Christchurch could be in for more flooding next week, with rain forecast to return to a city on track for its wettest July on record.
After surprise flooding on Thursday, which Christchurch City Council attributed to getting more rain than forecast, staff are giving residents an early warning to prepare, and say they will be monitoring the already wet ground across the city.
Christchurch received 44mm of rain in the 24 hours to 2pm on Thursday, and Akaroa received 79mm.
The rain caused the Ōpāwaho/Heathcote and Ōtākaro/Avon rivers to break their banks in sections, causing closures on several roads in low-lying areas for much of the day.
* Residents say river continues to flood properties despite remedial work
* Evacuations, slips and roads closed despite less rain than feared for Canterbury
* Flood protection work pays off as Christchurch homes stay dry
MetService said this July is shaping up to be the wettest one on record for Christchurch, with a severe weather warning for the Garden City likely in the days ahead.
The city has already received 157 millimetres of rainfall this month, more than double the monthly average of 61mm. The wettest July on record is 1977 with 180.8mm of rain and MetService predicts this could be toppled in 2022.
“It is likely this total will be surpassed before the month comes to an end due to this next event starting on Monday,” a MetService spokesperson said.
Christchurch can expect a dry weekend, but from Monday, rain is expected to return throughout Canterbury.
“We expect the rain to be quite persistent for Christchurch from Monday until the end of Wednesday.
“This is likely to be a significant event due to the wet weather you have been experiencing down there over the last month and the basin being saturated.”
Heavy rain has caused flooding around the Heathcote River in Christchurch.
The criteria for a rain warning was for widespread rainfall greater than 50mm in six hours or 100mm in 24 hours.
Council’s hhead of Three Waters, Helen Beaumo said the council would be monitoring the already wet grounds across the city when next week’s rain hits.
“Our advice to residents is to clear your own gutters and ensure your cars are parked well away from areas that are vulnerable to flooding.”
She said the council’s approach would be to mitigate the effects of flooding through the operation of storage basins and stormwater networks.
“For these larger events we will proactively clear critical stormwater grilles, check and excavate beach outfalls, deploy pumps to vulnerable locations across the city, let the general public know what to expect, communicate with Civil Defence and make staff available to monitor the event and supervise the response across the city.”
Beaumont said the basins would reduce peak water levels, but in cases of significant rainfall, rivers could spill out onto streets.
“For moderate events this generally means that we need to close the low-lying roads, often disrupting access to houses and commercial buildings, and many residential properties close to our rivers and streams will see their sections and some garages flooded.”
On Thursday, the flooding caught many people by surprise and some residents were left stuck inside their homes, while several cars were waterlogged, especially those parked next to the Heathcote River.
Asked why residents weren’t warned about the possibility of flooding, Beaumont said the council relied on weather forecasts from MetService just like the general public did.
“Whenever there is a forecast rain event, we issue warnings for people in coastal and low-lying area
“That implies people who live in flood prone areas take responsibility for things like shifting vehicles,” Beaumont said.
“We ask them to prepare in the normal way that they would for the coming event.”
Fire crews were called to a fully submerged vehicle in the Heathcote River just after 9.30am on Friday.
Spreydon fire station officer Cory Stewart said when they arrived no-one knew if a person was inside the vehicle.
Two crew members entered the river to check the vehicle but no-one was found inside.
“We don’t know who it belonged to or how it ended up there,” Stewart said.
The city council has spent tens of millions of dollars in the past six years protecting low-lying areas of the city from floodwaters.
Along the Heathcote, it has dredged the river to increase its capacity, stabilised banks and the council was building four stormwater storage basins in the upper Heathcote catchment at a cost of about $96m.
The basins were designed to store water and slowly release it into the river over time rather than all the water being channelled down the river at the same time. Once completed, these basins would have the capacity to store more than 2 million cubic metres of water.
Two of three storage basins are completed, however one of the completed basins – Curletts Rd – has not been operational since February when the council discovered leaks in the concrete structure that contains the flood gates.