If the sluices and locks contain the most massive concrete on the project, one of the most complex mechanisms is in the £7M fish pass. Specialist Fishway Engineering came in to work on the design of the fishway gate and Dennil pass, which follows Environment Agency specifications.
An 8m wide structure, this begins with a ladder of pools from the low tide point to the top. Fish accumulate in a start pool area entrapped by a weir around the stilling basin of the fifth sluice gate. At the ladder top is a gated entrance into the lake beyond with a specially shaped Dennil pass welded on to on the back of it.
When the gate is open, water flows in this pass which comprises a relatively narrow steel channel with fins on the inside to slow down the water flow, which is vital to allow fish to swim upstream.
But to attract fish to the entrance they must 'smell' the sweeter water so a continuous flow is essential. Complementing the 10m3/s ladder flow is a further 5m3/s capacity pipe and culvert outlet system.
Fresh water flows are maintained near the level of the sea by an unfolding multi-leaf gate at the ladder base, which deploys upwards as the tide rises to prevent salt water ingress. The ladder gradually floods with fresh water.
At highest spring tides the top of the ladder is overtopped. The gate is then sealed, but this prevents fish passage. So an alternative system is provided using a pump to maintain a 3m3/s fresh water flow against the high tide head. The pump entrance is screened and another entrance to the lake is provided leading to a small Dennil pass on one side which is supplied with a downstream of turbulent pumped fresh water. The fish bypass the main fish gate by swimming up this to a down pipe on the bay side like a water theme park ride, sliding them into the lake.
It is hoped the pass will allow passage for hundreds of salmon and especially trout which could re-colonise the newly clean Taff and Ely rivers. 'But,' added one engineer 'they will have to be bloody intelligent if they are to understand it.'