Types of fish pass

Pool and weir passes

These passes consist of a series of pools, which divide the large fall of water at the structure into several smaller falls. These pools perform the dual function of dissipating the energy of the falling water and providing resting areas for ascending fish. Typically the gradients achievable with these passes are of the order 10-15 per cent.

 

 

 

Baffled passes

Fish passes of this type generally use a sloping rectangular channel with a series of precisely positioned and shaped plates or 'baffles'. These baffles redirect the water flowing down the fish pass channel, thus dissipating power and reducing the average water velocity dramatically. These fish passes can operate at gradients of 15 or even 25 per cent.

There are two principal types, those with a relatively narrow and deep channel and those with a wide shallower channel. The former, known as denils, use side and bottom baffles to reduce water velocity. They generally use limited amounts of water. Super-active bottom baffle only passes onthe other hand can consist of many justaysed units being very wide and carrying high proportions of river flow.

 

 

Fish locks and lifts

Sometimes the size of the obstruction excludes the use of the previous types of fish pass. In particular, hydro-power dams can form obstructions many tens of metres in height. At gradients of 10-25% the size of the fish pass can become very large and therefore make it difficult to place the entrance of the fish pass close to the natural gatherings of ascending fish.

To overcome these problems, active methods of passing fish have been developed. Fish are drawn into a collection area by an attraction flow and then periodically a downstream gate or sluice is closed to allow the water in the chamber to fill to the upstream level.

Once the upstream level is achieved the fish may swim out or, in the case of a lock, the upstream gate is opened. After a short time the lower gate is opened and the process is repeated. These fish passes have been successfully used for a number of years throughout the world.

Pre-barrages

In many cases, fish passage at small obstructions, in terms of the vertical height which has to be traversed, can be helped by provision of a small weir or weirs downstream of the main obstruction. These have the effect of splitting the distance to be traversed into smaller leaps or traverses.

Such weirs can often be made of local materials and can look much more natural than some of the 'technical' civil engineering structures.

Natural type by-passes, rock ramps and easements

Some of the most efficient fish passes have been found to be man-made substitutes for river channels. Such artificial river channels normally have a low gradient and extend from below the obstruction to a considerable distance upstream.

Typical by-pass gradients range from 1 to 3 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 characteristics are built into the channel, these include rock ramps in various forms. In other cases various type of baffle may be used for example in culverts.

In summary, if an in-river structure is being substantially modified or rebuilt, or a new structure is built on a river frequented by migratory fish, then the local area office of the Environment Agency should be contacted.

There are many modern solutions to fish passage problems but expert advice should be sought as simple mistakes at the design stage can incur substantial costs to rectify at a later stage.