Draft minutes for the TWG Water Quality and Hydrodynamics Meeting on 14 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: Pierre de Villiers, Cassandra Sheasby, Dalton Gibbs, Martin Thompson, Candice Haskins, Robert Siebritz, Mulalo Tshikoti, Thilivhali Meregi, Ken Findlay, Andy Killick, Lara van Niekerk, John Fowkes, Sandra Fowkes, and Lynn Jackson.
Observers: Kim Adams, Allisen Okeyo, Ryno van Schoor (interns at Scientific Services).
Apologies: Alan Boyd.
1. TOR for the TWG.
The initial TOR for the group read as follows:
i) To revisit the current monitoring programme to consider where adjustments could be made so as to have an “early warning” system in place for such events in the future;
Based on these, as well as submissions from the other TWGs and some individuals, it was suggested that the meeting should focus on the following priority issues:
A further submission recommended that the heavy metal load in fish in the estuary should be determined, particularly if fishing of alien species for consumption was going to be encouraged.
2. Contingency Plan.
It was reported that there had already been some internal discussions within the City (between CSRM and Scientific Services) with respect to the constraints regarding the analysis of algal samples as had been highlighted during the recent Golden Algae bloom and associated events. The existence of a protocol for these analyses had already been noted in a previous meeting. However, it was suggested that the envisaged Contingency Plan would have a much broader scope than this and should also address issues such as:
During the subsequent discussions the following points were made:
3. Water Quality and Nutrients.
It was noted that both the previous TWG discussions had pointed to the fact that the high nutrient levels were contributing to many of the other problems in the estuary, including pondweed growth, algal blooms, low oxygen etc. It was therefore felt that addressing the nutrients should be a priority. The meeting concurred with this assessment, and the discussions were focussed on a number of potential solutions which had been put forward.
3.1 Zandvlei Trust proposal on Westlake Wetlands
John Fowkes introduced a proposal to use the reed beds of the Westlake Wetlands as a filtering mechanism for the water of the Keyser’s and Westlake Rivers before this water enters the main body of the Zandvlei. It was noted that in the past, the water from the rivers had flowed through the reedbeds, but that in recent years the edges of the rivers had been hardened and effectively “canalised”. The proposal entails redirecting the flow of the Westlake River through the southern part of the wetlands, and that of the Keysers River through the northern part (but excluding the important flood-plain-marsh vegetation identified by Azorin). In addition, the proposal recommends a series of controlled burns of the reedbeds to reduce the fuel load – and thus the risk of an uncontrolled fire – and to generate new reed growth and uptake of nutrients. The full proposal is attached as Annex A.
The subsequent discussion showed that there was significant support for the proposal to redirect the influent waters through the reedbeds. The only significant concern was that in some parts, where a more sophisticated intervention involving weirs was required, it could result in flooding of the upstream areas – specifically on the Keysers, and the northern bank of the Westlake River. It was suggested that for these, an engineer should be engaged to undertake a thorough evaluation of the hydraulic implications of this proposal. It was also noted that prior to implementing such a proposal, it would also have to go through an EIA. There are therefore funding implications. On the other hand, breaking through the banks on the southern side of the Westlake channels to allow high flows to be diverted through the southern part of the wetlands could be done with minimal extra cost and could be implemented on a trial basis next summer.
With respect to the proposed burning of the reedbeds, questions were raised as to the effectiveness of this in terms of removing nutrients from the system. It was suggested that a fairly low percentage of nutrients are stored in the foliage, with most being in the root system. The reeds therefore need to be dug out to remove a significant percentage of the stored nutrients. This needs to be further investigated.
3.2 Sewage spills
It was noted that sewage spills are one of the sources of nutrients into the system. There was concern that the data provided on spills is not comprehensive and the reporting thereof needs to be improved.
It was also reported that although all the pump stations now have telemetry so that faults can be picked up relatively quickly, this does not necessarily mean that repairs can be effected in time to prevent sewage from entering the estuary. It was therefore suggested that retaining walls or impoundments should be built around the pump stations to contain any spills until the sewage could be pumped back into the sewerage system.
Concerns were also raised about the fact that although ratepayers are paying for their sewage to be pumped to the sewage works, when a spill occurs there is no access to these funds to compensate for the damage done.
