Sediment management in Zandvlei

Why is it necessary to manage sediment in the Vlei? The benefits of sediment removal

By dredging or other methods.

Zandvlei’s location in an urban environment has necessitated that the water level be maintained between certain limits to meet the needs of the residents and recreational users, but should not unduely disturb the function of the Vlei as an estuary. Unfortunately, the infrastructure put in place by the City Council to manage the water level has had the unnatural consequence of trapping large volumes of sediment in the lower reaches of the Vlei. A study done in 2014 showed that the sheet of accumulating sediment was advancing away from the mouth into the Vlei at an average rate of more than 2 metres per month, leaving behind a shallow area much of which is above water level at low tide.

Too much sediment in the Vlei is bad for recreational activities – canoeing and yachting in particular, as well as for fishing. It is also bad for some of the creatures calling the Vlei home. The Zandvlei estuary is the last remaining functional estuary in False Bay, and as such represents the only nursery for the fish that live in this area. If the estuary fills up with sediment, not only will the fish in the Vlei be threatened, but there is also likely to be a negative impact on the fish population of False Bay.

It is generally accepted that the silting up of the Narrows is due to the artificial raising of the base level of the Vlei by a rock weir constructed just below the Royal Road bridge to protect a sewerage pipe located just above the bridge. The long-term solution is to re-engineer the mouth and relocate the pipe and remove the rock weir, thereby enabling tidal ebb to cut a channel back down to the low water level. This will avoid the accumulation of sand in the narrows.

As the re-engineering of the mouth could not be funded by the Council in the near future, options for remedial action in the interim was to;

  1. Lower the rock weir to try and reduce the level of the sand sheet behind it.
  2. Dredge a channel through the accumulated sediment to the mouth.

Passive sediment removal through mouth management: reduce the weir or more dynamic measures?

From an estuary management perspective, free flowing tidal movement with an unconstrained, meandering mouth is the ideal scenario, for fish activity as well as sediment movement. Due to the current constraints on the vlei, being built up at all sides, this is not currently possible. Keeping the weir to maintain a stable water level is also not good because it cuts off access for fish to the nursery, and it gives no option for the coarser fractions of sediment (coming from upstream or introduced through the mouth by flood tide) to leave the estuary into the sea. Furthermore, removing any constriction to the mouth would minimize flooding that might arise as a result of heavy rain in the catchment area to Zandvlei.

The next best thing is to manually manage the mouth at certain times of the year, depending on rain and tidal movement, which is the current practice. With the prolonged drought this is difficult.

When the mouth is open, there may be some sediment removal through tidal ebb and rainstorms in the catchment. Unfortunately, there is a weir in the way that hampers this action and, on balance, the tidal inflow will deposit more sediment above the weir than is removed. Lowering the rock weir increases the volume of saltwater entering the Vlei and also reduces the level to which the sand sheet will accumulate. But it will not stop the advance of the sand sheet into the Vlei.

The rock weir has now been lowered to the minimum level at which it still affords acceptable protection to the sewer line. The sewer line could be moved, but currently it only relies on gravity to transport the wastes and any alternative option would require pumps and introduce more risk of failure. With the lowering of the rubble weir, there has been increased tidal inflow which has pushed the stability of the building foundations of Marina da Gama to the limit.

Before the drought there was a small net sediment removal in response to lowering of the weir. With the extended drought there has been insufficient rain to create enough flow to remove the sediment that arrives in suspension and it is collecting in the body of the vlei. Even in periods of good rain the weir stops bed-load sediment from being removed from the vlei.

The benefits of dredging

Sediment removal specifically through dredging.

Considering all the options, the need for dredging is clear. It provides all the benefits of working towards Water Sensitive Design except water supply including:

  • Flood prevention
  • Improved water quality
  • Improved Biodiversity
  • Increased property values
  • Maintaining blue flag beach status
  • The maintenance of a well utilised, truly multi-racial public space

Dredging is not regarded as the optimal remedial action to the unnatural build-up of sand in the lower reaches of the Vlei and should not be regarded as a long-term solution. However, if re-engineering the mouth is not presently affordable, it does offer some relief to the problem in the shorter term.

