EGL Ref: 9216 22 June 2022 Page 7 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. location is shown in Figure 2 and was determined from review of different breach locations. 10.4. Volume of Breach Outflow Discharge of the supernatant pond water in a breach event would mobilise tailings through erosion (i.e., the tailings would be entrained). For the Sunny Day scenario the outflow volume associated with Process I is assumed to be 0.480 Mm3. This includes 0.240 Mm3 of supernatant pond water and 0.240 Mm3 of eroded and entrained tailings. A volume of 0.937 Mm3 of tails and embankment is assumed to be deposited as part of Process II. For the Rainy Day scenario the outflow volume from Process I is 1.976 Mm3. This includes 0.786 Mm3 of supernatant pond water and 1.194 Mm3 of eroded and entrained tailings. A volume of 0.391 Mm3 of tails and embankment is assumed to be deposited as part of Process II. The assumed volumes released in Sunny Day and Rainy Day breach scenarios are summarised in Table 1. 10.5. Breach Modelling 10.5.1. Breach Parameters Several models for estimating the ultimate breach dimensions (width, side slopes, volume eroded, etc.), as well as the breach formation time, have been developed (Refs. 2 & 12). They are empirical models derived from breaches of water storage embankments, which are either homogeneous or zoned earthfill, or rockfill dams. Ideally, models developed from tailings dams that are representative of the embankments at the Waihi Operation Site would be preferred. However, there are no comparative models available that simulate the formation of the breach of tailings dams. Therefore, the models used for water storage dams to estimate the breach dimensions and formation times have been used. This is considered acceptable by the CDA Technical Bulletin (Ref. 2). The different models give different estimates, which have been used to model a range of possibilities. Five models have been used to estimate breach parameters. They are: • Froehlich (1995) – Ref. 13 • Froehlich (2008) – Ref. 14 • MacDonald and Langridge-Monopolis (1984) – Ref. 15 • Von Thun and Gillette (1990) – Ref. 16 • Xu and Zhang (2009) – Ref. 17. The breach parameters leading to the worst case scenario, i.e., that give greatest maximum flood depth and shortest time of arrival, are adopted for this assessment of the breach consequences. 10.5.2. Breach Outflow Hydrograph 10.5.2.1. Sunny Day Breach Scenario The Sunny Day breach scenario is assumed to be a collapse type failure caused by instability associated with weak embankment foundations under strong earthquake shaking. Breach parameters were determined by using the
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