Screw jacks give Rother Salmon a helping hand

The river Rother in Sussex, like many other rivers, has its natural flow interrupted by man-made constructions.

However, fish such as salmon need to get past these structures to reach their spawning grounds upstream.

For this reason, salmon ladders are used to allow salmon to pass over or round these obstacles.

On the river Rother at Midhurst the salmon ladder gives extra help to the salmon by using screw jacks to alter water flow over the ladder.

Each year between April and October, thousands of salmon fight their way upstream from Atlantic feeding grounds to spawn in the upper reaches of rivers.

When they encounter an obstacle such as a dam, a salmon ladder provides access.

The ladder can take various construction forms, such as a 'weir-pool' design, however all allow the salmon up the ladder while gravel and river sediment pass down the ladder.

In front of the ladder there is a concrete box, which allows the salmon to rest prior to attempting their ascent of the ladder.

The velocity of the water over the ladder has to be great enough to attract the salmon to the ladder, but not so great that it washes the salmon back downstream or causes exhaustion to the point that they cannot continue their journey upstream.

The ladder at Midhurst on the river Rother differs from most as it uses a screw-jack system to raise and lower the complete salmon ladder so that the water velocity can be adjusted for river conditions.

This way the salmon are given the best water flow-rate for swimming up the ladder.

The jacking system has operated successfully for a number of years and uses two screw jacks to lift the ladder.

The 50kN E-Series rotating screw jacks were selected with stainless-steel lead screws.

A drive shaft mechanically links the jacks' worm shaft so they can be driven synchronously by one electric motor.

The ends of travel for the screw jack are signalled by an electro-mechanical limit switch in each direction.

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