“This brochure is very misleading. The information is quoted out of context and misinterpreted.
In the dams industry, a dam break hazard assessment is a rigorous process:
• To determine the potential downstream impacts from hypothetical dam failure
• To guide standards for design, construction and operation – the key point is to ensure where a high consequence exist the design and construction standards adopted ensure an extremely small probability of a probability of failure even in unlikely combinations of adverse events
• To provide base information for the development of an Emergency Action Planning, such as flood hazard, inundation extents, warning times.
Such studies are hypothetical in nature, and entirely divorced from the remote chances of a dam failure ever occurring. Our 2009 report (dam break hazard assessment) in its conclusion states “The risk of failure occurring for a dam engineered, built, maintained and monitored to appropriate standards, as would be the case for the Lee dam, would be extremely low.” We engaged with the Lee Valley and Brightwater communities and presented findings from our dam break study in a series of community meetings/open days in 2010 and 2013.
Nowhere does our analysis say Brightwater would be hit by a “tidal wave up to 8 m high.” Rather than depth the map shows the product of depth and velocity, which we use as an indication of hazard. Our 2012 report (Emergency Action Plan, EAP) actually states that in the unlikely case of a rapid dam break “At the peak of the flooding approximately half of the town of Brightwater would be inundated to depths ranging from 0.5 to 3 m”. The elapsed time for the peak water level to occur is 61 minutes, not 20 minutes as implied by the brochure. While normally unmanned, the dam will be fully instrumented and constantly monitored to detect any unusual occurrences and potential emergency situations enable actions to be taken to reduce the risk of a dam break. Early warning systems and evacuation procedures are integral parts of the Emergency Action Plan.
People should be very cautious of claims made in an anonymous brochure. We urge people to read the full Reports that are publicly available on the Tasman District Council website. Seismic risk, dam safety and hydrology are very serious issues on which communities need sound scientific and engineering skills to make good judgments.”
Statement from Dr Mike Johnston GNS on anonymous brochure 26 November 2018
“The active faults have been recognised and the dam designed in consequence of this. The dam is not on an active fault but some distance from the Alpine Fault and WFFS. The GNS reference to the Alpine Fault is to the southern section and not to the northern section, sometimes referred to as the Wairau Fault, which has a much lower level of activity. Therefore what is quoted is at best, mischievous.”