Zandvlei – Factors concerning Water Level Mangement
A Z– fold pamphlet produced by Martin Thompson for the City of Cape Town. Content is below.
The control of the vlei water level has to take a number of issues into account, the most important of which are:
Needless to say, this is a primary concern and because Zandvlei is located in a winter rainfall area every precaution is taken during the winter months to prevent flooding. The level of Zandvlei can be regulated by two mechanisms at the mouth downstream of the Royal Road Bridge, namely the rubble weir and the sand bar, both of which require manual intervention in order to alter the status quo. In 2004 consulting engineers undertook a sophisticated hydraulic modelling exercise for Zandvlei, using revised inflow figures for the various river catchments.
This revised model indicated that the rubble weir would need to be at a level of 0,6 m MSL to enable a 1 in 100-year flood to pass through the outlet accompanied by a vlei level of 1,78 m MSL. The rubble weir is presently at a level of 0,7 m MSL although requests have been made to raise it to 0,8 m MSL or even 0,9 m MSL. With the rubble weir at 0,9 m MSL (200 mmm above its present height) the predicted 1 in 100 year flood level will result in extensive damage to a great many properties in the Marina and surrounding areas. For this reason, during the winter months, it would be extremely unwise to raise the rubble weir.
However, in order to avoid a low-water operating level throughout the winter, it has been decided to periodically close the sandbar when the existing vlei water level is low, when the inflow from the feeder rivers is not excessive and when there is no impending cold front. While this modus operandi increases the risk of flooding 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 when the conditions are acceptable.
During the summer months, when the risk of flooding is low, the sandbar is closed to raise the vlei water level. No alterations are made to the rubble weir. However, because there is an environmental need to keep the salinity levels of the vlei at a reasonable level, the sandbar is opened approximately 5/6 times per summer season to allow the high spring tides to enter the vlei. Each mechanical opening and closing cycle of the sandbar costs approximately R20 000 (2007 figures) While climate change predictions for the Western Cape are for less overall rain, the prediction indicates that more intense rainfall events are expected. If this prognosis materialises a more conservative approach may have to be adopted regarding the opening and closing of the estuary mouth. Any new information in this regard will be made available to all interested and affected parties (IAAP’s).
It has been shown that the maintenance of good water quality in Zandvlei depends, to a significant degree, on an ambient salinity of between 5 and 10 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 ensure enhanced precipitating capacity of the water, resulting in improved water clarity (Harding, 1999).
The only method of keeping salinities high is by removing the sandbar during the summer at spring high tides. The large quantities of freshwater from the influent rivers in winter preclude the possibility of any salinity management during the winter months. During the last few years it has been possible, with timeous manipulation of the sandbar, to keep salinities in the vlei at a reasonable level.
1.3 Revetment stability
The revetments are the concrete gravity units separating the canal from the land in the Marina da Gama residential area and were provided by the developer- along all canal frontages. Investigations into the original design undertaken by Hill, Kaplan, Scott indicate that the assumed operating water level at the time was 0,7 m MSL. The stability of the revetments was checked for water behind the revetments to the top of the revetments which were set at a top level of 1,0 m MSL. Varying water levels were considered and in all cases the resultant forces fell within the middle-third of the revetment, which indicates that the revetments are stable under these conditions.
Factors which will result in the instability of the revetments are:
1.3.1 When property owners request the weed harvester operators to trim the weed close up to the revetment, thereby removing much of the pondweed that would otherwise protect the sandbed in front of the revetment from erosion. In the past some property owners have been known to make this request.
1.3.2 When an increase in operating water levels frequently leads to the overtopping of the revetments (i.e. water levels in excess of 1,0 m MSL), increasing the hydraulic pressures behind the revetments. In order to protect the stability of the revetments the harvesting of pondweed should not take place in close proximity to the revetments and the operating water level should not be allowed to frequently exceed 1,0 m MSL nor to stay at this higher level for long periods of time.
1.4 Fish migration
Fish generally make up a large component of the fauna of an estuary, as estuaries offer abundant food, temperatures that are warmer than the sea, lowered salinities compared to the sea and a lower number of predators. However, fish need to be able to move between estuaries and the sea at certain critical times. In most cases, this involves a movement from the sea into the estuary during the larva stage of their life cycles or immediately thereafter, followed by a return migration to the sea within one to three years. In order to be available to marine species, estuaries need to have free exchange of water between the estuary and the sea at certain critical times during the year, mainly from September to November. There also needs to be a sustainable flow of seawater into the estuary during these critical times as most species have poor swimming abilities at the time of first entry, and therefore generally move passively in the estuary with the inflowing water. Strong seasonal (winter) flushing of the estuary is also required as it enables faunal species to exit the vlei. The connection with the sea during the critical periods need not be an uninterrupted one, and can take the form of an intermittent connection during a longer period (e.g. during spring high tides only).
From the various water level issues described above it can be seen that there is limited scope for the retention of high water levels in the vlei. During the winter months the avoidance of large-scale flooding is of paramount importance with the rubble weir at a low level (0,6 m – 0,7 m MSL) and the sandbar open when the river inflow is high and/or when a cold front is impending. During dry spells in winter it will be possible to raise the water level above 0,7 m MSL by manipulating the sandbar, although with good river inflow and the sandbar open the vlei water level is likely to hover around 0,8 m MSL.
During the summer months the sandbar is closed to keep the operating water level high and is only opened when the water level becomes too high or when there is an adequate spring high tide (five to six times per summer period).Ideally the water level should not be allowed to exceed 1,0 m MSL for long periods of time and this should not be allowed to occur too frequently. Recreational users will need to take cognisance of these operating procedures.
The yachting fraternity have undertaken to ensure that all their main events are programmed for non-spring high tide periods. However, the situation is not as easy for the canoeing fraternity, since a large component of their season occurs during the winter months, although it is expected that the closing of the sandbar during dry winter periods will go some way to alleviating their concerns regarding the level of the water.
For any further information please contact
Roads & Stormwater Department