Fish passage systems and fishways

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.

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.

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.