'Urgent' meeting called to revise New Brighton flood modelling discrepancy

The Christchurch City Council said previous flood modelling for floor levels did not account for the effect of flooding ...
DON SCOTT/FAIRFAX NZ

The Christchurch City Council said previous flood modelling for floor levels did not account for the effect of flooding in the lower reaches of the Avon River.

Floor levels in some New Brighton properties could be "significantly lower" than the minimum height required to protect against flooding, a community organisation says.

The issue, which prompted an urgent meeting between Empowered Christchurch and the Christchurch City Council, led to the council updating its flood modelling for floor levels in an area north of Bridge St, along the Avon River.

The council conceded previous modelling did not account for the effect of flooding in the lower reaches of the river.

Christchurch City Council consenting and compliance general manager Peter Sparrow.
SUPPLIED

Christchurch City Council consenting and compliance general manager Peter Sparrow.

It would review project information memorandums for rebuild projects in the affected area. The number of properties affected by that task was yet to be determined. 

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It would also notify affected homeowners, councillors and the area's community board.

Empowered Christchurch believed minimum floor levels for some New Brighton properties were set 50 centimetres lower than they should be.

Secretary Seamus O'Cromtha​, whose property was next the the Avon River, said the problem was due to incorrect flood modelling by the council and could extend to Earthquake Commission repair work.

"Basically a surveyor was doing a survey of two different properties and he had another job in Southshore.

"The level [at the Southshore property] was 11.81 metres and then he was doing me and found it was 11.34m," O'Cromtha said. 

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The full extent of the issue was yet to be determined, but Empowered Christchurch believed several rebuilds had been completed at the wrong height, while others remained in the building stage or were halted due to insurance battles.

Council information stated minimum floor level requirements did not apply to homes only undergoing repairs, unless the repair work required a resource or building consent, which would trigger reassessment of the floor level.

O'Cromtha thought the council did not know how many properties were affected.

"There are lots of other categories, like myself. They haven't got in contact with me, who is affected, or my neighbours.

"We had everything settled for the plans of the new house and then when I realised they wanted to rebuild at the original [floor] height I said 'that's nonsense'." 

The group held an urgent meeting with the council on April 26 to seek a resolution.

"Their explanation was that they had assumed a tidal flood bank would be built on Bridge St which would affect properties to the north from flooding. They factored that in for the flood planning."

Consenting and compliance general manager Peter Sparrow said the council ordered surveys of five properties that might be affected by the change in modelling to determine floor level issues.

"We have also advised insurers through the Insurance Council of New Zealand to use the updated information for any assessment or building work in this area."

Insurance Council chief executive Tim Grafton said affected homeowners should contact their insurer to find what, if any, implications there were for their insurance. 

O'Cromtha said affected homeowners, himself included, were "angry" following the discovery. 

In a statement, the council said assumptions for modelling in the affected area had been revised, taking a more conservative approach to the calculation of floor levels needed to withstand a 50-year flood.

"New builds in this area are not at an immediate risk of flooding in a 1-in-50 year flood, as they have a 0.4 metre freeboard.

"The new floor level calculations take into account 0.5 metres of sea level rise, which is the assumed increase over the next 50 years."

Freeboard was an addition included in the floor level to compensate for factors that could increase the water level beyond the calculated flood level.

"The previous modelling did not fully account for the effect of tidal flooding in the lower reaches of the Avon River in that small area as a result of future predicted sea level rise over the next 50 years."

The council continued to investigate options for long-term river and tidal protection for the Avon as part of its land drainage recovery programme.

Decisions on those options were closely linked to decisions around the future use of the residential red zone.

 - Stuff

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