Fish Screen
In May 2022, a new $18m fish exclusion facility was commissioned approximately 1.6km below the main Rangitata River intake. This state-of-the-art facility ensures any fish that enter the scheme through the intake are not lost down the RDR canal, but instead are diverted safely back to the river.
Testing the Fishscreen
The resource consents issued by Environment Canterbury to RDRML, in relation to the new fishscreen facility, have conditions that specify a number of design aspects of the screen (e.g. 2mm wedgewire for the screens) as well as requirements for testing the facility once it is commissioned. There are two key areas of testing – the water velocities around the screens must be within certain limits and we also need to test and prove that the fish are not harmed before being returned to the river. Both of these test criteria are challenging to measure, but they have been successfully completed. The velocity testing was only necessary once to prove the design, but the fish health testing will be conducted over three to four years at least.
Velocity Testing
The design of the fishscreen facility requires that the water flows through the screens at a slow rate, to avoid small fish being ‘sucked’ onto the screens. The flow along or past the screen is required to be high enough to encourage the fish to be swept along in the current. These two parameters are called the Approach Velocity and the Sweep Velocity. The consent conditions require that the average Approach Velocity is not more than 0.12 m/s, and the Sweep Velocity is at least twice the Approach Velocity and, ideally, “in the order of 4 to 5 times the Approach Velocity”. The adjacent photos show the method used to measure the Approach and Sweep Velocities at a point 76mm from the screen face. To measure this, acoustic doppler velocimeters (ADVs) were attached to the cylindrical screens at four locations along each screen, and the screens were then rotated to measure eight points around the circumference. The testing confirmed that the average Approach and Sweep Velocities across the whole screen area meet the consent conditions. The average Sweep Velocity was well over five times the average Approach Velocity, which was less than 0.10 m/s.
Fish Health Testing
To confirm that the fish were being returned safely to the river and were not being injured by any interaction with the fish screen facility, a method was required to be able to carefully capture the live fish after the fish screen so they could be physically examined.
A collaborative effort by staff from RDRML and Fish & Game, along with assistance from local anglers involved in the McKinnons Creek Salmon Hatchery, resulted in an innovative and impressive facility being constructed in the bypass channel that carries the fish from the main screen facility back to the Rangitata River. The fish capture facility has flat panel wedgewire screens (the same material used in the main screens) to ‘filter’ the small fish from the water flow and divert them into a holding basket that sits off to the side of the bypass channel.
A fish capture session is normally run overnight with the catch basket checked every hour and the screens cleaned to avoid the build-up of aquatic weeds and debris. The captured fish are then inspected under a microscope for any injury and any evident scale loss.
The testing results have been very satisfying, with the extent of scale loss (if any) being generally indistinguishable from what would be expected to be found on any wild salmon in the river.
As well as capturing small wild salmon for testing, the facility has proven to be a very effective method of capturing fresh broodstock for the salmon hatchery.
Fish Screen Project Information Signs
