Erosion Rectification and Prevention

Case Study by Sean Altern – Consultant: Biodiversity Conservation Services

A portion of the 100m long embankment showing deep gulley formation (Altern, May, 2022).

NCC Environmental Services was contacted by one of our long-term clients, Vergelegen Wine Estate and requested to rehabilitate a large embankment adjacent to one of the farm dam spillways that had, for many years, been slowly eroding away.

Erosion can be the result of environmental impacts and conditions elsewhere. As such before any work was undertaken or suggested, careful site assessment was done to analyse the root cause of the problem which entailed the following factors:

  • Above the embankment there were large patches of hard, barren soil in which shallow rills from water runoff had formed.
  • Clues to the cause of these barren patches and subsequent rills were seen in the remains of large burnt out cut stumps of Eucalyptus and pine trees – These trees have allelopathic properties (Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms) which in the case of gums and pines prevents the indigenous vegetation from growing and resulting in denuded and weakened soil. Secondly these large alien trees burn much hotter than the indigenous vegetation and subsequently bakes this denuded soil into a hard surface in which water does not absorb but rather runs off.
  • This run off water picks up force and sediment which cuts into the soil, first as rills, and then as gulley’s as it digs deeper, especially in this case as the water moved over the sharp edge of the quarzitic loam embankment, a very erodible soil type and scenario.
NCC’s off season wildland firefighting crew members not only tackle the summer blazes but excel in repairing the damages (Altern, May, 2022).

IMPLEMENTATION

Understanding the root cause of the erosion enabled the design of a solution to not just fix the resultant embankment erosion but more importantly to address the cause (any rectification work would continually be undone, and the erosion would keep occurring if the problem was not dealt with at the source).

As such NCC designed a series of silt nets which were dug into the ground above the embankment. These biodegradable structures function in a series by:

  • Interrupting flow of silt and water thereby reducing its strength and erosive capability;
  • Increase plant germination along these lines to aide soil stabilisation; and
  • Assist in holding mulch in place.

Between these nets, indigenous plant cuttings were planted, and additional fynbos seed (sustainably harvested) was spread out before the oncoming rains to help revegetate the area and subsequently reduce the problematic runoff.

The embankment, now somewhat protected, needed to be reshaped and stabilised. This was achieved first by utilising large scale machinery which was followed up by hand to break apart all the gulley’s and rills and produce a more gradual and smoother shaped embankment.

Goutte soil saver cloth was then secured into a channel and finally pegged in across the entire area. This helps to:

  • Stabilise the embankment;
  • Protect seeds and plant cuttings; and
  • Reduce direct raindrop impact erosion.

On top of this cloth, for added stabilisation and to create a berm step like effect, three rows of old vineyard poles gathered from the farm were laid.

Finally, a light covering of straw was spread over the site to assists as a temporary mulch whilst the site re-establishes.

Local Erepsia cuttings planted into the Geojute clothe (Altern, May, 2022).

It is important to understand the cause

It is vital to always understand and address the root cause of a problem before deciding on how to fix the result.

Erosion, as with many other problems, are often multi-faceted and work in conjunction with other elements – in this instance, invasive species and fire triggered the barren patches which resulted in rills, which when encountering a steep slope of erodible soil created massive gulley’s.

As the causes and series of events that follow problems are interconnected, so to, are the remedies. Whilst temptation is often to address a large problem with an equally large solution it is rather that a combination of small, well thought out solutions to an understood cause, can work equally as well, if not better. In this case the solution was derived from understanding the cause and to rectify this a combination of soil reshaping, anti-erosion structures and ecological restoration practices (indigenous revegetation) was utilised, none of which on their own would suffice, but in combination and working together as a whole achieve the desired result.

The result highlights Vergelegen Wine Estate’s continual drive to partner with NCC Environmental Services to restore and improve the unique natural habitat of the farm.

“This work provides valuable out-of-season work and new skill sets to our ever-improving wildfire crews who are learning to see the full range of negative effects from invasive alien vegetation and subsequent fires.”

The site will be continually monitored and maintained with the expectation of a much more naturally vegetated area resulting and reduced erosion occurring.