Too Much Water, But Not A Drop To Drink

We investigate the impact of poor drainage systems in South Africa, and how it affects infrastructure and sustainability. 

- Françoise Gallet

Memories of the chaos caused by the flash flood that hit Johannesburg on 9 November 2016 are still fresh in the minds of many South Africans. The images of sunken cars, collapsed roofs and stranded motorists have left many with one question: are the city’s stormwater drainage systems being properly built and maintained? The answer is a complex yes and no, especially if you’re hoping for our country’s water resources to be sustainably managed, experts say.

ENGINEERING, ECONOMICS, WEATHER 

“Where they are properly designed and built, which is more or less always the case, South African roads are built for floods. But, it’s not possible to design for the maximum flood that can occur,” explains engineer Pine Pienaar.

It is accepted that severe weather outside of the norm is going to cause some flooding. To keep the socio- economic impact of any flood to a minimum, the roads are built on a risk-based principle. So, roads of key national importance are built in line with international standards. These include drainage pipes, culverts and bridges designed to pass floods with a return period of between 10 and 100 years (generally storms so big that they happen only once in a 10-year or 100-year period). Meanwhile, lower-order residential roads are generally designed for the power of the storm that is most likely to occur once every two to 10 years (a less severe weather event).

Hourly rainfall data from the OR Tambo International weather station for the massive rain we had on 9 November shows that the day’s rain was likely a 1-in-200-year event, explains Elsa de Jager, unit manager of climate information at the South African Weather Service.

THE ODD FLOODPLAIN