Weir upgrade to cost $1.7m
Opuha Water Ltd will spend almost $1.7 million upgrading a downstream weir.
Chief executive Tony McCormick said they hoped to complete the project by autumn and planned to start work as soon as the lake level lowered.
This would reduce the risk of water spillage from the dam during construction, he said.
‘‘After a lot of time and effort considering the option to build a new separate spillway structure, we have reverted to a previous option which involves modifying the existing concrete spillway and installing some new control gates.’’
A weir is a barrier across a river designed to alter its flow characteristics.
In most cases they take the form of obstructions smaller than most conventional dams, pooling water behind them while also allowing it to flow steadily over their tops.
Weirs are commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help render rivers navigable.
The weir upgrade option is more than $1m cheaper than the replacement, which would have cost up to $3m.
The upgraded structure would not only be more resilient, it would also be a lot safer, he said.
‘‘As well as reducing the frequency of the overflow, and thereby protecting the rest of the structure and embankment, one of the other advantages will be that it reduces the risk we have always attributed to operating the lake at a very high level.’’
The Opuha Lake was sitting at 89 per cent capacity, a statistic McCormick described as sobering.
‘‘We were 92 per cent full this time last year. We intend to operate under a managed regime right through spring.
‘‘What we are trying to make sure is we get through the high demand period and have a reasonable storage.
One hundred per cent full at the start of January, would be a great position to be in.’’