Stormwater infrastructure under investigation

Flooding on Aubrey Road at the entrance to Northlake 22/5/18 Photo: Nikki Heath

The heavy rain event in Wanaka on Tuesday May 22 has once again highlighted the issue of surface flooding and pollution of creeks and lakes from construction runoff.

Last week the Wanaka Sun reported on the dirty water polluting Lake Wanaka in Bremner Bay, and this week it has come to light that sediment has entered Bullock Creek for the seventh time.

There was also significant surface flooding from new developments onto Aubrey Road and into Hikuwai reserve, while the entrance to Three Parks was also under water.

Queenstown Lakes District Council (QLDC) said it would be investigating the surface flooding and Mayor Jim Boult said the event was on the radar for him and fellow councillors.

“It was a reasonably extreme event following heavy rain in the days leading up to it, but the resulting level of surface flooding in some areas leaves some questions that we need to be answered,” he said.

QLDC chief engineer Ulrich Glasner at the peak of the downpour the hourly rainfall rate was 5.2mm.

“This was a very heavy event on top of already saturated ground. On the face of it, in this event the performance of some of the stormwater infrastructure did not meet our expectations, particularly in some new developments,” Ullrich said.

He said developers were required to implement stormwater management measures to QLDC’s  standards, but that some localised erosion and sedimentation took place across the district.

QLDC’s Monitoring & Enforcement team was investigating and developers had been contacted and were working with QLDC to implement appropriate controls.

“Should our investigations highlight the need for improvements to the way council processes new developments in the future then these will certainly be addressed,” Ullrich said.

Alpha Series subdivision stormwater retention pond. Photo Nikki Heath

Fish & Game (F&G) was disappointed with the inadequate action taken by QLDC to address the repeated flooding of Bullock Creek from the neighbouring subdivision.

Water flowing out of the Alpha Series subdivision retention pond and into Bullock Creek 22/5/18 (Photo: Supplied)

F&G officer Paul Van Klink said, “We again alerted the QLDC to the site when the stormwater system failed. It is clearly inadequate. This latest event could not have come at a worse time. Bullock Creek is a major spawning tributary of Lake Wanaka and it is the peak of the brown trout spawning season. Silt can smother the eggs.”

Paul said it was a disappointment for F&G, and the wider community, that the issue remained unresolved and said they had exercised “considerable patience” with the developer and the QLDC.

“We are still awaiting confirmation from the QLDC on what action is to be taken against the developer to stop what is happening or if they are proposing to prosecute for repeat offending,” he said.

“The community of Wanaka should not be putting up with this and the environment deserves more protection.”

When asked to comment on the impact of sedimentation on Lake Wanaka, Otago Regional Council director of engineering, hazards and science, Dr Gavin Palmer, said the ORC doesn’t take measurements of the lakebed profile

“However the process of sedimentation is natural and it appears the changes we have seen are as normally expected. The water level in Lake Wanaka can fluctuate by several metres depending on inflows from the rivers with much of the sediment likely coming from the large rivers feeding the lake.”

Touchstone Project member Eddie Spearing said the increased amount of sediment in the stormwater at Bremner Bay last week was the result of more developments in the catchment.

“When Wanaka was a sleepy little hamlet, stormwater and the associated levels of sediment from this catchment was fine,” Eddie said.

“The Touchstone Project is in the early throws of a new stormwater project, titled 'Down the Drain' for which we have very recently received funding and together with schools and community and council, to create stormwater ponds which will trap sediment with plantings and clever design.”

Bremner Bay is the first stormwater drain Touchstone will look at but the group also intends to work on other drains, in particular the more active stormwater drains.

“Part of the project is not only mitigating the effects of storm water 'after the fact' but also awareness of what goes down stormwater drains ends up in the lake. That's from all of us, from homeowners to builders to developers,” Eddie said.

QLDC advised as we went to press that results of tests taken from the Bremner Bay stormwater drain last week were not yet to hand.

Cover image: Flooding on Aubrey Road at the entrance to Northlake 22/5/18 Photo: Nikki Heath


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