Pahīatua flood: 60mm of rain falls in seven hours

"Lakes" formed on farmland, showing the extent of Sunday's heavy rainfall, which caused flooding in Pahīatua. Photo / Leanne Warr

Almost 60mm of rain fell over Pahīatua in a seven-hour period, leaving residents in several streets with a clean-up last weekend.

Heavy rain fell within 24 hours flooding several homes and keeping both the fire service and Downer crews busy.

Environmental data from the Niwa station near the township recorded roughly 75mm for the 24 hours over Sunday, with close to 60mm falling in a seven-hour period between midnight and 7am.

MetService meteorologist Dan Corrigan says at that rate, the rainfall was so high, the soil would have had trouble keeping up with it.

Closer to the Tararua Range, about 133mm was recorded at the Mangahao at Kākāriki site.

Corrigan explains that there would have been less over the town than in the Tararua Range due to the flow over air up and over the mountain range.

“Rising air is one of the ingredients that helps with heavy rain,” he says.

“That’s why we get so much heavier rain in the Tararua Range typically, compared to other places around the region.”

Corrigan says there can be rainfall events where it “clocks up” slowly and steadily, but this wasn’t the case over the weekend.

“It’s these times when you get a lot of rain falling in a short period of time that can cause the most trouble.”

Local country roads were also flooded. Photo / Leanne Warr

By Monday, the rain had mostly cleared up, with cooler air coming in from the south.

That meant widespread frosts and classic winter clear skies.

More rain was expected by Friday and Saturday but it was predicted to move on relatively quickly.

The weekend’s deluge flooded several streets in Pahīatua, including Huxley and Wilson, as well as Tutaekara Road at Mangamaire and Carisbrook Road in Mangatainoka.

Some businesses on Main Street were also flooded as well as properties near the Community Garden.

Tararua District Council closed several roads and the Pahīatua Track was also closed due to slips, but most were reopened after the water levels went down.

Fire and Emergency New Zealand said crews attended multiple call-outs related to the weather.

Eight call-outs related to flooded houses and buildings were received, as well as trees downed on roads and burst riverbanks.

The service continued to monitor the situation throughout the day on Monday.

This photo, taken from the site of the Polish memorial, shows the floodwaters hadn't receded on Monday morning. Photo / Leanne Warr

Tararua Riding for the Disabled in Mangatainoka had to move two of its ponies to another property and grazing land was flooded.

The organisation was also not able to run sessions over the week due to damage caused by the flooding.

Some residents in the flooded streets felt the problem was exacerbated by blocked drains and not enough maintenance.

There were various comments and images on social media which suggested the problem was something that had been present for some time before the flooding.

Tararua Alliance manager Daniel Erard says in anticipation of the weather, crews were out inspecting and cleaning out the drains on Friday afternoon.

It’s believed that some debris was caught up in the water as well as other obstacles, which contributed to the pressure on the system, blocking the drains and causing the flooding.

Erard says teams will have done a debrief to discuss the issue and see what could have been done better.

• A search of environmental data on the Horizons Regional Council website recorded 141mm for the period between 6pm Friday and 6pm Monday at the Mangatainoka River near the Pahīatua Dairy Factory.
• Greater Wellington Regional Council also records environmental data and a site near Eketāhuna shows 122mm of rainfall for the same period.