Zandvlei Trust

Draft minutes for the TWG Pondweed Meeting on 10 May 2012 16:00 – 18:00 at Zandvlei Sports Club.

CAPE Estuaries Programme: Implementation of Selected Components of the Estuary Management Plan (EMP) for the Zandvlei Estuary, Cape Town.

Present: Cassandra Sheasby, Dalton Gibbs, Bob Craske, Ken Findlay, Rob MacLean, Damian Gibbs, Peter Kruger and Lynn Jackson.

Apologies: Pierre de Villiers, Chandre Rhoda, John Ridley and Janine Adams.


1. Objectives of pondweed management.

In general, the meeting agreed that the objectives as set out in the 2010 EMP are still valid. These read as follows:

“Given the ecological importance of pondweed, the overall objective of management efforts must be to achieve a sustainable level of pondweed in the system while at the same time preventing the development of nuisance conditions. More specific objectives include:

  • Improved understanding of pondweed dynamics and especially the role of temperature, salinity, nutrients and depth in controlling its growth;

  • Ongoing maintenance of a “pondweed reserve” consisting of closely connected beds throughout the system;

  • Refinement of the harvesting protocol based on a monitoring programme which allows effort to be linked to harvesting needs;

  • The main channels in the recreational areas should be free of pondweed.

On the logistical side, objectives include:

  • Reduction of downtime of the weed harvester through improved maintenance planning;

  • Improved working conditions for the drivers/operators.”

There was a proposal that another objective, namely the purchase of another harvesting machine, should be added. However, after the discussion on the logistics of harvesting (see below), this was dropped.

2. Logistical problems.

There was extensive discussion around the logistical problems being experienced in the implementation of the harvesting protocol/schedule, which it was agreed are the primary cause for the pondweed being “out of control” at present – apart from the increasingly high nutrient levels in the estuary which contribute to the high growth rates.

Points raised included:

  • In addition to the two machines owned by the City, there is a privately owned one at Intaka Island in Century City. However, it was felt that it would not be possible for the City to “borrow” this;

  • It was noted that Councillor Purchase had reported to the recent Public Meeting that funds had been requested against the City’s budget for the next financial year. However, there is no guarantee that this request will be approved given other priorities in the City. A new machine costs around R 2.36 million;

  • Investigations had been done into the possibility of building a new machine. Ray Whitenberg (?) still has the blueprints which were used for the existing machines. But it was found that it would be too expensive to produce them locally;

  • It was also noted that the City’s Invasive Species Unit had two tenders out at present, although it was unclear as to what extent these would deal with pondweed;

  • In the opinion of Bob Craske, it is not necessary for the City to get a third machine as the work can be done by two machines if there are used in a more efficient manner. Under the present arrangement, the second machine – which is operated by City Parks – is deployed at Little Princess Vlei where far less work is required than at Zandvlei (some 90% of the weed needing to be harvested is in Zandvlei). The second machine is therefore currently being under-utilised;

  • Part of the problem at present is that the two machines are operated by different departments within the City, and it seems to be difficult for them to co-operate;

  • Biodiversity do not have sufficient capacity to manage the harvesting eg. only one driver. Their current budget for harvesting is around R 600,000/year;

  • Another problem is that the machines break down on a regular basis, and the repairs and maintenance are the responsibility of a third department – namely Fleet Management, who are responsible for all capital (?) equipment in the City.

  • It was felt that the frequent breakdowns – at least in the recent past – were not being caused by the operator. Moreover, the static items of the harvesters should have a lifespan of 50 years. The problems arise primarily from the fact that some of the parts have been incorrectly engineered and operating errors.

