Council assesses potential impact of flood model change

Christchurch City Council investigations show a change in flood modelling for a small area of New Brighton potentially impacts five homes.

Last week, the Council revised its flood modelling for a small area of New Brighton, north of Bridge Street along the Avon River, taking a more conservative approach to the calculation of floor levels for a 50 year flood.

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 predicted sea level rise over the next 50 years.

The Council has looked at all building consents granted in this area since 2011, and established all but five have been built with floor levels above the revised 50 year flood level.

General Manager Consenting & Compliance Peter Sparrow says, "Our priority now is to focus on these five property owners who may be affected in some way by this change in modelling. We are fast tracking surveys of land and floor levels for each of these five properties.

"Each individual site will be different, so until we can analyse the survey results, it's too early to say what the next steps will be. We will work closely with the property owners through each step of this process."

Mr Sparrow says it is important to understand that this change in modelling only has a potential impact on consent building work that triggers a new floor level assessment – a full replacement of the foundation, or a new build.

"Minor repair work does not have a floor level requirement, so people in this area who have had minor repair work done are not affected by this change in modelling," Mr Sparrow says.

The Council is also going through Project Information Memorandum (PIM) records to check floor level information that might have been provided for any building consent applications in this area.

"And we will be informing any PIM applicants who haven't progressed through to a consent about the change in modelling so they know to use the new floor levels in any assessments or future building consent applications in this area."

"I want to reassure affected residents, the New Brighton community and Christchurch residents we are treating this with the highest importance and will continue to update the community as further information comes in. If anyone in the area has concerns about how their property is affected, please get in touch with the Council on 941 8999" Mr Sparrow says.

New builds in this area are not at an immediate risk of flooding in a 1-in-50 year flood, as the modelling calculations take into account 0.5 metres of sea level rise, which is the predicted increase over the next 50 years. The floor levels also include a 0.4 metre freeboard – which is an extra allowance in flood levels to provide for wave action, uncertainties in determining land levels, imprecision in modelling, and construction tolerances.

The Council continues to investigate options for long-term river and tidal protection for the Avon as part of the Land Drainage Recovery Programme. Any decisions on these options are closely linked to decisions around the future use of the Residential Red Zone.

Floor level background

The Council sets minimum floor levels to protect properties from the risk of flooding. This is a requirement under the Building Act.

The floor levels are assessed on the best available flood model information and take into account the impact of predicted sea level rise over the next 50 and 100 years.

Floor levels under the Building Act are required to be above a 50 year flood, plus 0.4 metre freeboard, and include 0.5 metres allowance for predicted sea level rise over the next 50 years. Existing flood protection works (such as stopbanks and tide gates) are taken into account when determining the extent and depth of a 50 year flood. In the East of Christchurch, the floor level is largely defined by tidal flooding.

Floor levels within Flood Management Areas (FMAs), mapped in the Christchurch District Plan, are required to be above a 200 year flood, plus 0.4 metre freeboard, and include 1.0 metre allowance for predicted sea level rise over the next 100 years. Flood protection works are ignored when determining the extent and depth of a 200 year flood, as a flood of this magnitude may breach these.

ENDS

© Scoop Media