EGL Ref: 9216 22 June 2022 Page 5 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. 7.0 TAILINGS CHARACTERISATION In-situ tests, including Cone Penetration Tests (CPTs), Seismic Cone Penetration Tests (SCPTs) and Seismic Dilatometer Tests (SDMTs) have been undertaken on the tailings in Storage 2. The results indicate that the tailings have contractive behaviour (Ref. 5). Tailings stored in Storage 3 are expected to have similar characteristics. Consequently, the tailings are assumed to be liquefiable in the TDBAs. The runout analysis of the tailings slurry is based on the residual strength inferred from insitu tests and empirical models considering a continuum approach reported in the literature, such as Lucia et al. (1981, Ref. 6) and Olson and Stark (2002, 2003, Refs. 7 & 8). 8.0 HYDROLOGIC ANALYSIS Two different events have been considered for a breach of Storage 3. They are Sunny Day and Rainy Day breach scenarios. In a Sunny Day breach scenario, the supernatant decant pond water is assumed to be at a normal operational level. A volume of 0.240 Mm3 is adopted as the normal decant pond water volume. Unlike water storage dams, passing a flood through an active TSF during operation is not a common practice due to the requirement to manage, reuse or treat supernatant water on top of the tailings. Direct rainfall and runoff from the catchments above the TSFs will be fully stored. Water collected on the TSFs at the Waihi Operation is pumped to the Processing Plant or to the Water Treatment Plant for treatment before being discharged into the Ohinemuri River, until the impounded water reaches an acceptable quality to be discharged without treatment. The Waihi TSFs do not have permanent spillways that allow direct discharges to natural watercourses until closure. However, designs and defined emergency spillway locations for controlled release of water are included in the EAP. The inflow design flood (IDF) for the Waihi TSFs is the runoff arising from a Probable Maximum Precipitation (PMP) storm event. This is generally viewed as the Probable Maximum Flood (PMF). It is the theoretical maximum flood that could plausibly occur at a particular time of year in a design watershed (WMO, 2009, Ref. 10). The TSFs are designed to contain the runoff from a 72hr-PMP (i.e., PMF) with 1 m freeboard. The Rainy Day breach scenario further assumes that water has filled up the 1m freeboard as well and is at the dam crest level, i.e., a water volume of 1.2 Mm3. This crest full water level is assumed to occur when the TSF is at its maximum height and the tailings are at their maximum level. The NZDSG (Ref. 1) requires that reservoir inflows and levels, and downstream watercourse flows, should be those most likely to occur coincident with an assumed potential dam failure mode. For the Rainy Day PMP event at Storage 3, the concurrent downstream river flows across the catchment can easily vary. As it is the incremental change in effects which determines PIC, sensitivity analyses have been undertaken for downstream watercourse flows with different return period intervals, including 1 in 100, 1 in 1,000, and 1 in 10,000 Annual Exceedance Probabilities (AEPs), and for the PMF. The 1 in 1,000 scenario resulting in the greatest incremental effect from a dam was adopted for PIC assessment. This approach is consistent with the methodology in the CDA Technical Bulletin (Ref. 2).
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