A “big cold front” caused the recent heavy rainfall in Stellenbosch, the South African Weather Service (SAWS) told SMF News. At its peak, rainfall gauges in Jonkershoek recorded 124 mm of rainfall on 19 April, according to Dr Jan de Waal, a researcher of water resources in Stellenbosch University’s department of geography and environmental studies.
Why the flooding?
On 19 April there was widespread flooding in and around Stellenbosch.
“It’s always the first rain of the season that floods,” said Professor Neville Sweijd, associate professor at Stellenbosch University’s (SU) School for Climate Studies. The first floods of the rainy season, he added, are often due to “management issues” related to the stormwater systems.
According to SU scientists, the combination of sudden rainfall, impenetrable surfaces, and backed-up drainage systems often causes flooding of Stellenbosch’s urban spaces.
“Stormwater [systems] were the problem,” said Dr Andrew Watson, senior researcher of hydrological modelling at SU, who noted that Stellenbosch had experienced “urban flooding, and not natural flooding”. Watson explained that the natural water system had not been overwhelmed by the rain, while the urban systems had. The fact that the Eerste River did not flood is evidence of this, he said.
The town has “a high concentration of impervious areas”, Watson added. In other words, Stellenbosch is covered by a lot of tar, concrete, and bricks that do not allow water to drain through these surfaces.
Over the dry summer months, debris builds up in the stormwater and drainage systems, and “when build-ups of leaves and mud happen, floods are gonna happen”, said Watson. “There is no way of stopping them.”
“As part of our ongoing winter preparedness programme, municipal teams continuously work to clean and clear stormwater drains of debris and obstructions,” said Stuart Grobbelaar, spokesperson for Stellenbosch Municipality.
However, he also pointed out how “due to the sheer volume of rainfall” debris enters the stormwater draining systems, and “can overwhelm systems and lead to temporary blockages”. He added that the municipality cleans drainage systems to “quite a good extent” before the rainy season, but struggles to keep up due to the size of the municipality that stretches to Klapmuts and Franschhoek.
Winter is coming
Sweijd said this storm was merely an indication that the wet Western Cape winter had arrived. “A storm makes winter,” he said. “People have a short-term memory for weather. We often have a storm around Easter – the first winter storm.”
Statue of Jan Marais on the Rooiplein of Stellenbosch University against a backdrop of stormy clouds, only metres away from the underground library entrance, which flooded over the weekend due to heavy rainfall. PHOTO: Janco Botes
During the transition from summer to winter, “atmospheric rivers” (bands of moisture/clouds) move northwards as the southern hemisphere tilts away from the sun. “The cold fronts that usually pass underneath us [dropping rain over the oceans south of Africa] during summer now come onshore,” Sweijd added. These cold fronts bring with them the “wet weather, and moisture from the southern oceans to South Africa”.
The SAWS said that more of these cold fronts would make landfall throughout this winter, creating the ongoing wet conditions typical for the Western Cape.
During this particular storm, Watson explained, Stellenbosch had been the epicentre of the storm. He added that according to the most recent data, while river levels surged in Stellenbosch, the peaks were significantly lower around Franschhoek, Paarl, and Somerset West.
Global warming’s possible imprint
Sweijd told SMF News that globally “the chances of extreme rainfall events are greater” due to rising temperatures.
“Oceans are much warmer than they used to be,” said Sweijd, which means that evaporation from the ocean increases, meaning more moisture in the atmosphere. On top of that, because the atmosphere is warmer it expands, creating “greater carry-capacity”, which means it has more room to hold more moisture. This starts brewing the perfect storm as “our weather systems are carrying more water”. In practice, this means that “there is more water to fall in any rain event”, amplifying rain events across the world, he noted.
“With more energy, and more water vapour in the atmosphere, […] flooding has become the biggest change if we look at extreme events,” said Sweijd. “[Climate change] exaggerates seasons, […] and pushes you into new territory.”
