Zandvlei Trust

Draft minutes for the Meeting on 11 November 2013.

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


Draft Report of the Meeting on Issues Related to the Management of the Mouth 11th November, 2013.

A meeting on the management of the mouth of Zandvlei was held at the Cape Research Centre, Tokai.
It was attended by:

Name. Position /Representing.
Sandra Fowkes Chair of ZEMF
Piet Huizinga retired consultant (previously with CSIR)
Martin Thompson Catchment Management, CoCT
Justin Smit Catchment Management, CoCT
Candice Haskins Catchment Management, CoCT
Julia Wood Biodiversity Management: CoCT
Joshua Gericke Biodiversity Management: CoCT
Sandra Setati CapeNature intern
Gavin Lawson Cape Bird Club and Zandvlei Trust
Andy Killick retired geology consultant and Zandvlei resident
Gordon Laing engineer, canoeist and Marina da Gama resident
David Walker scientist and lecturer at CPUT
Lynn Jackson Consultant: Coastal & Environmental Consulting.

Sandra Fowkes welcomed everyone to the meeting and noted that the purpose was to brainstorm around issues related to the management of the mouth, and in particular, concerns raised by Andy Killick and others about the significant sedimentation which had taken place over the past 10 or so years (as shown in the aerial photographs in AK’s article).

Lynn Jackson then shared her thoughts in relation to the process of updating the EMP, namely that although the sedimentation issue had been recognised in the original Situation Assessment and EMP, the emphasis in relation to mouth management had been around salinity. The measures adopted were intended to promote seawater intrusion to increase salinities within the estuary as well as fish recruitment and had, in fact, been reasonably successful. However, insufficient consideration had been given to sedimentation, and this needs to be addressed in the updated EMP. Any proposal emanating from this meeting would thus be incorporated into the updated EMP.

Piet Huizinga then gave a presentation on the mouth dynamics of the estuary, and the impacts thereon of both the weir and the canalisation of the mouth through a channel only some 20m wide. The implications of these structures in combination with the current mouth management regime on the natural functioning of the estuary include:

  • A significant reduction in the variation of water levels which, under natural conditions would have been between 0 to 2.5/3 m AMSL. They are now restricted to between 0.7 and 1.4;
  • Restriction of flow in the outlet channel by the creation of a bottleneck (channel limited to 20m width);
  • Sediments transported by incoming waves accumulate behind the weir;
  • Mouth is open for shorter periods than under natural conditions.

In the subsequent discussion, the following points arose:

  • The mouth can be flushed by two processes: mouth breaching and river floods;
  • Under natural conditions, the berm across the mouth would be 2-3 m high which would create the conditions for significant breaching, but this is no longer the case;
  • The present management arrangements allow conditions that are reaso0nably healthy for fish populations;
  • In the past, there was periodic dredging of the channel (every 4 – 5 years). This was stopped in around 2001 (which links to the current build-up of sediments based on the photos provided by Andy Killick) because dredging activities now require an EIA in terms of the regulations.
  • Dredging cannot be done in-house so the City would need to go out to tender;
  • The caisson walls on the south side of the channel are close to collapsing and are likely to need to be replaced (with the same or alternative solution) within 5 – 10 years;
  • Sedimentation is not limited to the mouth, but is also happening in the channels of the Marina (where an organic sludge has accumulated on the bottom), and at the points where the various influent rivers enter Zandvlei.
    However, it was noted that:
  • The formation of deltas from sediment being carried down the influent rivers is a natural process (even if it is being exacerbated by activities in the catchment), whereas the sedimentation at the mouth is being caused by the weir and is completely unnatural;
  • Measures aimed at decreasing sediments coming down the rivers are included in the Catchment Management Plan (but have not been implemented);
  • The purpose of this meeting is to address sedimentation around the mouth.
  • If the weir is lowered far enough, the sewer line – which is enclosed in a concrete caisson – effectively becomes a second weir;
  • It was noted that the sandbanks consist of unconsolidated material. AK has had to rescue people on a number of occasions. The resultant risk to the City could be used as part of the motivation for dredging;
  • An analysis of the sediments for this purpose could be undertaken by UCT students, or by Stephen Lamberth as part of the proposed bathymetric survey;
  • Any steps that are taken should be communicated in advance to the public and particularly recreational users and residents of the Marina and other surrounding areas;
  • The mouth opening programme could be adjusted to take account of wave activity. Waves higher than 2 metres would increase the transport of sediment into the estuary mouth, so it should not be opened under such conditions. Wave direction (SE or SW) is also important.
  • Increasing the water levels could cause flooding of those marina boathouse structures which have been built below the legal building line and which are probably not covered by insurance. However, since they are illegal, this should not have to be taken into account when setting optimum water levels.

