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• SSR165 - Temporal trends in erosion and hydrology for a post-mining landform at Ranger Mine (PDF - 2,101 KB)

Executive summary

An important part of rehabilitation planning for mines is the design of a stable landform for waste rock dumps or spoil piles, at the completion of mining, which minimise erosion and environmental impact offsite. To successfully incorporate landform designs in planning, there is a need to be able to predict the surface stability of the final landform using erosion and landform evolution modelling techniques.

In the long term, weathering, soil forming processes, ecosystem development and even climate change may affect the surface characteristics, and hence the stability, of the rehabilitated landform. In this study, changes to the surface characteristics of a landform in time can be quantified in terms of erosion parameters. Since a prediction of the stability of the rehabilitated landform is required over the long term, temporal changes in these erosion parameters are incorporated into landform evolution modelling of a post-mining landform.

The landform evolution model SIBERIA was used to predict the stability of the proposed rehabilitated landform at Ranger Mine, Northern Territory. Previous landform evolution modelling at Ranger used input parameter values derived from data collected from areas of the waste rock dump at the mine and these input parameter values were assumed to remain constant throughout the period that was simulated. In this study, natural rainfall event data were collected from various sites considered to be representative of the surface hydrology and erosion characteristics that would exist at Ranger at various stages after rehabilitation. The sites were located on: a batter slope on the waste rock dump at Ranger; the waste rock dump of the abandoned Scinto 6 mine in the South Alligator River valley; a natural, undisturbed area at Tin Camp Creek, Arnhem Land; and a natural area near Pit 1, Ranger Mine. SIBERIA input parameter values were derived for each study site to determine the rate of temporal changes in parameter values under both concentrated flow and sheet flow conditions.

There is a very clear temporal effect on SIBERIA input parameters that reflect the erosion rate likely to occur on the Ranger landform (m₁ and β₁). The change is rapid and occurs within the first 50 years after mining is completed, at which time the parameter values approach that of an old, natural landform. This study has quantified temporal changes of erosion processes in terms of these input parameter values.

SIBERIA landform evolution simulations of the proposed rehabilitated landform at Ranger were conducted incorporating the rate of temporal change in input parameter values due to ecosystem development. The erosion rate and valley development on the simulated landforms with input parameters that change with time decline relatively quickly in the short term, particularly on the landform with sheet flow conditions where sediment movement stabilises almost completely after 50 y of simulation. The incorporation of these temporal changes in parameter values into the SIBERIA model has provided a best estimate of the stability of the landform at Ranger over a 1000 y simulation period.