Simulating the long-term erosion equilibrium of a rehabilitated mine landform to evaluate the dynamics of land restoration

Land, which is the basis for human survival, is a non-renewable resource that should be used sustainably. Over half of the land area on Earth is modified by anthropological activities. That can have consequences beyond the area directly affected. One such indispensable activity is mining as it provides goods for human survival and endeavours, however, it can result in land degradation and desertification if the site is not rehabilitated properly. Rehabilitated post-mining landforms should behave similarly to the surrounding stable, undisturbed areas. There is a lack of knowledge about determining when rehabilitated landforms return to equilibrium with their surrounding undisturbed areas. A sensitive approach to analyse post-mining landform stability in tropical regions is to assess the quantity of fine suspended sediments (FSS) leaving the catchment where the mine resides and entering the receiving streams. Following a rainfall event there is an expected background relationship between rainfall event discharge and event FSS loads in the receiving streams of a catchment. If there is landform disturbance in the catchment, the event FSS load rises above the background relationship and returns to the expected levels once the landform has stabilised or when it is in equilibrium with the surrounding undisturbed areas. This paper aimed to use this approach to analyse landform stability following mining in tropical northern Australia. Since geomorphological processes are very slow, a rehabilitated mine landform is expected to stabilise after several hundred years. Thus, the upstream and downstream of the catchment where the mine resides were modelled using the Hydrologic Engineering Centre’s Hydrologic Modelling System (HEC-HMS) for continuous stream discharge and FSS quantities. The calibrated model was run for 1,000 years to predict continuous stream discharge and FSS quantities for predicted rainfall scenarios. The increased event FSS load following a rainfall event was compared to the expected FSS quantity for a specific event discharge based on the background relationship determined for upstream and downstream of the catchment, respectively. It was found that event FSS loads following a rainfall event upstream were always within the prediction limits since there was no disturbance caused upstream of the catchment. However, the event FSS loads downstream of the catchment where the mine resides were above the prediction limits for about 600 years after mine closure. After 600 years, the event FSS loads at the catchment downstream of the mine returned to within the prediction limits of the background erosion rates. Thus, the study evaluated the approach of using stream-suspended mud as an indicator of landform stability in long-term erosion equilibrium simulations. It determined the time period until a disturbed landform would be restored to equilibrium with its surrounding catchment. Short-term erosion and deposition across the mine catchment were also evaluated using a calibrated landform evolution model, CAESAR-Lisflood.