Consultants recommend pondweed harvesting, other interventions for Rietvlei remediation

15 March 2023

The City of Cape Town has received the final assessment reports from specialist consultants on the proposed short, medium and long-term remediation interventions for Rietvlei, located in the Table Bay Nature Reserve. The City is studying the reports, and notes the recommendation that dredging of Rietvlei is not advisable at this point in time; and that a range of alternative interventions – key among which catchment level interventions, and the mechanical and manual harvesting of pondweed – could be implemented to improve the overall water quality and health of the waterbody. We are pleased to announce that a mechanical weed harvester for Rietvlei will be delivered by mid-January 2024, if all goes as planned. Although dredging is not recommended for the immediate future, the City will still go ahead with the environmental and other approvals needed for dredging as these processes can easily take up to three years to finalise.

Specialist consultants were appointed in June last year to advise the City on possible remediation options, as well as the potential risks and benefits of dredging Rietvlei to improve the water quality for recreational use.

Apart from the removal of organic material by dredging, the consultants investigated a range of remediation options including: the capping of organic material, water circulation and aeration, retrofitting of sewage grinders in nearby sewage pump stations, pondweed management through mechanical harvesting, and planned treatment reedbeds at the Bayside Canal outlet.

In summary, the specialist consultants' recommendations for Rietvlei are as follows:

The high costs and significant ecological disturbance associated with dredging of this particular waterbody do not warrant this intervention at present
The ongoing build-up of organic and inorganic sediments from catchment inflows and in-lake processes can be contained within the deep-water basins in which they occur
Artificial water circulation and associated aeration would not address the water quality concerns given the particular nature of this waterbody, and could potentially worsen them
Catchment interventions would improve water quality over the longer term
Catchment interventions should be complemented with improved in-lake management, through the controlled growth and sustainable harvesting of pondweed
The installation of sewage grinders in nearby substations should reduce the frequency of blockages at these stations
Improving bunding capacity within or around pump stations, to allow for sewage retention and prevent overflows while the City effects repairs
Identifying sewer manholes that are low-points from where sewage is likely to flow when there are downstream blockages or when there is a pump station failure – the manholes should be raised where possible and/or their design adapted to allow diversion of overflows back into the sewer rather than into stormwater
The Bayside Canal discharges stormwater from the Table View area into the north-western part of Rietvlei. The construction of sediment ponds and treatment reedbeds at the downstream end of the Bayside Canal with litter and sediment trapping upstream of the reedbeds, would improve the quality of water passing into the Rietvlei area

The assessment reports recommend the active management of pondweed in identified key areas of the vlei as one of the alternatives to dredging to improve water quality.

While pondweed is a nuisance to recreational users of Rietvlei due to it clogging motors, rudders and centreboards and making passage through parts of the vlei difficult for small craft, it also offers ecological benefits, including:

Oxygenation of shallow submerged waters
Stabilising of bottom sediments, making wave-induced turnover of bottom sediments and resuspension of their associated nutrients less likely
Provision of habitat complexity, making the dominance of a few nuisance aquatic faunal species such as midge larvae or mosquitoes, less likely if there is adequate habitat for aquatic predators such as fish larvae and some macroinvertebrates
The uptake of nutrients including phosphorus, makes pondweed a potentially significant store for this nutrient, which can be removed through harvesting.

'I am pleased to say that the City has awarded a contract for a mechanical weed harvester for Rietvlei. If all goes as planned, the harvester should be delivered by mid-January 2024. Thus, we are already on track with this deliverable, and note the finding that rather than deepening Rietvlei through dredging, the City should actively manage pondweed growth by reducing it in areas where recreational activities take place, while allowing it to expand in other sections.

'The mechanical weed harvester will assist us to undertake frequent, effective and controlled pondweed management. This is a priority for the City, as are interventions to reduce the inflow of nutrients and contaminants from the catchment feeding Rietvlei,' said the City's Deputy Mayor and Mayoral Committee Member for Spatial Planning and Environment, Alderman Eddie Andrews.

The reports state that pondweed should be removed from areas where boat launching takes place. These could be identified through engagement with the Milnerton Aquatic Club, while allowing pondweed growth in other areas, in particular the southern portion, where it will not interfere with, or can be managed to minimise interference with recreational activities.

It is recommended that the City uses a mechanical weed harvester as the harvesting of pondweed and other selected aquatic plants will assist in removing nutrients (particularly phosphorus) from the ecosystem, provided that it is carried out on a sustainable basis.

'The specialist consultants' findings will complement the City's Diep River Estuary Management Plan that was adopted by Council in December last year. As it flows towards the sea, the Diep River meanders through the Rietvlei and Milnerton Lagoon. The revised Diep River Estuary Management Plan identifies pragmatic and realistic management interventions, and combined with the recommendations from the consultants, it will guide the City's actions going forward in improving the water quality at Rietvlei. Also, while dredging is not recommended right now, we will still go ahead with the environmental and other approvals needed as these processes can easily take up to three years to conclude,' said Alderman Andrews.

End

Published by:
City of Cape Town, Media Office