My 121 Ha property, “The Granites” at Carapooee, consists of a granite outcrop with sloping granitic sand terraces at the foot. There is about 7 km of erosion gullies on the property, which represents a serious loss of habitat, resulting in a significant sediment load in the Carapooee Creek and ultimately the Avoca River.
The sandy soil of the terraces has very little clay to bind it and so erodes readily. There is an underlying soft sandstone layer which is eroded slowly by the process of falling water.
The photo below shows typical erosion.
The Kara Kara Conservation Management Network secured a grant for some erosion control work on this property. The grant paid for 60 tonnes of rock, gravel/sand mix, filter cloth and seeds for regeneration.
Half of these materials have been used to control the erosion in a 400 m section of one gully, where about ten leaky weirs have been placed using the rocks across the eroding sections and a further five areas of undercut have been stabilised.
The objective of the work is to
• stop the spread of drops from propagating upstream
• stop widening of the gully floor
• stop further undercutting
• Slow the water speed in the bottom of the gully
The slope of the steep sections and drops were jack hammered until the slope was about one in five. Filter cloth was laid across the gully floor and covered with the sand/gravel mixture. Rocks were the placed over the filter cloth to form a water race and leaky weir to slow the water flow and collect sediment.
This has effectively stopped all erosion in the bottom of the gully. Any further erosion of the sides of the gully results in reducing the slope of the wall. Eventually the walls will become revegetated. The sediment from this erosion is collected by the leaky weirs and does not leave the erosion area.
Other leaky weirs were placed across the gully floor to slow down the water flow.
A typical undercut is shown below. By placing a rock wall beside the undercut, eroding water flow is kept away from the undercut. The undercut will eventually collapse, but will not widen further.
The work requires at least annual inspections and minor repairs. As the sediment builds up at each wall, additional rock will be added to raise the height.
Natural Healing of some gullies
Some of the gullies have self-healed just by removal of stock (in 1995) and reducing rabbit numbers. The slope of the sides of these gullies has become less steep and grasses (mainly weeds) and trees are regenerating there, thus stopping further erosion.
The photo below illustrates this effect.