3.3 Riparian buffers
Another point raised was that the riparian buffer zones, especially along the lower parts of the estuary, had been lost and there was a need to try and rehabilitate these. It was reported that attempts had been made to do this in the cookie cutter area , but that they had not been that successful. Moreover, an EIA was required in order to undertake this kind of work. It was suggested that this could possibly be circumvented by incorporating it into maintenance plans under the EMP.
3.4 Heavy metals
A point raised under the TWG on Fishing was that if fishing – and consumption – of alien fish from the estuary was going to be encouraged, there was a need to undertake a heavy metal analysis of the fish as there is very little data on toxins in the system at present.
4. Hydrodynamics / Circulation
There are a number of factors which have affected and continue to negatively affect the circulation in the estuary. Amongst others, this gives rise to stagnant conditions in parts of the estuary and particularly the Marina, thus exacerbating the development of low oxygen conditions, algal blooms etc.
4.1 Coral worm
One of these, which was raised in the Pondweed TWG, is the extensive coral worm growth particularly in the vicinity of bridges, although it grows wherever there is a suitable substrate and even on pondweed itself. This growth has significantly reduced water flow into the Marina, and 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. It was noted that removal of some of the coral worm colonies, in addition to enhancing circulation, would improve the ability of canoes to move through areas that have become constricted by dense worm colonies. However, removal needs to be planned on a scientific basis as it has been estimated that the coral worm filters the entire vlei in just over 24 hours thus playing a beneficial role by improving water quality (clarity is improved mainly through removal of suspended material).
4.2 Sedimentation and the Weir
Andy Killick introduced his concerns as outlined in his letter of 18 April, 2012, a copy of which had been circulated to the TWG members: in particular he expressed concern that alterations to the basin of the estuary itself were not adequately addressed in the EMP. Moreover, it seems that not only is the basin silting up, but that some of the current management interventions may be accelerating the process. Sediments are coming into the estuary from various sources, and the issue may have been under-stated in the Ninham Shand report. Of particular concern is the weir which has effectively raised the base level of the estuary to that of the weir itself (namely 0.6 mamsl). Without any intervention, this means that the estuary will eventually silt up to that level.
Similar concerns had been raised not only in the recent TWG meetings, but also in the EMP which recommends the relocation of the sewer line below the weir to enable the removal of the weir on a permanent or seasonal basis. The main problem in this regard is the cost of relocating this line which is conservatively estimated at R 12 million. However, all TWGs feel that given the value of the estuary, the required funds are a small price to pay and efforts should be made to secure political support for this. It was also noted that the costs of dredging – one of the alternatives for sediment removal – were likely to be higher than this, especially since it would have to be done on a regular basis.
There was also considerable discussion around the possibility of manipulating the water levels in the estuary to effect a “natural” flushing of the system – to remove both poor quality water and sediments. This would be done by increasing water levels at an appropriate time of year (possibly August/September) by closing the mouth until the water reached a set depth, then removing or reducing the elevation of the weir before re-opening the mouth. This would not only have a scouring effect, but if done on a regular basis (eg. annually) would re-introduce a more dynamic element to the system – as opposed to the current fairly static water levels which promote, for example, the growth of pondweed. However the scouring above the weir could not go below the artificial base level as long as it or the sewage pipe remained in place. This flushing would remove limited sediment but could not counteract the silting up caused by tidal influx and would therefore not approximate a natural flushing. Furthermore, in view of the presence of the Marina, this could only be done within certain limits: the water levels could not be increased above 1.6m because of possible flooding, or decreased below 0.5m because of potential damage to the revetments in the Marina.
Despite these limitations, it was felt that this should be done on a trial basis. However, in light of the risk to the sewer line, it was agreed that this could not be done until the sewer line had been moved. For the short to medium term, therefore, the focus must necessarily be on trying to get support and funding for this. On the other hand, it was suggested that, as an interim measure, the weir be lowered by an additional 5 – 10cm.
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. However, it was acknowledged that, given the passage of time, there could be merit in reassessing the options available.
There was also a proposal to create a funnel at the estuary mouth so as to increase the wave height and thereby the amount of seawater entering the estuary on high tides. However it was noted that this would also increase sedimentation behind the weir.