Some suggested solutions included:

  • Constructing a barge to pull behind the machine to enable it to collect additional material before having to return to shore to unload. However, it was felt that this was not practical;

  • Returning the responsibility for managing the machines to City Parks, but with Biodiversity determining which areas should be cut;

  • Outsourcing of the responsibility for harvesting the pondweed to an agency or Section 21 company. It was reported that the Marina da Gama Association had submitted a initial proposal to the City to do this in 1998, and more recently offered to set up a Sect 21 Company if the machines were sold to the Company for one Rand and acceptable rate per Hectare agreed with City, but had had no response. Subsequent discussions with a commercial operator of suitable equipment revealed that this had also been turned down by City. Bob Craske agreed to circulate this proposal to the group.

  • Marina residents had also volunteered to act as observers on the harvester to ensure that the protocol was being properly implemented.

  • The possibility of using pondweed commercially eg. to make fertiliser was raised, but it was noted that this had been explored in collaboration with a farmer in Phillipi and was not commercially viable.

3. Legal basis for City’s responsibility with regard to the Marina.

It was felt that one way of trying to strengthen motivations to the City to improve the harvesting operations would be to look at the legal basis for the City’s responsibility in this regard i.e. was there an agreement between Anglo-American and the City during the construction that the City would take on the function of maintaining the waterways? Bob Craske undertook to try and locate this document.

4. Harvesting Protocol.

In general it was felt that the harvesting protocol as outlined in the EMP (which was based on the Guidelines developed by Harding in 1999, and which in turn incorporated recommendations from recreational user groups regarding the areas required for their respective sports) was still acceptable. The main problem is that the protocol is not being implemented because of the logistical problems outlined above. Nevertheless it was agreed that the representatives of the yacht club and canoe club would revisit the maps provided to Harding, and if necessary submit new maps to the TWG.

A related question was about whether there is a set depth to which the harvester cuts i.e. in relation to the machine. If so, then water depth will affect the extent of the weed harvested. The machine therefore needs to be adjusted based on the water levels on the day of operation.

It was reported that suggestions had previously been made to paint simple depth markers on the arms of the cutter section of the weed harvester to give a guide to the operator – this should make an allowance for the loading of the machine. Further, that if possible the weed should be cut in relation to the bottom of the vlei/canal rather than from the water level. The City had been offered free use of a GPS which, together with a simple depth indicator, would have allowed a map or GPS chart of the vlei to be generated. This offer has not been taken up.

5. Coral worm.

Another problem which affects canoeists to some extent, but which has a significant impact on water flow and thus contributes to stagnation is the extensive coral worm growth particularly in the vicinity of bridges. This is exacerbated by rafts of floating pondweed which get stuck at the bridge narrows when the NW or SE wind blows. It was requested that the removal of accumulated coral worm growth around bridges – and particularly the Park Island bridge – be considered a priority. Previous attempts by residents to do this, both using a machine and by hand, had proved disastrous and it needs to be addressed in a professional manner.

6. Water quality.

It was noted that it is highly likely that the pondweed problems are being exacerbated by increasing nutrient levels, and that the relevant TWG needs to address this. Some practical suggestions included:

  • That the influent water from the Keyser’s River be channelled through a reed-bed (or existing reed-beds be expanded ) to remove some of the nutrients;

  • That the rubble weir be removed and that flushing of the system be undertaken by increasing water levels at an appropriate time of year (possibly August) by closing the mouth until the water reached a set depth, and then opening the mouth. The problem regarding the sewer line below the weir was raised – including the costs of relocating this line – but it was felt that given this is one of the main problems affecting the estuary (exacerbating water quality and many other problems) and that Council need to be convinced that given the value of the vlei, the required funds are a small price to pay.

  • The possibility of a sluice gate to regulate water levels was also raised, but it was noted that this had been previously discussed and it was felt that it would require too much maintenance.

  • It was noted that removal of some of the coral worm colonies would also improve water circulation and thus water quality.

  • Poor water quality also impacts on the suitability of the estuary for recreational use.

7. Other.

It was suggested that the possibility of biological controls for pondweed be revisited – acknowledging that the alien Chinese grass carp referred to in the Harding report are not acceptable.

It was also suggested that an exercise be undertaken to compare current water canal depths with what was originally designed for the Marina (assuming that a water depth / water current / water movement report / plan formed part of the Marina design).


