Authors: Mengler, FC; Gilkes, RJ


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Mengler, FC & Gilkes, RJ 2006, 'Thresholds, Triggers and Time ⎯ Erosion Risk on Evolving Reclaimed Landforms after Bauxite Mining in the Darling Range, Western Australia', in AB Fourie & M Tibbett (eds), Proceedings of the First International Seminar on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 587-597,

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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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596 Mine Closure 2006, Perth, Australia
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