Wednesday, May 18, 2022

RDR fish screen first of its kind in NZ

A key piece of water infrastructure for the Ashburton District could lead to a flow of similar designs across NZ that will keep fish in the waterways where they belong.

RDRML chief executive Tony McCormick says it is important to ensure there is no impact on the fish species that reside in the river.  

A key piece of water infrastructure for the Ashburton District could lead to a flow of similar designs across NZ that will keep fish in the waterways where they belong.

The $18 million high-tech screen installed on the side of the Rangitata River just below the Rangitata Diversion Race (RDR) irrigation intake is specifically designed to keep young salmon, trout and native fish from being trapped in the main irrigation diversion race.

Young fish that are swept into the intake will be diverted by the fish screen back into the river where they can continue their life journey. 

The first of its kind in NZ, RDR Management Ltd (RDRML) believe it has the potential to lead the way for fish screens on irrigation schemes around the country.

Set for launch in mid-May, the fish screen and its associated supporting concrete infrastructure and canal construction recently opened for a public viewing before it all goes under water.

RDRML chief executive Tony McCormick says interest exceeded expectation, with close to 1000 visitors turning out to see how it will work.

McCormick says while it was made in Australia, with the main onsite assembly and construction undertaken by Ashburton company Grant Hood Contracting, the overall project has resulted from the successful collaboration of local, national and international companies and individuals.

Engineering specialists were engaged in the design process to ensure the fish screen performs as required by the consent conditions and returns fish safely to the river.

“The RDR diverts a significant amount of water from the Rangitata River, so it is important that all reasonable efforts are made to ensure this does not impact the fish species that reside in the water.  

“Very complex computer modelling was undertaken to ensure the water velocities were low through the screens but high enough along the channel to direct fish back to the river,” McCormick said.

The total project cost is about $18m, breaking down to $5.5m for the fish screen, $9.5m for construction and $3m for engineering design and management costs. 

The main fish screen structure, at 105 metres long and five metres high, looks a bit like a bullet train.

It is designed to pass up to 33.5 cumecs of water, with the fish bypass operating within a flow range of three to five cumecs to enable fish to return safely back to the river.  

The actual screen covers 370 square metres, enough to wrap a one-metre-high barrier around a rugby field.

Once the water has entered the canal it passes through the fish screen, which consists of seven fine mesh covered cylindrical T-screens that rotate to divert young fish away from the irrigation diversion race and channel then back into the river.

Downstream of the T-screens are a series of flat panel screens made of the same 2mm spaced wedge wire, with all screens having control baffles to regulate the flows through the screens and ensure an even flow across the screen area.

“The screens are designed such that the flow velocity through is low to ensure small fish do not get sucked onto the screens, while the sweep velocity downstream remains high to enable fish to continue to swim downstream and back to the river via the fish bypass channel.”

The screen is also an essential part of the consent to take water, which is vital for the region’s economy as well as the ecosystem of the river.

The consent specified key design aspects of the facility, including having it located as close as practical to the Rangitata River.

The fish screen has been built in a new section of race excavated in an area of scrubland about 1600m downstream of the main RDR intake.

The RDR, referred to as the lifeblood of Mid Canterbury, is a 67-kilometre-long canal scheme that has been in operation since 1945, diverting water from the Rangitata River for multiple uses.

It delivers water to three community irrigation schemes covering 100,000 hectares of highly productive farmland, two hydroelectric power stations and supplies stock water races.

Over time technologies have continuously been improved and environmental aspects investigated with the fish screen, while no easy feat, one of the biggest projects set to meet environmental regulations well into the future.

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