There has been increasing concern about the sediment build up that is occurring in Zandvlei estuary. One area of particular concern is the advancing sandbar above the bridge on Royal Road, near the mouth of the estuary. The Zandvlei Estuary Management Forum (ZEMF) is exploring a range of initiatives to address this issue.
Winter provides an opportunity to test one way of reducing the sandbar. We want to conduct an experiment to see if outflows from the winter rains can remove at least part of the sandbar. Winter is the time when the mouth is kept open continuously for several months. In summer, when the mouth is opened artificially for a few days each month, the tidal surge carries marine sand particles further into the estuary than it would in winter. In winter inflows of marine sand are usually countered by larger outflows of the river.
As soon as the water has passed the weir on its inward journey, the velocity slows significantly and much of the marine sand is deposited upstream of the weir. The ebb flow is unable to remove this material because of the barrier formed by the weir and this has resulted in the development and upstream advance, of a sandbar. There is also sediment that comes in via the rivers that flow into the vlei but most of this is probably dropped at the upper end of the vlei. In winter with a greater amount of water flowing out of the vlei mouth than flowing in from the sea, we want to see if this natural energy can be used to remove at least some of the sandbar
Clearly the winter outflow has not stopped the summer advance of the sandbar over the past years. Something additional needs to be done to help the outflow to remove sand. ZEMF has explored the possibility of lowering the rubble weir (which is there to protect a sewerage pipeline) by up to a maximum of 200 mm to determine if this will enable a stronger outflow of rain-fed water to carry more sand out of the estuary. A lower rubble weir will also have the added advantage of increasing the volume of sea water that penetrates into the estuary at high tide.
This initiative will be followed by a dredging operation which will probably be undertaken over two to three years, and will hopefully commence during January/February 2015. An application has been submitted to the provincial authority (Department of Environment Affairs and Development Planning) for authorization to undertake the dredging operation. No work of this nature may take place in the absence of such authorization.
In any thoughtfully and competently planned initiative, it is important to think through the consequences of what is proposed. Where there might be any risks, preparation needs to be made to take steps to minimise that risk. If any unexpected problems arise, the weir can be returned to its original level within a day, as changes to the weir level are reasonably quickly and inexpensively done. ZEMF will maintain close contact with the representatives of recreational users and local residents who serve on ZEMF. The preparatory work has also established that Marina revetments are stable at these slightly reduced water operating levels.
How will we know if the weir lowering is effective? The City’s Rivers, Catchment and Bulk Stormwater Branch has done surveys of the floor of the estuary. This provides a baseline against which it will be possible to measure the nature and extent of any change.
The best case is that a significant amount of sand will be flushed out; that if any unexpected problems occur, the weir can quickly be rebuilt; there will be quantitative measures to evaluate the effectiveness of the weir lowering; and finally, the cost of the exercise is relatively low.
Any comments should be emailed to all of the following: Manager of Zandvlei, Joshua.firstname.lastname@example.org; Head Southern Catchments: Rivers, Catchment, and Bulk Stormwater email@example.com; Chair of ZEMF Sandra@metaplan.co.za; Vice chair of ZEMF firstname.lastname@example.org.
Zandvlei Estuary Management Forum May 2014