Sewage spill

23 March 2020

Please be advised that a sewage spill was detected to be coming down the Langevlei Canal this morning (23/03/20) and entering the Sand river canal. The issue was traced to a pump station upstream and rectified however untreated effluent has entered the system and will soon enter the main Zandvlei Estuary Nature Reserve waterbody.

Bioenzymes are being applied to the effluent in the Sand river and we shall continue monitoring the spill to ascertain it’s extent and severity but in the interim, we are advising users to exercise caution and avoid the northern sections and shoreline of the Zandvlei waterbody until further notice.

We apologise for any inconvenience caused. Please distribute the above message to your constituents and contacts for dissemination.

Kind regards,

Kyran Wright

Reserve Manager: Zandvlei Estuary Nature Reserve

PROTECTED AREAS NOTICE:

Please be advised that, as the Management Authority for Zandvlei Estuary Nature Reserve and according to R6(e) of the Regulations for the Proper Administration of Nature Reserves (GNR 99 of 8 February 2012), we shall be CLOSING THE GATED ACCESS SECTIONS OF ZANDVLEI ESTUARY NATURE RESERVE to all members of the public until further notice, as per the direction of the City Manager. 

The closure shall apply to the Northern section of the Reserve (Bird Sanctuary), Park Island and the Zandvlei lookout (formerly known as the Rendezvous). The purpose of this closure is as a precautionary measure to deal with the global COVID-19 pandemic. These areas are indicated in red on the map overleaf. 

This decision has not been taken lightly and we are aware that the above actions will cause disruptions and inconvenience to Reserve users. We are entering an unprecedented time that calls for greater awareness and teamwork. It is incumbent upon all of us to support the national effort to reduce infections. 

It is important to note that the Open Access sections of the Reserve and waterbody remain open for use by the public. 

NOTICE: CLOSURE OF ZANDVLEI ESTUARY NATURE RESERVE

Please be advised that due to the global pandemic of the COVID-19 virus, the President of the Republic’s pronouncement on 15 March and directives from the City Manager, we are unfortunately closing the following sections of Zandvlei Estuary Nature Reserve to members of the public, effective immediately, until further notice:

–          Northern section (Bird Sanctuary)

–          Park Island

–          Zandvlei Lookout (formerly known as the Rendezvous)

This decision has not been taken lightly and we are aware that the above actions will cause disruptions and inconvenience to Reserve users. We are entering an unprecedented time that calls for greater awareness and teamwork. It is incumbent upon all of us to support the national effort to reduce infections.

It is important to note that the Open Access sections of the Reserve and waterbody remain open for use by the public. Please note the official press release below and Protected Area Notice attached.

Kyran Wright

Reserve Manager: Zandvlei Estuary Nature Reserve

Biodiversity Management Branch

Environmental Management Department

Physical Address: Coniston Park, Coniston Avenue, Steenberg

Dream Zandvlei visioning exercise

Workshop held at the Zandvlei Sports Club on Thursday 7 February 2019, facilitated by Alan Cameron from Places Plus. The take home message is included here and the full notes is included as a pdf.

The requirement for security of tenure for the Zandvlei Sports Centre, to enable a pivot in how Zandvlei is managed was the single most supported idea at the visioning exercise held at the sports centre. About 22 area stakeholders attended, representing a variety of interest groups and active citizens from the area.

The visioning exercise asked individual participants to share and then prioritise their hopes, but also their fears, for the Zandvlei area so that goals could be set and risks identified. Pink and green dots showed individuals’ primary and secondary priorities while yellow and orange dots showed likely risks and those with a large potential impact.

Several themes arose from the exercise, namely:

  1. Governance
  2. Community involvement
  3. Environmental and ecological

1. Governance

The idea of having security of tenure (5 pink, 5 green) was shared as it may provide a base of operations, a consistent presence in the vlei that could generate activity and be used to host activities and raise funding. This stability would provide a base for more adequate and effective management by the city (4 green) of the whole area.

Adequate funding (2 green) for projects and improvements was seen as necessary, and tied to participants’ level of participation and energy (2 green). Research-driven policy and management would improve meaningful implementation. Using a public private partnership model to effect change was also mentioned. Crime (3 orange, 1 yellow) and traffic congestion (2 yellow) were seen as plausible risks.