  • Bob Craske reported that at their request, Councillor D’Alton was currently in discussions aimed at trying to get the harvester based in Princess Vlei relocated back to Zandvlei. These efforts would continue.

  • The possibility of forming a Section 21 company to take over harvesting operations would be further explored using the proposal developed by the MdG Association as a starting point.

  • Bob Craske would share a copy of the Marina’s proposal with the group and would try to locate a copy of the City’s agreement with Anglo-American.

  • The recreational user groups would submit updated maps of their requirements.

  • Lynn Jackson would undertake an internet search to see if there were any recent developments in terms of biological controls for pondweed.

  • Relevant proposals from this group would be forwarded to the TWG on Hydrodynamics and Water Quality.

Next Meeting:

It was agreed that another meeting of the group should be held in about a month.

Annex A.

Additional comments received from Bob Craske. 

1. In response to the stated objective that “The main channels in the recreational areas should be free of pondweed.”

“This has always been objected to at the public meetings held on ZEMP – it goes against the advice in reports commissioned (Harding et al + Southern Waters) by Council. In the Marina it would lead to properties on one side of, say, Admirals Walk (Cannon Beat) having the weed cleared at greater frequencies than those on the other side (Cannon Cove) this would apply to numerous other streets in the Marina. If Thibault Beat were to be cut all of the uncollected weed and litter would end up in De Lille Cove and Baalen Cove. It should be borne in mind that property rateable values are, in part, predicated on the proximity to water access (Turpie & Joubert 2001 and ‘Sustainableoptions + others 2009’ Zandvlei in particular). The blind canals would become an overgrown morass of rotting weed and litter. It is highly probable that users would take the law into their own hands and use a herbicide with the disastrous consequences experienced when could did this at another local vlei. The need to harvest the blind canals – to remove the nutrient saturated weed - is more important than the other channels. ERM in their April 2012 report express concern over these matters. With weed lying on the surface wind circulation and mixing of the fresh and saline water is not possible with detrimental temperature gradients forming.”

See also  for details of the Parks and Gardens Weed Harvesting programme.

2. On the logistical objectives:

  • Reduction of downtime of the weed harvester through improved maintenance planning;
    “The reduction in downtime through improved maintenance planning would only be possible if linked to other factors – spares holdings – additional drivers – knowledgeable maintenance staff.”

3. On the proposal to look at commercial uses of pondweed:

“This should continue to be explored – payment is made to the City Dump to take the weed perhaps it would be cheaper to pay to have it removed and reused. (Harding states clearly that it should not be left to drain back into the vlei but removed as soon as possible) Parks and Gardens had that capability – perhaps they could mulch it and use within their department.”

4. On the causes of the frequent breakdowns of the harvesters:

“No mention is made of the attempts to correct the operating errors of the machine at the present;

  • Incorrectly adjusted cutting blades ( trying to cut paper with scissor blades 5mm apart rather than a blade to blade shearing action)

  • Least efficient underwater cutting blades – grain/grass rather than rice – see weed cutter blades fitted to the cutter bed of reed cutter (stepped pyramid blades). Different blades may be required for Water Hyacinth.

  • Incorrect forward speed of the harvester in relation to the speed of the cutting blades – causes some of the weed to be ripped out and float away under the machine - GPS would give an accurate speed indication for the driver– see 1 above also. The ratio of uncollected weed appears to be very high compared to other years - this results in additional collection runs and rotting surface weed in the blind canals + lost time harvesting – to collect the uncut weed - a high pressure water jet was installed in the 1990’s to help with the loss of cut weed from the conveyor belt – is it still operational? “


Please find attached area that IYC need in order to run a proper sailing course. At the moment we are using perhaps half to a third of the available area. Although, historically, IYC have had a race through the Marina, this overlaps with the paddlers requirements and I have not included in this map as it is only one race in the season. If the canals are cleared well enough for the paddlers, it will probably be fine for the sailors.

Created with Skitch - 


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