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