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*

While 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 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

Automatic control gate releasing a fixed flow into a canal

A regulating control gate prior to installation

and/or crest gates can be fitted, which will open automatically as the water level rises to discharge a safe flow downstream, while surplus water is stored in the water feature, thereby attenuating the flood. When the flood recedes, the gates will automatically close to retain the full supply level in the water feature. It also releases sediment, which would otherwise build up in the water feature.

By providing retarding facilities on many smaller streams and watercourses, the peak flows can be held back, and the water released at a safe rate over a longer period of time. These will then considerably reduce the impact of the combined peak flows and minimise damage downstream, as well as deposit litter in an area where it can more easily be cleaned up.

Large dams

For the large dams on major rivers such as Shongweni, Inanda, Hazelmere and others, there should be large bottom outlets to lower the water level ahead of predicted heavy rains. This will provide additional capacity in the dam to attenuate the incoming large flood.

Unfortunately, these dams have fixed spillways and do not have the flexibility to provide additional storage to provide protection to downstream properties or lives. However, such dams can be modified to provide this additional protection against future flood events like that which KwaZulu-Natal has just experienced.

*Managing director, Amanziflow Projects

FIGURE 1 Routing through automatic control equipment

FIGURE 2 Larger catchments and rivers automatic canal gate

A section of the Robinson canal in Johannesburg. Installing litter traps in rivers and stormwater systems is a highly effective way to capture and remove plastic and other waste