Wetland Rehabilitation

by Liz Day

Duneslack wetlands in the Cape Flats

Large areas of the Cape Flats in Africa’s second Ramsar Wetlands City  were once characterised by expanses of seasonal wetlands, often found in between dunes. Infilling and drainage for urban and agricultural development means their ongoing, and usually permanent, loss. 

Most of the remaining wetlands have been severely degraded and are now poor shadows of their natural habitat quality and type. Many have been entirely disconnected from surrounding natural areas and are slowly losing biodiversity value. 

Some systems, however, lend themselves to rehabilitation, and can provide great opportunities to bring back, or at least improve, highly threatened habitats and ecosystem connectivity, function or habitat quality. 

A wetland that lends itself to rehabilitation is one whose current water quality can still be improved. Among the things to consider are the wetland’s hydroperiod (that is, how much water it holds; where that water comes from and when; and how long it stays) and how it has changed compared to a natural wetland; its topography (the wetland profile, including the steepness of its banks; depth; diversity of depth types and different habitat zones); its links to groundwater (is it fed by groundwater or is it perched above the water table, separated by a lens of clay or rock?); the remaining indigenous vegetation; the type and ease of removal of alien vegetation; and its links to other natural ecosystems. 

Parts of the Van Blommestein Park wetlands have recovered well after removal of gums. Credit: Theo Stock

The most successfully rehabilitated wetlands are usually those where the indigenous seedbank and wetland topography (shape) remain intact, but where alien vegetation has established and literally sucked them dry, affecting Cape Town’s water security.  Removing or killing water-thirsty trees allows the water table to re-establish and the wetlands to rehabilitate naturally.  Parts of the Van Blommestein Park wetlands in Zeekoevlei on the Cape Flats have recovered well after removing some of the alien gum trees that previously covered this area.  Removing more gums will allow more of the wetlands to recover. 

Infilled wetlands can also be rehabilitated by removal of fill and attention to reshaping and removal of any waste or contaminants.  Natural wetland seedstock can remain viable in infilled areas for many years.  The rehabilitation of Moddervlei in Rondevlei Nature Reserve and Skilpadsvlei near Kommetjie are both examples of very successful wetland rehabilitation projects achieved by the City of Cape Town through the removal of fill and the introduction of some appropriate locally indigenous plant species.

Local is better

When the existing indigenous seedbank comprises a low diversity of indigenous plant species compared to natural, the re-introduction of some key species may be desirable. Using plant propagules (seeds or cuttings from natural areas nearby or within, ideally, the same catchment) is important, to ensure that the genetic integrity of indigenous plant communities is not compromised.

Moddervlei – Rehabilitation undertaken by Rondevlei Nature Reserve.

Skilpadsvlei wetland rehabilitation – before (left top and bottom) and after (right top and bottom). Rehabilitation undertaken by City of Cape Town’s Biodiversity Management Branch. Credit: Suretha Dorse, Project Manager

More difficult wetlands to rehabilitate are those where there have been permanent changes (increases) in the local water table.  In circumstances where the original habitat type cannot be restored, improvement in habitat diversity might be an achievable objective.  The establishment of seasonally flooded wetlands along the margins of Zeekoevlei is an example of this kind of rehabilitation. Scarce safe breeding habitat has been created for endangered Western Leopard Toad, and breeding and feeding grounds for other wetland animals. These kinds of habitats are missing in the large, open water vlei.

Deep erosion gulley or headcut creating an incised channel that drains the surrounding wetland. Credit: Hans King

Erosion threatens many wetlands. It needs to be addressed urgently to prevent soils from washing away and to stop wetlands from drying out because the water table has been drawn down to the bottom of the gulley.  The flat gradient of most areas of the Cape Flats means, however, that erosion is not a pervasive issue in these wetlands. 

Wetlands where the soils have been infilled by contaminated material, or which have a long history of receiving polluted water, are often the most difficult to rehabilitate without expensive excavation and disposal of contaminated soils, sometimes to hazardous waste sites.  One motivation for this kind of rehabilitation expense might be to reduce the risk to water users or downstream systems.

Measuring ground water level in a seasonal wetland, using a piezometer.

As an alternative or supplement to wetland rehabilitation, sometimes areas of land lend themselves to the creation of wetland habitat.  Many wetlands on the Cape Flats are fed by groundwater, being inundated in winter when the water table rises and floods low-lying areas.  Excavating into the sand just enough to expose shallow groundwater in the wet season and allow it to dry out in the summer, can be achieved quite easily in these areas.  Attention needs to be paid to shaping such artificial wetlands so that they look natural, with gentle undulations grading out to the surrounding terrestrial habitat.  Nutrient-rich soils might also need to be removed, as would alien plants such as kikuyu grass and other species, and locally indigenous wetland plant species would need to be introduced because there is unlikely to be natural seed stock in such habitats. 

Getting the right depth profile is also difficult – too deep and it will be bulrush-invaded; too shallow and it won’t provide high quality wetland habitat. Measuring changes in the water table over a year can inform decisions as to the right depth of excavation.  This measurement can be done using a slotted pipe (or piezometer), inserted 1–2 m into the ground – the water table rises up and down inside the pipe and its depth below ground can be measured over time. 

“When thinking about wetland management and rehabilitation opportunities, it is important to remember the first rule of wetland management – it is quick and easy to destroy wetlands but difficult to rehabilitate them once degraded. The first prize is not to destroy them in the first place.” Adapted from Rutherford et al 2000