Predicting Erosion Impacts to Mine Waste Facilities

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 finalizing 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 modeling 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.
 
Recently, SRK developed an erosion assessment work plan for a closed mine site in the southwestern U.S. The work plan featured both a retrospective and prospective approach to developing parameters for predicting erosion impacts. The retrospective approach included examination of three different as-built closure designs, in terms of the original erosion design criteria and the actual performance of the design after 10 years of post-closure maintenance. The approach also included instrumenting a sub-basin for an as-built closed facility and measuring the rainfall-runoff relationship for a “mature” cover system. The facilities chosen for the retrospective assessment are in the same region and of the same scale. Insights from this program will guide erosion-related decisions related to closure planning at new sites. The prospective approach included new field methods for estimating erosion parameters and allowing use of more sophisticated landscape evolution models.
 
The SRK global library of mine closure projects offers valuable experience in assessing erosion impacts for our clients involved in planning mine closures.

Terry Braun - Principal GeoEnvironmentalistTerry Braun, MSc, PE
Principal GeoEnvironmentalist (SRK Denver)
 
Terry Braun has over 25 years of professional experience in mining and environmental projects. His practice incorporates various elements of environmental compliance activities as well as engineered solutions to potential environmental liabilities at mining and other industrial operations. He develops and often implements projects that require negotiations with regulatory agencies and other stakeholders to achieve client objectives. Terry leads multi-disciplinary project teams tasked with producing engineering studies in accordance with public disclosure requirements (i.e., Canada and U.S.) as well as internal due diligence reviews on behalf of clients. His project experience includes planning, permitting and implementing the large-scale closure program for the BHP Copper San Manuel mine and plant sites in Arizona. He has also led teams in mitigating legacy mining impacts at active and inactive mine properties in the western U.S.

 

SRK North America