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Historical impacts on current hydrology at Zandvlei Estuary explored ahead of its rehabilitation

City signage about Zandvlei Estuary pollution.

City signage about Zandvlei Estuary pollution.

Published May 19, 2022

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Cape Town - The rehabilitation and possible redesign of Zandvlei Estuary mouth has brought into focus how pollution, poor water quality and algae blooms are being dealt with at the estuary.

The Zandvlei Trust said the vlei’s current problems were the result of historical developments and urbanisation.

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Spatial planning and environment Mayco member Eddie Andrews said: “The present Zandvlei Estuary mouth was fixed by a concrete canal many years ago and prevents the natural functioning that an estuary should have.

“As a result, Zandvlei estuary suffers from an influx of sand, hampering recreational activities and ingress of fish from the sea.

“The flushing of the estuary is hampered by the canalisation at the mouth and prevents effective flushing of the estuary system during the winter months. A more natural and less canalised estuary mouth will lead toward more natural functioning and a more healthy ecosystem,” Andrews said.

Recently, Andrew Killick, a retired geologist and specialist member of the Zandvlei Protected Areas Advisory Committee (ZPAAC), advised on how historical impacts such as the concrete canal and development of the Marina Da Gama residential area affected the current hydrology and water dynamic at the Zandvlei Estuary.

Environmental scientist and Zandvlei Trust chairperson David Bristow said Zandvlei was an artificial lake in an environment that would usually run dry through several months of the year and was originally viewed as a sandy valley because of its extensive dune fields and shrubland that arose during summer months.

Bristow said it was originally named Zandvallei – the vlei came later after human developments such as the weir.

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Killick said that historically, a strong outgoing flow during winter would have scoured out the mouth and returned to the sea whatever sand had accumulated in the estuary mouth in summer, but in the late 1800s the vlei was artificially dammed. This was when most of the vlei’s current troubles began.

Bristow and conservation activist David Rogers said another historical development was when the northern end of the vlei was dredged to facilitate the development of Marina Da Gama in the early 1970s.

“As wonderful a residential area as it has turned out to be, it greatly reduced water reticulation and increased stagnation of the water body, and with it the build-up of algae and weeds among other ailments. The Marina has increased the vlei’s water volume by 44% and its troubles by at least double that,” Rogers and Bristow said.

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Bristow said the City ordered for the crumbling concrete casement walls downstream of the Royal Road bridge to be reinforced and the engineers tasked with this job opened discussions with ZPAAC and Anchor environmental consultancy, among others, to offer additional options to address this.

This was part of a larger project that looked at the coastal infrastructure in the Muizenberg area.

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