Why is it necessary to manage sediment in the Vlei? The benefits of sediment removal
By dredging or other methods.
Zandvlei’s location in an urban environment has necessitated that the water level be maintained between certain limits to meet the needs of the residents and recreational users, but should not unduely disturb the function of the Vlei as an estuary. Unfortunately, the infrastructure put in place by the City Council to manage the water level has had the unnatural consequence of trapping large volumes of sediment in the lower reaches of the Vlei. A study done in 2014 showed that the sheet of accumulating sediment was advancing away from the mouth into the Vlei at an average rate of more than 2 metres per month, leaving behind a shallow area much of which is above water level at low tide.
Too much sediment in the Vlei is bad for recreational activities – canoeing and yachting in particular, as well as for fishing. It is also bad for some of the creatures calling the Vlei home. The Zandvlei estuary is the last remaining functional estuary in False Bay, and as such represents the only nursery for the fish that live in this area. If the estuary fills up with sediment, not only will the fish in the Vlei be threatened, but there is also likely to be a negative impact on the fish population of False Bay.
It is generally accepted that the silting up of the Narrows is due to the artificial raising of the base level of the Vlei by a rock weir constructed just below the Royal Road bridge to protect a sewerage pipe located just above the bridge. The long-term solution is to re-engineer the mouth and relocate the pipe and remove the rock weir, thereby enabling tidal ebb to cut a channel back down to the low water level. This will avoid the accumulation of sand in the narrows.
As the re-engineering of the mouth could not be funded by the Council in the near future, options for remedial action in the interim was to;
- Lower the rock weir to try and reduce the level of the sand sheet behind it.
- Dredge a channel through the accumulated sediment to the mouth.
Passive sediment removal through mouth management: reduce the weir or more dynamic measures?
From an estuary management perspective, free flowing tidal movement with an unconstrained, meandering mouth is the ideal scenario, for fish activity as well as sediment movement. Due to the current constraints on the vlei, being built up at all sides, this is not currently possible. Keeping the weir to maintain a stable water level is also not good because it cuts off access for fish to the nursery, and it gives no option for the coarser fractions of sediment (coming from upstream or introduced through the mouth by flood tide) to leave the estuary into the sea. Furthermore, removing any constriction to the mouth would minimize flooding that might arise as a result of heavy rain in the catchment area to Zandvlei.
The next best thing is to manually manage the mouth at certain times of the year, depending on rain and tidal movement, which is the current practice. With the prolonged drought this is difficult.
When the mouth is open, there may be some sediment removal through tidal ebb and rainstorms in the catchment. Unfortunately, there is a weir in the way that hampers this action and, on balance, the tidal inflow will deposit more sediment above the weir than is removed. Lowering the rock weir increases the volume of saltwater entering the Vlei and also reduces the level to which the sand sheet will accumulate. But it will not stop the advance of the sand sheet into the Vlei.
The rock weir has now been lowered to the minimum level at which it still affords acceptable protection to the sewer line. The sewer line could be moved, but currently it only relies on gravity to transport the wastes and any alternative option would require pumps and introduce more risk of failure. With the lowering of the rubble weir, there has been increased tidal inflow which has pushed the stability of the building foundations of Marina da Gama to the limit.
Before the drought there was a small net sediment removal in response to lowering of the weir. With the extended drought there has been insufficient rain to create enough flow to remove the sediment that arrives in suspension and it is collecting in the body of the vlei. Even in periods of good rain the weir stops bed-load sediment from being removed from the vlei.
The benefits of dredging
Sediment removal specifically through dredging.
Considering all the options, the need for dredging is clear. It provides all the benefits of working towards Water Sensitive Design except water supply including:
- Flood prevention
- Improved water quality
- Improved Biodiversity
- Increased property values
- Maintaining blue flag beach status
- The maintenance of a well utilised, truly multi-racial public space
Dredging is not regarded as the optimal remedial action to the unnatural build-up of sand in the lower reaches of the Vlei and should not be regarded as a long-term solution. However, if re-engineering the mouth is not presently affordable, it does offer some relief to the problem in the shorter term.
Although there are potential negative impacts to dredging there are several benefits;
- It will enable salt water to penetrate further into the estuary, and this should improve the flushing of the Vlei and also discourage pondweed growth.
- Research (reported in Whitfield, 1998) has shown that estuary mouth channels with depths greater than 1.5m have greater diversity and numbers of piscivorous fish. Thus, the deteriorating fishing experienced could be due to the shallowing water for hundreds of metres above the mouth. Dredging may ameliorate this situation.
- Canoeists will be able to resume paddling, without getting stuck, from the vlei to the sea.
- The Narrows, one of the most popular fishing sites in the Vlei, would be reinstated and once more expanded as a fishing venue.
Evaluating the potentially negative consequences of dredging
Water stratification is an undesired possibility, due to the more dense (heavier) sea water taking up the deeper channels. On the other hand, estuaries are dynamic: seawater will fill the channels in the Narrows, but it will be exchanged with every tide when the mouth is open. Furthermore, our prevailing winds, induce currents in the Vlei and these result in both horizontal and vertical mixing of the water body. Nevertheless, dredging should be undertaken in a manner so as not to create pockets which may become anaerobic. The front-end shovel used in 2015 is not an optimal tool in this regard. However, in the Narrows these pockets will be short-lived and ironed out by redistribution of the sediment by tidal and fluvial currents, which was indeed observed with the holes created by the front-end shovel in 2015. The only area where dredged depressions are likely to persist is in the middle section of the Vlei, distant from fluvial and tidal action.
The sandprawn population and other benthic organisms would clearly be negatively impacted by removal of the sand bodies. If everything is dredged all at once this could be devastating to the prawns. Consequently, the current dredging plan leaves undisturbed areas of sand flats adjacent to the dredged channel, where the prawns and other benthic fauna would remain. Anecdotally, the prawn population of Zandvlei has been found to be remarkably resilient. This is not surprising as sand prawns live in the dynamic lower reaches of estuaries and have to cope with rapid changes. Not to dredge for the sake of a few prawns and allow degradation of the remainder of the ichthyofaunal nursery does not seem wise.
Dredging is not an optimal solution, but would combat the advance of sediment from the mouth into the basin, thereby keeping the estuarine system functional in the meantime. To be effective dredging would have to be undertaken at regular intervals until the infrastructure at the mouth has been re-engineered.