Presentation regarding initial research on managing nutrients, silt and flows in Zandvlei and Westlake Wetlands, prepared by Bernelle Verster and friends, and presented at the ZPAAC meeting on 18 January 2017.
The presentation covers the project background and dredging options. Because dredging would not make sense in isolation of an integrated nutrient management plan, the presentation touches on the need for nutrient inflow mapping as well as wetlands rehabilitation. A challenge on what to do with the dredgings made the presentation conclude on a way forward, and includes the request to ‘dream for Zandvlei’. Images used in the presentation is from the Zandvlei parkrun, and can be found on the Zandvlei parkrun Facebook page.
Background on the speaker
I’m Bernelle Verster, recently completed a PhD in Bioprocess Engineering (UCT). I describe myself as a post-environmentalist, and at heart I am a biotechnologist. I am interested in how to use biology to provide value, and I consider economic and other sorts of value of roughly equal importance. I am a resident of Lakeside. I am passionate about water, sport-STEM (science, technology, engineering, mathematics (& design) interface, and the connectedness of things. I am involved in orienteering, and the Zandvlei parkrun, and am passionate about building community. This year I am figuring out where to go, and my dayjob is split into 4 equal parts: The Water Hub in Franschhoek, The new Future Water Institute at UCT, connecting and communicating around water in general, and lastly, my own stuff.
The project background
“water environments have one of the greatest potential values among urban open spaces (Turpie et al., 2001).“ – WRC report, van Zyl et al
“I sail at Zandvlei – and dodging sandbanks (coral?) is proving to be quite challenging.“ – Neil Armitage, 2016
“the Vlei is getting shallower in the southern part (referred to as the “Narrows“), destroying what used to be one of the better fishing venues and an enjoyable paddle down to the sea.“ – Andy Killick, 2014, 2016
Water Level Management
Martin Thompson (City of Cape Town) compiled a flyer in 2004. It outlines the complexity of managing the level in the vlei. The rubble weir at the ‘narrows’ needs to be lower (about 0,6 m MSL (mean still water level)) for 1 in 100-year flood to pass through the outlet accompanied by a vlei level of 1,78 m MSL, but residents want it higher, about 1.0m MSL for recreational activities. The working solution for this is to mechanically open the sandbar following the tides and season. Each mechanical opening and closing cycle of the sandbar costs approximately R20 000 (2007 figures). The content of the flyer can be accessed via the Zandvlei Trust website.
Reality checks: Zandvlei & Westlake
There is not a lot of money available from the City to make changes, or even do extensive maintenance. Thus there needs to be a maintenance plan, which needs to be self-funded.
The lagoon and wetlands are no longer natural, so restoring to a ‘natural’ state is not possible with the current urban scenario. Thus, the stakeholders need to consider all the factors we consider important, identify some ideal state as a standard and compare against that to measure progress.
Dredging is needed to remove sediment clogging up the vlei. This sediment contains nutrients which affects the health of the vlei. It also reduces the ability of the vlei to reduce flooding, and affects recreation.
Where does the sand come from?
- From the sea, a mouth clearance issue, BUT, this is local, does not affect entire vlei – does likely affect ‘the narrows’
- Windblown sand, BUT with the development of the area, this source is greatly reduced
- Major contributor: streams coming in: Sandriver, Diepriver and Westlake river – preventative measures are possible!
- Does coral worm contribute?
- After the meeting, Andy Killick rightfully added the impact of construction sites, and stormwater depositing sediment.
The biggest challenge: dealing with the sediment once dredged – where does it go?
How much material needs to be moved?
There are two areas to be considered, the main area and a smaller one near ‘the narrows’. Taking a rough estimate for the main area, using the length of vlei as 1300m, and the width as 300m, and the depth of removal (on average): 0.5m (this needs more investigation) gives 1350 x 300 x 0.5 = 195 000 m3.
The Killick report estimates the required area at ‘the narrows’ needing dredging at ~35740m2, and expects this area to need a deeper dredging, about 1.5m. This also needs further investigation. For the first rough calculations we just used 0.5m depth removal, giving ~ 17 870 m3.
Using estimates from a company we contacted, we budget the dredging to cost about R10 million. Transport or dealing with the dredgings would likely be another R10 million at least.
Preparing the presentation I did not see any options to make use of this dredging, but this was where the magic started happening. People mentioned it could be used as filling for a road reserve (area 1 in the image below), Two Rivers Urban Park substrate (but they need to dredge their own wetlands and could use that themselves), Sand bag buildings, material for renovating the caravan park, hard landscaping for example building wind breaks for picnics. The meeting really opened up with possibility here!
