Post-mining landform stability
Without proper design and appropriate management, above-grade waste rock dumps (WRDs) and other landforms (eg capped tailings dams) produced by mining have the potential to cause severe environmental impacts, both in the short and the long term. One potential form of impact is the pollution of waterways through erosion of post-mining landforms, including WRDs, and movement of sediment into streams and rivers. Impacts on water quality by excessive levels of suspended sediment an be reduced by designing post-mining rehabilitated landforms in such a way that erosion will be minimised. The ability to quantify sediment generation from a landform, through predictive modelling, takes the 'guess-work' out of landform design.
Fundamental to any assessment of the stability of post-mining rehabilitated landform is the ability to quantify soil erosion rates. The extent of gullying or erosion may be used to indicate how effective a structure encapsulates waste material such as low grade mineralised rock or tailings. If erosion results in gullying which could expose tailings and other waste material, enabling contamination of the environment, then the design of the landform obviously needs to be re-assessed.
Recent research has addressed the use of of landform evolution modelling (incorporating hydrological, topographic and sedimentological components) for evaluating the likely performance of landforms being proposed for rehabilitation of mine sites. Important components of this process include the derivation of appropriate model input parameter values; and the assessing the ability of the models to correctly predict behaviour in the field. A schematic diagram of the landform design and modelling process is shown in Figure 1.
Figure 1 Schematic representation of the landform design and modelling process
A range of different landform evolution models (LEMs), of varying degrees of complexity may be used. These require different forms of site-specific data for calibration.
The choice of model may be determined by a combination of factors. These can include the likely environmental impact; the cost of data collection for model parameter value derivation; the process regime; and the rehabilitation standards that will be applied. At the less complex end of the scale, if a standard requires a maximum allowable sediment loss from rehabilitated areas then the empirical Revised Universal Soil Loss Equation can give an indication of soil loss rates applicable in the initial stages of assessment and design. At the other end of the scale the complex topographic evolution models such as SIBERIA and CAESAR can be used to assess gully development and incision and landform containment design.
Figure 2 Three-dimensional representation of a proposed final landform for the Ranger mine
SIBERIA is a sophisticated, 3-dimensional topographic evolution model that simulates runoff, erosion, and deposition. It predicts the long-term evolution of channels and hillslopes in a catchment and can be used to map the location and quantify the rate of development of gullies in a landform. This provides a method for predicting how long contaminants would be retained in an encapsulating structure.
Using the SIBERIA software, it was possible to identify areas of potential erosion/deposition by subtracting the 1000-year modelled surface from the proposed surface, for separate 'best case' and 'worst case' scenarios (figure 3).
Figure 3 Areas of potential erosion / deposition on proposed landform after 1000 years under (a) best case; and (b) 'worst case' scenario
A longitudinal profile of the differences between the two modelled surfaces is shown in figure 4.
Figure 4 Comparison of different landform profiles at Ranger mine
Until recently the Siberia landform evolution model (LEM) was being used to predict long term erosion on the ERA (Energy Resources of Australia) Ranger Mine rehabilitated landform. Siberia erosion simulations use an average area-discharge relationship to determine average sediment loss per year and do not use the time series hydrology of a single rainfall event or series of events. Consequently, the average long-term erosion assessments conducted to date have not implicitly addressed the impact of an extreme rainfall event or a series of events comprising an 'extreme' wet season. Concern about the potential impacts of climate change have strengthened the need to understand the effect of extreme rainfall events on the landform to assess the possibility of large sediment influx to catchments. To address this issue the CAESAR LEM (Cellular Automaton Evolutionary Slope and River) developed by Professor Tom Coulthard of the University of Hull is currently being trialed. A particular strength of CAESAR is that it can use input data from discrete rainfall and runoff events, enabling the effect of extreme rainfall events on landform stability to be simulated. Hancock et al (in press) have described the application of both the CAESAR and Siberia models to a study site in Tin Camp Creek. Evaluation of the suitability of the CAESAR LEM for assessing the stability of rehabilitated landforms at Ranger and the effects of extreme rainfall events on landform erosion is continuing.
- Assessing Ranger rehabilitated landform geomorphic stability using landform evolution modelling and GIS technology.
- The impact of extreme events on Ranger rehabilitated landform geomorphic stability using the CAESAR landform evolution model.
- Extensions to SIBERIA for the study of extreme events.
- Assessment of the significance of extreme events in the Alligator Rivers Region.
