• failure to achieve planned post-mining land use(s);
• mobilization and dispersal of hazardous mine materials;
• increased potential for acid and metalliferous drainage (AMD)/acid rock drainage (ARD);
• negative impacts on soil and water quality affecting ecological integrity and community health; and
• landscape aesthetics that do not meet expectations.

A recent technical paper published by the Queensland Government explores an iterative approach to using erosion and landform evolution models (LEMs), sometimes referred to as landscape evolution models. LEMs can be used to enhance the conceptual model for erosional stability by predicting erosion rates for differing conditions, identifying vulnerable areas, and thereby supporting improved landform design.

As experts in integrated mine closure, we recognize that effective landform design is key to long-term stability and thus, functionality of the landform. In this article, we explore common landform modelling tools, their practical applications in different jurisdictions, and how they can be used to develop resilient, sustainable mine landforms. By leveraging advanced modelling techniques, we help enable post-mining landscapes that meet social, environmental, and regulatory expectations while achieving an acceptable level of long-term erosion risk.

Geomorphic Principles in Landform Design

Designing landforms using geomorphic principles is essential for creating stable landforms to demonstrate reduced erosion and enhanced hydrologic response to major storm events. This approach draws inspiration from natural and mature landforms that have achieved equilibrium with local environmental conditions over extended periods. Integration of landform design with managing surface water runoff is also an important aspect to supporting long-term stability of a landform (INAP, 2017).

Landforms designed using geomorphic principles exhibit greater long-term slope stability as they mimic slopes that are in equilibrium with the local conditions of rainfall, soil type, and vegetation cover (Ayres et al., 2006). The complex topography of these landforms includes micro-topography beneficial for promoting revegetation, which contributes to reduced erosion, improved slope stability, and more effective site closure (MEND, 2007).

Landform evolution models simulate and predict changes that occur in the surface topography of a landform over time due to erosion. The models are particularly useful in assessing potential environmental risks over time to help mining companies develop accurate, adaptable, and responsible closure and post-closure management plans.

Regardless of the LEM model, it is essential to ‘start at the start’ with a robust conceptualization of performance that identifies physical, chemical, and biological mechanisms, and the site-specific controls on these mechanisms that influence performance (and why). From this starting point, the conceptual model can then guide appropriate focus of the LEM model to enhance the conceptual model of performance across multiple factors.

Understanding the Different Landform Modelling Tools

Several prominent erosion modelling tools are utilized internationally, each with distinct advantages and limitations. In jurisdictions like North America, the Revised Universal Soil Loss Equation (RUSLE) is a widely used model developed to estimate long-term average annual erosion rate on sloped fields. It considers factors such as rainfall patterns, soil type, topography, crop type, and management practices (Wall et al., 2002). However, a key limitation of RUSLE is that it only considers soil loss from sheet or rill erosion on slopes, and does not account for soil losses that might occur from gully, wind, or tillage erosion (Wall et al., 2002).

When Okane was engaged to evaluate the erosion potential of a tailings storage facility at a mine site in Arizona, United States, RUSLE modelling estimated erosion risks ranging from moderate to high, with the highest erosion rates occurring at the lower portions of the slope. This assessment informed various mitigation strategies to reduce erosion to acceptable levels, such as applying surface covers using natural or engineered amendments (e.g., erosion control blankets) or introducing surface roughness to stabilize disturbed areas while vegetation establishes.

The Water Erosion Prediction Project (WEPP) is another process-based modelling tool that predicts soil loss across multiple slope configurations while factoring in surface materials and climatic conditions. It accounts for variables such as rainfall-runoff, freeze-thaw cycles, and their effects on surface stability. Okane applied WEPP at a mine site in Northwest Territories, Canada, where a risk framework was developed to establish acceptable erosion rates for mine closure and enable long-term encapsulation of potentially acid-generating materials.

In Australia and New Zealand, SIBERIA and CAESAR-Lisflood are commonly used landform evolution modelling tools. SIBERIA simulates long-term geomorphic evolution of landforms by integrating widely accepted hydrology and erosion models influenced by runoff and erosion processes (Taylor et al., 2016). This approach involves calibrating relationships between runoff, rainfall, drainage area, and slope to predict sediment transport over extended periods, focusing on average annual hydrological and erosion parameters. In contrast, CAESAR-Lisflood predicts erosion based on specific storm events and sequences. This model accounts for initial landform geometry and surface material properties, enabling detailed analysis of short-term, high-intensity storm impacts, as well as long-term performance (Taylor et al., 2016).