2. Community involvement

Community involvement (6 green); adequate, active stewardship and a sense of ownership by user community (4 green) tie directly into the call for: an overall masterplan (2 pink, 2 green); how these communities’ impact on irrigation as well as how development and sewage influences water quality (2 pink). This community involvement would then contribute to a sense of ownership and pride in young people, which would especially be visible in how they engage with visitors. Important tasks were identified as: re-engineering of the mouth and sewage line, solutions to backyard-dwellers’ waste management, and creating paths for cycling, walking and canoeing.

The most plausible risks identified were the City losing the ability to monitor ecological quality (4 yellow) leading to severe pollution (5 yellow) and surprise at the crash of the False Bay fishing industry due to the fish-nursery properties of the Zandvlei estuary being destroyed (4 yellow).

3. Environmental/ Ecological

A functional estuary (3 pink, 4 green) with the situation under control (6 green) characterized by a plastic-free Zandvlei (4 green) and interconnected and extended water ways (3 green) with circular access routes to and from all communities (1 pink) characterize a Dream Zandvlei.

This environment is impacted most by:

  • research-inspired action (3 green),
  • a mix of habitats and beautiful urban design (2 green),
  • sewage systems that have an alternative to dump into the vlei,
  • and a nature reserve system that works to address over-supply of nutrients, invasive species and plastics.

The fish nursery is nurtured and regular patrols of wetlands prevent and clean weed encroachment.

With a broader biodiversity (2 green) and a more diverse flora and fauna (3 green), integrated sporting facilities (as well as for night-time activities), Zandvlei is well frequented and very popular with a shared sense of ownership among users.

Next steps and recommendations

  1. A meeting with the ward councilor to discuss progress and ask for advice.
  2. A meeting with community members not reached in this first meeting – contacts available through Berenice (ZENR) and Sharon McCallum (Dream Zandvlei group)
  3. Better understand the current tenure of the Zandvlei Sports Centre and investigate alternative arrangements.
  4. Look at donor funding bodies that are aimed at a broad array of interests (youth and sporting development, environmental stewardship and conservation, infrastructure upgrades (re-engineering mouth, sewer line, promenade, sports and toilet facilities).

Water quality monitoring

HELP NEEDED: Calling citizen scientists: We are working on sourcing and interpreting data to give a weekly updated ‘dashboard’ of the water quality of the vlei. We need help on generating the data and a way to embed it in an useful way on the website. Keen to help? Please get in touch: admin@zpaac.org.za. Check out other ways to get involved here.

Something along the lines of 

E.coli levels (big circle coloured red/orange/green)

link to what this means

(one or two other parameters of importance)

Bottom line: Smiley face or sad face

Short description

Salinity measurement method (pdf, 300 kB)

Sewer spill in Sandriver canal

Update: 4 May 2018

CITY OF CAPE TOWN MEDIA RELEASE

Zandvlei water area re-opened to public

The City of Cape Town’s Environmental Management Department has advised that the Zandvlei water area is now open to the public following a short period of closure. As water quality testing results have shown a significant improvement, the City is re-opening the Zandvlei water area to the public for recreational activities, with the precautionary recommendation that water users stay about 50 m away from the inlet of the canal north of the marina. Read more below:

The City of Cape Town has undertaken ongoing water quality testing within the Zandvlei water body since last Friday, 27 April 2018, when the temporary closure of the vlei was effected. Test results made available today, 4 May 2018, has indicated a significant improvement of the water quality to levels that are acceptable as per the national recreational water use guidelines.

The precautionary closure of the water area has therefore been revoked as of today, Friday 4 May 2018. Recreational activities such as canoeing, windsurfing and sailing can therefore continue with the precautionary measure of staying about 50 m away from the inlet of the canal north of the marina (Sand and Landelvei River canal).

The City apologises for any inconvenience caused as a result of the temporary closure of the water area and thanks the public for their cooperation during this period.

End

Issued by: Media Office, City of Cape Town

Media enquiries: Julia Wood, Manager: Biodiversity Management Branch, Environmental Management Department, Tel: 084 464 9153, Email: Julia.Wood@capetown.gov.za.

