Prognosis is dire for dry dam
● Cape Town’s biggest dam is now so low that it will take a major flood just to saturate the sand — and officials have been urged not to count on significant storage for years to come.
Theewaterskloof dam near Villiersdorp provides about half of Cape Town’s water. This week it was just 12.9% full. Farmers have exhausted their allocation and officials are preparing to dredge channels to get access to the remaining water.
Experts said the once-overflowing dam risked becoming obsolete in the short term due to the enormous amount of sediment now exposed to the elements.
“We have a dam that is now desiccated,” said Anthony Turton, a water scientist and former specialist at the Council for Scientific and Industrial Research.
“You will need something on [tropical storm] Domoina scale to resaturate the desiccated sand pile, to bring it to the point where you can start having liquid water again.”
Photographs of Theewaterskloof have circulated widely on social media, vast sandy plains and dead trees dominating a vista once covered with water.
To augment its dwindling surface water supply, Cape Town is building four temporary desalination plants and extracting groundwater from two aquifers via a network of boreholes.
Gavin Smith, water spokesman for the Greater Cape Town Civic Alliance, said water managers had now entered uncharted terrain in terms of balancing supply and demand. “This is all new — we have never been through this before. There is no historical data to work on,” said Smith, a former member of the Berg River Catchment Area users forum.
He pointed out that Theewaterskloof dropped perilously low in 2005 after a fouryear drought cycle, but a 100mm cloudburst helped it recover. “It takes a while to get to saturation point. You have so many millimetres of rain before you see anything coming up,” Smith said.
But Department of Water and Sanitation deputy director-general Trevor Balzer said rain falling in a dry catchment area “was not lost to the system” and added up over time. “With the first rains you might not get as much runoff as you would in a saturated catchment — that’s a normal issue after a dry period,” he said.
“The water that saturates into the soil layers is not lost, it rejuvenates the grass, trees and groundwater. It just takes a bit longer for us to reach the type of [dam] levels you would get out of a wet catchment.”
He appealed to residents not to panic or be taken in by doomsday headlines.
Climate scientists are refusing to make any detailed rainfall forecast for the coming winter, claiming such seasonal predictions are unreliable. They said such forecasts should not be used as the basis for planning decisions — as appears to have been the case in Cape Town.
The Climate System Analysis Group at the University of Cape Town recently stopped publishing its forecasts due to fears about how they could be misconstrued, according to scientist Chris Jack, who was a key figure at a climate science summit last year when scientists resolved to urge water managers to plan for drier conditions.
Turton said it appeared water managers were aware of the oncoming disaster but had reacted too slowly. “I’m afraid in my professional opinion a disaster is unavoidable because we’ve simply left it too long.”