Flood review clears DCC staff findings

Cars create a wash along a flooded Bay View Rd, Dunedin, during the June 2015 floods. Photo by Stephen Jaquiery.
Cars create a wash along a flooded Bay View Rd, Dunedin, during the June 2015 floods. Photo by Stephen Jaquiery.

Dunedin City Council staff have been vindicated by peer reviews which backed their findings over the cause of last June's devastating floods.

The reviews, carried out by infrastructure consultancy firm Opus, backed the council's findings South Dunedin and other parts of the city would have been flooded even if the city's stormwater system was running at full capacity.

Mayor Dave Cull said the peer reviews revealed council staff had nothing to hide.

"I think the peer reviews confirm the investigation our staff did and the conclusions they drew were right and I think it is a credit to our staff that they didn't try and hide anything.''

Council staff had shown "diligence'' while operating during such a stressful time.

"The flood was horrific for the people of South Dunedin and there was subsequently a lot of criticism of our staff, and some of it, in terms of supervision of maintenance contracts, was justified.''

Opus principal environmental engineer Warren Bird agreed with the council's conclusion failures at the Portobello Rd pumping station contributed to flood levels and duration and a lack of mud-tank maintenance probably contributed to localised flooding.

But, like council staff, he concluded the magnitude of the rain event exceeded the capacity of the Portobello Rd pumping station and there would have been flooding even if it was running at full capacity.

"Such a storm, had it occurred anywhere else in New Zealand, would still have overwhelmed the primary stormwater system.''

The first of two council reports published on last year's flood was "commendable for its frankness in addressing issues, and is a useful starting point for a wider debate'', Mr Bird said.

The two peer reviews - one looking at overall infrastructure performance during the floods and the other investigating the influence of mud-tanks on flooding - raised questions about the need for further investment in South Dunedin's infrastructure, but the council yesterday said it was following up all recommendations made.

Council infrastructure networks general manager Ruth Stokes said the findings were a vote of confidence in council staff.

"I think it confirms that we have been open and transparent with the council and the public and that our analyses have been robust and our conclusions appropriate.''

It further confirmed the council had "really good staff who know what they are doing''.

"The reality is there was a lot of rain and we have been open and honest about what didn't go so well.''

When it came to future investment in South Dunedin, Mr Bird recommended the council revisit the "design parameters'' of the stormwater system, taking into account changes to the ground's ability to soak up floodwater, updated rainfall data, climate change and the lack of secondary flow paths.

It should also look at ways of enhancing pipe inlet capacity.

He said the council and Fulton Hogan's methods for determining whether mud-tanks were blocked were flawed and recommended a new performance standard.

Poor mud-tank maintenance could have played a "much more significant factor'' in flooding had the Portobello Rd pumping station been running at full capacity.

Mr Cull said the council was already carrying out all the recommendations in the report.

"I think that they are bang on the mark ... and those recommendations are exactly what we are already doing.''

Council chief executive Sue Bidrose said she did not imagine the report would silence critics who said the council had been hiding the real causes of the floods, but hoped some people would be reassured by an independent party reaching the same conclusions.

All recommendations in the report were being acted on and the new screen being put in at the Portobello Rd station would ensure it ran at full capacity, which would make a substantial difference if the weather event of last June was repeated, Dr Bidrose said.

A project, firstly to ensure the council's infrastructure was performing at full capacity and then looking at how it could be improved to meet challenges created by climate change, was under way.

The council's new road maintenance contract with Downer, which replaced Fulton Hogan, includes a new system to monitor mud-tank maintenance based on Auckland's system, widely seen as the best in New Zealand.

vaughan.elder@odt.co.nz

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