Abstract
Mining causes environmental disturbances, including erosion and landform instability, and thus mined lands must be managed properly to minimise future detrimental impacts. A major focus when evaluating mine site rehabilitation in the Australian tropics is determining when excessive erosion of waste rock landforms is no longer occurring and when the landforms are in equilibrium with the surrounding catchment. Knowledge of the dynamics of returning waste rock landforms to an erosion equilibrium relative to the surrounding catchment will enable rehabilitation engineers to assess and estimate when the land will be rehabilitated and can be returned to the Traditional Owners of the land. This study aims to identify and critically assess an approach to determine the duration and dynamics of rehabilitated mine landforms achieving stability/erosion equilibrium. This approach involves quantifying and modelling fine suspended sediment flowing from the disturbed land to the receiving waters of the mine catchment.The research developed a framework of steps that could simulate the long-term erosion equilibrium of a rehabilitated mine landform. The project initially evaluated stream fine-suspended sediment (FSS) as an indicator of post-mining landform stability by developing stream discharge and corresponding fine-suspended sediment relationships for the receiving waters of a mine catchment. The FSS corresponding to the stream discharge in the receiving waters as a result of a specific volume rainfall event was determined. This relationship defined a background level of FSS corresponding to a stream discharge in the receiving waters as a result of a specific volume rainfall event at pre-mining conditions. The FSS relationship was elevated when a mining-related landform disturbance occurred. The elevated FSS levels are expected to return to background levels once the catchment is stable and in equilibrium with the surrounding catchment.
Equilibrium may take centuries to occur and thus to analyse the FSS-stream discharge relationship for future years, a Landform Evolution Model (LEM) called CAESAR-Lisflood was calibrated and validated to available site data for the mine catchment. This LEM can simulate the temporal variation of landform characteristics at high resolution. However, it can take considerable time to simulate continuous discharge and fine suspended quantities when run at high resolutions, which are required for better accuracy. Thus, a hydrology model called HEC-HMS (Hydrologic Engineering Centre-Hydrologic Modelling System) was calibrated to available site data to potentially provide a faster solution in the subsequent chapter. Wetter and drier rainfall patterns, which are possible due to climate change, were also simulated to assess the impacts of climate change on achieving a stable rehabilitation equilibrium. In the final chapter, the calibrated HEC-HMS model was simulated for different rainfall scenarios for several years to centuries to the future. This enabled the temporal variation of fine suspended sediment-stream discharge relationship to be analysed for wetter and drier rainfall scenarios. Modelling the mine catchment with a landform evolution model (CAESAR-Lisflood) and simulating it for a short term provided insight into how the erosion and deposition features of a landform changed across the mine catchment after rehabilitation.
My research determined the time and dynamics until an elevated waste rock landform returns to stability after rehabilitation and closure. It determined that the time taken for a landform disturbed by open-cut mining to be restored to its background levels can take centuries. The methodology showed how a disrupted landform can move in and out of equilibrium until it reaches a steady state with the surrounding environment. It determined that the time and dynamics of achieving stability require hundreds of years and that the dynamics of returning back to equilibrium are influenced by even minor changes in climate factors. This methodology can be adopted and replicated in landform stability assessments of other disturbed tropical landforms. It increases the sensitivity of assessing post-mining landform recovery and assists rehabilitation engineers in choosing management options for healing the land. It provides an approach for developing more accurate assessments of disturbance timeframes and this benefits owners of the land to whom it is bestowed after rehabilitation. It assists in enabling better assessment and regulation of landform disturbances.
| Date of Award | Mar 2024 |
|---|---|
| Original language | English |
| Supervisor | Sean Bellairs (Supervisor) & Ken Evans (Supervisor) |