Opening of Zandvlei's estuary mouth to the sea.
On a monthly basis, at full moon and new moon, the mouth may be opened depending on the level of the water in Zandvlei. There are numerous and sometimes complex issues which have to be considered when doing so.
The mouth opened at the end of January 2006, as an example.
Who is responsible?
The City of Cape Town, Engineering Department at the South Peninsula offices.
Who else has input?
Catchment, Stormwater, River Management,
under the Transport, Roads, Stormwater Directorate.
Sand River Catchment Forum
(made up of City officals and civil society groups)
, meet regularly to discuss relevent issues. Zandvlei Trust is an original founder member of the forum. The Yacht and Canoe Clubs are represented. The Boy and Girl Scouts. A number of other Environmental Groups and Home Owner Groups within the Catchment are also represented. The Citys
Zandvlei Nature Reserve Management
are also present. Also various City Councillors. Scientific Services
also have a large input with their recorded water analysis on a monthly basis together with ZIMP volunteer helpers taking readings.
The dozer has to move the large amount of sand to both sides of the estuary mouth.
What issues have to be considered?
Rising water levels in the estuary from the Catchment source (Peninsula Mountain range)
Annual fish breeding cycles
The cyclic season for all the natural water organisms
Seasonal and forecast weather patterns
Seasonal user requirements (yachting, canoeing, windsurfers and beach users)
The water can be difficult to cross at high tide, for people walking along the Muizenberg beach, so the mouth is kept closed during high use summer months
Natural and exotic waterweed conditions in the vlei
Zandvlei is a shallow water body
The management of the water height of 0.7meters above mean sea level. (The shoreline in the Marina da Gama has artificial concrete linings and can be undercut if too low or backcut if the levels are too high). The critical management levels are only 0.3 meters!
Salinity levels have been quite well managed since 2000 as records indicate, attaining an annual average of 10 ppt for the total vlei area. At the mouth of the vlei area the salinity levels are much higher. Sea water is 35 ppt.
(ppt = parts per thousand)
Salinity levels are higher in summer (evaporation - with very little summer rain) and lower in winter (ingress of fresh water from the Catchment in the annual winter rainfall period)
The seasonal balance for the Sago Pondweed (Potamogeton pectinatus)
a water filterer of the also the polychaete worm (Ficompomatus enigmatica)
a filter feeder and water cleanser
There are even more issues to consider, which indicates this estuary, wetland and reserve with its many users, is a complex and at the same time dynamic system and has to be managed as such. The artifical concrete lined canal from the Royal Road bridge also has a sewerage pipe crossing the span and impedes the natural flow of the tides.
These are some of the reasons why the system has to be artifically managed to help maintain a balance.
This allows the sea water in at peak high tides and allows an outlet of some of the estuarys water at the low tide.