5 Hydraulic modelling The modelling software TUFLOW was used to develop a hydraulic model for the dam break assessment. TUFLOW software can dynamically simulate linked one-dimensional (1D)/two-dimensional (2D) hydraulic models and has the capacity to represent complex changes in topography, hydraulic structures, boundary conditions, land use classifications, floodplain storage and floodplain/channel interaction.
5.1
Modelling Approach and Inputs
In addition to the inputs and considerations discussed in previous sections, the following modelling assumptions and approach were adopted for hydraulic modelling: — Terrain surface file was developed based on combining the following available terrain data covering the study area, obtained from different sources: — Light Detection and Ranging (LiDAR) survey dataset covering the immediate vicinity of the mine site, sourced from Heritage Minerals (1m Digital Elevation Model (DEM) dated 2021). — Publicly available DEM data covering the township of Mt Morgan and downstream receptors, Australia New Zealand Land Information Council (ANZLIC) [23] (1m DEM dated 2015)
— A grid cell size of 4 m.
— The model extents cover an area of approximately 6 km x 10 km, including the mine infrastructure and PAR locations within the township of Mt Morgan, extending downstream to the Dee River and Boulder Creek confluence, where flood attenuation is expected. — A constant surface roughness coefficient (Manning’s n) of 0.03 was adopted across the model extents, considering it representative of an average condition of the surface (that includes natural streams, constructed areas, ponded water, among others). — Inflow boundary conditions, representing the breach failure hydrographs, were assigned at the breach locations for each scenario. — The downstream outflow boundary was defined using a stage-discharge relationship considering the local terrain geometry and slope of the final reach to estimate normal depth at the outlet (free outflow). — The water level in the OCP was conservatively assumed to be at its spillway invert level (273.1 m RL) for all assessed scenarios. — Mobilised tailings were assumed to be discharged and fully contained within the OCP, displacing water through the spillway. Minimal tailings flow is expected downstream, and therefore, only Newtonian flow characteristics were considered for the analysis.
5.2
Hydraulic modelling results
5.2.1
Inundation mapping
Flood maps were generated from the hydraulic results and are provided in Appendix A. The following outputs were mapped:
— Maximum flood depth: maximum flood depth of the outflows routed through the modelled domain.
— Maximum flood velocity: maximum flood velocity of the outflows routed through the modelled domain
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 20
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