Lake Forsyth during heavy rain in Canterbury on March 1, 2025. Photo: Jan Daffin / Supplied
Christchurch City Council says it will be reviewing the decision not to open Wairewa/Lake Forsyth sooner, amid heavy rain last week.
The level of Lake Forsyth has dropped since it was opened to the sea on Friday, while the nearby Te Waihora/Lake Ellesmere level remains high, despite three openings and the high tide twice forcing its closure.
Homes and businesses in Little River were swamped by floodwater late last week after more than 200mm of rain fell on the peninsula.
Residents have been critical of the decision, saying if the Lake Forsyth was opened earlier, flooding could have been mitigated.
Seven homes in the Selwyn District have been yellow-stickered since the flooding.
Christchurch City Council general manager of city infrastructure Brent Smith said the total rainfall was significantly higher than forecast, the Wairewa Lake Forsyth didn't reach the minimum required level for opening until Wednesday.
He said the conditions remained unsuitable for opening until Friday.
The council holds a joint resource consent with Wairewa Rūnanga for the opening the lake.
In order for it to be opened due to flood risk, Wairewa/Lake Forsyth needs to be 2.7m above mean sea level, or there needs to be rainfall in a storm event predicted to raise the lake to that level.
It can also be opened when there is an inundation risk, between September and April, when the lake level reaches 2.3m above mean sea level.
Photo: Jan Daffin / Supplied
Smith said last Monday, the lake level was just under 2.6 metres and with 100mm of rain forecast it would have reached the required height, but wave and swell conditions weren't favourable for opening, and attempt to do so was likely to have failed.
The forecast had changed by Wednesday to indicate a significant increase to 200mm of rain, but conditions still weren't suitable for opening and posed a safety risk for staff.
The conditions for a successful and safe opening came on Friday, and the lake was opened in the afternoon.
Since then, the lake level had dropped significantly was expected to continue to drain for a number of days.
Smith said the level of the lake did not usually have a direct impact on flooding in Little River. At a level of 4m, there was still downhill fall, but it did mean that water drained away more slowly.
"If conditions had allowed us to open the lake earlier, it may have resulted in a temporary improvement on drainage, but given the existing stormwater network through the township and volume of rain it is likely it still would have seen flooding impacts on the community."
He said the council would review its response and work with the community to look into factors that contribute to flooding in Little River, which included examining the lake levels.
Selwyn mayor Sam Broughton said the seven yellow-stickered homes in the district were among 48 that had been assessed by the council, mostly in Doyleston, Leeston and Southbridge and others in isolated locations across the Springs and Ellesmere wards.
"People are still certainly feeling the effects of last week's emergency... There was a lot of property damage across farms and fields, around people's homes."
He said despite several days of fine weather, there was still a lot of ponding and surface water the district and the full extent of flood damage was not yet clear.
A recovery manager had been appointed to co-ordinate the council response with other agencies and with mana whenua. He said property and welfare support were available to anyone affected by flooding.
Broughton said the level of Te Waihora/Lake Ellesmere meant a lot of water remained around properties in the area, and they were waiting to see that drain away.
The decision to open the lake is jointly managed by the council and Ngāi Tahu and governed by a National Water Conservation Order and a suite of resource consents.
A joint decision was made on 29 April to open the lake when conditions were suitable. It reached the required level for an opening the same day, at 1.13m, but the council said it was not physically possible to carry out due to sea conditions.
Te Waihora/Lake Ellesmere was first opened on Friday after the heavy rain, then again over the weekend and it closed both times.
It was opened for a third time on Monday and had remained open for more than 24 hours, scouring out from 15m to 65m overnight.
The regional council said it hoped the opening would widen further over the next 24 hours, enabling more outflow. Sea conditions were currently favourable and machinery remained onsite to help the process along.
Broughton said on Monday afternoon, the Te Waihora/Lake Ellesmere level was still above 1.5 metres.
"Obviously, that's causing a lot of the water to still sit around people's properties and we want to see that drained away... We can only do so much with bulldozers and diggers, in the end, we need the force of the water to open that mouth out into the ocean and that will then see waters recede far more quickly."
He said it would take some time for enough water to move out to sea and the lake levels drop.
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