Although there are potential negative impacts to dredging there are several benefits;

  • It will enable salt water to penetrate further into the estuary, and this should improve the flushing of the Vlei and also discourage pondweed growth.
  • Research (reported in Whitfield, 1998) has shown that estuary mouth channels with depths greater than 1.5m have greater diversity and numbers of piscivorous fish. Thus, the deteriorating fishing experienced could be due to the shallowing water for hundreds of metres above the mouth. Dredging may ameliorate this situation.
  • Canoeists will be able to resume paddling, without getting stuck, from the vlei to the sea.
  • The Narrows, one of the most popular fishing sites in the Vlei, would be reinstated and once more expanded as a fishing venue.

Evaluating the potentially negative consequences of dredging

Water stratification is an undesired possibility, due to the more dense (heavier) sea water taking up the deeper channels. On the other hand, estuaries are dynamic: seawater will fill the channels in the Narrows, but it will be exchanged with every tide when the mouth is open. Furthermore, our prevailing winds, induce currents in the Vlei and these result in both horizontal and vertical mixing of the water body. Nevertheless, dredging should be undertaken in a manner so as not to create pockets which may become anaerobic. The front-end shovel used in 2015 is not an optimal tool in this regard. However, in the Narrows these pockets will be short-lived and ironed out by redistribution of the sediment by tidal and fluvial currents, which was indeed observed with the holes created by the front-end shovel in 2015. The only area where dredged depressions are likely to persist is in the middle section of the Vlei, distant from fluvial and tidal action.

The sandprawn population and other benthic organisms would clearly be negatively impacted by removal of the sand bodies. If everything is dredged all at once this could be devastating to the prawns. Consequently, the current dredging plan leaves undisturbed areas of sand flats adjacent to the dredged channel, where the prawns and other benthic fauna would remain. Anecdotally, the prawn population of Zandvlei has been found to be remarkably resilient. This is not surprising as sand prawns live in the dynamic lower reaches of estuaries and have to cope with rapid changes. Not to dredge for the sake of a few prawns and allow degradation of the remainder of the ichthyofaunal nursery does not seem wise.

Conclusion

Dredging is not an optimal solution, but would combat the advance of sediment from the mouth into the basin, thereby keeping the estuarine system functional in the meantime. To be effective dredging would have to be undertaken at regular intervals until the infrastructure at the mouth has been re-engineered.

Littertrap progress

This piece was written for the Marina News and The Zandvlei Trust Newsletters and submitted to them on 17 August 2017. It was written by the ZPAAC chair at the time – Bernelle Verster, in a volunteering capacity.

The issue of litter is a highly emotive topic and a very difficult one to solve. The Zandvlei Protected Areas Advisory Committee (ZPAAC) has been trying to address this issue for some time.

Apart from the bigger problem of how we as a society live and dispose of waste, two more immediate issues make dealing with litter hard: As Councillor Aimee Kuhl reiterated in a ZPAAC meeting in October 2016, problems like litter management have to be balanced against the needs of the massive influx of people into the Cape Metropolis, who needed housing, water and sanitation. In short and as can be seen in this article, the City simply does not have the funding to build and maintain litter traps to the extent that it is required. Secondly, to the credit of members of the public who live, work and enjoy the vlei and its surroundings, many people are contributing in their private capacity through for example litter clean ups. This is, however, of limited value. Litter is an easy and visible thing for people to complain about, but it is also easy to misjudge the scale and scope of the interventions required. We are dealing with immense inequality issues and poor design decisions from the 60’s and 70’s which result in what we see in the waterways.

As a recently appointed chair to the ZPAAC, I had a look over the minutes of the ZPAAC since inception (mid 2015) to track the progress towards better litter management. It is important to note here that ZPAAC is a statuary advisory body to the City Council and that as such it does not deal with members of the general public. The ZPAAC is not obliged to furnish any information to individuals but connects with the public through the organisations represented on ZPAAC.

Note: the minutes of ZPAAC meetings are available on this website.

The timeline of addressing the litter problem since June 2015:

June 2015:                    Bob Crask of Marina da Gama Association (MDGA) tables the state and future plans of the litter traps in Sand River canal. Heavy rain results in the litter bypassing the existing litter traps. It is acknowledged that funding is a challenge. MDGA would like cities backing to retrofit the existing litter traps.

September 2015:           Abdullah Parker (City of Cape Town) met with Bob Crask, and supports the initiative.

February 2016:              The meeting discusses the repair, cleaning and maintenance of litter traps along the river(s) feeding into the vlei. More traps were needed, and an awareness campaign was ongoing. The incidents of dumping were increasing. Dredging and rubbish collection were ongoing. A long term solution to improving the river was proposed of proactive environmental education. Currently approximately 6000 children per year attend environmental education at Zandvlei. It was also noted that the cleaning of litter traps is not always done correctly, resulting in litter escaping over the top or through the bars.

