McCullough, CD, van Rooijen, A & van Maren, DS 2019, 'Process-based erosion modelling for shoreline rehabilitation design of a coal mine pit lake', in AB Fourie & M Tibbett (eds), Mine Closure 2019: Proceedings of the 13th International Conference on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 75-88, https://doi.org/10.36487/ACG_rep/1915_07_McCullough
Mine pit lakes are a common feature of open cut mine closures that extend below the local groundwater levels. Due to the relatively steep lake bottom profiles compared to natural lakes, shoreline stability remains a significant limitation for the long-term sustainability of many pit lakes, potentially impacting upon highwall geotechnical stability, revegetation success, water quality and proposed end uses including wildlife habitat, aesthetics and recreation. However, although a potential issue for long-term pit lake closure planning and management, there is currently very little information available on pit lakeshore erosion, including potential erosion rates and extent.
The ENGIE Hazelwood Coal Mine project is located in the Latrobe Valley, Victoria. The site has ceased operations and is currently in closure work phase. With approximately 16 km of pit lake perimeter, the effect of the final pit lake shoreline design on wave-induced shoreline erosion was identified as a key area requiring further study.
This innovative study used the process-based hydro-morphodynamic models Delft3D to predict final pit lake shoreline erosion at the Eastern Batter (EB) domain region of the pit void. Shoreline erosion modelling was undertaken for three erosion treatments over a total of 10 scenarios.
2. Effect of shore slope angle (design angle of 17° from horizontal, to a lower 12° and steeper 22°).
3. The effect of prevailing winds compared to episodic storm winds.
Pit lake shoreline erosion modelling was found to be a useful design tool for to study pit lake shoreline developments. Further model findings were that relatively gentle (12°) slopes did not show substantially decreased erosion rates compared to design (17°) slopes. However, relatively steep (22°) slopes showed substantially greater erosion.
Erosion rates were strongly influenced by sediment properties, which constituted the biggest uncertainty in the modelling. Lowest erosion rates were for cohesive grey clays, highest erosion rates for sands and erosion rates of mixed sediments were in-between. Eroded sands were also deposited nearby at slightly deeper water, thereby reducing shoreline slopes over longer timescales. However, eroded clays were dispersed throughout the lake, preventing an erosion/deposition equilibrium being reached.
Short-term storm events simply mimicked erosion patterns of longer duration prevailing wind events. Consequently, slope and sediment characteristic choices to mitigate prevailing wind effects may be similar to those used for mitigating event-driven erosion.
Keywords: mine closure, pit lakes, waves, erosion, numerical modelling, Delft3D
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