Meridian is proposing to realign the Waiau River towards the Manapouri lake control structure (MLC).
It aims to excavate 220,000cum to create a new channel that would enable controlled floods to clear the river of didymo — particularly for nuisance periphyton — and flush it out to the sea.
Meridian Energy statutory advocacy manager Andrew Feierabend said didymo effectively suffocated normal organisms and invertebrates that lived in the riverbed.
Meridian had an obligation to provide four or five flushing flows every year for that purpose, he said.
That requirement was put in place in 2012 but because of the nature of the channel at the Waiau arm Meridian was unable to take action.
The excavation would help satisfy a host of Meridian’s other compliance obligations with the flow regime more consistent when Lake Manapouri was at low levels, Mr Feierabend said.
"When you get to low operating range you are relatively constrained in terms of what sort of flows you can put through.
"But this excavation will mean that even with low range we’ll have the ability to put through flows that are useful," Mr Feierabend said.
The excavation would mean Meridian would not have didymo build up behind its gates at the MLC, he said.
No disturbance would be caused to sediment flow to the spit protecting cribs in Bluecliff Beach Rd, a tiny Southland settlement on a seawall facing Te Waewae Bay.
"Our science says sediment is still moving through that system as it would naturally with or without the [Manapouri power] scheme.
"I can say that pretty confidently."
Didymo was not an environmental concern when it was out in the sea because the sea would break it up and destroy it.
"I’ve never heard of didymo being a problem with salt water. In fact, it wouldn’t tolerate salt water because it’s a freshwater organism.
"By the time it gets to the ocean it will basically be cut up and broken up into small pieces and it just begins organic debris."
The $5 million to $7 million project was contingent on obtaining approvals under the Resource Management Act and was expected to be completed in either 2024 or 2025.
Eighty percent of the work creating the channel would be done on dry land so it would be easier to manage sediment during excavation.
