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Tshwane / Rietvlei View Map The Tshwane project implements wetland rehabilitation intervention in an area that is increasingly pressured by urbanisation. The project spans two quaternary catchments in Gauteng in the south-west of the City of Tshwane Metropolitan Municipality. They are linked to the Jukskei and Crocodile rivers. Wetlands in this area are of high importance for biodiversity conservation as well as water quality and flood attenuation.
The area includes habitat of high biodiversity importance, such as Egoli Granite Grassland. There are still significant areas that are undeveloped, with some open spaces being included in conservancies.
The health of wetlands in this area impacts on water quality well beyond the boundaries of the Municipality, most notable affecting water quality in the Hartebeespoort Dam in the North West Province which experiences significant water quality challenges. Water quality is therefore a key selection criteria for wetlands targeted for rehabilitation intervention.
Impacts on the wetlands in the area include storm water outflows, roads, pipelines, railway crossings, infrastructure encroachment, canalisation, dumping and mining. Some wetlands were also modified in the past by ridge and furrow ploughing for cultivation, and subsequently used for livestock grazing. Many wetlands are still used for grazing.
Rehabilitation intervention includes the construction of concrete and gabion structures, and clearing of invasive alien plant species. The project provides temporary employment and skills development for 62 people.
Rietvlei
Previous work in the Municipality focused on the Rietvlei wetland which was considered a priority for rehabilitation, given the high rate of development in its surrounding dolomitic landscape and the fact that it contributes significantly to Tshwane’s water supply.
The wetland lies upstream of the Rietvlei Dam within the 4,000 ha Rietvlei Nature Reserve just outside Pretoria. It is a diverse wetland system that performs wonders in contributing to keeping much of the city’s water clean. The Rietvlei Dam has been used by Pretoria since 1934 for drinking water, producing about 41 million litres per day, or about 3 percent of Tshwane’s current drinking water requirements. The reserve was established to protect the dam’s immediate catchment and is also an important tourist destination, providing sanctuary to many game species including the hippopotamus, rhino, zebra, cheetah, buffalo and impala.
When under private ownership in the past, the wetlands were drained for cultivation and access, with water flow diverted away from the main wetland, a peatland, at the lower end of the system. The artificial drainage ditches resulted in erosion of the main channel upstream and destabilised the river banks, leading to further erosion. Road crossings and berms across the wetland altered its hydrology, and a sewage treatment plant upstream periodically discharges into the wetland.
Rehabilitation intervention included gabion, concrete and earth structures to control erosion and ensure even distribution of water across the wetland. Clearing of invasive alien vegetation was an important part of the work.
By early 2007 rehabilitation intervention at Rietvlei showed great success and the wetland was considered to be nearly fully recovered. Reeds re-established throughout the wetland and there is even distribution of water with no longer any channeling. It is suspected that peat growth may have resumed, but this is still to be confirmed.
Kosmosdal Wetland : The 15 ha wetland on the Rietspruit is affected by several erosion gullies that seem to be the result of stormwater flow into the wetland from the N1 highway and concentration of flow by a road and a pipeline. There is evidence of past cultivation in the wetland, and some sections of the wetland have been developed. Its catchment is heavily developed in sections, but also includes natural grasslands. The wetland has a gentle gradient, which reduces its sensitivity to erosion. It has remained intact for the most part and there is only a small amount of desiccation. Gabion structures and earth works are used to stabilise erosion and raise the water table within erosion gullies, as well as restore functionality to a desiccated section of the wetland.
Doornrandjies Wetland 1 : This is a 3ha wetland that was modified as a result of historical cultivation and grazing in the wetland. Its catchment is mostly grassland in good condition. Headcut and gully erosion has desiccated the wetland and resulted in faster, more erosive water flow through channels. An eroded dam wall has affected part of the wetland, and desiccation has allowed for encroachment by invasive alien species, which cause further desiccation and degradation. Rehabilitation is aimed at stabilising erosion, raising the water table and restoring wetland functionality.
Doornrandjies 3 Wetland : This 8 ha wetland was modified significantly by gully erosion. Overgrazing and a concentration of water flow have created headcut erosion and a gully that extends along its length. The wetland’s catchment is in a natural condition for the most part. Rehabilitation is aimed at stabilising headcut erosion and raising the water table in the incised gully.
Contacts
For more detailed information such as project implementation plans and progress reports, please visit www.thelapa.com, click on Working for Wetlands and enter view for both username and password.
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