City notes consultants’ recommendations for dredging of Zandvlei

23 March 2023

The City of Cape Town has received the final assessment reports from specialist consultants on the proposed remediation interventions for Zandvlei, located in the False Bay Nature Reserve. The City is studying the reports, and notes the recommendations and considerations related to the imminent dredging of Zandvlei. 

Specialist consultants were appointed in June last year to advise the City on the proposed dredging of Zandvlei to improve the water quality for recreational use, and practical remediation options to address pollution in the catchment. The study included hydrodynamic modelling to evaluate the sedimentation patterns, tidal influences, projected climate change implications, and dredging scenarios.

The reports state that Zandvlei hosts inorganic and organic sediments and that fine sediments contain the greater part of contaminants and nutrients contributing to the poor water quality. Of critical importance is to also address the sources of pollution from the catchment to ensure the ongoing health of Zandvlei's ecosystem, and to reap the benefits of dredging in the long-term.

'According to the experts, the dredging of Zandvlei would have an overall beneficial effect on the water quality of the estuary. However, there are several considerations the City needs to take into account, among which available funding and how we can use the budget that we have to achieve the best possible outcome. In this sense, the reports are very useful as the consultants also advise us on an incremental way forward as far as dredging is concerned,' said the City's Deputy Mayor and Mayoral Committee Member for Spatial Planning and Environment, Alderman Eddie Andrews.

The reports recommend the following dredging scenarios:

• Contaminated sediment is spread over the entire Zandvlei, and the ideal is to dredge the whole waterbody to the same depth – this would cost an estimated R202 million for dredging, dewatering and hauling to a disposal site
• If funding is limited, the priority area to be dredged must be the upper part of the estuary as it has a higher rate of sediment, the highest nutrient loading in the sediments, and higher rates of pollutant build-up than the lower parts of the vlei
• The dredging of the tidal channel will improve water exchange in the estuary with tidal driven ocean water and should be implemented in conjunction with dredging of the upper part of the estuary
• The estimated cost for dredging the upper part of the estuary is R67 million, and will take about 13 months
• The minimum dredging scenario is to dredge only the areas dredged in the 1970s to the same depth
• Intermediate dredging could be focussed on increasing the ecological benefit, as well as recreational use areas of the main waterbody
• Further ecological, financial, and logistical considerations must inform the final decisions around the most suitable dredging scenario(s) and this will be subject to the outcomes of environmental approvals
• Dredging would remove poor quality sediments from the estuary, resulting in improved dissolved oxygen concentrations, and reduced algal growth

Mitigation measures are recommended to limit the negative impact of dredging, among which:

• To minimise the duration and extent of disturbance to the estuary, start upstream and work downstream, during the dry period when the mouth is closed
• Dredging activities in the lower estuary should not take place at the peak of wader presence in summer
• Dredging should not take place during estuarine fish recruitment peak in spring and early summer (September to December)
• Dredging should not happen during peak leopard toad breeding activities in late winter (August)
• Water quality should be monitored in the vicinity of the dredging area

Given the high nutrient load in Zandvlei, the estuary is facing challenges with diverse and abundant invasive alien plants, among which water hyacinth. Overgrowth of algae and the excessive expansion of reedbed stands changing terrestrial habitat diversity is also a result of the eutrophication of the system.

'I am happy to say that the City's procurement of a mechanical weed harvester for Zandvlei is well under way, and we anticipate the delivery of the vessel by the end of November this year. In Zandvlei in particular, a weed harvester is used to cut pondweed and also to remove excessive filamentous algae build-up in the marina,' said Alderman Andrews.

Next, the City will commence with the necessary statutory processes related to dredging, among which an Environmental Impact Assessment and Water Licence. It is foreseen that these processes will commence in the 2024/2025 financial year.

End

Published by:
City of Cape Town, Media Office