Bypass channels

Description of measure

A bypass channel provides opportunity to wholly circumvent the barrier to fish migration and should aim to resemble, in form and function, a side channel or natural tributary of the main river system.  For other types of fish pass please see Fish passes in river systems.


Bypass channels are particularly well suited to small scale barriers such as low head weirs (<2 m), where the height difference between up and downstream does not lead to an overly long bypass section. Typical by-pass gradients range from 1 to 5 per cent making them particularly suitable for the passage of species that are difficult to accommodate with other types of fish pass (such as juvenile and smaller coarse fish). The gradient of the channel may be increased to nearer 5 per cent if energy-dissipating features are built into the channel.  Various type of baffle may be also used, for example in culverts.

The aquatic ecosystem should be carefully evaluated to assure that bypass channels do not adversely impact other aquatic biota and stream corridor functions.  It is advisable to consult an ecologist or fisheries scientist when considering bypass channel application. Bypass channels can improve connectivity across in-stream barriers, effectively mitigating morphological impacts associated with this type of habitat fragmentation. Land acquisition adjacent to the barrier is a consideration.

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 bypass channels can help deliver objectives of the Water Framework Directive, by:

  • Ensuring that fish can move freely between different parts of the river system or between the river and coastal waters in order to access breeding, nursery of feeding grounds.
  • Ensuring the passage of other mobile aquatic species, such as invertebrates and plankton.
  • Providing additional habitat. A bypass channel can replace a portion of flowing water habitat that has been lost due to the impoundment. The restorative function however is limited to those species (plant, invertebrate, fish, and mammal) that use these channels as natural habitat. Where there is a deficit in flowing water habitat, the restorative function of the channel can be widespread. Effective migration can generally be provided for all fish and invertebrate species in quantities that ensure the maintenance of all populations.

Bypass channel can also provide a number of other benefits. For example, fish passage is also an important factor supporting the other designations, such as river Sites of Special Scientific Interest. Unlike other types of fish pass, a bypass channel is unlikely to become blocked with debris, and requires little maintenance. A bypass channel can also offer benefits in terms of visual amenity if incorporated with planting and landscaping.

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