27 APRIL 2018

CITY OF CAPE TOWN MEDIA RELEASE

Zandvlei water area closed to public until further notice

The City of Cape Town is temporarily closing the Zandvlei water area from today, Friday 27 April 2018, as a precautionary measure following water quality concerns. The closure only applies to recreational activities in the Zandvlei water area, including fishing. Access to the Zandvlei Estuary Nature Reserve and other visitor facilities in the area remain open to the public. Read more below:

The City of Cape Town has undertaken water quality testing at various points within the Zandvlei water body due to water quality concerns, and has concluded today that the water body should be closed to the public as a precautionary measure until further notice.

The closure only applies to recreational activities in the Zandvlei water area, including fishing, but does not prohibit access to the Zandvlei Nature Reserve and other visitor facilities in the area.

The test results showed high levels of faecal coliforms (Escherichia coli) within the water body, which indicates an elevated risk to human health.  The public is therefore advised to avoid all contact with the water at Zandvlei until these levels fall back within national recreational water use guidelines.

The high E. coli count is a result of a sewage spill earlier in the week due to a bulk sewer main collapse, which has subsequently been contained. The City has been monitoring the effect of the sewage spill into the water body and the additional pollutants entering the vlei from the catchment area. The additional pollutants are as a result of the flushing effect of the recent rain after an extended dry period. The results of these water quality tests dictated the decision to apply a precautionary approach for the safety of the residents of Cape Town.

The City will continue to monitor the water quality and expects to see an improvement in these levels soon. In the meantime, signage has been erected advising visitors to the vlei to exercise due caution. The public will be informed once the vlei is safe for recreational use. The City apologises for any inconvenience caused as a result of the closure of the water area and thanks the public for their cooperation during this period.

End

Issued by: Media Office, City of Cape Town

Media enquiries: Julia Wood, Manager: Biodiversity Management Branch, Environmental Management Department, City of Cape Town, Cell: 084 464 9153 or Email: Julia.Wood@capetown.gov.za

GroundUp article: Sewer pipe collapses and destroys homes (26 April 2018)

Update: Wednesday 25 April 18:00

Water quality
Preliminary results show EXTREMELY high counts of E coli. This is to be expected. We are waiting for an official call from Env Health. In the main vlei water is such that contact recreation should be avoided.

Volumes of spillage
We do not have exact figures but given the size of the pipe this is likely to be large volumes. The Zandvlei mouth was opened on Sunday in response to the spill; from the reserve side there is not much else that can be done.

Sand bank at the mouth / rubble weir
Due to the sand build up at the mouth – the rock weir has been totally buried by sand – there is likely to be minimal flushing with sea water, this will also depend on the rainfall we receive. At this point we estimate at least a week; but E coli levels will drop during this time.”

Update: Tuesday 24 April 10:00 am

The sewage have stopped and the water is looking clear.

As of yesterday at approximately 16h00 over pumping of the collapsed sewer commenced – this eliminates the need for the tankers that were apparently the source of the issue.
The spillage from the sewer was therefore almost completely eliminated from that time onwards, a recent check has confirmed that the overflow has stopped completely.
The contractor is currently sourcing at least one additional pump to act as a standby in case of a breakdown, also to assist with keeping the sewer flowing at peak times.

Whatever contamination found its way into the Sand River canal has long been washed downstream by the rain – it is therefore not feasible to pump out the canal.
We have however make plentiful use of bio-enzymes to attempt to minimise the environmental impact downstream.

The spill was pretty much confined to the Sand River canal – all of the stormwater drains in the area discharge there.

We are constantly monitoring the situation.

Further to this the structures that were over the area of the sewer that has collapsed are being moved, the appointed contractor is currently on site and is commencing repairs.
The primary aim is to make the sewer operational and reduce the dependence on temporary pumps to keep it so.

Also noting with gratitude that the new litter nets managed by residents of Marina de Gama seem to have done a good job of preventing the bulk of the oil slick relating to the spill from entering into the main waterbody.

Notice sent to stakeholders, Monday 23 April 2018.

Dear ZPAAC stakeholders

Brace yourself, this is very bad news.

