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Some rehabilitated bauxite mines in southwestern Australia have accelerated gully erosion while the majority
have little erosion. Anecdote suggests that gully erosion in rehabilitated forest is controlled by slope gradient
but not all steep areas erode and conversely, some gentle slopes do. Mining companies aim to achieve
erosion behaviour in rehabilitated areas similar to that of the surrounding forest - where large gullies are rare.
We surveyed 26 eroding and erosion-prone rehabilitated hillslopes and developed descriptive models to
predict the occurrence of gully erosion. A model of gully triggers implies that triggers and threshold effects
are as influential as slope gradient and length in determining both the occurrence and severity of gully
erosion. Many pre-existing triggers that predispose critical parts of a landscape to gully erosion activate only
under threshold-excess conditions. Pinched concavities (thalwegs), shallow topsoil and gravel cover,
erodible subsoil, high groundwater level, misplaced fauna habitats and irregular rehabilitation boundary
edges are common erosion triggers. Slope angle and slope length, upslope catchment area, landscape
position, soil storage and infiltration capacity, and rainfall (duration and intensity) are threshold variables.
Many of these triggers and some thresholds can be identified and hence mitigated at the pre-mining stage.
Topographic thresholds for gully erosion determined by the relationship between the critical slope (Scr) and
contributing area (A) at Boddington, Huntly and Willowdale bauxite mines are: Scr = 0.2A-0.39, Scr = 0.05A-1.66
and Scr = 0.02A-1.59. Additionally, at the minimum catchment area for gully incision (0.3 ha), critical pre-
mining slopes are 14° for Boddington, 10° for Huntly and 6° for Willowdale. Landforms exceeding these
conditions may need site-specific designs to mitigate gully erosion risk. The rate of cumulative erosion and
gully development measured by erosion pins on selected hillslopes closely follows the trend of cumulative
precipitation at least during the first three seasons of rehabilitation growth. After this time, most gullies reach
stasis. Cumulative erosion of non-mined, natural slopes also closely follows cumulative precipitation but at
much lower rates (about 30 times lower than gullied sites). A proposed model of site erosion potential versus
contributing area suggests that sites with the biggest gullies are above a threshold separating low- and high-
state erosion. The effect of fire and maturity on the stability of gullied, rehabilitated sites is unknown.
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Thresholds, Triggers and Time ― Erosion Risk on Evolving Reclaimed
Landforms after Bauxite Mining in the Darling Range, Western Australia
F.C. Mengler, R.J. Gilkes
596 Mine Closure 2006, Perth, Australia
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