The capability of CAESAR-Lisflood to assess short-term, high-intensity storm events is particularly useful, as such storms frequently impose substantial influence on rapid erosion processes, including gully formation, slope failures, and mass movement. These intense events are typically critical site-specific controls that must be considered as they affect landform stability, especially in early rehabilitation stages when vegetation coverage is limited. Short-term event analysis using CAESAR-Lisflood, therefore, provides critical insights into the resilience of landform design and helps determine whether a site’s risk profile might be one that is adaptable or resilient. Thus, this analysis enables proactive identification and mitigation of vulnerable areas and the improvement of initial rehabilitation strategies.

At a mine site in Queensland, Australia, Okane leveraged CAESAR-Lisflood to project that erosion rates of a mine rock stockpile’s top-tier cover system would significantly decrease after five years, once vegetation reached 60% of surface coverage. The model supported Okane’s conceptual model of performance, illustrating that erosion risk would be within acceptable limits, helping refine the overall landform design.

At the former Rum Jungle Mine in Northern Territory, Australia, Okane used SIBERIA and CAESAR-Lisflood to assess erosion rates and depths of several landforms to enhance understanding of long-term landform performance. These models were particularly useful in analyzing erosion processes while considering factors like runoff accumulation and changes in catchment morphology (Taylor et al., 2016). They enabled visualization of landform changes over time to illustrate anticipated erosion patterns based on thresholds for gully formation and development (Taylor et al., 2016).

Among the models discussed, SIBERIA and CAESAR-Lisflood explicitly evaluate three-dimensional (3D) landform surfaces. These models simulate landscape evolution by dynamically modelling spatial variations in terrain morphology and accounting for 3D surface processes, such as runoff accumulation, infiltration, sediment transport, and erosion patterns.

In contrast, models such as RUSLE and WEPP primarily provide analytical estimations of erosion rates on slope segments or two-dimensional (2D) slope profiles and, therefore, do not inherently simulate complex 3D surface interactions or evolving landform geometries.

Okane’s Approach

Regardless of whether numerical models (SIBERIA, CAESAR-Lisflood) or analytical models (RUSLE, WEPP) are used, as mine closure practitioners, our team of expert engineers and scientists prioritizes landform designs based on geomorphic principles, while always ‘starting at the start’ with a conceptual model of performance. We conduct observational testing programs for rainfall and materials to further refine erosion model parameters while also enhancing the conceptual model of performance. These tools help identify key factors contributing to erosion risks, including slope length and gradient, rainfall-runoff relationships, surface compaction, and surface treatments.

At times, a landform with a steep angle of repose may appear unnatural, i.e., not necessarily be designed with a concave profile. However, this landform can still be stable if designed according to geomorphic principles, where energy and evolution have shaped that specific form. We collaborate with a wide range of subject matter experts to interpret and integrate geological and environmental factors into our modelling processes, ensuring site-specific results and outcomes.

Whether modelling is used to optimize erosion rates for a site’s preferred landform design, support an environmental impact statement submission, or visually illustrate landform performance to meet stakeholder expectations for landform aesthetics, our expertise in landform evolution modelling empowers clients to make informed decisions about effective closure landform designs. By understanding how landforms evolve over time, we help stakeholders assess acceptable levels of residual risk and implement ‘fit-for-purpose’ mitigation strategies.

References

Ayres, B., Dobchuk, B., Christensen, D., O’Kane, M., and Fawcett, M. (2006). Incorporation of natural slope features into the design of final landforms for waste rock stockpiles. In Proceedings of the 7^th International Conference on Acid Rock Drainage, St. Louis, MO, USA, March 26-30, (pp.59-75).

International Network for Acid Prevention (INAP). (2017). Global cover system design technical guidance document. INAP.

Mine Environmental Neutral Drainage (MEND). (2007). Macro-scale cover design and performance monitoring reference manual (MEND Report 2.21.5): Prepared by: Okane Consultants. MEND.

Taylor, I., Kemp, A., O’Kane, M., Walker, S., Barteaux, M., & Laurencont, T. (2016). Designer waste landform modelling and design – Rum Jungle Mine. In Fourie, A. B. & Tibbett, M. (Eds.), Mine Closure 2016: Proceedings of the 11th International Conference on Mine Closure (pp. 405-417). Australian Centre for Geomechanics. https://doi.org/10.36487/ACG_rep/1608_29_Taylor.

Wall, G. J., D. R. Coote, E. A. Pringle, & I. J. Shelton (editors). (2002). RUSLEFAC — Revised Universal Soil Loss Equation for application in Canada: A handbook for estimating soil loss from water erosion in Canada. Research Branch, Agriculture and Agri-Food Canada. Ottawa. Contribution No. AAFC/AAC2244E. pp.117.