Post-closure site management at mine sites typically includes measuring surface reclamation success, monitoring surface water and groundwater quality, and maintaining active or passive water management systems. The effects of water erosion on engineered cover materials for regraded slopes and surfaces often pose a long-term and costly maintenance challenge for the owner. If the owner had known about the negative (i.e., underestimated) financial impact of this long-term maintenance before finalising the closure design, the trade-off between initial capital and long-term maintenance costs might have changed the original design.
While reclamation and revegetation practices are firmly established and widely adopted throughout the mining industry, the ability to improve predictions of long-term erosional stability of closed mine waste facilities is an emerging and interesting field. Born in the agricultural sciences and adapted to land management practices, erosion modelling methods range from simplified empirical relationships to multi-variable finite element models. Empirical methods offer meaningful comparisons of average annual erosion estimates between different cover designs and slope angles; however, empirical models lack the accuracy needed to estimate long-term maintenance costs and extreme (high or low) precipitation events. The emergence of Landscape Evolution Models offers the promise of increasing robust predictions of erosion impacts and associated costs over timescales of decades or longer.
As always, models are only as good as the data available and the judgment of the practitioner. More sophisticated models incorporate new variables that characterise the rainfall model and the physical properties of the soil. The industry is moving toward new analytical methods in terms of precipitation models, field testing and interpreting the erosion impacts to the undisturbed surrounding landscape. SRK continues to develop useful analytical tools to simulate precipitation dynamics over designated design periods. SRK also collaborates with researchers and specialists focused on new ways to measure key parameters for the predictive models.
SRK teamed with a specialist soil erosion firm to co-develop an erosion assessment work plan for a closed mine site in the southwestern U.S. The plan included field-scale testing of erosion resistant soil/rock covers constructed from locally-available materials. The team applied innovative on-site rainfall simulation techniques to estimate the rainfall-runoff and sediment-yield parameters for each cover system. The results of the testing program informed a screening analysis of the different erosion covers.
The team then developed a range of 3D surfaces of different slope profiles for an existing tailings embankment. The range of 3D surfaces included simple linear profiles, compound slopes and natural analogs. The team combined the short-list of erosion covers with the different 3D surfaces and analysed the long-term erosional performance of the integrated cover system. This process provided the client with a robust technical basis for decision-making.
The SRK global library of mine closure projects offers valuable experience in assessing erosion impacts for our clients involved in planning mine closures.
