Waipā urban flood maps to be available early 2022

Waipa District Council is undertaking a district-wide urban flood hazard mapping project to help plan for New Zealand’s changing climate, and the adverse weather events climate change could bring to its urban areas.

The project is in the final stages of undertaking a visual survey and assessment of the small number of dwellings identified as potentially at risk of flooding in a 1-in-100-year, severe rainfall event.

Waipā Mayor Jim Mylchreest said councils across New Zealand are required under the Resource Management Act to take a proactive approach to plan for adverse weather events and the flood mapping exercise was part of that.

“Severe weather events could become more common as a likely impact of climate change and with more frequent intense rainfall, this may increase the likelihood of flash flooding when urban drainage systems are overwhelmed.”

“The district-wide flood hazard mapping project will help us better understand the areas that may be at risk of flooding in a severe 1-in-100-year rainfall event, which has a 1% chance of occurring in any given year,” Mylchreest said.

Information gathered during the project will also identify if the current stormwater infrastructure meets Councils Levels of Service and is fit-for-purpose. Areas identified that don’t meet levels of service will be assessed by stormwater engineers for priority and Council’s water services team will undertake improvement works through the stormwater programme.

“Understanding the flood risk helps Council to help protect future development areas against flood risk through a range of mechanisms like the District Plan and building and resource consents,” added Mylchreest.

The project models severe rainfall events using the best industry standard data. Council has also undertaken an additional visual survey of dwellings identified through the modelling as potentially at risk of flooding to ensure the information was as robust as possible, said Strategic Planning and Asset Management Team Leader Robin Walker.

“We’re looking at the capacity of the current stormwater network and other infrastructure in the urban areas of Cambridge, Leamington, Hautapu, Te Awamutu, Pirongia, Kihikihi and Ōhaupō and have notified approximately 1000 property owners across these areas that their dwellings were identified as potentially at risk through the modelling,” Walker said.

“Through undertaking a secondary assessment using a visual survey we have already been able to remove approximately 90 percent of these dwellings from the original list.

“The visual surveying element was unfortunately delayed due to COVID-19. Once all of the final visual surveying has been completed, properties still at risk of flooding will be notified individually with maps showing water depths and additional information in response to frequently asked questions,” said Walker.

Each property that remains ‘at risk’ will be those indicated through the modelling and visual assessment that over-floor flooding could occur in a severe rainfall event. Property owners will be provided with a water depth map showing their individual property’s flooding extents.

“We were anticipating to have publicly-available maps showing where the modelling indicates rainfall from the severe events will pond and flow over-land for all of our urban centres ready by the end of 2021, but due to COVID-19 this has also been delayed. We now expect to have them available in early 2022 to allow all owners of dwellings at risk to be notified first.”

“The map layers that we will be developing will show the extent of overland flow paths and ponding that we can expect in a severe rainfall event for all properties, however this will not include flood-depth information to maintain confidentiality.”

“Individual property owners who don’t receive a confirmation letter that their property is ‘at risk’ but who can see on the public map flood layer that water overlaps their property will be able to request further information for their own property from Council if they wish,” Walker added.

Once all properties have been notified, the final urban flood hazard maps will be added to the Council’s Maps Online software on the Waipā District Council website.

For more information about the flood hazard mapping project, click here.