A new power scheme was "likely to significantly increase" didymo on the lower Waitaki River between the Waitaki dam and Stonewall, something which could not be mitigated, Dunedin environmental consultant Bill Chisholm told the Environment Court yesterday.
Mitigation measures proposed by Meridian Energy Ltd for its proposed north bank tunnel concept power scheme could even exacerbate the problem, he said.
Mr Chisholm was subpoenaed by the Lower Waitaki River Management Society, which is opposing resource consents for the new scheme.
More didymo on the lower Waitaki River would cause significant adverse effects on a wide range of environmental and amenity values, he said.
"Didymo is not only invasive but also pervasive - it adversely affects every value in the Waitaki River."
The lower flows in the river between the dam and Stonewall as a result of the scheme would increase habitat suitable for didymo, allowing it to occupy a greater proportion of the river.
Meridian proposed up to seven flushing flows a year of 450cumecs each to mitigate the effects of enhanced didymo growth, but Mr Chisholm said it was unlikely they would remove significant amounts.
Flows had to be sufficient to move the bed material.
Flows that sheared (broke off) didymo would not remove enough.
There was also the potential for adverse effects, with didymo "slugs" shifting downstream, washing into backwaters, clogging irrigation intakes and making the river unpleasant for some time.
"It could be the cure is worse than the disease," he said.
Meridian also proposed a flood flow of 900cumecs or more, when in-flows into the upper catchment provided the water.
These were estimated to occur every 1.9 years on average. That flow would remove significant amounts of didymo but could not be guaranteed to occur every two years.
It could be five years or longer between such events, Mr Chisholm said.
Because didymo grew very fast, the flood flow would provide only limited respite.
Mr Chisholm said there were no control agents (such as chemicals) at present which could be used against didymo in a river the size of the Waitaki.
