Tools for designing post-mining landscapes with acceptable erosion risk and discharges on the receiving environment

Abstract

Open-cut coal mining in Central Queensland involves the breaking up of overburden, generally of Permian origin, that overlies the coal seams using explosives, followed by removal with draglines which results in the formation of extensive overburden spoil-piles with steep slopes at the angle of repose. These spoil-piles form long multiple rows, with heights of up to 60 or 70 m above the original landscapes. They are generally highly saline, dispersive and highly erosive. Legislation requires that these spoil-piles be rehabilitated into a stable self sustaining ecosystem with no off-site pollution. In contrast to agricultural ecosystems where slope gradient is a fixed characteristic of the landscape, the first stage in the rehabilitation of these landscapes is the modification and lowering of slopes to create a landscape that is stable against geotechnical failure (landslides) and erosion. This is followed by revegetation with rapidly establishing grasses as pioneer vegetation to further reduce erosion, and an appropriate mixture of native shrubs and trees which are slower to establish. Excessive erosion and on-site discharges of sediment into mine working areas may result in the temporary cessation of mining operation with significant financial consequences, while off site discharges may breach the mining lease conditions. The average cost of rehabilitation is high (average of $ 22,000/ha) and with approximately 100,000 ha of spoil-piles in Queensland at present, the total cost of rehabilitation facing the industry is very high. Most of this comprised the cost of reshaping the landscape, associated with the large amount of material movement necessary to achieve the desired landscape. Since soil and spoil-piles vary greatly in their erodibilities, a reliable and accurate methodology is needed to determine cost effective combinations of slope length, slope gradient and vegetation that will result in acceptable rates of erosion. This paper reports on the development of a set of two user friendly computer packages, MINErosion 3.1 and MINErosion 4.1 that were designed to assist minesites to achieve stable postmining landscape reconstruction that will result in acceptable rates of erosion. MINErosion 3.1 is a user friendly predictive hillslope computer package useful in the selection of suitable combinations of landscape design parameters (slope gradient, slope length and vegetation cover) that will result in acceptable rates of erosion. The slope length and gradient are parameters required by the mining companies as inputs into their landscape design software to cost effectively construct suitable landscapes that will meet the required erosion criteria. MINErosion 3.1 predicts with reasonable accuracy, both the annual average soil loss under the prevailing climatic conditions as well as the potential erosion from individual rainstorms with known annual recurrence intervals. A database of 35 soils and spoils from Central Queensland are imbedded in the model. MINErosion 4.1 is a user friendly catchment/landscape based predictive erosion package useful in the estimation of erosion rates (mean annual or individual rainstorm) from the proposed reconstructed postmining landscape. It combines MINErosion 3.1 with Arc-Gis v9. The outputs are maps of erosion and deposition on the landscape as well as discharges from the catchment/landscapes.