It was also reported that sedimentation in the northern part of the Marina was extensive and that there is very limited water movement in this area, and it was noted that this was where the recent bloom had started. It was proposed that perhaps the incoming Sand River could be diverted into this section. However, this was not supported because of the very poor water quality of the Sand.
With respect to the accumulation of sediments in general, it was suggested that it would be very useful to have a bathymetric survey done and to compare this to the original hydrological plan for the Marina (assuming there was one). Such a survey would in any event be required prior to any dredging.
It was agreed that another meeting of the group should be held in about a month.
A proposal to use the Westlake Wetlands to filter the water flowing into the Zandvlei.
The proposal recognises that there are important plant communities in the northern area of the wetland and that these should not be prejudiced.
For those who do not know the Zandvlei, Appendix C sets out its locality and composition.
2. THE IDENTIFIED PROBLEMS.
All of these leading to high, on-going maintenance cost.
3. PROPOSED SOLUTION
It is proposed that consideration be given to redirecting the flow of the rivers through the associated wetlands. It is proposed that this be addressed in two parts: 1. The Westlake River and southern wetlands and 2. The Keyser’s River and northern wetlands.
3.1. The Westlake River and southern wetlands.
3.2. The Keyser’s River and northern wetlands.
It is proposed that a weir be constructed across the Keyser’s River at a point opposite the permanent pond that exists, marked A on Appendix B, to cause water to flow into an existing permanent water body. That this water body is then joined with an east-west channel to the two existing channels marked B and C on Appendix B. These channels contain permanent water so no levels would be required in ensuring water flow. That the southern channel, marked D on Appendix B, be opened by means of low lying culverts to allow water to pass out into the Westlake River channel.This action will have no impact on the important saline Flood plain-marsh vegetation area identified in the northern areas.
4. CLEARING OF THE REED BEDS.
Submitted for consideration on behalf of the Zandvlei Trust by John Fowkes, Project Co-ordinator Westlake Wetlands Project, Zandvlei Trust.
Appendix A - were not available for this document.
Appendix B - were not available for this document.
Appendix C - were not available for this document.
LOCATION AND COMPOSITION OF THE WESTLAKE WETLANDS.
The Westlake wetlands lie at the north west end of the Greater Zandvlei Nature Reserve. They are bounded on the east by the Cape Town - Simonstown railway line, on the west by the Main Road through Lakeside. To the north is Steenberg suburb and on the south part of the Lakeside suburb with Rutter Road and Spinnaker Avenue.
The wetlands mainly comprise reed beds containing phragmites with some typha. There are two river systems which feed into and pass through the reed beds before entering the main body of the Zandvlei estuary. These rives are the Keyser’s, feeding from a catchment rising below Silvermine and the Wynberg Ridge, and the Westlake, rising on the Silvermine area of and Muizenberg mountains. There are many subsidiary channels and pools. To the north of the wetlands there is an area that contains mud flats. These are subject to seasonal flooding and have important plant communities.
The wetlands have been substantially modified. Originally the reed beds were linked to the main body of the Zandvlei with water passing easily under a railway line raised on wooden poles. This railway line was then placed on a solid earthen embankment with a narrow bridge through which the Keyser’s and Westlake Rivers flow. This effectively cut off the Westlake Wetlands from the main body of water in the Zandvlei.
With the proposed development of the Eastlake and Westlake Marinas in the 1970s a substantial amount of earth was dredged from the main water body and placed on the east and west of the railway line. Channels were dug through the Westlake reed beds. In the late 1980s a “hard embankment’ was put in place alongside one side of each river to allow access by a drag line and heavy vehicle.
Many of the water channels in the wetlands have become blocked by clumps of Typha or Phragmites. In places growth of land based creepers has moved across water weed mats, closing the water surface.
Recently the Westlake and Keyser’s Rivers and the southern part of the wetlands have been infested with Parrot’s Feather, Myriophyllum aquaticum; Red water fern, Azolla filiculoides; Water Lettuce, Pistia stratiotes; Yellow hybrid Water Lily, Nymphaea; Water Cress, Nasturtium officinale; Duck Weed, Lemna sp. Concerted efforts have cleared these but constant monitoring and management is required.