April 2016:                    Abdullah Parker provided a feedback email about the litter traps

May 2016:                     This is the first meeting I attended. Litter traps were not being maintained due to insufficient funding.

August 2016:                Litter Traps were still ‘not working’; in the absence of Bob and Abdullah there was no further feedback. In this meeting I agreed to look into options for sediment removal, which in my view was the most pressing need. Through discussions about the management of the vlei I became aware that all the matters are related, and thus became more involved in the litter trap discussions.

During this time the Zandvlei Trust received an anonymous donation to investigate an improved litter trap.

November 2016:            No specific mention of litter traps. At this meeting the Friends of the Liesbeek were invited as it was hoped to learn about their strategy, seeing the success in managing to rehabilitate the Liesbeek river. They have also more recently suffered from funding shortfalls.

During this time Anchor Environmental did a report for the Stormwater and Sustainability Branch Planning Department – Transport for Cape Town, which is to be appended to the Integrated Reserve Management Plan (IRMP).

January 2017:               Litter traps feedback had been received from Abdullah Parker.

March 2017:                  Joshua Gericke, the Zandvlei Reserve Manager, had a preliminary design for a litter trap and needed a pilot study done. He had spoken to two students, but there had been little interest shown.

May 2017:                     An emotional discussion about the litter traps ensues, but with not enough personpower to address. A few people had a workshop and worked through Prof Neil Armitage’s 1998 WRC report as well as Kyle Kriel’s 2014 report (a student of Neil Armitage). From this it was established that the ground work has been done, but that a student is not appropriate for the final design. The next step is to obtain quotes/expression of interest from consulting engineers.

Litter reduction upstream: The conversation then went towards reducing the problem at source, either through recycling or through global initiatives. Peter Kruger has looked at this at a local level with a nearby recycling depot and Angus Hemp was advised to get in touch with him.

At this meeting I am also elected chair of the ZPAAC.

July 2017 (the most recent meeting):     Following the last meeting and working with Prof Neil Armitage (Future Water Institute and Urban Water Management Group at the University of Cape Town) (who has been contributing pro bono) quotes were sent to several engineering companies. Two companies replied with quotes, JG Afrika and ZAA Engineering Projects & Naval Architecture. ZAA then met with Joshua and myself discussing the project scope and limitations. Following this ZAA revised their quote and informed us that they have selected the Sand River Litter Trap as a ZAA Corporate Social Investment Project, to be executed as a  team effort between all parties involved.

The approach was further discussed, including approaching the litter from multiple interventions, and modifying the current trap to only capture large items, allowing another litter trap downstream to be designed to capture smaller items with less potential for damage. Neil considers the cascade of interventions as a good idea, but expressed concern that the current litter trap is bad design. Neil kindly offered to advise during the project.

At this meeting the concerns expressed by some members of the public of an ‘over-engineered approach’ was also discussed. There was some concern that the litter trap is expensive and will take long to build, which does not address the immediate problem. I took input from all members attending. Neil’s response “If you are not going to do it properly you can’t do it at all” and that “it will have to be an engineering solution as we can’t deal with the social source of the challenge”, was met with agreement all round.

The committee further agreed that “You are not going to solve the problem in one go, it needs to be an iterative solution” particularly as it relates to larger and windblown litter.

Litter reduction upstream

A social approach to litter reduction around the Blue Route mall involving the Keysers river, a WESSA project in the 1990’s were mentioned. While it was noted that there is very little in that river that contributes to the litter in the Sand area, it was lauded as a good approach that we should consider for the catchment at large.

Neil also noted that the litter can be an order of magnitude higher than what we see. Nets can work but it really is about the maintenance and only works at low flow. The better litter traps is where a machine can empty it: it’s not a nice job, people start to ‘pretend not to clean it’.

As part of an integrated and iterative strategy, we need to find ways to reduce litter at source. One low hanging fruit is to get a working relationship with a recycling centre. There is apparently such an initiative underway in Noordhoek, and we are busy investigating this.

Dream Zandvlei: Meeting with Sarah Chippendale

Sarah was project manager of the Source to Sea project a year ago. We met on the 4th of March to talk about how her time there influenced what she would like to see in the Zandvlei catchment.

Continue reading “Dream Zandvlei: Meeting with Sarah Chippendale”