New Zealand – Record Rainfall Causes Flooding in Otago and Canterbury

Heavy rain and high tides between 21 to 23 July 2017 combined to cause widespread flooding in Otago and Canterbury on the South Island of New Zealand.

Some areas of Dunedin recorded 225 mm of rain – 4 times the total for the whole of an average July.

Hundreds of homes were evacuated and major highways cut off. A regional state of emergency was declared in Otago on 21 July. A state of emergency was also declared in Selwyn District, Canterbury, on Saturday 22 July.

Overall, about 220 homes were evacuated overnight 21 to 22 July across Otago region from Oamaru to Milton and inland to the Ida Valley. Around 100 of those are in Outram.

Civil protection officials said that some of the 100 families evacuated around Dunedin, most of them in the Henley and Taieri areas, are unlikely to be able to return home for several days.

A large landslip has cut off residents on the Otago Peninsula north of Harwood. There have been more than 25 slips on roads on the Otago Peninsula.

Three bridges have been damaged by the weekend’s flooding. The affected bridges are Sutton-Mount Ross, Mount Stoker and George King.

Selwyn river floods in Canterbury Region, New Zealand. Canterbury Region, New Zealand. Photo: Civil Defence Emergency Management Canterbury
Floods in Dunedin, July 2017. Photo: Dunedin Civil Defence Emergency Management
Selwyn river floods in Canterbury Region, New Zealand. Canterbury Region, New Zealand. Photo: Civil Defence Emergency Management Canterbury

Rivers, Rain and High Tides

Rivers

Otago disaster authorities said earlier today that levels of the Taieri river at Outram were at the second highest level ever recorded, at 1825 cumecs.

The Manuherikia River peaked at Ophir on 23 July, at the fourth highest level on record. Canterbury disaster officials said that the Selwyn river overflowed at several points, prompting the state of emergency in Selwyn district.

Rainfall

Otago Civil Defence Emergency Management Group Controller Scott MacLean said that at one Dunedin site, 225mm of rain fell in less than 24 hours, compared to the average rainfall of 45-50mm for the whole of July.

National Institute of Water and Atmospheric Research (NIWA) said that Oamaru recorded its wettest day on record on Friday 21 July with 161.2 mm of rain falling in 24 hours, beating the previous high of 128mm in April, 2006. In a normal July Oamaru’s total rainfall is 40.6mm.

NIWA also said that Dunedin had its wettest July day on record since 1918 on Friday when 89mm fell – beating the 2007 record of 75.8mm.

At Winchmore, South Canterbury, 111.8mm of rain was recorded in a 24 hour period to Saturday 22 July, making it Winchmore’s second wettest day on record.

Christchurch airport weather station recorded 88.6 mm in 24 hours to 22 July. Christchurch normally sees a total of 64.7 mm for the whole of July.

High Tides

NIWA said that the heavy rainfall over the weekend coincided with some very high storm tides for Christchurch.

NIWA added: “The  peak storm tide about about 3:15 pm on Saturday was equal highest recorded in last 13 years at NIWA’s Sumner gauge, which exacerbated flooding in low lying areas.

NIWA scientists were able to forecast a drop in the next tide at 3am on Sunday but further high tides combined with rainfall caused more flooding. Fortunately for Timaru and Christchurch, the storm arrived just ahead of peak king tides yesterday and today.”

Recovery

The state of emergency has been lifted in Otago and the region has officially transitioned into recovery.

Dunedin Civil Defence Emergency Management said earlier today that the flood recovery phase underway.

Dunedin City Council Recovery Manager Simon Pickford says, “We are now in the information gathering phase. We don’t believe anyone is facing immediate danger now as a result of the heavy rain and flooding and our attention is on gathering key information.”

Mr Pickford says the first priority is getting an accurate picture of the situation for residents who have been evacuated, whether this was done as part of the civil defence response or whether people self-evacuated.

“At this stage, it will be several days before the area is dry enough for us to assess flooded properties, so residents in these areas will have to remain out of their homes for at least the next few days, possibly longer.”

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Flood Summary

Last updated: July 26, 2017
Event
Otago and Canterbury Regions, New Zealand, July 2017
Date
July 21 to July 26, 2017
Type
Landslide, River flood
Cause
Extreme rainfall, High tide

Locations

A - Dunedin
B - Outram
C - Selwyn

Magnitude

River discharge
1825 cubic metres per second
Taieri river at Outram - July 22 to July 23, 2017
Rainfall level
225 mm in 24 hours
Dunedin - July 22 to July 23, 2017
Rainfall level
161.2 mm in 24 hours
Oamaru - July 21 to July 21, 2017
Rainfall level
111.8 mm in 24 hours
Winchmore - July 21 to July 22, 2017
Rainfall level
88.6 mm in 24 hours
Christchurch - July 21 to July 22, 2017

Damages

Evacuated
1,000
Otago - July 22 to July 24, 2017
estimated figure. Civil protection said that 220 homes were evacuated.