Washed away bridges
Sandy Grant | Wednesday January 18, 2017 16:28
With this kind of money spent on their construction, it is rare for a bridge to collapse, not least because they are designed so that such disasters do not happen. So what happened to the Moshupa bridge?
Why on earth did it so dramatically collapse? The obvious first answer is that it was not a bridge, it was a culvert. To use different words for the same thing may seem picky but the fact is that they are not the same. A bridge allows water to pass through its piers.
A culvert, large or small, has no piers, it is a drop-in corrugated iron casing, an enlarged drain if you wish.
It follows that culverts are more prone to collapse than bridges but that said, it needs to be understood that the principal weakness of both may prove to be the approach roads. With the Moshupa ‘bridge’, however, it appears from Mmegi’s dramatic photo that, unusually, it was the culvert itself which was simply opened up and pushed aside, the wrecked metal being just about visible.
But for those in the Gaborone area to get a better angle on such matters I recommend a peep at the remarkable new bridge over the Notwane on the Morwa-Mochudi road. In fact, it is not the centerpiece bridge itself which is of interest, it is the extraordinarily long approach roads designed as an S to cope with a very wide flood plain. It is a pity that we know nothing about the considerable perceived problems that confronted those who designed the bridge and how, variously, they worked out the best way of overcoming them.
My photo shows what can happen when floodwater finds the weakest point. It works away at it until it finally bursts through taking everything with it.
This is very much what happened in 1966 to Mochudi’s new bridge over the Notwane. The bridge itself, a concrete structure, was, I believe, unaffected by the flood waters but the photo shows that it was isolated in the middle of the water and with no means of approach – as the goats had just discovered!
Noting that those who design bridges and those who decide on priorities are no fools, I do wonder, given the lack of information, why the Ramotswa bridge was given such a priority and why it had to be so extraordinarily large?
The Notwane River floods there only infrequently and for short spells. Could there have been a cheaper solution to what cannot have been a huge problem? Yes, immediately across the river is the border with South Africa and traffic is always held up when the river floods.
But traffic is also held up when the Segoditshane in Gaborone floods. Why was it decided that Ramotswa represented the greater need?