Authors: Slingerland, N; Dressler, S

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Slingerland, N & Dressler, S 2022, 'Evaluating construction tolerances and tailings dam shape for closure using the CAESAR-Lisflood landscape evolution model', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2022: 15th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1117-1130,

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Variability in landform construction for closure has long been an uncertainty: downstream tailings dam slopes are designed with precision, while the earth moving equipment that build them vary in their ability to accurately replicate designs. As such, performance outcomes of the closure landform can be tied to the equipment used and/or operator experience, for better or worse. Unintended surface variations can lead to concentrations of surface water flow, gully formation, and excess erosion of the landform surface. A key concern is therefore how much variability in construction, or ‘surface roughness’ is acceptable, and does this level of acceptability change based on the complexity of the landform being constructed? While many closure landscapes seek to maximise surface roughness and micro-topography for the associated microclimatic and biodiversity benefits, a concern with respect to surface roughness at closure, particularly on steep slopes, is that too much may lead to excess erosion. Controlling factors lend this problem well to investigation using a landscape evolution model (LEM); in this case the CAESAR-Lisflood LEM. Using four different downstream sand dam designs with identical overall dimensions, one digital elevation model (DEM) of each was created with (a) low surface roughness and with (b) high surface roughness. A standardised precipitation database, grain size distribution, and parameters were used for all LEM simulations, such that variations due to surface roughness could be isolated. LEMs are often used to test computer-generated designs (i.e. smooth contours with no surface roughness), as well as existing landforms whose surfaces are replicated using light detection and ranging (LiDAR)-based DEM inputs (i.e. surfaces with substantial roughness). These two extremes have not previously been compared but have implications for the way LEM inputs are generated and assessed, the assumptions made during design, and for the degree of precision required in closure earthworks construction. Results of the LEM testing indicate landform-scale topography provides superior erosion mitigation compared to micro-topographic variations, and that geomorphic landform designs are more resilient to surface variations than traditional designs, providing greater ‘room for error’ during construction.

Keywords: CAESAR-Lisflood; construction tolerance; erosion; landscape evolution model; tailings dam; risk assessment; surface roughness

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