After the first Dream Zandvlei presentation (18 January 2017), Barry Clark from Anchor Environmental offered to discuss the merits of dredging the vlei, following work they did for the Department of Water Affairs, in collaboration with Aurecon and Southern Waters, on environmental flow requirements for the Zandvlei estuary along with several other systems in the Western Cape. Following this meeting some of the findings from that report was also presented at the Estuary Management Workshop, as reported at a Dream Zandvlei presentation on 15 March 2017.
As part of this work, Barry and his team looked at a number of options for restoring ecological health of Zandvlei including considering merits of dredging the vlei. Bernelle met with him on 27 February 2017.
Barry works in marine and estuary management. The work they were doing was required as part of a Department of Water Affairs classification study on the significant water bodies. The classification study broadly means to decide on a future desired state of health of these bodies, which takes into account its environmental, human, agricultural and industrial impacts. Significant bodies broadly fall along the Catchment Management Agencies (CMAs) and Zandvlei falls in the Berg/Steenberg area. This study will be publicly available once published.
The bodies are classified along 3 levels:
2. Moderately used
3. Fully developed.
Further, the bodies are classified along their current ecological health, where A corresponds to largely natural, and E being completely trashed.
“Zandvlei is probably the third most important estuary in the Berg catchment area”
(First Berg, then Langebaan, then Zandvlei, then Diep…)
“It is critically important that we conserve Zandvlei”
– Barry Clark, Anchor Environmental
Generally water abstraction is a large factor in water bodies, with about 60% of the Berg taken out for human use, for example. Zandvlei, in contrast, does not have a large water use, with it’s current runoff about 95% of the reference (or only about 5% of the water taken away). All the degradation seen in Zandvlei is human related, with a large portion recreational. But it is an estuary that is improving. In 1995 when Barry did his PhD on it, the vlei, especially in the Narrows, was filled with dark sludge. Now it has clean marine sand and an abundance of fish. This is due to good management, and specifically to Joshua Gericke’s input.
“Encouragingly, the transformation of Zandvlei from 1995 to now has been most amazingly good”
– Barry Clark, Anchor Environmental
As part of the classification study, reserve determination studies were done, which include water quality records. The present health of Zandvlei is classified as a ‘D’, about 50% degraded, and the best attainable state is advised as a “C”, given the pressures on the system. This is to acknowledge that the vlei will always have a large human component and cannot return to its natural state for a variety of reasons.
Improvement strategies included four scenarios (to be included in the Dream Zandvlei project):
1. Removing the weir
Removing the current rubble weir is desired for better tidal flow into the estuary. This is predicted to have a modest positive impact on water quality. The importance of water depth for recreational events are acknowledged, but Barry recommends that the weir be managed in a more dynamic way, similar to the mouth opening. It should be removed most of the time, for example in the week when people are less likely to be sailing. An option is to have a movable weir (like a sluice?). This would allow for (even) higher water levels when recreational activities take place, and extensive flushing more often per year, which would improve the water quality and reduce the silt buildup – it’s a potential win-win.
Another alternative to maintain a higher level constantly, especially for the rivets of the Marina da Gama residences is to install weirs where the Marina channels join the main water body.
One complicating factor is a sewer pipe that is currently protected by the existing rubble weir.
2. Restoring habitat
The area most requiring this is the shallow intertidal salt margin, which is currently lined with concrete. Rehabilitating this is not expected to really have an impact on water quality, but it would contribute to the biodiversity of the region.
3. Improving water quality
Difficult because of diffuse urban water runoff, but we spoke about the potential for biological means, biomass cultivation rather than chemical or physical means, – and the importance of harvesting the biomass as a means to remove the nutrients from the system.
Dredging did not have a positive outcome in this study, but the scenario that was considered was a deep, 1.5m channel through the middle of the water body. The main function of this is to entrain more salt in the system to reduce the growth of invasive grasses. But, through modelling this is predicted to cause stratification in this channel of the more dense salt water, leading to hypoxic conditions.
In retrospect, however, Barry comments that dredging 0.5m everywhere rather than a single deep channel should still invoke a tidal prism allowing for better flushing. This, in combination with a re-think of the weir may yield better results, but needs more hydrodynamic modeling.
A point to mention is the potential impact of dredging on prawn stocks. Dredging sections over longer periods of time would allow the prawns time to adapt and move to new areas. “Artisinal dredging”. – See Port Owen’s set up in the Bergriver.
“The Zandvlei estuary has huge potential as a fish nursery and in terms of biodiversity once the system is restored”
Closing remarks touched on monitoring stations, linking to nutrient inflows into specifically the Westlake Wetlands. The City apparently has about 20 monitoring stations in the Zandvlei catchment. – listed in Candice Haskins’s presentation at the Estuary Management Workshop (notes at the feedback meeting). From Josh, this data gives an indication on the concentrations, but not the total flows and hence total amounts of nutrients incoming. Flowmeters could be a good project here.
We then briefly discussed rehabilitation of the Westlake Wetlands generally, and Barry noted that the city’s preference for the concrete canals is to reduce the risk of backflooding. But he agreed on the need to have the wetland meander to remove nutrients and design for sediment traps, as the sediment holds most of the Phosphorous. Barry agreed that the nutrient levels are a large potential risk, and agreed that if the larger plants (polymechetons??) are successfully removed, this opens up opportunities for algal blooms which are potentially more dangerous. The nutrients need to be captured into biomass and then removed from the system (harvested). If this can be done with economic yield, even better. Bernelle mentioned her work on wastewater biorefineries.