As most of you know at this stage, there is a sewer spill into the Sandriver canal.

A main sewer pipe collapsed in the Vrygrond Area on Thursday. It is 3m underground. Above ground there is an informal settlement.

The situation was being maintained by pumping with trucks and so no sewage was entering the canals. However there has been violence at Vryground and the trucks which were pumping had to be withdrawn for safety. Sewage is now entering the canals. A contractor has been appointed to repair the canals but we have no details of how long the repairs will take.

This is a sectional pipe en route eastwards; it is a big pipe. It is basically the Southern Suburbs’ sewage.

All the people involved are currently trying to find ways to address this. For the moment the sewage will keep flowing. The mouth was opened on Sunday morning around 09:00. The sand plug at the mouth however means that there will be limited outflow to remove sewage.

Another option is to try dam the canal, but with the volumes involved the advice Dalton received is that it may not be possible.

Please advise all water users to be cautious, this is going to be a LOT of sewage.

Zandvlei in the drought

The drought is affecting the Zandvlei estuary in two major ways: the water quality and quantity as a potential source of water.

The important message here is that Zandvlei is under stress, like all other local wetlands, due to the low water levels from the drought. The nutrients that have accumulated in wetland systems over time can now be expected to be manifested in algae blooms , excessive plant growth and in this case a new weed species coming to the fore. Wetland systems can be expected to manifest water quality issues in response to the stress imposed upon them by years of city runoff, pollutants and the drought conditions. What we are seeing are symptoms of underlying stresses placed upon our wetlands.

Water quality

The condition of the Zandvlei estuary is currently a cause for closer monitoring. It is not as dire as some may suggest, considering what it has to accommodate:
1.         There is a larger than usual amount of stringy algae. This is due to the hot temperatures, and also a nutrient build up because there is not enough flow to flush the system, because of the extreme  drought.   Lower water levels concentrate nutrients already in the system – high temperatures spurn algal growth.
2.         The drought also means that there is a larger build-up of nutrients higher up in the catchment. When rain does come it washes large amounts of nutrients off the catchment into the estuary in one or two events, as opposed to continued rain with small amounts of nutrients over a long period.
3.         The biggest challenge the system currently faces is sediment build up which is impairing the functioning of the estuary as a nursery. The annual maintenance dredging has been redirected to drought emergency measures.
4.         The mouth opening depends on flow into the system to manage the salinity, and the flow has been low due to the extreme drought. The build-up of sand in the mouth of the estuary means that when there is tidal interchange the amount of water exchanged with the sea is limited. In addition, the budget for this is also constrained due to drought emergency measures. The sand bar, coupled with the long drought, means a decrease in sea water exchange and a decrease in flushing of the system. Thus the conditions seem to be set for poor water quality issues in the summer ahead.

The top priority of the ZPAAC currently is to dredge the estuary. While this will have a positive impact on the nutrient balance and hence pond weed growth, the pond weed management is not the main topic under scrutiny. Any help obtaining funds or assistance in getting any amount of dredging done will be welcomed.

Water extraction

There has been reports of water being extracted from the estuary. It is illegal (see below), but people removing water by hand from Zandvlei wouldn’t be a huge ecological disaster; there may be health consequences but this would probably be out of the ambit of the City to resolve.

In terms of the Marina; the first 2m of water is owned by the adjacent property and is not Council property. Thus Council permission is not needed as they are not the land owners. The person however still needs a water use licence from National Government and be registered as a water user.

The Zandvlei water has a high salt content (about half sea water) so it won’t be too good as drinking water.

Below is an extract of the present legislation – it is old and outdated but still in effect. If you witness water extraction, please call the reserve ranger staff (24/7) at 083 499 1717 (please note this is for ENVIRONMENTAL emergencies only).

 

 

Dream Zandvlei – members wishlist

An excerpt from the minutes of the ZPAAC working meeting held on 17 January 2018. In due course this will be regrouped per topic and worked into a detailed, timelined subsidiary plan to the management plan. Please add your thoughts and comments below!

The reasoning is that we need significant amounts of funding, which can only realistically be obtained through private-public partnership and that requires significant economic incentive. This requires a strategy that likely includes commercial development.

