The opening of the weir at Zeekoevlei on Freedom Day, Tuesday April 27, marked the 24th annual drawdown of the wetland.
A weir is a barrier across the width of the vlei that alters the flow characteristics of water as well as the water level and a drawdown allows nutrient rich water to be flushed out of the vlei
Explaining the need for the drawdown, Bronwen Foster, people and conservation officer, for the City of Cape Town’s spatial planning and environment department, said: “This intervention is carried out to allow the vlei to be drained of excess nutrients during the winter rainfall period. The lowered water levels also allow for easier access, to remove litter and excess reeds from the vlei.”
The Zeekoevlei weir will be closed on June 30.
“However, this date is flexible to accommodate a potential cold front and heavy rain that may be present at that time,” said Ms Foster.
The False Bay Nature Reserve members, Friends of Zeekoevlei and Rondevlei (FOZR) and residents of surrounding areas attended the event.
Sidney Jacobs, chairperson of the FOZR said the residents were excited about the drawdown.
“The residents loved to see how the water was released. It started just after 9am, with amazing, breathless weather. We were shocked as we expected about 25 people, but (there were) over 60 attendees – and even a couple of boats were there.”
Freshwater ecologist Dr Liz Day who described the event as “great”, with lots local community engagement, shed some light on the role of the drawdown in managing Zeekoevlei.
She said this year’s drawdown marked 24 years since the first one. “It is easy to forget how much the vlei has changed since then, and what enormous strides have been made in rehabilitating the system as a result of drawdowns.”
She said In 1995, following decades of inflows of polluted water from the upstream catchment, as well as some 40 years of direct inflows of treated sewage effluent, Zeekoevlei had been regarded as one of the most polluted water bodies in the country.
“The abundance of phosphorus nutrients carried into and stored within the vlei promoted plant growth, and since the 1950s, when pondweed growths in the water were eliminated, the vlei waters have been characterised by blue-green algae.”
Dr Day said the thick green blooms covering large areas of the vlei were common in spring and summer, and at times when the algae produced toxins, the vlei had to be closed to users..
“Over time, the rain of dead algae dropping to the vlei bottom created a deep layer of sludge.”
She added that, in 1983m 200 000 m3 of sludge had been dredged from Home Bay – the protected area in front of the Zeekoevlei Yatch Club – but the nutrients remained in the system.
“The algal rain continued, while reed beds encroached ever further into the vlei,” she said.
“Water hyacinth also made its appearance, and enormous rafts of this weed drifted across the vlei, at times blocking the yacht club entrance and preventing access to parts of the vlei.
“Reedbeds advanced and were managed from the shorelines with difficulty,” she added.
“In 1997, following years of pressure from the (then) Zeekoevlei Environmental Forum (ZEF), the original weir was modified to allow the vlei to be partially drained, and the first drawdown took place. The first drawdowns exposed decades of accumulated litter, thick sludge throughout most of the vlei, and even the odd body.
“It is easy to forget all this, because it has been a long time since the vlei was last closed because of toxic algae (Covid lockdown closures being another issue entirely) and the water hyacinth was eliminated for several years by the False Bay Nature Reserve team, and their vigilance has kept it well under control today,” she said.
How drawdowns help the vlei
If the vlei is drawdown-ready, then polluted inflows from the catchment can be flushed through the system – the first rains of the season usually bring the most polluted water into the vlei.
- They provide an opportunity to remove polluted water. Over the past year, for example, there has been additional nutrient loading as a result of sewage inflows, and the drawdown creates the opportunity to “reset” at least some of the pollution load.
- Drawdowns increase the contribution of cleaner inflows from the aquifer. A 1.2m head of water is removed during drawdown, and if the vlei is refilled after a good wet season, when the aquifer is full, then it fills relatively quickly with this water.
- They provide management opportunities for the removal of litter, reeds, water hyacinth and other invasive plants.
- They provide short-term shocks to an ecosystem that was previously too stable … and which favoured domination by a few (usually nuisance) plant and animal species.
- They allow aeration of sediments and their exposure to sunlight, which dries out sludge, speeds up decomposition and results in a decrease in organic sludge over time. The southern and eastern shores were, in early post-drawdown years, characterised by deep sludge which has since given way to sandy shores.
- They also provide shallowly inundated habitat for wading birds, increasing biodiversity for a few months.
- They provide opportunities to inspect the shoreline for point-source or seepage inflows of polluted water. Seepage water from the Waste Water Treatment Works settlement ponds was observed during early drawdowns, and subsequently addressed with a cut off drain in 2008.