Here is a detailed review of a paper called Thresholds, Triggers and Time –Erosion Risk on Evolving Reclaimed Landforms after Bauxite Mining in the Darling Range, Western Australia by F.C. Mengler and R.J. Gilkes, both of the Center for Land Rehabilitation, The University of Western Australia. The paper is available in Mine Closure 2006.

They describe their observations of gully development at reclaimed bauxite mines. Here is how they summarize their observations:

A combination of concentrated flow and trigger points caused gully headcut incision. Ponding, crusted soil and silt-like furrows were present upslope at many gully headcuts---providing a setting for “fill and spill” overland flow. Small pipe-like openings 2- to 15-cm wide on the face of tilled furrows upslope of a gully at Taipan [one of their sites] indicate that piping and subsurface flow may have contributed to some gullies at this site. The most common setting for gully erosion was at points in the landscape where water run-off from upslope forest areas was concentrated onto the steep shoulders of rehabilitated hill slopes. A poor standard of rehabilitation tillage (off contour, incomplete, or not seamlessly tied into adjoining areas) led to run-off concentration. Gullies incised wherever this concentrated run-off met a trigger such as concavities, thalwegs, knickpoints, boulders, poorly-installed fauna habitats or the remnants of waste rock dumps. Insufficient depths of returned topsoil and overburden (less than 20 cm combined) also triggered gully erosion. Steep gradients ort long slope lengths made gullies more severe but did not trigger gullies.

The authors propose that the critical factor in gully initiation is the relationship between critical slope gradient (Scr) measured in degrees, and the upslope drainage area (A) measured in hectares for the three sites they studies. They express the relationship as:

Scr = a A-b

These are the values of “a” and “b” for the three sites they studied:

a b
Boddington 0.2 0.39
Huntly 0.05 1.66
Willowdale 0.02 1.59

On the basis of their observations, the authors propose two types of gullies that develop as follows:

Type A Gully: The gully is caused by the concentration of fast-moving, surface water in excess of infiltration capacity during “fill and spill” episodes involving the overflowing of ponds within contour furrows upslope. These gullies form when rapid flows cut down into weak topsoil and overburden materials. Once formed, they act as conduits for more upslope and lateral surface flow.

Type B Gully: The gully is caused by substrates that are saturated either by inherently slow-draining subsoil or by geological impediments to groundwater flow. Some Type B gullies develop from small pipe-like seepage outlets in upslope contour mounds. Type B gullies form more slowly than Type A gullies, mainly by seepage, slumping, and/or mass wasting. Once formed, Type B gullies evolve similarly to Type A gullies, but can act as conduits for both surface and subsurface flow.

The authors do not bring all this insight together. Rather they simply conclude that a combination of these major triggers can cause gully erosion:

  • Excess off-site waster supply into the eroding area
  • Poor surface completion resulting in concentrated flow and/or poor infiltration
  • Insufficient depth of returned topsoil and overburden
  • Fauna habitats displacing contour lines
  • Shallow groundwater.