Flushing is effective in managing the sediment in Lake Roxburgh and two-yearly lakebed monitoring is an ''unnecessary cost'' given the slow and gradual changes to the lakebed, Contact Energy says.
Contact has applied to the Otago Regional Council to vary the monitoring condition in the suite of resource consents it gained in 2007 to continue operating the Clyde, Roxburgh and Hawea hydro dams.
The matter was discussed by the Central Otago District Council yesterday as the district council was one of seven parties who submitted to the consent hearing.
Contact has applied to reduce the frequency of monitoring the Lake Roxburgh lakebed from every two years to every five years. The change was based on studies that showed the erosion of sediment from the lakebed was expected to gradual and the level of sediment into the lake was low.
Flushing took place when Contact lowered the level of Lake Roxburgh during high flows to increase the velocity of the lake and enhance the erosion of the lakebed. That meant sediment was scoured from the upper reaches of the lake into the lower reaches or downstream of the dam, with the aim of decreasing the risk of flooding at Alexandra.
There would be no change to that process, which would continue at regular intervals when flows were at a suitable level, as required under the resource consent conditions.
To vary the lakebed monitoring condition, the regional council said Contact had to gain the written approval of the parties involved in the original hearing.
Contact's hydro operations environmental adviser, Daniel Druce, told district councillors yesterday if the company was unable to gain approval from all parties, the application would become a notified consent.
He said the two-yearly survey of the lakebed was the same as taking photos of a landslide site, monitoring a slow and gradual activity.
Contact's application said monitoring every two years was required in the resource consent, as at the time of the hearing there was some uncertainty surrounding the effectiveness of flushing. However, studies completed since 2007 confirmed flushing would remain effective for the next 20-50 years.
As well, the Clyde dam would continue to ''intercept'' sediment from the upper Clutha catchment and Contact would continue to extract gravel from the Manuherikia River, preventing it from entering Lake Roxburgh.
Four consulting firms involved in the Lake Roxburgh assessments agreed five-yearly monitoring would be appropriate, Contact said.
The last lakebed survey was carried out in 2011 and the survey and subsequent sediment and backwater analysis cost about $80,000.
Because of the advice from the consulting firms, Contact thought this cost, every two years, was ''unnecessary''.
It offered to add a new proposed condition - to prepare an annual report comparing the water level of the Clutha River at Alexandra to the Lake Roxburgh inflows (on inflows greater than 850cumecs) and an interpretation of the data and any change in the flood hazard at Alexandra.
One of the positive effects of the proposed condition would be to provide a more frequent and regular assessment of any change in the flood hazard, it said.
The district council agreed to support Contact's application. Deputy mayor Neil Gillespie, who works for Contact, did not take part in the vote.
