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Dredging at estuary starts to help fish

Posted on July 29, 2015

A dredging operation of the sand bar at the mouth of the Zandvlei Estuary Nature Reserve has started as part of the necessary rehabilitation action of this estuary which is an important estuarine nursery area for fish.

The build-up of sand in the estuary has occurred over the last 40 years. Although the accumulation of sand has at times been interrupted by past dredging operations, it has been building up, especially over the past decade.

The extra sediment has reduced flow and depth to the point where it restricts movement of fish into and out of the estuary and may increase the risk of flooding upstream. It also reduces the flushing of excess nutrients out of the estuarine system.

Ensure estuary functionsThe accumulation of sand is the direct result of the rubble weir at the mouth of the estuary which, amongst others, had to be constructed to protect the sewer line.

In a natural system, sand brought in by wave action tends to accumulate during summer. In winter, scouring during storms far outweighs sand input by wave action. Estuary mouths also tend to migrate laterally as part of the process. The result is that the depth of an estuary, particularly near the mouth, tends to fluctuate between deep and shallow phases.

The weir slows the speed of flow at the bottom of the estuary quite severely, thereby reducing scouring and causing a net accumulation of sediment. The dredging operation, if continued every few years as needed, will assist to ensure the estuary functions.

The dredging operation will have a short-term negative effect on benthic creatures such as prawns, crabs and bloodworms. The strategic parts of the sand bar will be left as holding areas for the purpose of the re-colonisation of any affected creatures. It is expected, however, that any populations that could be affected will recover quickly, as has been the case in past dredging operations.

Stored, then usedSediment will be stored on both banks while drying and will then be removed for use in projects elsewhere as needed. A rehabilitation plan is also in place for banks that are damaged during operation of the dredging equipment, and they will be restored to a better condition than at the start of the project.

The dredging activity has been approved through the general river maintenance environmental impact assessment authorisation given to the City of Cape Town for various activities in rivers and wetlands.

Dredging will be phased across a number of financial years. The first phase will include the removal of approximately 2 000m³ of sand.

Zandvlei Estuary Nature Reserve is one of 16 protected areas managed by the City. These areas conserve a variety of exceptional biodiversity and provide numerous recreational and economic opportunities

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