How to reduce future flood damage KwaZuluNatal, in particular its coastal regions, suffered its worst flooding in decades in April this year. The unprecedented rainfall over a few days caused wide-scale damage to property and infrastructure, as well as loss of life. A major tragedy, it provides crucial insights for future flood mitigation across South Africa. By Peter Townshend, Pr Eng*
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hile this was an extreme event, we now know from climatologists that, due to global warming, such extreme events will occur again. Lessons have been learnt about the inadequacies of the stormwater infrastructure, the need for routine maintenance to clean out large stormwater conduits, and the importance of keeping canals clear of vegetation. Preventing human settlement encroachment into streams and flood plains is an equally important priority. Going forward, measures need to be taken to minimise the adverse impacts of future floods, including environmental pollution. This includes concerted efforts to eradicate plastic litter and, in the immediate term, install litter traps in rivers to prevent it being strewn on South Africa’s Blue Flag status beaches and along attractive river banks inland and on the coast.
Robinson canal An example of a successful litter trap intervention inland is the Robinson canal in Johannesburg. The Robinson litter trap has a catchment of 8 km2 within the Johannesburg business district. On average, 80 bags of litter have been removed after even moderate rain events. Such litter traps can be installed on minor urban waterways, as well as waterways serving rural and peri-urban settlements. In terms of operation, the waterway or canal is closed off with an automatic control gate so that water flows over side weirs and through
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a series of specially designed screens to trap flotsam and other debris and then returns to the water course relatively free of litter. The screens are designed for easy cleaning. Most of the litter comprises plastic bags, bottles, tins, polystyrene containers, tyres, clothing, and the sort. There are also dead cats, rats, dogs and other organic items caught up in these traps. Furthermore, oils are trapped and adhere to the waste. However, these oils can be removed with oilabsorbent booms.
Dry season During the dry season, when the waterflow is low, it is possible to divert low-flow, highly polluted waters into a sewer, as in the case of the Robinson canal, or in some cases through an artificial wetland to remove most of the biological waste before returning it back to the watercourse. For floods greater than a 10-year event, the regulating control gate will then open automatically, sufficient to maintain a high water level in the canal. This is to protect the debris screens so that the litter is not washed away in heavy floods, as well as to minimise upstream flooding. Even with the gate slightly open, flotsam on top of the water is still diverted through the screens. A series of these litter traps should be constructed downstream of dense urban and informal settlement areas. The collection of this litter could also be privatised to earn income from the recycled materials.
Stormwater management It will also be important to retard the stormwater peak flows through a series of retarding or retention facilities. These would be situated on open public spaces such as parks, golf courses and on sports grounds that can accept occasional flooding. Most retarding facilities are earth embankments with pipe or culvert outlets. These fixed, orifice-controlled structures increase the outflow with the rise in water level. These systems require a large space to accommodate the incoming peak flow. An alternative, which will reduce the area for flood attenuation, would be to install an automatic self-regulating gate as the outlet. These gates will open to discharge a safe maximum flow downstream so as not to cause flood damage. As the incoming flood increases above the safe flood capacity of the waterway, the gate will close slightly to maintain the safe outflow, while the excess flood water is stored temporarily behind the embankment. This results in less area being used to attenuate the flood and is therefore more effective than fixed-size outlets. These automatic control gates will also allow sediment to pass down the watercourse and will deposit litter on the retarding facility banks where it can be more easily removed than along the banks of the watercourse.
Automatic scour and crest gates For retention facilities such as urban lakes or large water features, automatic scour gates