- The impact of tailings subsidence on rehabilitated landform erosional stability.
Relevant Supervising Scientist reports
Willgoose G & Riley SJ 1998. Application of a catchment evolution model to the production of long-term erosion on the spoil heap at the Ranger uranium mine: Initial analysis. Supervising Scientist Report 132, Supervising Scientist, Canberra.
Evans KG, Willgoose GR, Saynor MJ & House T 1998. Effect of vegetation and surface amelioration on simulated landform evolution of the post-mining landscape at ERA Ranger Mine, Northern Territory. Supervising Scientist Report 134, Supervising Scientist, Canberra.
Erskine WD, Saynor MJ, Evans KG & Boggs GS 2001. Geomorphic research to determine the off-site impacts of the Jabiluka Mine on Swift (Ngarradj) Creek, Northern Territory. Supervising Scientist Report 158, Supervising Scientist, Darwin.
Boggs GS, Devonport CC, Evans KG, Saynor MJ & Moliere DR 2001. Development of a GIS based approach to mining risk assessment. Supervising Scientist Report 159, Supervising Scientist, Darwin.
Moliere DR, Evans KG, Willgoose GR & Saynor MJ 2002. Temporal trends in erosion and hydrology for a post-mining landform at Ranger Mine. Northern Territory. Supervising Scientist Report 165, Supervising Scientist, Darwin NT.
Evans, K.G., Willgoose, G.R., Saynor, M.J. and Riley, S.J. 2000. Post-mining landform evolution modelling. I. Derivation of sediment transport model and rainfall-runoff model parameters. Earth Surface Processes and Landforms. 25(7), 743-763.
Evans, K.G. and Willgoose, G.R. 2000. Post-mining landform evolution modelling. II. Effects of vegetation and surface ripping. Earth Surface Processes and Landforms. 25(8), 803-823.
Evans, K.G. 2000. Methods for assessing mine site rehabilitation design for erosion impact. Australian Journal of Soil Research. 38(2), 231-248.
Hancock, G.R., Evans, K.G., Willgoose, G.R., Moliere, D.R., Saynor, M.J. and Loch, R.J. 2000. Long-term erosion simulation on an abandoned mine site using the SIBERIA landscape evolution model. Australian Journal of Soil Research. 38(2), 249-264.
Boggs G, Evans K, Devonport C, Moliere D and Saynor M 2000. Assessing Catchment-wide, Mining Related Impacts on Sediment Movement in the Swift Creek Catchment, Northern Territory, Australia, Using GIS and Landform Evolution Modelling Techniques. Journal of Environmental Management, Special Issue. 59(4) 321-334.
Hancock GR, Willgoose GR & Evans KG 2002. Testing of the SIBERIA landscape evolution model using the Tin Camp Creek, Northern Territory, Australia, field catchment. Earth Surface Processes and Landforms 27 125-143.
Boggs GS, Evans KG & Devonport CC 2004. Rugged Plateaus and extensive floodplains – modelling landform evolution in a northern Australian catchment. Australian Geographical Studies 42(2), 260-273.
Hancock GR, Grabham MK, Martin P, Evans KG & Bollhöfer A 2006. An erosion and radionuclide assessment of the former Nabarlek uranium mine, Northern Territory, Australia. Science of the Total Environment 354, 103-119.
Hancock GR & Evans KG 2006. Channel head location and characterisitics using digital elevation models. Earth Surface Processes and Landforms 31(7), 809-824.
Hancock GR, Evans KG. 2006. Gully position, characteristics and geomorphic thresholds in an undisturbed catchment in Northern Australia. Hydrological Processes 20, 2935–2951.
Hancock GR, Loughran RJ, Evans KG & Balog RM 2008. Estimation of soil erosion using field and modelling approaches in an undisturbed Arnhem Land catchment, Northern Territory, Australia. Geographical Research 46(3):333-349.
Hancock GR, Lowry JBC, Moliere DR & Evans KG 2008. An evaluation of an enhanced soil erosion and landscape evolution mode: a case study assessment of the former Nabarlek uranium mine, Northern Territory, Australia. Earth Surface Processes and Landforms 33(13), 2045-2063.
Hancock, GR, Lowry JBC, Coulthard TJ, Evans KG & Moliere DR 2010. A catchment scale evaluation of the SIBERIA and CAESAR landscape evolution models. Earth Surface Processes and Landforms, Vol 35 Issue 8 863-875.