Canterbury floods May/June 2021

Flooding is New Zealand’s most common natural disaster and we expect that climate change will intensify rainfall resulting in more frequent, high intensity events.

NIWA’s New Zealand Fluvial and Pluvial Flood Exposure report notes that total exposure in Canterbury to river and rainfall flooding is some of the greatest in New Zealand.

This is another reminder that our flood protection plays a big part in protecting the health and wellbeing of communities and critical infrastructures such as roads, community water supplies and power lines.

What happened?

On 28 May, MetService issued a red alert for the Canterbury region forecasting 200–300 millimetres of rain in the high country which they warned could cause dangerous river conditions and significant flooding.

What followed was an extreme rainfall event, both in terms of volume and duration of rainfall. It caused wide-spread out-of-river flooding and resulted in the declaration of a region-wide state of emergency from 30 May to 10 June. In some inland areas the flooding was so extreme that it might only be expected to happen once in 200 years.

This was the largest 24-hour event on record for most of our foothills rain gauges (eg Ashburton, Opuha, Mt Somers, Rangitata, Ashley, Selwyn and Waimakariri), causing significant damage to our hydrometric network. 

Major incidents

  • Our rain gauge in Mt Somers recorded its largest 48-hour rainfall ever, at 526mm.
  • At State Highway 1, this was the biggest flood on record for the Ashburton River/Hakatere.
  • At least 19 flow recorders from our hydrometric network were destroyed or damaged. The majority are now operational again although some remain in a vulnerable state for now.

You can continue to monitor rainfall and river flow data on our website.

Damage to flood protection infrastructure 

Our flood management systems were certainly tested in this weather event. Region-wide, the damage to Environment Canterbury-owned flood infrastructure included 20 holes in stopbanks, significant stopbank scour and weakening, erosion to river banks and large sections of flood protection vegetation which plays the critical role of slowing down floodwater was lost. There was also significant deposition of shingle on private property and throughout rivers.

Waimakariri-Eyre-Cust scheme

Flooding in the Waimakariri-Eyre-Cust scheme was moderate with a peak flow in the Waimakariri River of about 2500 cumecs, which is about half of the design capacity of the stopbank system.

The primary stopbanks were not breached in either the Waimakariri River or the Ashley River/Rakahuri.

Ashburton/Hakatere area

In Ashburton, one of the hardest hit areas, the $2.5 million investment made a decade ago to build stopbanks on either side of the river, proved to be money well spent, protecting assets worth hundreds of millions of dollars and prevented many homes from flooding. In this catchment the weather event was an order of magnitude greater than everywhere else in the region. 

The stopbanks are formed from compacted gravel covered with grass and paid for with targeted rates from owners of properties alongside the river.

Unprecedented flows washed out our rain gauges and so several of our engineers took turns to camp out and be eyes on the ground at Jessop’s Bend, just west of Ashburton in case the stopbanks were breached.

The event on the Ashburton/Hakatere in the upper catchment was “over-design” meaning the event was much larger than the flood protection measures were ever designed to withstand. 

Other areas of flooding

Very high flows in the Temuka and Lower Opihi Rivers put significant pressure on the stopbanks protecting the southern end of Temuka township and the Levels Plains/Opihi River Hut communities. The Temuka River broke out to the southwest (where there are no stopbanks), causing serious flooding of the Arowhenua area. In the rest of the scheme, flood protection infrastructure did its job despite being close to capacity.

Repairs were also required on the Selwyn/Waikirikiri, Hinds/ Hekeao, Orari and, Waihi Rivers.