he suggested, I became more and more concerned.

   The first thing I was sure we needed to worry about was trees. Apparently, trees on an earth dam are a disaster waiting to happen. Mandavu didn't have trees - it had a forest........

   The road across the wall - dam to the left, forest to the right!

   Next concern was termites and animal burrows. We have both of those all over Sinamatella so probably at Mandavu too but had anyone ever checked? Almost certainly not.

  Prof Google guided me to one website that gave a series of stages to look for to show any deterioration in a dam wall. First stage, in which there is nothing to worry about,  is a flow of clear water on the downstream side and the final stage, probably best labelled 'panic',  is a whirlpool in the dam itself and a strong flow of muddy water and debris downstream. As far as I could see, at that stage all you can do is 1. Run for your life and 2. Call the emergency services. I was pretty sure that we were far from that situation but not at all sure of anything else so Courage Mutema, the Sinamatella Parks Ecologist and myself went out to have a look.

   It was, as you would expect, a mixture of good news and bad news. The good news was that there was a small flow of clear water through the wall, there were no active termite nests (though there were some large ant nests) and no obvious animal burrows. The bad news items were that the wall was covered in trees, that many of the rocks which are meant to resist wave action (the technical term seems to be rip-rap) were out of place and that the road across the wall has eroded, causing rain water to pour down the back face of the wall where it has cut some fairly disturbing gullies.

Part of one of the gullies - not something you want to see in a dam wall!

   The gullies were obviously something we should be dealing with very soon so we started work on them first. A proper solution to the problem will mean rebuilding the top of the wall to direct rainwater into the rip-rap but that will take some organising. For now, we have simply thrown tons of rock and brush into the deepest gullies and we'll try to cover them over with soil so that grass can grow and bind it all together. It would have been nice to have heavy earthmoving equipment to do this but that's way beyond our resources. Instead, we have manpower, and of course a Land Rover ....

   Removing the trees turned out to be a huge task. If they had been Mopane, as they would have been in most parts of Sinamatella, it would have been difficult enough but in fact they were mostly Acacia tortilis, which comes armed with both hooked and straight thorns..........

   ..........and strongly objects to being handled. It took us two full days to fell all the trees, cut them into reasonably sized pieces, winch them up to the top of the wall and stack them. By the end we were all heavily scratched and thoroughly fed up with the task but satisfied to have got it done. I will go and kill all the tree stumps with a strong alkali to stop re-growth and poison the ants whose nests seem to go deep into the wall. We will eventually re-build the eroded crest of the wall and spread the branches over it to protect the loose soil and encourage grass-growth. All being well, this is a lesson learnt and regular dam maintenance will become part of the annual work schedule at Sinamatella. We are grateful to Colin Gillies for pointing out what needed to be done - but I have to say, there were times as I pulled yet more hooked thorns out of my skin, when I wasn't quite so grateful!