Pahīatua flooding: A one-in-30-year event

Flooding on one of the streets in Pahīatua on August 18. Tararua District Council says the flooding was a one-in-30-year event.

The rain that flooded parts of Pahīatua was a one-in-30-year event that the infrastructure simply couldn’t cope with, according to Tararua District Council.

Council chief executive Bryan Nicholson said that in a two-hour period, more rain hit the township than it had in 10 years.

“It was a significant rain event that council systems aren’t designed to cope with.”

Stormwater systems backed up, due to the sheer amount of water.

According to environmental data, one station, owned by the National Institute of Water and Atmospheric Research, recorded 75mm of rainfall in the 24 hours of Sunday, August 18.

While Tararua Alliance gave assurances that the drains had been inspected prior to the weekend in anticipation of the heavy rain, many comments on social media raised concerns over the issue.

Nicholson said that the infrastructure simply wasn’t designed to cope with that level of heavy rain in such a short period and it was designed for a one-in-5-year event.

He said clearing vegetation from open drainage systems wasn’t going to solve the problem and that weeds wouldn’t have stopped the water going through.

The council did not declare an emergency, unlike last year when Cyclone Gabrielle decimated coastal areas of the Tararua District.

A rural property just out of Pahīatua on August 18.

While some residents did have issues with water coming in, Nicholson said no one had to be evacuated and no homes were declared unsafe as a result.

Services such as internet and phone were also not interrupted, he said.

The rain event may be a sign of things to come.

Flood modelling from Horizons Regional Council shows hotspots in Pahīatua in streets where the August flooding occurred, with more severe hotspots on the western side in the locale of the Mangatainoka River.

Nicholson said for the moment there was very little the council could do to mitigate future flooding events of future or greater intensity.

However, it would continue working with the community to ensure that everyone was as prepared as possible.

He said such a solution would cost in the “tens of millions” and would likely be years before something could be done.

According to Tararua Alliance, the problem with drains such as the one in Huxley St, was that they were open channel natural water courses located near properties and a number of utilities crossed the drains that could snag debris.

However, the main cause of the flooding that weekend was that the upper catchment of the town was contributing more water than the drains could handle, and no amount of additional maintenance would have made any difference.

The year’s weather was also a contributing factor as it had been relatively dry for much of the year and the surface dryness coupled with the intensity of the rainfall would have affected the drainage rate.

Tararua Alliance confirmed that the required maintenance of the Huxley St drain had been carried out, and there were limitations to how often vegetation on the banks could be sprayed and how much vegetation could be removed from the drain, due to environmental considerations, as well as the challenges with the utilities crossing the drain and proximity to properties.

There were no plans to upgrade the drain, but the council was considering long-term options to manage stormwater flow into the town. However, these options were likely to cost millions, and there was no guarantee they would work. Extensive investigations would be required.

'Lakes' formed on farmland showing the extent of the heavy rainfall which caused flooding in Pahīatua on August 18.

Infrastructure group manager Hamish Featonby said under the recently adopted Long-Term Plan the council had allocated some funding for stormwater renewals and planned to address ongoing issues across all the district’s main towns.

“This figure is limited because we know that what we have isn’t fit for purpose. The funding is for the next three years’ stormwater asset failures.”

However, he said there was also funding in the plan for flood modelling work that would enable detailed modelling to be done for stormwater over the whole of the Tararua District.

“This study is being led by an in-house expert in hydrology based at the Tararua Alliance.

“The findings of the study will inform the strategy for what kind of stormwater systems should be built and where.”

It was expected that in three years a detailed flood simulation model for each town would be available and findings from the study would also help inform decisions that needed to be made in the near term.

While Pahīatua was a priority, each town had its own stormwater issues, as in many rural areas it was the least developed network.

The issue was not limited to the Tararua District as there were other rural communities throughout the country facing similar problems.

Leanne Warr has been editor of the Bush Telegraph since May 2023 and a journalist since 1996. She re-joined NZME in June 2021.