The fish pass at Ardnacrusha is a Borland type vertical fish-lift. The bottom chamber and the vertical shaft are built in an area which was left for a 6th turbine at Ardnacrusha (there are currently 4 turbines installed). The vertical shaft is 4.57m in diameter and 34.44m high. The horizontal chamber at the bottom of the pass is 4.57m in diameter and 14.9m long, and has two openings from the tailrace. Flows from the openings of the lower horizontal chamber vary between 0.2 – 0.5 cubic meters per second (cumecs), depending on tailrace water levels. Fish have to find these flows when up to 400 cumecs are being discharged from the turbines.
Although at least 49,000 salmon should be passing upstream on the River Shannon each year if the river was reaching its “conservation limit”, in reality only a few hundred salmon pass upstream here each year
A particular opening is selected by an automatic control actuated by the water level in the tailrace. When the pass is fishing this chamber is fed with water from the headrace canal, through a pipe terminating in a dispenser at the bottom of the vertical shaft. An open channel leads from the top of the shaft into the headrace. It is 3.04m wide and 4.87m deep with a 1.06m wide walkway on either side. It is approximately 27m long.
The piping consists of a 0.68m diameter pipe from the top channel terminating in two branch pipes. One leads into the vertical shaft and the other terminates in 4 nozzles in the tailrace, which can be used to discharge down the tailrace to attract fish into the pass. A cooling water discharge, which was diverted to the base of the fish-lift in the 1970’s, provides additional attraction. When the pass is fishing, one of the two gates in the tailrace is full open and the over-flow gate in the top of the vertical shaft is closed. The water is supplied at the bottom of the vertical shaft into the bottom chamber at a rate of approximately 0.5 cumecs. As the water level in the shaft rises, any fish already in the bottom chamber are ‘lifted’ until they reach the top, when they can swim out through a flume into the channel and headrace.
The fish-lift operating cycle has a duration of 4 hours and there are six cycles each day (cycle one commencing at 00:00hrs) throughout the year. Each cycle involves approximately 120 minutes fishing time, 40 minutes filling, 70 minutes full and 10 minutes emptying and are typically operated automatically
Great technical details but does it work?
Although at least 49,000 salmon should be passing upstream on the River Shannon each year if the river was reaching its “conservation limit”, in reality only a few hundred salmon pass upstream here each year. Indeed, so embarrassing are the numbers here that the ESB no longer make these counts widely available, although we know that the number of salmon that passed though Ardnacrusha in 2013 was less than 500. It is clear that this fish pass has been a failure, and it is time to start looking at new ways of helping salmon (and other migrants such as eels and lampreys) reach the upper Shannon.