Cape Town - Last week the Berg River Dam reached full capacity for the first time in five years. Capetonians were relieved to see the first of their supply dams overflowing and releasing water into the Berg River.
However, this raised the question of whether overflowing dams were a waste of water, said Dr Kevin Winter, a senior lecturer in the Environmental and Geographical Science Department at UCT. With more rain ahead, it looked likely that parts of the Berg-Olifants catchment would receive average winter rainfall this year when compared to historical records.
“Signs are encouraging that the major dams supplying Cape Town and the surrounding region will be 80% full by November 1, which signals the start of the hydrological year, compared with 34% in 2017,” said Winter.
However, he warned that managing dams might be a lot harder than we think. Dam managers clearly understood the storage part, but must deal with a complex set of requirements, such as groundwater recharge, abstraction of water (legal or otherwise), maintenance of aquatic life and discharges to release the build-up of pollutants, in addition to supplying sufficient water for downstream users.
“During the Western Cape drought, water was deliberately stockpiled in the Berg River Dam. It made sense to pump water from the Theewaterskloof Dam, a shallow dam with a large surface area exposed to evaporation, to the Berg River Dam, a deeper dam with a smaller surface area.
“Storing water in the Berg River Dam during 2017 and 2018 also meant that water could be released to meet the needs of the ecological reserve,” said Winter.
He said managers of the dam could justify the position that they took, but what effect did this have on the ecological reserve, and what indicators were available to tell the story?
Studies of the Berg River upstream from the dam showed that the stretch between Paarl and Wellington were permanently hypertrophic (very high in nutrients), resulting from failed or poorly functioning municipal sewerage systems and contaminated storm water, but mostly from run-off from informal settlements. Releasing more water from the Berg River Dam helped to flush out the build-up of nutrients in the Berg River, but was by no means an effective way of managing a deteriorating river system.
Following the logic of the role of water release in servicing the ecological reserve, the data showed that when more water was released from the Berg River Dam during winter rainfall (2010-2014), there were signs that trophic levels were reduced. By contrast, low-flow conditions during the dry period (2015-2018) caused trophic levels to rise above the hypertrophic threshold, notably in the months of August and November.
But the data didn’t give a conclusive analysis. The number of times that sampling occurred at the Hermon site (2015 to 2018) showed a 40% reduction by comparison, with no sampling taking place during the month of December for the entire period.
The only conclusion that could be drawn was that data and information informing catchment and dam managers was so limited that it would be impossible to balance the needs of water supply against the needs of the ecological reserves of the river.
Winter concluded: “It seems that there is a long way to go before new dams are considered for storing excess water without understanding the effect on riverine ecological goods and services.
“The blame cannot reside entirely with the management of the Berg River Dam, considering the limited feedback that they receive.
“What is urgently needed is a comprehensive monitoring plan that includes indicators about the state of the ecological reserve and offers timely information to managers involved in a range of functions for maintaining and regenerating the river.”