Fish passes at tidal outfalls

Description of measure

Easing fish passage at tidal outfalls relates mainly to those species migrating regularly between the sea and freshwater or visa versa as part of their life cycle. Species include salmonids, eels, lamprey, and shads. For many species, modifications will need to ensure both adults and juveniles can pass structures.

There are many types of tidal gate used including flap gates, swing gates, slide gates, self regulating tidal gates (SRT) and pinch valves. They work by a positive head differential against the downstream side of the plate face which forces the plate against the rim of the culvert to seal it. A positive head differential against the upstream side of the gate will force it open to release water.

Refer to the measures Bypass channels and Mitigation for pumping station intakes for additional techniques for fish passes.

Implementation of the measure

Before attempting to improve a situation where fish access is poor, consideration should be given to the wider ramifications of permitting tidal incursion and inundation. This is because it may not be environmentally or socially acceptable for reasons not associated with fish migration. Presuming that it is acceptable then potentially there are a number of possible solutions, or at least improvements that can be made to enhance fish passage.

Benefits for Water Framework Directive

The free passage of migratory fish is a key requirement of the Water Framework Directive, and is being used as an indicator for assessing whether water bodies are meeting Good Ecological Potential or Status. Initial assessments suggest that many waters throughout the UK are at risk of failing to achieve Good Ecological Potential as a result of barriers to fish migration. Well designed tidal outfalls can help ensure that fish can move freely between the river and coastal waters in order to access breeding, nursery of feeding grounds. Tidal outfalls can also offer benefits to the passage of other mobile aquatic species, such as invertebrates and plankton.

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