Authors: Nair, D; Bellairs, SM; Evans, K

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DOI https://doi.org/10.36487/ACG_repo/2215_78

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Nair, D, Bellairs, SM & Evans, K 2022, 'An approach to simulate long-term erosion equilibrium of a rehabilitated mine landform', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2022: 15th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1063-1074, https://doi.org/10.36487/ACG_repo/2215_78

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Abstract:
A major focus for evaluating mine site rehabilitation in the tropics of Australia is determining when excessive erosion of above-grade waste rock landforms is no longer occurring and that they are in equilibrium with the surrounding catchment. The Ranger uranium open cut mine in the Northern Territory, Australia, lies between Magela Creek and Gulungul Creek. These creeks are sand-bed, ephemeral streams draining into the East Alligator River and wetlands of international significance. Landform evolution modelling (LEM) and assessments of streamflow fine suspended sediment (FSS) discharge are being used to evaluate erosion and determine whether FSS exported to Magela and Gulungul creeks during rainfall-runoff events is in excess of natural background levels. LEM provides an avenue for simulating how a landscape may evolve over extended time periods of thousands of years. The CAESAR-Lisflood LEM is being used to assess the proposed final landform morphology by simulating how the mine landform and the landscape in the Magela Creek catchment and Gulungul Creek catchment would evolve over a 1,000-year period. The model assesses gully formation and bulk sediment export to the creek systems. The challenge is how to assess when a landform has reached equilibrium with the surrounding catchment and therefore whether the site can be considered rehabilitated with respect to landform stability. FSS-stream loads following a rainfall event can be used as an indicator of landform stability, as the studies in Gulungul and Magela Creeks have shown. The aim of this project is to validate a new approach using an event-based stream FSS discharge relationship in combination with a LEM to determine when erosion of a rehabilitated landform is at equilibrium with the surrounding catchment. An event-based FSS/stream discharge relationship was previously developed using stream monitoring data. When disturbance occurred in the catchment, this FSS/stream discharge relationship changed to reflect a system change. For a small disturbance, monitoring has shown that the relationship returned to the predisturbance condition after one or two years. The CAESAR-Lisflood model is being used to predict FSS values for a given discharge for a large disturbance across the whole catchment. The hydrology and FSS discharge for the whole Gulungul Creek catchment is being calibrated and validated using the available data. Future rainfall events will be input into the model. The FSS values from the current observed relationship (expected FSS loads for a given discharge) will be compared with that obtained from the CAESAR-Lisflood modelled landscape (predicted FSS loads for a given discharge) to find out when and why the system moves in and out of equilibrium during the 1,000 year period. A variation in event FSS loads beyond the confidence intervals of the best-fit line in FSS (expected from current relationship) versus FSS (predicted from CAESAR-Lisflood model) indicates future landform instability. Such a study will throw light on how a disrupted landform can move in and out of equilibrium until it reaches a steady state with the surrounding environment, which is considered stable.

Keywords: mine rehabilitation, landform stability, erosion equilibrium

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