Fish passage systems and fishways

For upstream migration, Hydrostadium designs fishways that enable migratory fish to overcome obstacles by attracting them downstream of the facility and encouraging them to move upstream.

Three factors determine the functionality of a fishway:

  • Attractiveness : the fishway entrance must be easily located by the fish,
  • Hydrodynamic conditions : the upstream migration facility must be adapted to the swimming capabilities and behaviour of the target species (velocity, drop height, flow power),
  • Fishway maintenance : the fishway must remain functional throughout the migration period.

Numerous parameters must be considered when designing an upstream migration. There are therefore several types of fishways, depending on the height of the obstacle, the space available and the target species.

Pool-type fishways

Pool-type fishways

The pool-type fishway is the most commonly used type. The principle consists of dividing the total head difference into a series of small drops (generally 15 to 30 cm), creating a “staircase” compatible with the swimming capacity of fish and enabling upstream migration.

It consists of a series of pools extending from the toe of the obstacle to the upstream side. The partitions separating the pools are fitted with weirs, orifices or vertical slots through which the water flows and which control the water level in each pool. The pools dissipate the energy of the drops while providing resting areas for the fish.

Pre-barrages

Pre-barrages

Pre-barrages consist of several walls or weirs creating, downstream of the obstacle, large pools that divide the height of the drop to be negotiated. On small watercourses, they can be installed across the entire width of the passage.

The advantage of pre-barrages lies in their attractiveness, since a large proportion of the river’s flow can pass through them. This attractiveness sometimes comes at the expense of fish comfort, as the drops are generally higher than in successive pool fish passes (generally 0.3 to 0.5 m), in order to limit the number of pre-barrages required.

Baffle fishways

Baffle fishways

In these fishways, baffles are arranged on the bottom of a straight channel with a relatively steep slope and a rectangular cross-section, in order to reduce the average water velocity during upstream migration.

Due to the highly turbulent flow, these fishways are intended for large fish species. They can also be designed for mixed use, allowing canoes or kayaks to pass through.

Natural fish passes

Natural fish passes

Natural fish passes recreate a more or less wide channel in which energy is dissipated and velocities reduced through the roughness of the bed and banks, or by a succession of more or less widely spaced rock armour, recreating a natural watercourse.

Two main types of natural devices are distinguished:

  • Bypass rivers : low-gradient channels positioned along the bank, meandering and recalling the course of a natural river. Suitable for sites with sufficient available space,
  • Ramps or weirs : generally installed on part of the obstacle or against a bank, with a straight alignment for ramps, or spanning the full width for weirs. The roughness elements can be arranged in three ways: large rocks distributed at regular intervals, rocks arranged in rows to create pseudo-pools, or adjacent rocks forming a continuous passage.
Fish lifts

Fish lifts

Unlike other devices, fish lifts enable the mechanical passage of fish. They are used at obstacles where available space is limited and/or the head drop is significant (> 8–10 m).

Eel-specific fish passes

Eel-specific fish passes

Eels do not have the same swimming and passage capabilities as other species. For adult eels, it is sometimes possible to adapt multi-species fish passes, but for young individuals (glass eels and elvers), specific structures based on their crawling ability are preferred.

These specialised devices take the form of wet ramps fitted with various crawling supports:

  • Brush type : mats covered with bundles of bristles,
  • Stud type : slabs with more or less truncated-conical studs.

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