Dam design and safety

The Dam will be a concrete-face rock fill dam. A rock fill dam is a very stable structure that reuses indigenous material excavated from the site and is appropriate for absorbing seismic loads. The Waimea Community Dam will be approximately 53m high, 220m long and 6m wide at the crest. About 430,000m3 of rock will be used to build the dam or be recycled on site. The reservoir lake created by the dam will contain approximately 13.4 million cubic metres of water.

The Waimea Community Dam is designed to the latest international design standards under the NZ Dam Safety Guidelines and has been and continues to be peer reviewed by technical experts.

Design Parameters

Features
  • 13Mm3 (13 billion litre) reservoir; 53m high; 220m crest length; 430,000m3 rock
  • Rockfill embankment selected as most robust to seismic loads and uses indigenous material
  • 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

Measures to ensure the dam’s resilience and early resolution of issues:

  • Real time monitoring of inflows, outflows and reservoir levels.
  • Leak detection and measurement of water within the embankment.
  • Seismographs to measure earthquake loads.
  • Regular surveying for deformation.
  • Onsite cameras for remote monitoring.
  • Trained observers on site for monitoring when significant floods are
  • expected, or after earthquakes.
  • A comprehensive and regular schedule of inspections.

Click here for information about the dam’s Emergency Action Plans.