Significant damage from August floods

Some of the Bay of Plenty’s flood schemes suffered “significant and expensive” flood damage in August’s flooding, this weeks Bay of Plenty Regional Council meeting heard.

The Regulation, Monitoring and Operations committee heard that a comprehensive inspection is being undertaken of all rivers and drainage schemes to assess flood damage.

The storm generated up to 349 mm of rain in some eastern catchments, with river flows ranging from five year return floods in most rivers to a 30 year flood in the Whakatāne River. However there were no houses flooded, no stock losses and floodwaters were quickly removed.

Rain had reached totals of up to 260mm by Saturday evening on 14 August in the Eastern Ranges, with Metservice updating their forecast and predicting another 120mm to 180mm in the ranges overnight and into the next morning.

While the event was not notable in terms of rainfall in isolation, there had been significant rain in
August on top of a relatively wet July for some catchments, and catchment soils were saturated.

The flood team was activated on Friday 13 August to ensure systems and staff were available to deal with flood-related issues. Stoplogs were installed at sites on the Whakatāne River and tributary canals on Saturday night in preparation for the high tide on Sunday morning coinciding with the flood peak in the Whakatāne River.

On Saturday Trustpower began spilling from the Matahina Dam to reduce the dam to its minimum
operating level. The peak outflow from the dam was reduced from a 10 – 20 year event to a 5 – 10 year event in the Rangitāiki River by controlling the dam. Low level stopbanks on the Waioeka and Whakatāne Rivers were over topped, flooding low lying land.

A stopbank on the Te Rahu Canal that leads into the Whakatāne River was partially breached on Sunday morning, scouring to about 15 metres wide, Group Manager Rivers and Drainage Ken Tarboton said.

“Seven families were evacuated in the Fortunes Road basin due to the breach, which was repaired by early evening through the ingenuity of our staff and contractors. The cause of the breach will require investigation, however it may have begun as a rabbit hole. Additional animal pest eradication and monitoring may need to be undertaken to prevent this in future.”

Low level stopbanks on the Whakatāne River within the main flood protection stopbanks overtopped, flooding paddocks behind them.

There were no reports of flooding to homes across the Eastern Bay of Plenty, but some garages flooded due to localised drainage problems. Farmers acted on warnings early and moved stock to higher ground.

The committee heard that a number of lessons had been learned from the flood.

Staff initiated an exercise with Lake Aniwhenua while flows were high to gather information on the effects of lake level on upstream flooding.

ENDS

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