New flood level building guidelines

Nicole Mathewson Reporter from Northern Outlook

By David Hill, local democracy reporter

The risk of flooding in the Waimakariri district has caused a rethink of building guidelines.

The Waimakariri District Council has endorsed new flood mapping freeboard and floor level guidelines for very low, low and medium risk areas as it plans ahead for future flood events.

The new guidelines recommended floor levels of 300mm to 500mm above the flood level.

But staff were recommending no building in areas considered to be high risk flood hazard areas.

‘‘It is really important that our district continues to be built in a way that provides minimal risk to people and property,’’ mayor Dan Gordon said.

‘‘There has been a significant amount of work undertaken to ensure any risk of flooding has been mitigated the best it can be and that the process is as streamlined and cost effective as possible to developers.’’

In a report to April’s council meeting, utilities and roading general manager Gerard Cleary said the freeboard level was the height above the designated flood level.

In the Waimakariri district, the previous standard was a freeboard level of 300mm.

‘‘If the flood level is one metre above the ground at a house site and a 300mm freeboard level is applied, then the building’s floor level would need to be 1.3 metres above the ground,’’ he said.

Guidelines varied throughout New Zealand, but the freeboard allowance was generally between 300mm and 500mm.

In Christchurch the freeboard allowance was 400mm.

Cleary said the Waimakariri district was located in a large and predominantly flat floodplain, which meant a 300mm freeboard allowance was sufficient in a number of areas.

But there were several factors supporting the raising of the freeboard allowance to 500mm.

These included vehicle wash, survey error and inaccuracy, fences impeding flow, less public and insurance industry acceptance of flooding, modelling error and uncertainty, minor earthworks, ongoing movement due to seismic activity, climate change and catchment
changes.

National guidelines for new greenfield developments recommended a 500mm freeboard allowance above the flood level, Cleary said.

‘‘Where the land has a low risk of flooding, there is little or nothing that needs to be done by the developer to achieve the freeboard requirement.

‘‘In areas of medium or higher risk there may need to be additional filling or careful design considerations given to achieve the desired level protection.’’

Cleary said the guidelines took into account variation 1 to the Waimakariri District Plan, required by the Resource Management (Enabling Housing Supply and Other Matters) Amendment Act 2021.

Variation 1 identified areas of Kaiapoi as unsuitable for further housing intensification due to flooding risk.