Working on a project in the Sacred Valley Peru, I came across an opportunity to install Gabions to reduce soil erosion on a steep slope. A Gabion is a porous dam wall made from rock and small stones free standing or packed into a wire basket. They combat soil erosion by slowing the flow of water and dropping sediment and organic material behind the rock wall as water slowly leaks through it. They can also be used as retaining walls and in drylands used as a water harvesting feature, see Geoff Lawton’s article here https://www.permaculturenews.org/2010/11/25/gabions-water-soaks-in-the-desert/
To the left of a steep (35-40 degree) area planted to food forest is a growing erosion gully caused by water erosion during the wet season. This was probably initiated by disturbing the natural slope when paths were cut in, creating conditions for a large flow of water to spill out over the path above the point of the gully. In some cases the erosion is back to bear rock, and furthers rains threatened to erode the edges of the newly planted food forest.
It was decided to install 3 wire cage rock gabions to slow the flow of runoff, capture sediment to provide a bed for plants to colonise and further stabilise the Gully. Small stone walled earth back swales were also installed to catch any potentially overflow from the gabion, runoff, and also to provide access to the steep food forest.
The swales were installed first by marking out contour lines using an A-frame (see article here: https://www.permaculturenews.org/2015/02/13/contour-beds-peru/), these contours intersected with the proposed gabion positions (In Hindsight it would have been better to install the gabions first to ensure an accurate level, however in the end our levels were fine).These positions were chosen based on an equal distance to one another down the gully and also the ease of which the earth below the gabion wall could be levelled.
Trenches were then dug to about 200mm to create a foundation for our stone wall which were then built to around 400-500mm and backfilled and mounded with soil to create a shallow swale. The swales were then levelled by flooding with water and adjusting with a shovel.
To install the gabions the earth was levelled and the size of the wire boxes measured based on the depth of the gully at the point of the gabion and the height of the swale mound.
The wire mesh was cut in such a way to allow for the minimum amount of joins such that 4 sides of the box were cut from one piece and folded into shape, with another piece making up the other 2 sides. All joins were joined together threading galvanised wire through the mesh.
2 months later and a small amount of silt has built up at the base of the gabions and plants have began to colonise this space.
Respect!
Looks good! Densely planted vetiver grass planted on contour is often used to stabilise steep sites. Might be worth planting some adjacent to and below the gabions to help keep them in place and stabilise the nearby bank.
Good suggestion Tim. Will try and source some vetiver.