The input from the meeting participants are noted below. Going forward, DG noted that many of the desired things are already legally required and built into the management plans. BV noted that this is encouraging for investment. The risk of irresponsible development is limited because of extensive legislation and a vibrant local community. Enforcement of basic rules like fishing equipment and public order policing is challenging.

DG suggested after the meeting that the management plan is vague, and that more detailed subsidiary plans are needed with timelines and defined objectives. These include specific topics, like invasive flora, development, sediment management schedules, littertrap and litter management etc. DG, BV and the new reserve manager will meet and formulate these with input from the ZPAAC.

Obtaining funding for these plans to supplement city budgets is a full time exercise, GL noted we need a full time fundraiser. GL developed a job description for this. To pay the salary for this person, BV suggests entering the Dream Zandvlei into global competitions, for example. GL shared the #cocreate design festival as example, after the meeting.  It would be great to get a landscape architect volunteer on board for the visualisation.

Preamble: Andy: sewer line: if moved make a dramatic change, but doesn’t bring it back to natural, because it’s so constrained. But it is likely the best we can do.
Can we put the sewer pipe below the low spring tide? Drop 2m? May cause small leaks, would require an additional pump station. The most expensive part is the power line to the pump station. The sewage from Clovelly, St. James, Muizenberg, Lakeside goes through this pipe.

Cherry:

Rebuild estuary roadbridge – traffic, restore canals,

More natural estuary mouth – seem to fit well with the estuary roadbridge

Build the Steenberg extension at the bottom of the M5. If this road is landscaped properly, it would still be an environmental asset. – bridge over railway line there. Allow to develop the northern part of the reserve better, currently underutilized.

Environmental reserve, platformed tented camp. – Can get to this nature reserve by train!

Perhaps café, open on weekends.

The old campsite revamped, including a very special amenity centre, special sort of restaurant (like Harbour house)

Source to Sea pathways – connecting the catchment (good already in Constantia valley, Tokai), have started in Muizenberg and Lakeside, need to connect them in Westlake, and consider issues of safety. It’s a short distance to connect to Kirstenbosch

Water quality,

An enhanced aesthetic value to Cape Town

A financial audit of what the estuary is worth from a fishing perspective.

Educationally worth optimized.

Marketing programme for students and learners, learn, understand, take ownership of this area.

Set of behavioural rules – a code of conduct for how people should behave in this environment.

Better signage

 

Pam:

Investment, investors

A mini waterfront, having a market, restuarants, not too upmarket

Nothing noisy

 

Lathif:

Look at what is already happening, working with that rather than designing something new

Investors look at maximizing profit at the exclusion of the people who live here, so need to manage that, keep the people and council involved.

 

David Roux

The banks around the sea cadet base, the eastern side of the yacht club, something permanent. The sand bag idea worked well.

Better utilization of areas – e.g. next to caravan park, e.g. light environmental centre, camping centres

Move on the bowling green

 

Anthony

Source to Sea is very important, creating connectivity out of the vlei, drawing people in.

Security concerns follow this,

Also False Bay source to sea efforts. But we need to open up the community. The more people using it, the safer it will become, and yes we need to upgrade the security.

Improving awareness. Not just this community, but everywhere, catchment management, catchment and upwards. Strong community involvement.

Improved amenities, making use of space that isn’t used properly Look at what GreenPoint park has achieved.

Sufficient budget from council for management. Not only from a staffing perspective.

Office facilities, education centre (co-working spaces) that budget need to be allocated

Naturalisation of the litter traps

Don’t: harden the edges of the vlei. A hybrid system is important.

Connectivity to the mountain, from a natural flow of flora and fauna.

 

Dalton

The management plan – 2011. The proclamation has now formally been proclaimed. These ideas are only ideas until they are encapsulated into the plan.

“Zandvlei usage is a valuable component of the CT network. Appropriately used for education … “

These ideas are not short on thought or on plans – these are in place! Institutionally we are one of the better managed in the Western Cape.

