EGL Ref: 9215 23 June 2022 Page 32 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. operation, and performance of a dam and all the systems and procedures that affect dam safety and a comparison against current dam safety guidelines, standards and industry practice. The CDSR was undertaken in accordance with the recommendations in the NZDSG (Ref. 15). The NZDSG recommend that a CDSR should be undertaken every five years for High PIC dams. Recent designs to allow raising of Storage 2 to RL160.7 and Storage 1A to RL182 were reviewed by Damwatch Engineering as part of the Building Consent process (Refs. 16 and 17). 4.9. Performance The piezometers, subsurface and leachate drain flows and deformation monitoring points measured confirm that the performance of the Storage 1A and 2 embankments are as intended by the design. 4.9.1. Seepage As the embankments were raised, there was a reduction in the seepage into the ground beneath the embankment. This is due to the low permeability of the liners and tailings. This is seen in the records of the subsurface upstream, and initial embankment toe drain and drains under the tailings, which have shown a general decreasing flow with time and raising of the embankments. Figure 26 shows this effect for Storage 2 from 1989. This same effect is evident in the Storage 1A upstream cut off drain and drains under the tailings in Figure 27. Flows from the final downstream toe drains show relatively consistent trends historically, with generally similar flows over time with peaks in the winter months. This indicates ground water recharge beneath the embankment is affected by rainfall, infiltrating from the hills behind the facilities and groundwater levels downstream of Storage 1A and Storage 2. FIGURE 26: STORAGE 2 TAILINGS UNDERDRAIN AND INITIAL TOE DRAIN HISTORIC FLOWS
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