Mōkihinui River. Photo:
Managed retreat and restricting new developments have been flagged as potential ways to help protect a West Coast township into the future.
Mōkihinui was hit hard by flooding in July 2021 and February 2022, resulting in several of its roughly 50 buildings being yellow-stickered.
It was caused by runoff that overwhelmed the culverts, running through Mōkihinui River stopbank and ponding in the township.
The West Coast Regional Council sought advice from the National Institute of Water and Atmospheric Research (NIWA) on how to mitigate flooding impacts after receiving a Ministry for the Environment funding grant in 2022.
Last month, NIWA released a report that found the township was highly vulnerable to flooding, exacerbated by sitting on the lowest-lying part of the floodplain.
"Flooding from the Mōkihinui River or sea due to failure of the defences has the potential to cause higher flow velocities and greater risk to life than flooding from the local catchments. In the event of stopbank overtopping or failure, deep and high velocity flood water could enter the township," the report said.
Sea level rise was a major threat as it exacerbated coastal erosion, and the likelihood of flood protections being overwhelmed while also reducing how effective culverts were at allowing water to drain away.
The report recommended modifying or removing a twin barrel farm culvert that was vulnerable to blockages, upgrading and increasing the capacity of a culvert under the river stopbank, and undertaking more inspections and maintenance to reduce the risks from local catchments.
Longer term recommendations included restricting new development in the low-lying areas and focusing on nearby lower risk areas where land was 1.5 metres or higher, and for property owners to consider raising floor levels.
"The cost and feasibility of this is likely to be dependent on building design. Raising floor levels can be a very effective way to reduce the damages of flooding," the report said.
Managed retreat was suggested in the report as another long-term option.
"Planning ahead such that if or when properties require major renovation or repair (either because of a general need for maintenance, or because of flood damage), the option to move is available, would help minimise the costs associated with retreat," the report said.