$17m screen protects Rangitata fish stocks

Signing the fish screen installation contract (from left) RDRML chair Richard Wilson, director Neil Brown, Grant and Carolyn Hood, of Grant Hood Contracting, and project manager James Greene.

A giant state-of-the-art fish screen is soon to be installed near the Rangitata River, as part of a $17.2 million project to protect salmon, trout and native fish stocks in the river.

The fish screen has been commissioned by Mid Canterbury’s largest water supply company and the mechanical screens are being built in stages in Australia. It will be constructed by Ashburton company Grant Hood Contracting.

It is likely the largest and most complex of its kind in the world.

Rangitata Diversion Race Management Ltd has consent to take water from the river for irrigation, stockwater and hydrogeneration and distributes it via a 67km canal that runs across the district to the Rakaia River. It is a legacy piece of engineering, built 80 years ago to stimulate agriculture in Depression times, and remains vital to the district’s economy.

Components of the giant fish screen under construction in Australia. PHOTO SUPPLIED

RDRML chief executive Tony McCormick said conditions of the water-take required the company to prevent young fish from entering the irrigation canal.

A bio-acoustic fish fence previously installed was not working as hoped and will be decommissioned as soon as the new mechanical fish screen is up and running by May next year.

Grant Hood Contracting will be on site this March, undertaking earthmoving, then back again in spring to complete construction and begin installing the seven 8.5m long cylindrical screens, each 2.1m in diameter. They are each covered in stainless steel mesh with 2mm gaps and fitted with electrical motors that turn the drums to clean off sticks and other debris. It has been scientifically designed to allow water through but ensure young salmon and other fish are swept past the screen to an offtake that runs back into the river.

Water from the RDR’s Klondyke intake runs along a headrace canal to the fish screen. RDR has consent to take up to 35 cumecs (enough water to fill 50 big swimming pools in an hour), but is subject to restrictions when the Rangitata drops below 60 cumecs.

Water not used for irrigation is used by Trustpower to generate electricity at Montalto and Highbank power stations. Part of the consent is also to provide stockwater for the Ashburton District Council; water for the managed aquifer recharge network is also conveyed in the RDR.

Mr McCormick said fish needed to be safely returned to the river so some will be caught and checked. Wild salmonoids currently caught in the bio-acoustic fish fence were retained and used by the McKinnons salmon hatchery to boost the vigour of its hatchery stock.

The new mechanical rotary fish screen will lead the way for future fish screens all over the country and follows five years of intensive research, study and design.

One of the seven cylindrical screens, each 2.1m in diameter and 8.5m long, that will make up the mechanical fish screen at the Rangitata River. PHOTO SUPPLIED

The first screen arrived by ship from Australia recently and is being stored in Christchurch by Smith Crane and Construction, who are sub-contracted to assist with the construction. Victorian manufacturer AWMA will send the other six as they are completed, with the last one due in June.

Mr McCormick said the new fish screen will be built beside the existing canal, which would be shut for two weeks next year to allow the new hook up. Total construction time was estimated at eight months.

He is keen to show the scale of the work and have one of the fish screens on display in Ashburton, before it is fully assembled on site.

Grant Hood said it was one of the Ashburton company’s biggest projects to date and would involve managing groundworks and complex installation with sub-contractors. “It certainly is up there as one of our biggest and most technical.”

James Greene will be the construction project manager.