Horizons completes regional flood vulnerability assessment

Horizons Regional Council has completed a regional flood vulnerability assessment, giving a high-level overview of potential flood risk for more than 50 population centres across the region.

At a meeting today, Council received a summary of the assessment and its findings.
 
Horizons group manager catchment operations Dr Jon Roygard says the assessment gives Horizons its first high-level regional overview of relative risk from flooding, with 52 population centres ranked in terms of modelled risk.
 
“The Horizons Region is home to many large river catchments. We know people connect to rivers throughout the region for a range of reasons, and population centres and businesses have been built around waterways.
 
“This connection to rivers does heighten risk from flood events. While we have done some flood modelling before, this assessment fills the gaps where there was no modelling done previously. This allows comparison at a high level about relative flood risk for population areas.”
 
The findings follow a logical pattern, with flood scenarios increasing in severity in areas with more clustered buildings and population bases, says Dr Roygard.
 
“This is why Palmerston north and Whanganui rank first and second, respectively, in modelled risk when it comes to this assessment.

“The assessment also looks at the proportion of an area which would be exposed. For example, the assessment modelled nearly half of Levin’s population being exposed to flooding in a 0.5% Annual Exceedance Probability (AEP) flood event. That is why Levin ranks highly in terms of modelled risk.”
 
Given the assessment is at a high level, people should not use the findings to assess their individual property risk, says Dr Roygard.
 
“It is important to note the modelling used was not calibrated against previous actual floods, and the contribution of stormwater networks to alleviate flooding was not considered. The second point is especially important for some areas which rate highly for modelled risk in this assessment, such as Levin.
 
“Actual flooding could be quite different to what was modelled in the regional assessment.
 
“We also already have modelling for some areas, such as Palmerston North and Feilding, which is more detailed than what the regional flood vulnerability assessment provided.
 
“The regional flood vulnerability assessment gives us a broad overview of relative risk from 0.5% (AEP) flood events – also known as 1-in-200-year flood events – and we would need to do further work to get more detailed information.
 
“The risk rankings are there to help to inform strategic planning, and cannot be used to assess things such as insurability of properties.”
 
The assessment will help Council to make decisions on where investment could be made into flood resilience works in the future, says Dr Roygard.
 
“Having this regional assessment will provide important information for Council’s decisions during the upcoming Long-term Plan process, as they identify priorities for future flood resilience activity.”
 
The assessment also highlights the importance of regional councils working with district and city councils on flood resilience to ensure models are as accurate as possible, policy responses are aligned, and flood resilience work is done in tandem, says Dr Roygard.
 
“We are already doing this in the Horizons Region through projects such as work to improve Foxton’s and Feilding’s flood resilience, Pūtiki flood modelling, and the joint works programme in parts of Ruapehu District. District and city council staff did have input into the regional flood vulnerability assessment.”
 
If people do have concerns about their flood risk, Horizons’ District Advice team can help, says Dr Roygard.

“The District Advice team provides natural hazards information to the public and councils in our region. The most common advice sought is around the risk of flooding to guide development.”
 
The regional flood vulnerability assessment was one of a range of projects Horizons was able to complete after receiving more than $4.2 million from central government’s Cyclone Recovery Flood Resilience Co-investment Fund, to build resilience to the impacts of climate change and recover from Cyclone Gabrielle, says Dr Roygard.

“Other funded projects include upgrades to Horizons’ environmental data monitoring network, improvements to flood forecasting systems, and work on options to reduce flood risk to people and property throughout the Pohangina-Ōroua catchment.”
 
For more information on the study, see https://www.horizons.govt.nz/flood-emergency-management/regional-flood-vulnerability-assessment.
 
To contact Horizons’ District Advice team, either use the enquiry form on the Horizons website or freephone 0508 800 800.
  
ARI and AEP
Horizons uses ARI (Annual Return Interval) and AEP (Annual Exceedance Probability) ratings to measure and determine the level of service river management assets will provide in a given area.
 
A 1-in-100-year ARI means there is a 1-in-100 chance of a flood of that size occurring in any one-year period. The AEP level is the percentage version of ARI. A 1-in-100-year flood means it has a 1% AEP. In other words, in any given year, there’s a 1% chance of that level of flooding to occur in the area. It does not mean a flood of that level will only happen once every 100 years.
 
The regional flood vulnerability assessment considered the modelled risk in a 0.5% AEP flood event.

Image: The impacts of flooding in Feilding in February 2004, on a bridge near Norfolk Cres.