**6. Using the rainfall simulators' results for landform design and assessing the erosional stability of mining rehabilitation sites**

Open-cut mining involves a larger disturbance of surface area compared to underground mining. In Australia, explosives are used to blast the deep solid overburden above the mineral or coal seam, which is then mechanically removed using trucks and shovels or draglines. Draglines are the most commonly used methods in Central Queensland open-cut coal mines, which operate at high speeds and result in landscapes consisting of long parallel spoil-piles that are highly saline, dispersive, and erodible. These spoil-piles can be over 50–60 m high and have slopes at an angle of repose of around 75% or 37°. It is legally required for mining organizations to rehabilitate the land by law, and therefore, disturbed open-cut post-mining landscapes must be rehabilitated to an approved post-mining land use. The principal rehabilitation process is shown in **Figure 9**.

**Figure 9.**

*Pictorial representation of the process of rehabilitation of post-mining landscapes on open-cut coal mines is Central Queensland, Australia.*

The most expensive part of rehabilitating a mining site is creating a suitable landscape for vegetation growth by reshaping and preparing overburden dumps, which requires costly earthworks. The new landscape must be able to withstand geotechnical failure and surface erosion caused by rainfall and runoff. Steep slopes can lead to severe erosion, causing rehabilitation failures, gully erosion, acid mine drainage (AMD), and salt discharges. Mine sites must evaluate the potential annual erosion rates from rehabilitated areas and report them to

### *Using Rainfall Simulators to Design and Assess the Post-Mining Erosional Stability DOI: http://dx.doi.org/10.5772/intechopen.112240*

regulators, such as the Department of Environment and Science in Queensland, which monitor and enforce rehabilitation compliance. To minimize costs and prevent rehabilitation failures, it is essential to predict the soil erosion rates for the suggested landscape design before its construction. Since the soil and overburden materials used to build the engineered landform have varying degrees of erodibility, accurately measuring their erodibility values is necessary to predict potential erosion rates.

The mining rehabilitation industry has traditionally used predictive equations that rely on soil properties like texture to estimate erodibility values for soil/overburden materials used in constructing engineered landforms. While this approach was easy to use, its unreliability in predicting soil erosion rates led regulators to require actual measurements of soil erodibility values using rainfall simulators in either a field or a laboratory setting. Once these erodibility values have been measured, they are used in combination with more sophisticated erosion/deposition models such as WEPP, MINErosion, and SIBERIA to predict and assess the erosional stability of the proposed landform design.

As an example of using rainfall simulators to estimate and measure soil erodibility values, Sheridan, So [32] conducted a study in which they measured the rill and interrill erodibilities and slope adjustment factors of 34 soil/overburden materials using a portable rainfall simulator tilting flume (measuring 3 m long 0.8 m wide and with a slope adjustable from 0 to 50%) at the University of Queensland Erosion Processes Laboratory. The materials were exposed to a 100 mm/h rainstorm for 30 min (equivalent to a 1-in-20-year event in Central Queensland) at a 20% slope, followed by slopes of 5, 10, 15, and 30 for 15 min each. At these simulated rainfall intensities, a steady state was quickly reached. Data from rainfall simulation at 10% slope were used to determine interrill erodibility, and data from the overland flow experiments at 20% slope were used to calculate rill erodibility coefficients. Sheridan, So [32] found that soils were generally more erodible than overburdens, as many of the overburdens either contained considerable amounts of rock or had a strong sealing ability. The strongly aggregated high clay soils tended to be the most erodible, followed by the lighter textured sandy loams and loamy sands. Soils or overburdens with 20–30% silt tended to form strong, raindrop impact seals under rainfall and consequently had very low erodibilities. Khalifa [80] expanded the dataset obtained from this research by including information from 93 additional samples (46 soil samples and 47 spoil samples) gathered from six coal mines in Central Queensland. This was done to capture the diversity of spatially distributed samples across each of the selected mining sites. The data collected by Khalifa [80] were used to feed MINErosion 3 & 4 erosion/deposition models for post-mining rehabilitation [81], resulting in the creation of an embedded database file containing the rill and erodibility values for 34 open-cut mine sites in Central Queensland. Additionally, the model is capable of analyzing rainfall simulation data to calculate erodibilities and predict erosion rates on an annual or event-based basis.

Currently, an increasing number of environmental consulting companies operating in the mining sector have constructed and utilized rainfall simulators in a number of environmental rehabilitation projects. Their primary purposes include assessing the erodibility factors of materials intended for constructing the designated land cover and estimating final landform erosion stability. These rainfall simulators have been utilized in several Australian mines, such as the North Parkes Mine in New South

Wales, the Ranger Uranium Mine in the Northern Territory, the Carmichael Coal Mine and Mt. Rawdon gold mine in Queensland, and the Yallourn Coal mine in Victoria.

Furthermore, the utilization of rainfall simulators extends to the assessment of erosion control techniques. By providing a controlled environment, rainfall simulators allow for the testing and evaluation of various measures, such as terracing, vegetation cover, and mulching, to determine their efficacy in reducing soil erosion and improving landform stability [33, 82]. These experiments are valuable in assisting mining operators in selecting the most appropriate and cost-effective erosion control practices tailored to their specific site conditions.

Additionally, rainfall simulators are instrumental in evaluating and monitoring the success of rehabilitation efforts for a specific mine site. After mining operations cease, the restoration of ecosystems and the mitigation of long-term environmental impacts are of utmost importance. Rainfall simulators could be used to facilitate the assessment of rehabilitation effectiveness by comparing the erosional stability of reclaimed landforms with undisturbed reference areas. This evaluation provides invaluable feedback on rehabilitation techniques and offers guidance for future restoration practices.
