Another important factor in stabilising eroding slopes is to construct drains to divert the water that causes the erosion. Such drains must be built in conjunction with new pitched paths or steps to contain trampling, and reseeding or other revegetation work to help stabilise the slope. The drains may not be needed in the long term, but can be allowed to silt up or grow over as the slope revegetates.
Several large areas of erosion have been successfully contained in this way. The example below is the successful restoration of Ashop Head, in the Peak District, by the National Trust.
The area of erosion on the slope leading up to Ashop Head, a well-known viewpoint on the Pennine Way, was approximately 180m in length by 30-50m width. The slope was very unstable due to the three factors of trampling by walkers, waterflow, and trampling, grazing and ‘sheltering’ by high numbers of sheep.
All three factors causing erosion needed tackling. The entire area was first fenced against sheep. Cut-off drains up to 30m long were constructed of stone, to divert the flow of water to the vegetated, uneroded areas at the sides of the slope. The cut-offs were constructed of fairly regular size stone, in the manner described here. Management of walkers involved building a pitched path taking a natural curving line up the slope, to channel trampling onto a single line, where previously walkers spread over the entire slope. Here, and elsewhere, the concept of ‘hairpin bends’, is avoided, being difficult to construct, liable to erode and frequently avoided by walkers. The strength of a pitched path allows a steeper, more direct line to be taken. Large boulders were set in randomly to discourage walkers from taking routes other than the pitched path.
Very low nutrient levels and pH of 3.5-4.0 were typical of the slope, so the area was limed and fertilised, using a low nitrogen fertiliser, at a light application rate of 1 tonne/ha (10g/m2). It was reseeded with a general acid grassland mix, approved by English Nature. The mix is non-native, but is replaced by native species which spread naturally after a few years.
Within a few years, the whole slope was well vegetated, the scar almost invisible from a distance. Most walkers probably use the pitched path without noticing the drains, now weathered and softened with vegetation, although still functioning. As the vegetation recovers further, and run- off is slowed and absorbed into the slope, the drains will grow over as the need for them declines. The removal of grazing was vital to the success of the scheme. On other similar, though less severely eroded sites in the same area, the erosion process has been reversed simply by the removal of sheep, allowing slopes to stabilise and revegetate naturally.
Similar work had been done by the National Trust in Snowdonia using larger stone-lined drains of the type shown here.
