Erosion

The soils of this property are very fragile. They are derived from shale as witnessed by the presence of many reefs of shale. The reefs all turned on their side by about 70 degrees, run in a North/East – South/Westerly direction. It is this reef formation that gives the property it’s steep to undulating land form.

The layer of topsoil is very shallow and lies just above a pale yellow subsoil that is very prone to erosion. In some places it seems to be almost “soluble” and certainly easily dispersed.  A combination of cleared vegetation, overgrazing and extreme weather events (not uncommon here), soon lead to the exposure of the subsoil and an erosion domino effect.  The header image above shows country with quite gentle slopes and yet the effect of poor management have been devastating.

There exists evidence of “experimentation” by soil conservation “specialists” of the past. The experiment in question was the construction of a number of contour “absorption”  banks, one above the other, on a hillside with a 25 to 30 degree slope. The intent was that the banks, over 200 meters long, would hold the water under a storm event and then allow the stored moisture to be absorbed slowly into the soil. The original sheet erosion on this site was caused by over stocking and excessive clearing of ground cover/trees.

Unfortunately the soils were not very absorbent because of the soils high clay content. The result being that the banks filled, and overflowed, cutting the banks and washing away the disturbed soils.

The preferred solution to the effects of existing eroded gullies has been performed on this property in conjunction with the “The Corrowong Wallendibby Erosion and Sediment Control Project 1995-2001″. This was one of the largest of its kind in the state of NSW at the time. The earthworks carried out on Knockalong consisted of a series of diversions banks moving the concentrated water flows from the eroded gullies to be dispersed on the ridges. On one of the sites a dam was built to catch the gully sediment prior to the water being diverted.

The Corrowong and Wallendibby Landcare Group commissioned a “Benchmarking Report” in 2007 to evaluate the immediate and ongoing effectiveness of the erosion and sediment control structures. This would also provide the foundation for future erosion and sediment control strategies.

Diverting concentrated flows of water in this way is still relevant today and is incorporated into the management strategies on Knockalong. An example of this can be found on the Trees page where it has been incorporated into the establishment of trees.

Another form of erosion repair has been carried out on the “experimental” site mentioned above. The aim was to make as little disturbance to the site as possible and aid it in a process of “self healing”.

“Foot”netting was used across the hillside on a contour to build a series of terraced structures. In the deeply eroded sections bales of pasture hay were combined with netting. Short wood and iron posts were driven into the hillside to attach a top wire and secured the netting. Pasture hay was then spread in and along the lines of “fencing” to create a sediment trap. Grass and locally harvested tree seed was then spread over the area.

In addition to this we experimented on bared areas by pegging old fencing netting and coconut netting over a layer of pasture hay. The original report for the Terraced Erosion Control can be viewed as well as the follow-up report.Terraced silt traps