The dam is designed for resilience, from the reinforced concrete face to an erosion-resistant and flexible rockfill, drainage chimney and blanket. The flow-limiting filter layer under the upstream face reduces leaks and movement over time, and a grout curtain, which is a barrier that helps protects the foundation of a dam from seepage.
Resilience of the operational dam is managed through the implementation of a Dam Safety Management System, which meets the requirements of the new Dam Safety Regulations 2022 and international standards. There is regular surveillance and inspection of the dam, and regular engineering reviews of the dam.
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Design Parameters
Features
- 13Mm3 (13 billion litre) reservoir; 53m high; 220m crest length; 490,000m3 rock
- Rockfill embankment selected as most robust to seismic loads
- Concrete face membrane
- Zoning of material in embankment allows for drainage paths to mitigate internal erosion
- Grout curtain seals water migration around and beneath dam
Earthquake
- Operating basis: No non-superficial damage and fully operational: 1:150 AEP (years)
- Seismic evaluation basis: Retain reservoir contents: 1:10,000 AEP (years)
- GNS seismic assessment: 0.64g horizontal peak & 1.65g maximum spectral accelerations
- Active faults 8km and 12km from site
Flood
- Probable maximum flood (PMF): 1094 m3/s (18.6 Mm3 / 24 hour)
- 100 year flood: 375 m3/s
- 12 May 2019 1:50 year 250 m3/s (47 mm/hr)
Operational flow
- Operational flow: 2.2m3/s
- Environmental (min) flow and flushing flow: 0.5 m3/s and 5 m3/s
- Maximum: 17 m3/s (1.5 mM3/d = 10% reservoir / day)
Design Standards: NZSOLD / ANCOLD / ICOLD: Highest category (High PIC)
Resilient design
Key elements of the dam’s resilient design include:
1. Reinforced concrete facing and a grout curtain into the rock foundation to provide upstream waterproofing.
2. Erosion-resistant and flexible rock fill, which enables movement during an earthquake to minimise subsequent failure.
3. Joints to prevent leakage in between the foundation plinth and concrete facing have primary and back-up PVC waterstops.
4. A flow-limiting filter layer under the upstream face to reduce leaks and movement in the dam over time.
5. A spillway that is able to pass water in the maximum probable flood without overtopping the embankment and an upstream debris boom to keep out logs.
6. A system to ensure that if in the unlikely event of a leak, the release of water would be gradual, with water safely carried to the toe of the dam through an open-graded drainage layer of high-quality rock, without causing damage.
7. Dam design standards to ensure it will not release its contents in an uncontrolled manner in a 1:10,000-year earthquake. Modelling of such an event completed by New Zealand and international experts have shown that the dam’s planned drainage layers can accommodate resultant water flows even if there was significant damage to the concrete face and/or
movement of the fill.
Early detection of issues
Surveillance instrumentation and processes ensure issues are detected early, so they can be monitored closely and rectified as appropriate. For example, there is:
- Real-time monitoring of reservoir and spillway levels and outflows.
- Real-time monitoring of seepage and flows through the drainage layer and from beneath the spillway into the Monitoring Weirs.
- Real-time monitoring of water levels and flow through the dam and beneath the spillway using piezometers.
- Real-time leak detection at the plinth (the leading upstream edge) using thermistors.
- Observation wells from the dam crest through the embankment to measure seepage levels and monitor rockfill performance.
- Seismographs to measure earthquake loads.
- Regular surveying of dam for deformation, and surveying of reservoir for slope stability.
- Onsite cameras for remote monitoring.
- Regular comprehensive inspections.
Click here for information about the dam’s Emergency Action Plans.
