Skilled intervention keeps Zandvlei clean by Marian Shinn
(Extract article from the volume 2008/1 Newsletter)
Bulldozer opening the mouth of Zandvlei
Zandvlei, the only functioning estuary system on the False Bay coast, is a superb example of the skilled management required to keep it functioning optimally and addressing the needs of those whose lives it influences. In this, the first of two articles from a paper entitled ‘Zandvlei, factors Governing Water-Level Management’, by Martin Thompson, the City of Cape Town’s regional manager for catchment management in the south peninsula, the focus is on flooding and salinity.
Managing the water level in the vlei to support recreational users and safeguard the properties of those who live on the water’s edge is the most obvious task in managing Zanvlei’s water levels. But for the estuary’s health, the City of Cape Town closely monitors the water’s salinity levels, is proactive about the threat of flooding and is mindful of fish migratory needs.
The primary concern, says the city’s Martin Thompson, is flooding and every precaution is taken in the winter months to prevent this.
The water of the estuary is managed by a rubble weir just downstream of the Royal Road bridge and the sandbar at the mouth of the river. Both need manual intervention to manage the water level. The weir needs to be kept at 0,6 meters above sea level (MSL) to enable a 1 in 100 - year flood to pass through the outlet accompanied by a vlei level of 1,78 MSL. The weir is currently 0,7 MSL. Raising it higher could, in a serious flood year, result in extensive damage to many Marina da Gama properties. Thompson says it would be unwise to raise the rubble weir.
To avoid a low water operating level in winter the municipality has decided to periodically close the sandbar when the existing water level in the vlei is low, when the inflow from the feeder rivers is not excessive and when there is no impending cold front.
Piling the sand on on side
While this increases the risk of flooding, Thompson says, it is considered that, with careful monitoring of the water level as well as the impending weather conditions, a more suitable water operating level can be achieved.
During summer, when the risk of flooding is low, the sandbar is closed to raise the vlei water level but, because there is an environmental need to keep the salinity levels of the vlei at a reasonable level, the sandbar is opened between five and six times during summer to allow the high spring tides to enter the vlei.
Each mechanical opening and closing cycle of the sandbar costs about R20 000.
The mouth of Zandvlei open to False Bay
Climate change predicted for the Western Cape is less overall rain but when it comes, it will be more intense. Is this is true, then a more conservative approach to managing Zandvlei’s water levels by opening and closing the estuary mouth may be necessary. Should changes be necessary, Thompson advises that all interested and affected parties will be informed.
On the salinity front, it has been shown that the maintenance of good quality water in the vlei depends on an ambient salinity of between 5 and 10 parts per thousand (ppt); a healthy standing stock of pondweed and the filtration activities of the polychaete worm.
In addition to the maintenance of saline conditions to ensure the completive advantage of halotolerant biota (pondweed and polychaete worm) elevated salinities also enxsure enhanced precipitating capacity of the water, resulting in improved water clarity.
The only way to keep salinates high is by removing the sandbar during spring tides in summer. The large quantities of fresh water from the influent rivers in winter preclude the possibility of any salinity management at that time.
Thompson says that timely manipulation of the sandbar during the past few years has keep salinity in Zandvlei at a reasonable level.