A number of potential solutions were put forward for discussion, including:

  • Adjustment of the timing/frequency of breaching based on predicted wind and wave conditions;
  • Allow higher water levels (Note that the lowest allowable development is 1.85m above MSL).
  • Test effectiveness of further lowering the weir at times when good scouring can occur
  • Re-institute periodic dredging
  • Widen the mouth
  • Investigate the possibility of installing some sort of water level control system for the Marina at the two relatively narrow entrances at either end of Park Island
  • Widen the mouth
  • Remove the weir/sewer.

Other points made included:

  • Alan Boyd has installed two probes which are measuring salinity, temperature etc on an ongoing basis;
  • Pollsmoor Prison is a major source of pollution in the catchment;
  • The possibility of extending Kirstenhof and/or Westlake Wetlands to absorb some of the pollution from the catchment should be investigated. Any such initiative would need to include regular harvesting of the reeds;
  • Run off from vineyards is thought to contribute to influent sediments but no action has been forthcoming from farmers or the D of Agriculture to address the problem.

Conclusions

It was generally accepted that given the complexity of the issues and bureaucratic and budgetary constraints, a step-by-step approach (adaptive management) should be followed.

Summary of the Way Forward;

1. The weir will be lowered by another 10-20 cms during the next winter season on a trial basis (it is currently at 0.6m AMSL) with monitoring aimed at evaluating the impact. 
RESPONSIBILITY – MT

2. The sediments which have accumulated in the channel should be analysed with respect to their level of consolidation using a vibracore. This could possibly be done by UCT students during 2014. 
RESPONSIBILITY – JG

3. The possibility of including periodic dredging of the mouth into the mouth management protocol and/or list of pre-approved maintenance activities by Catchment Management must be explored with DEA & DP. If necessary, this could be supported by ZEMF (1a). Such discussions should be initiated as soon as possible. RESPONSIBILITY – MT and CH.

(1a). We did not really discuss this in detail, but the outcome of this meeting could be written up as a recommendation from ZEMF to the City in such a way that it could then be used to support a submission to DEA & DP.

4. In support of this initiative, a record of previous dredging in the Zandvlei mouth/channel should be compiled together with information on associated monitoring undertaken e.g. of sandprawns. 
RESPONSIBILITY: MT and JW.

5. Should periodic dredging of the channel be approved for inclusion as a maintenance activity (and thus exempt from an EIA), then such dredging should be re-instituted as soon as possible. It was noted that dredging need not occur annually but according to need. Steps should therefore be taken immediately to include provisions in the budget for this (expected to cost in the order of R 1 million). In this regard, it was noted that it is probably not necessary to dredge the entire area that has been sedimented up. If there are budgetary constraints, dredging could be limited to a central channel. It was also noted that the dredging should coincide with a lowering of the weir so that there is sufficient flow to carry the sand out of the mouth. RESPONSIBILITY – MT and JW.

6. Any actions implemented as part of mouth management (lowering of the weir, dredging etc) must be accurately recorded and the impacts thereof carefully monitored so that the effectiveness of the steps taken can be evaluated. 
RESPONSIBILITY – MT and JG.

7. An EIA to explore longer-term solutions for mouth management should be initiated as soon as possible. Such an EIA should include:

  • The relocation of the sewer line;
  • The widening of the mouth/inlet (in the context of the anticipated collapse of the caissons on the south bank) and possible restoration to a more natural condition;
  • An assessment of potential implications of climate change/sea level rise for the management
    options (2a).

The objective should be to produce a mouth management regime which addresses all the related issues including:

  • Ecosystem impacts
  • Safety issues (quicksands)
  • Collapse of caissons
  • Increased flooding potential (caused by the sedimentation).

Given the scope of the EIA, consideration should be given to undertaking it as a joint initiative of a number of City departments (ERMD, CSRM and Water and Sanitation). Mechanisms to facilitate this should be explored with the HODs. Support could be provided by ZEMF. 
RESPONSIBILITY: MT and JW.

(2a). It was noted that a high level catchment study is currently being undertaken and will include an assessment of the impacts of climate change on the system.


                                                                                                                                                   

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