Paradise Dam
Paradise Dam

More than 100 lives could be at risk in severe weather event

MORE than 100 people could be at risk if Paradise Dam failed in the event of a "substantial" flood, according to the Paradise Dam Preparedness Review.

Released to the public today, the review was tasked to the Inspector-General Emergency Management in September and highlights 33 findings and 17 recommendations relating to dam safety, disaster management arrangements, emergency communications, community readiness and lessons management.

>> VIEW PARADISE DAM REVIEW 

The dam was originally designed to safely pass a flood with an average recurrence interval of once in every 30,000 years.

"The current strength of the Paradise Dam spillway is calculated to be well below safety guidelines, and the life safety risk is also calculated to be well above the guidelines' limit for what society is expected to tolerate if there was a failure," the report read.

"Sunwater assess that the dam's stability may be marginal in a 1 in 50-year event and that the likelihood of failure is significantly increased in a 1 in 200-year event.

"The most likely failure mechanisms are sliding or overturning of sections of the primary spillway or undermining of the sections by the scouring effect of floodwater on the geology just below the dam."

Paradise Dam on October 17
Paradise Dam on October 17

 

The review makes clear that in the wake of the 2013 flood, Sunwater had taken appropriate investigative action in a timely manner and for all practical purposes, addressed all the actions from the Paradise Dam Flood Event of January to March 2013 - Review of Dam Safety Management Action Report for the Office of Water Regulation.

Paradise Dam is a roller compacted concrete dam which was built for water supply; it is not a prescribed flood mitigation dam and does not have flood gates.

And it has been dubbed   as a "referable dam" under the Water Supply Act 2008.

"It has a Category 2 failure impact rating, meaning that more than 100 people would be at risk if the dam failed," according to the report.

Delving into the complexities of the dam's safety, the review uses both the Factor of Safety methodology and the Life Safety Risk to highlight their findings.

The "factor of safety" is a measure of the sliding stability of the dam under particular failure modes, while the "life safety risk" relates to tolerable risk limits acceptable to society.

The review found that the strength of the Paradise Dam spillway is "well below the required factor of safety rating" under the Factor of Safety description based on analysis in September.

For a flood that has a 10 per cent probability, the Paradise has a factor of safety of greater than 1.5.

For some point of reference, a factor of safety of 2.0 means that a dam is twice as strong as the force applied from the water in it.; whereas a factor of safety of less than 1.0 indicates failure would likely occur, according to the report.

For rarer flood events the dam falls below the recommended minimum factor of safety.

And for "an event similar to the 2013 floods, classified as a 0.5% probability of happening in any given year (1-in-200-year event) the factor of safety is only around 1.1; it should be at least 1.3".

When it comes to the life safety risks and the risk of failure for Paradise Dam, the July assessment found concerning results.

Paradise Dam at Coringa.
Paradise Dam at Coringa.

"They were assessed at much higher than the tolerable limits in the Queensland Government guidelines on dam safety," the review read.

"The risk of fatality for a 0.1% (1-in-1000-year event) probability failure event has a probable loss of life of 65 people. The guidelines state this should be less than 1.

"For Paradise Dam a probable loss of life of 120 people was assessed for a 0.05% (1-in-2000-year) probability failure event in any given year. The guidelines limit for 120 fatalities is for a 0.001% probability flood (a 1-in-100,000-year event).

With this data set out, the findings were that the life safety risk for Paradise Dam exceeds the limits set in the guidelines and is more than what society would tolerate.

And the analyses for probable loss of life per has increased exponentially year-on-year.

This is a calculation based on the number of expected fatalities directly resulting from the failure multiplied by the probability of the failure event.

The figures have been calculated as a result of comprehensive risk assessments, mitigation works and better understanding since the 2013 flood of the geology, scouring risks and dam structure.

When these figures are graphed the calculated risk to life has grown significantly, 6-fold between January 2015 and September 2019 and 1500-fold between March 2011 and September 2019.

This is a stark comparison to the dam's 2012 reputation as one of Queensland's safest dams, which was severely impacted by the 2013 floods.

As a result of the 2013 disaster, severe scouring to the underlying geology occurred immediately downstream of the apron at each end of the spillway and the apron itself was damaged.

The apron sill was destroyed over almost the entire spillway width.

Earlier this year it was identified, by GHD, working for Sunwater, that the most likely cause of failure of Paradise Dam was a sliding within the primary spillway monoliths and the second was undermining of the primary spillway due to future scouring.

While these calculations are grim on the surface, in order for the dam to fail it would take a "substantial" flood event.
"An event similar to that experienced in late 2010 should not be a concern of failure, however the dam should no longer be considered safe to pass floods with a magnitude similar to that experienced in January 2013, when the dam reached an elevation of 76.25m," it read.

Throughout the review process 300 people downstream of the dam were surveyed and 93 per cent of residents were confident they were prepared for and could responded to a local disaster event, 87 per cent of respondents were confident they would receive adequate information or warnings about a potential local disaster event and 64 per cent of residents had prepared items for an emergency kit.

Overall, while small opportunities for improvement remain, this review rates disaster preparedness and planning of the Bundaberg LDMG for Paradise Dam as strong.

Nonetheless, the review findings suggest that the current risk of Paradise Dam is not obvious in the Emergency Action Plan or other easily available public documents, nor is it obvious how it has changed.

Among the findings and recommendations highlighted in the reviews, one is to enable the community to have a shared understanding of risks and enhance community engagement, flood scenarios in Emergency Action Plans be published, and in a format that can be used by public mapping systems.

The review found community understanding was mixed, with North Burnett's residents mostly concerned with water security rather than dam safety.

While for Bundaberg, the review team heard of little community conversation about the dam and disaster preparedness, but the telephone survey identified that flooding was the most commonly mentioned disaster risk.

One of the final recommendations in the review is to "enable the community to make informed choices about disaster management, and act on them, a joint Sunwater/council community engagement program be conducted to prepare the community downstream from Paradise Dam for events in the 2019/2020 storm and cyclone season, including the possibility of a Paradise Dam failure".


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