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:
On the logistical side, objectives include:
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:
Some suggested solutions included:
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:
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).
It was agreed that another meeting of the group should be held in about a month.
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 http://www.mdga.co.za/code/waterways/weedharvester.htm for details of the Parks and Gardens Weed Harvesting programme.
2. On the logistical objectives:
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;
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.
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