Flushed with

Re­lease of a large flow re­cently from the Opuha Dam, Macken­zie District, demon­strat­ed­how didymo and al­gae in­fes­ta­tions could be washed away.

The Press - Escape - - ESCAPE - Peter Shutt

Flush­ing to re­duce al­gal in­fes­ta­tions in a bid to­wards a good angling fu­ture was demon­strated sci­en­tif­i­cally by Opuha Water Ltd a few weeks ago.

Tony McCormick, the com­pany’s chief ex­ec­u­tive, said the op­por­tu­nity to es­tab­lish a base­line anal­y­sis of the Opuha Dam’s abil­ity to pro­vide a flush­ing flow has been crit­i­cal to know­ing what might be achiev­able in the fu­ture.

Opuha Water con­tracted Niwa sci­en­tists to con­duct flow ex­per­i­ments and ob­ser­va­tions of a test project that ‘‘saw us re­lease our largest-ever flow through the present weir spill­way and con­trol gate’’, he said.

‘‘Although it peaked at just 32 cumecs in­stead of the 40 cumecs we had planned, it still pro­vided a sig­nif­i­cantly stronger flush­ing flow be­cause we were able to sus­tain this flow for an ex­tended pe­riod.’’

It moved didymo and phormid­ium from the main­stream chan­nel, but side­braids prob­a­bly need some­thing sig­nif­i­cantly higher to mo­bilise the al­gae and carry it out to sea.

That can hap­pen only if the dam ex­pe­ri­ences flood in­flows that can be di­rected down­stream, and that’s a weather-de­pen­dent sit­u­a­tion that may hap­pen only ev­ery five years or so.

‘‘But the test was mod­er­ately suc­cess­ful.

‘‘We did es­tab­lish a lot of base­line in­for­ma­tion from which to go for­ward and we know we will be able to pro­duce higher flows than we achieved in our test.’’

An­glers will quickly ap­pre­ci­ate that the re­cent flush flow was a costly ex­er­cise for the com­pany. McCormick said it was close to $30,000 of Niwa prepa­ra­tion and staff time. But that’s only the be­gin­ning.

‘‘We pro­pose to es­tab­lish a new con­crete spill­way fea­tur­ing ra­dial gates on the true right bank within the em­bank­ment ad­ja­cent to the present weir struc­ture that will also pro­vide for a small hy­dro devel­op­ment,’’ McCormick said.

This project is be­ing dis­cussed with En­vi­ron­ment Can­ter­bury re­gard­ing the con­sent process, and if ap­proved, Opuha Water will be mak­ing a $1.7 mil­lion at­tempt to in­crease the abil­ity to pass higher flood flows and help im­prove con­di­tions for all river users.

‘‘The hy­dro in­stal­la­tion would be ad­di­tional to this cost.

‘‘Your read­ers will know that our Sparge sys­tem of water aer­a­tion in the lake en­sures water de­liv­ered down­stream through the ex­ist­ing tur­bine main­tains oxy­gena­tion con­trol of the river water as re­quired by Ecan, and we be­lieve the in­stal­la­tion of two 7.5 me­tre-wide ra­dial gates in the new em­bank­ment struc­ture will pro­vide the abil­ity to spill as much as per­haps 130 cumecs in a con­trolled man­ner to as­sist the re­duc­tion of al­gal blooms.’’

It may not be per­fect, but it’s Opuha Water’s per­sonal ef­fort to help the en­vi­ron­ment.

McCormick says that sup­port from Cen­tral South Is­land Fish and Game, Ecan, and the Opihi Catch­ment and En­vi­ron­ment Pro­tec­tion So­ci­ety has been en­cour­ag­ing, with dis­cus­sions fo­cused on so­lu­tions.

Al­gal blooms and didymo in­fes­ta­tion was not unique to the Opuha River, there be­ing many such is­sues through­out the South Is­land in both small and

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