Mitigating Wairarapa’s flood risk

A report says more than 40,000 people and almost 18,000 properties in Wairarapa are potentially at risk in a major flood, but planning is both in place and ongoing to help mitigate the impact of a severe weather event.

The Greater Wellington Regional Flood Exposure Assessment from Greater Wellington Regional Council [GWRC] estimates more than 43,000 people, and almost 18,000 properties, in Masterton, Carterton and South Wairarapa could be affected by a serious flood.

Wairarapa has been affected by major flooding in the past, notably in 1947 and 1976. Cyclone Gabrielle hit Tinui this year, causing widespread damage to the settlement.

The report showed 700 commercial properties and 17,000 residential properties are at risk across all districts in Wairarapa, with numbers expected to rise by 2110.

Lian Butcher, group manager, environment at GWRC, said there are a number of tools available to help manage flood risk, including data collection, flood hazard modelling, flood risk management planning, emergency management, engineering controls [such as stop banks], river management, and land-use planning.

“Greater Wellington maintains flood protection schemes in many of the Wairarapa rivers, working with riverside landowners to provide erosion protection,” Butcher said.

“We also have in place two floodplain management plans in Wairarapa that set out how Greater Wellington will manage flood risk to Greytown and the Upper Valley.”

Butcher said the organisation is working with communities in Masterton and Carterton to develop management plans, and they are being developed following the process set out in planning guidelines.

She said GWRC has been working with district councils in Wairarapa to develop flood hazard mapping for inclusion in District Plans.

“Preventing unsuitable development in flood-prone areas is the most effective tool we have to manage flood risk. Detailed flood hazard mapping, developed in accordance with our modelling standard, has been provided. The district plan is due to be released later this year.”

One example of a community-based flood mitigation plan and strategy is the Waiōhine River plan, which incorporates the Waiōhine flood plain management plan for the area in and around Greytown.

The plan proposes new flood protection structures and strategies. New inland stop banks are planned along the north side of Greytown’s North St to prevent flooding into the northern outskirts and, if required, across farmland to the northwest of the town, near Kuratawhiti St. Gradual improvement to existing riverside defences is also planned using rock groynes where the river could outflank existing defences and threaten to set a new course.

A second example in the Masterton district is the plan for the Te Kāuru catchment, which includes the Upper Ruamāhanga River upstream of the Waiōhine confluence, and its tributaries, the Waipoua, Waingawa, Taueru, Whangaehu and Kopuaranga Rivers. The catchment has a history of flooding. Processes for managing flooding and erosion there have been in place in the catchment for some time and were reviewed in 2019.

GWRC manages risk from major rivers. Communities could be additionally at risk from surface water/stormwater flooding managed by district councils, and in some areas of coastal flooding could be an additional issue.

– NZLDR
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