Farmer's Weekly (South Africa)
Eroded watercourses on Vergelegen wine estate rehabilitated in R12m project
Vergelegen wine estate in Somerset West recently completed an extensive programme to rehabilitate eroded watercourses at five sites on the estate. The undertaking saw the removal, temporary storage, and replanting of some 15 000 indigenous plants.
The R12 million project has saved rehabilitated wetlands and critically endangered Lourensford alluvium fynbos vegetation, which might otherwise have been badly affected by sediment washing onto the area.
In addition, the venture has halted sediment contamination of the Lourens River, part of which runs through the estate. This is the only river in South Africa where a section of adjoining land is considered a formal protected area.
CAUSES OF EROSION
By 2019, Vergelegen had completed the largest privately funded alien vegetation clearing project in South Africa. Around 2 200ha of the 3 000ha estate were cleared of alien vegetation.
It is likely that the woody alien vegetation laid the foundation for erosion by displacing the indigenous deep-rooted species that protect the soil against degradation in the event of heavy rainfall. Structures that concentrate the flow of water, such as roads and road culverts, also contributed to the erosion.
Leslie Naidoo, risk and commercial manager at Vergelegen, said: “We had completed a large portion of the alien vegetation clearing when, in 2013, we experienced heavy rains and severe flooding. This increased peak flow rates of water run-off and created gullies in many of the watercourses. In some areas, rocks were washed out and blocked the natural watercourses flowing to the river, forcing an unnatural distribution of higher flows to be channelled onto the estate.
“The resulting erosion was initially not easy to see, because the land was still covered with vegetation. A wildfire in January 2017 exposed the areas and signs of damage could be identified; without the standing vegetation to stem the water flow, the situation worsened during the winter rainfall. We immediately brought in experts and, in due course, received reports from an environmental consultant, a specialist engineer, and a wetland expert.
“It was agreed that, without suitable intervention, there would be degradation resulting in ‘irreversible ecological damage and huge financial costs’.”
TACKLING THE PROBLEM
Of the five zones, one was especially challenging. Specialist engineer Hans King visited the estate in 2017 to stabilise a 40m gulley. He noted “a strange, very narrow (about 500mm) but very deep (about 2m) groove in the soil extending for maybe 100m upstream of the gulley”.
A few months later, the groove had become part of the gulley, which itself had become much wider and deeper.
An examination of the soil’s structure revealed why it was eroding so rapidly. The top 50mm was reasonably resistant to erosion, but beneath it lay deep, highly decomposed granite. Once the erosion was deep enough to penetrate this granite, vertical erosion was swift.
There was the danger that a wide area around the gulley, a large portion of which is home to the Lourensford alluvium fynbos, could be covered by a deep layer of weathered granite and silt.
In August 2020, the rehabilitation got under way. Thirty erosion-control weirs were constructed, sediment was removed, and rocks and other materials were placed in the watercourses.
REPLANTING THE VEGETATION
As part of the rehabilitation programme, local indigenous vegetation was prepared for replanting after the erosion-control weirs had been constructed and other containment measures put into place.
The team erected a 10m x 30m tunnel, in which plants from the eroded areas were stored for replanting, while new plants were also propagated.
In addition, green, well-rooted palmiet [ Prionium serratum] was gathered from a dam spillway stockpile and placed vertically in the weirs.
“This project has been a major venture, but it has ensured that we have controlled the erosion that would have had an extremely damaging impact on the land,” says Naidoo.
“With 1 900ha of the estate promulgated as a private nature reserve, we are committed to the long-term sustainability of the estate so that it can be shared with future generations.” Vergelegen wine estate
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