Open Cut » Environment
The initial field study at Curragh assessed runoff, erosion and water quality from rehabilitated land at two scales: field plot (0.01 ha) and contoured catchment (0.58, 0.41 and 0.91 ha). Three contoured catchments with an average slope gradient of 10% were monitored from 1994 to 2003. These represent current rehabilitation practices at Curragh mine. The plot study terminated after 1999 but the catchments were continued until 2003.
Similar to the plot study, the greatest window-of-erosion risk occurred in the early years after rehabilitation and before >50% vegetative cover was established. Most of the sediment lost during the study was due to a large proportion of surface area being exposed to the erosive forces of rain and runoff. Pasture growth reduced soluble salt concentrations at the surface of spoil material and reduced the risk of salt movement on-site and off-site.
In contrast to the spoil erosion plots, vegetative cover has progressively colonised the spoil on the catchments. The weathering and breakdown of spoil laid down in strips has enabled pasture species to colonise the spoil areas in the catchments. Soil migration from upslope and leaf litter expressed as organic carbon also improved the quality of the material, contributing to pasture growth and survival.
The 10-year study has provided a unique opportunity to monitor the condition and trend of rehabilitated catchments over 3 distinct climatic cycles. The first 3 years were dry, with well below Curragh's median rainfall of 565 mm, receiving an average of 315 mm. The following 3 years were relatively wet with an average of 686 mm experienced. This period saw buffel grass flourish and cover levels attain >80% of surface area. The final 4 years saw a shift back towards drier conditions - average rain was 335 mm. During this period buffel grass survived and showed its resilience during unfavourable rainfall conditions.
Long-term research into the success of rehabilitation at Central Queensland coalmines was conducted at Oaky Creek Coal Mine. The study measured wetter, dryer and median years of rainfall and recorded the performance of topsoils, spoils and landform designs. In 1993, field plots 0.1 ha in size, with slope gradients of 30% were established, measuring runoff, sediment loss and runoff water quality. From 2002, box-cut style slopes with 30% gradients, mid-slope berms or benches and sediment ponds were monitored using erosion pins. Rainfall was measured at the experimental sites using pluviometers connected to data loggers.
A major result from the long-term study was the importance of vegetative cover in rehabilitation. On steeper slopes - 30% gradients - it was found that the greatest window of erosion risk was before vegetation reached a minimum 80% surface cover.
Another important finding from the long-term study was the critical role topsoil played in reducing erosion. High vegetative cover levels were attained on topsoil treatments with spoil treatments performing poorly in regard to vegetative establishment.
Findings from box-cut study sites at Oaky Creek identified the need for these designs to be stabilised soon after establishment to control bench integrity and rill erosion. The effectiveness of benches or berms, designed to dissipate runoff velocity down a slope, was reduced due to low pasture establishment on spoil treatments. Benches exposed to rainfall and runoff, deteriorated rapidly and are now largely ineffective in fulfilling their design purpose. On the other hand, topsoil treated benches performed better with reasonable cover established, and significantly less erosion evident. .
The performance of the sediment pond designs on the modified box-cut sites featured at the new sites ranged from reasonable to poor. Ponds were formed from spoil and tended to have low hydraulic conductivity and permeability, resulting in sealing of the base thus preventing infiltration. Infiltration was further inhibited due to fine clays and silts deposited in the ponds reducing the ponds ability to capture subsequent runoff events. If sediment ponds are to be used in the future greater consideration on the permeability of their base is required. Successful design of sediment ponds requires taking into account the runoff coefficient of the material and catchment area. Pond wall and spillway design also needs to be considered to account for occasions when ponds overtop.