EGL Ref: 9216 22 June 2022 Page 6 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. 9.0 TAILINGS DAM BREACH ASSESSMENT CASES The CDA Technical Bulletin (Ref. 2) recommends that there are two main factors that are expected to have an important impact on the character and volume of the outflow from a TSF during a breach event: • The presence of fluids on the surface of the impoundment near the dam; and • The potential of liquefaction induced flowability of the tailings material, which may be due to various trigger mechanisms, including the breach itself. CDA Technical Bulletin (Ref. 2) uses these factors to define four types of tailings dam breaches. It is assumed that the tailings are liquefiable in Storage 3 and that there will be supernatant water present during operation and post closure. Therefore, it is assumed for this study that a breach of Storage 3 will result in release and flow of fluids and eroded and liquefied flowable tailings. On this basis, Storage 3 is classified as a Case 1A type of breach according to the CDA Technical Bulletin (Ref. 2). This case is characterised further in Section 10.0. 10.0 BREACH ANALYSIS 10.1. Physical Processes for Tailings Dam Breaches For Case 1A defined by the CDA Technical Bulletin (Ref. 2), a breach can be assumed to consist of two processes: • Process I: discharge of supernatant pond water eroding part of the dam, and carrying eroded tailings and dam fill materials entrained in the water flow, and • Process II: the tailings mass undergoing flow liquefaction resulting in a discharge of liquefied tailings, including part of the failed section of the embankment, together as a mud. 10.2. Type of Breach Outflows The breach outflow from Process I results in a water flood wave that includes entrained tailings that can propagate far downstream causing erosion and inundation of the downstream environment. Process I is modelled as a water breach using the HEC-RAS software (Refs. 11 & 12). In Process II, the liquefied tailings create a mud flood, which is less fluid compared to flows discharging during Process I. The tailings consequently deposit closer to the breach location when the slope of the downstream surface is less than 4 degrees. In this dam breach assessment, the extent of deposition associated with Process II is estimated using 3D volumetric modelling as the downstream slope is less than 4 degrees. 10.3. Dam Breach Locations Assessment of various dam breach locations for Storage 3 has been undertaken. The location that causes highest consequence was used in the detailed breach analysis. This
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