Three options for disposal
The bottom line is that trucking the dredgings anywhere would just be prohibitively expensive. One would need a place to treat like a slimes dam: build a berm, pump the dredgings, then let it drain. Once dry, landscape it. This landscape change may slightly affect wind patterns. If the dredgings can be utilised, this is still required as a handling site.
Option 3: Taking it out to sea (where it would have gone in nature), but while this could be the easiest option, in an industry member’s experience, this was unlikely to be permitted. It wouldn’t necessarily be cheaper because barge rental and pipes would still cost a lot.
Option 2: Using the dredging to rehabilitate the canal and landscaping what I called ‘Steenberg vlei’ (areas 23, 24, 25) might be an option. The dredging would be spread over a large area. During the meeting it was mentioned that there may be rare plants here. IF possible to gain access here, then it would make most sense to rehabilitate the canal to meander, allowing the water to slow down, become a silt trap, while also purifying the water. This piece would include the area next to the railway line ( * on the graph).
Option 1: The island near Marina da Gama is another option. At a size of approx 480m x 250m, the island would be raised approximately by about 2m if all the dredging went there. This is the most likely option, in our opinion.
In order to understand how to manage the nutrients in the vlei and implement the most effective interventions, we need to understand where the nutrients come from. The best way of mapping these is with continuous monitoring with probes. The basic things to measure include:
- pH (measuring the acidity of the water)
- Dissolved Oxygen, a measure of the health of the water
- Electrical Conductivity, a measure of the dissolved contaminants in the water
(the things we can’t see)
- Total Dissolved Solids, a measure of the turbidity of the water,
(the contaminants that we can see, that makes the water murky)
- Salinity (how salty is the water)
- Voltage: check the strength of the (solar powered) battery.
Kevin Winter (UCT) lends these out for a start, and ideally we would make our own ones. Kevin has been working to get robust probes that are much cheaper to make. We could possibly put probes on floating wetlands. Once-off / periodic analysis (wet chem) could give an idea of the more complex chemistry in the water.
‘Passive’ nutrient removal options
The conventional options include forebay structures consisting of earth embankment and vegetation. It traps silt and captures litter, but maintenance is difficult. Meandering flows through vegetation, like treatment wetlands are an option, where the nutrients goes into plant biomass and sediment, but it is hard to manage the biomass, it is really hard to remove the sedimentation. There is more interest in wetlands but practitioners don’t like wetlands. Also, it can be conflicting to both conserve a ‘natural’ wetland and try to use it as a treatment wetland.
I would like to propose piloting floating treatment wetlands. You have the benefit of a wetland, but it is physically distinct from the reserve wetlands and can be managed independently. It floats, providing access to the sediment underneath, and can be moved.
Wetlands provide ecosystem services (NB for flood attenuation!), they are our insurance against nature’s temper tantrums. In case studies done in 2004 already in Kirstenhof, it was found that the main costs were associated with flood prevention, but the main benefits were aesthetic and recreational (and increased property values) “Such low benefits indicate how the overall benefit cost ratio of the project could be negative if a narrow ‘engineering only’ approach is followed.”
The maintenance costs were about R200 000 per year (2004).
Reference: WRC report: The costs and benefits of urban river and wetland rehabilitation projects with specific reference to their implications for municipal finance: Case studies in Cape Town (2004). Authors: Hugo van Zyl (Independent Economic Researchers cc), Anthony Leman (University of Cape Town School of Economics), Ada Jansen (University of Stellenbosch)
- Get a plan: proposal
Get current dredging rates
- Meet with people who can advise on permits, EIA, public participation processes. Find out what is nature reserve, what can be worked with
- Meet with Catchment Management (CoCT). Argue for the benefits towards Water Sensitive Urban Design “This checks all the boxes except water supply”, including:
- Flood prevention (after a drought comes a flood!)
- Water quality
- Property rates
- Blue flag beach
- The maintenance of a well utilised, truly multi-racial public space
The plan for 2017: WHAT IS THE DREAM?
From email discussion with Friends of the Liesbeek (FoL, Liesbeek River Life Plan) Kevin Winter and Andrew Bennett: The value of images and architectural designs to sell the dream is a must-have.
- Examine the concept with a wider network of people, including:
- Make a very basic brochure with mock-ups.
- Finding people who can help with creating good conceptual drawings
- Iteratively fleshing out where we want to go: WHAT IS THE DREAM?
- Develop a fundraising strategy, for a start the funding to create the framework plan.
- Have a great brochure explaining the dream.
- 2018: Develop the “framework plan”, with the “design interventions at a framework level with associated principles and guidelines”
Follow up meetings arose from this presentation. The presentation (pdf, 2MB) can be downloaded here: Caring_for_Zandvlei_18jan17_Bernelle