Authors: Lowry, JBC; Evans, KG; Coulthard, TJ; Hancock, GR; Moliere, DR


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Lowry, JBC, Evans, KG, Coulthard, TJ, Hancock, GR & Moliere, DR 2009, 'Assessing the impact of extreme rainfall events on the geomorphic stability of a conceptual rehabilitated landform in the Northern Territory of Australia', in AB Fourie & M Tibbett (eds), Mine Closure 2009: Proceedings of the Fourth International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 203-212,

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The Ranger uranium mine located in the Northern Territory of Australia is the largest producer of uranium oxide in Australia. It is currently envisaged that the mine will cease production and commence rehabilitation after 2020. The mine lease is located in the catchment of Magela Creek and is surrounded by the world- heritage listed Kakadu National Park. The Supervising Scientist Division of the Commonwealth Department of the Environment, Water, Heritage and the Arts undertakes an independent physical, chemical and biological monitoring programme in the Magela catchment to check for any impacts, including stream sediment loads, by mine-related activities. During late February and early March 2007, a period of exceptionally heavy rainfall occurred throughout the entire Magela Creek system as a result of a monsoon trough that extended across the top end of the Northern Territory. In what has been categorised as a greater than 1-in-100 year storm event, total rainfall over the three day period between 27 February and 2 March 2007 at Jabiru airport near Ranger was 785 mm, the largest three day rainfall that has been recorded in this region. The general extent of sediment transport and flooding associated with this storm highlighted the need to have an understanding of the possible impacts of extreme rainfall events on the erosion of the proposed rehabilitated mine landform and subsequent sediment export to Magela Creek. An understanding of these effects can be gained through landform evolution modelling (LEM). One such model is Cellular Automaton Evolutionary Slope And River (CAESAR). In contrast to LEM packages such as Siberia that use data that has been averaged, CAESAR uses an hourly rainfall record and has the capacity to simulate the effects of discrete rainfall events on landform stability. Simulations were conducted using a digital elevation model (DEM) of the largest catchment of a proposed rehabilitated mine site using the Jabiru hourly rainfall recorded over a 21 year period. For the purposes of the simulation, the tailings dam catchment was attributed with the surface characteristics of waste rock. Data collected for the greater than 1-in-100 year rainfall event in 2007 were inserted into this rainfall record after an initial ten year simulation period, to allow the CAESAR model to simulate the impact of the storm event, and the model run for a further 11 years. The simulation showed a pulse of sediment moving out of the catchment immediately after the storm event and identified a denudation rate of 5.6 mmy-1 during the 2007 rainfall year time series. In contrast, previous studies in the region, using various methods produced a range of denudation rates for waste rock of -2–7 mmy-1 with a median of 0.04 mmy-1. Further research is required to identify the potential impact of extreme rainfall events on other possible surface treatment types in a catchment setting.

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