Fish passes in river systems

Design Guidance

The optimal design for a fish pass is likely to be site-specific and dependant on a range of different parameters, including the size of the channel and impoundment structure, geomorphological and hydraulic conditions and the target fish species. Extensive guidance currently exists on the design and application of fish passes. Early guidance included that provided by the former Ministry of Agriculture, Fisheries and Food (Beach, 1984).  More recent guidance is detailed in Clay (1995), Jungwirth et al. (1998) and Larinier et al. (2002).  For Statutory authorities, the Environment Agency has produced their own fish pass manual in 2004 (Armstrong et al., 2004).  The EA also has Management System Guidance Policy 606_06 on instream structures.  In Scotland, the Scottish Government has published guidance for river crossings and migratory fish (Scottish Executive, 2000).

A summary of the key design requirements for fish passes is provided here.

Identifying the problem

The need for a fish pass scheme should be clearly identified at an early stage in discussion with fisheries experts and ecologists. If a fish pass is deemed necessary, the specific requirements should be defined.  Fish passes can be designed to meet a number of different objectives and requirements (e.g. upstream migration, downstream migration, or both). Fish pass design should involve a multidisciplinary approach involving engineers, ecologists, geomorphologists and river managers.

Sufficient upstream and downstream access

For fish passes aimed at supporting upstream migration, the fish pass entrance should be located as close as possible to the upstream limit as fish prefer to move close to the bank of the river. Fish can be led towards the pass with barriers or other instream structures. The upstream exit should be in an area with low flow velocities so that that the fish will not be washed over the spillway, but not in dead zones with recirculation. The flow of water coming from the fish pass entrance should enter a relatively less turbulent area of water so that it is a distinguishable flow that fish can detect. Internal or external attraction water can be introduced to increase performance of the fish pass.

The water depth immediately downstream from the entrance must be a pool of sufficient depth at the foot of the fish pass to allow the fish to rest without any difficulty. Sometimes gravels and other sediment tend to accumulate downstream of long weirs making it difficult for fish to approach. In such cases a self-cleaning approach channel may be necessary.

Water quality conditions can be improved in the fish pass to facilitate fish ascending (increased dissolved oxygen content, increased temperature).

Flow velocity

The maximum velocity of the flow used for designing passes for salmonids can vary between 1-5 m/s, while considering other species 1 m/s is recommended (Szalay, 1967). Large fish such as adult salmon can ascend structures where the water velocity may be over 5m/s, since the maximum swimming speed and endurance of a fish normally increases with increasing length of the fish.

Turbulence

Within a fish pass turbulence should not exceed 200 W/m3 for fish pass for salmon, 100-150 W/m3 for species other than salmon (Mallen-Cooper, 1993). In the case of the pool type pass this can be achieved by constructing larger pools and avoiding rectangular underwater weirs.

Head loss between pools of the pass

The advised maximum head loss between the pools of the pass is 0.5 m (Environment Agency, 2009).

Other Considerations

The fish pass should be designed to ensure a minimum water depth at all times of flow. The minimum water depth may vary according the target species, however a guideline minimum depth of one metre is recommended in the fish pass as many species are reluctant to enter shallow water (Horv�th and Municio, 1998).

It may be necessary to install a fish screen at the impoundment structure; further guidance is provided on this in Mitigation for pumping station intakes.

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