Code of conduct – we have a recreational water use by-law Section 23

Economic study was done in early 2000 – in terms of property values, link economic benefits to natural systems

Suggest: A natural function estuary cycle bearing in mind we are in an impacted cycle. Fix the water levels etc. It will always be a ‘novel ecosytem’

Manage the impact – big one is water quality, solid waste and chemicals. Not that badly affected by water quantity.

The system needs to be better integrated in the surrounding environment, e.g. the caravan park has no connection to the external environment.

Zandvlei is very well studied.

All the work has been done! It is ripe for investment.

 

Andy

Operating as close to a naturally functioning estuary as possible. We need to do remedial dredging

Relocate sewer pipe

Allow water to fluctuate more

Maintain passage to allow fish

Water depth at about 1.5m at spring tide

The money and budget to achieve these

 

DON’T WANT:

The destruction of Zandvlei as a fishing nursery

Motorised craft

Urban or industrial development along the normal edge of the vlei

Litter

Nutrients, polluted entering from the North

Imbalanced weeds

 

Gavin

Similar to Cherry’s

Royal Bridge road needs to be triple its present span

The promenade walkway needs to come down – this is the part that constricts the mouth

Open up the mouth to four times it’s current width, allow it to self-regulate

Is in the plan, but Phase 2 onwards need to be implemented

Need a full-time fundraiser

Elegant, big windows restaurant that has great benefit from the sunset.

 

Peter

All my points covered

Having one representative in the city to deal with

 

Yvette

Children to engage with nature, sport

Adequate management

Sensitively and appropriate

Non-motorised boating

Eco-centre

Awareness – showcase eco stuff

The by-laws are there, but difficult to enforce them – need manpower and money – Dalton: the new by-law gives users the ability to manage their own by-laws

 

Lathif

Water reticulation of the ends of the waterways (Boksburg has a mobile unit to reticulate this water)

(Ask Tamsin about law of taking water from vlei to sell)

Fishing – endangered – Chapter 3. Better enforcement. More security officers, more staff.

More notice board. Better signage. (That doesn’t get stolen in two hours)

We need full time law enforcement – lobby local council! In Zandvlei and in Zeekoevlei. It is an important recreational space and need law enforcement (only the beaches seem to have them)

Walkway between Uitsig waterway and  the <> – Sandriver, that boom gate is not working to prevent undesirable traffic (only for emergency)

Agriwaste – diffuse pollution, pollutants in the ground – integrated nutrient management,

 

Angus

Not have Green algae!

Different weed harvester design? Dalton: tenderized model may have faster turnaround. Privately owned, leased to the city. If the machine is too specialized it may be useless in the near future.

 

Bernelle

World class water (sports) tourism destination

Vibrant fish

Vibrant birds

Nutrient managed

Softened banks, rehabilitated riverbeds

Softened catchment – WSD

Clear waters

Ferry fun stuff between caravan park and sports club

Zip line

Promenade

Dynamic mouth management – vibrant estuarine habitat

Promenade, night time walks, LED lighting (not light pollution), perhaps powered through wetland/microbial fuel cells (MFCs)

Dredged vlei, sediment used for award winning architecture and landscape architecture

Lots of otters!

A new “Otter trail” including sections of the Hoerikwaggo

Safe hiking

Vibrant community – also including the “homeless” to live the life they are comfortable with

Multifunctional, productive spaces

More animals, e.g. eland

Science! High tech integrated but also educational, it’s tech, but it’s not about the tech, it’s about the people.

Inclusive, resilient

Food gardens, allotment plots? => wider than Zandvlei, incorporate open areas everywhere, extend the green corridor, integrate with the cycle routes, down to Cape Point, up to various places, the V&A, the PHA

Link with the catchment – articulate and visualize the relationship with the catchment

Access and support for multiple activities – horses, dogs cycles, yacht, canoe, fish, integrated, responsible hiking.

Less lawns!

Win awards, world famous

Inclusive also to light industry – showcase the circular economy

Better circulation in Marina da Gama – connect with Capricorn business park, as per the original development plan

“liveable neighbourhoods”

Public-private research, engaged research

Park Island higher profile

Permanent orienteering course(s)

Build community between and within neighbourhoods – e.g. Muizenberg, Lakeside, Westlake and beyond.

Don’t want litter, motorboats

 

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.