The accumulation of unnatural amounts of sediment inZandvlei is a serious threat to it’s functioning, both in terms of recreational use, and for a healthy fish nursery, particularly near the mouth ‘the Narrows’.
This presentation by a resident retired geologist, Andy Killick, explains how estuarine sedimentation (pdf) works, and why with the mouth’s current design sediment gets stuck and accumulates in the vlei.
The infrastructure developed near the mouth of the vlei is a problem due to three main factors:
- A sewer line protected by a rubble weir that prevents the lower energy ebb flow from removing sediment introduced into the vlei by the flood tide, and
- A constrained mouth that cannot meander to deposit and erode sediments through natural migration.
- The constriction caused by accumulating sediment restricts the interchange between marine and fluvially derived waters, sometimes negatively impacting on water quality.
The vlei has a maintenance budget of R300 000 annually to manage the sediment through dredging, but this is not enough to do a good dredge to restore balance to the vlei. With the current water crisis (2017), this budget has also been revoked.
Removing the sediment is the most critical priority currently for the ZPAAC.
This post gives more information about sediment management in Zandvlei.