Planning

The individual specifications of a biopassage structure should be refined as part of a collaborative process. It is very important to bring together a design team that includes fish passage biological expertise, as well as engineering, construction and operation expertise, at the very start of the planning and design stage.

Location and siting

When deciding on the location of a biopassage structure at a particular site, it is important to consider the physical characteristics of the site, which might include:

• the nature of the waterway and flows such as ephemeral, intermittent, event-based, or perennial
• topography of the site including bends and flow pathways
• site geology including the presence of bedrock
• river depth and width
• stream discharge
• the size and width of the barrier
• the velocity of water over the barrier
• the operational range (i.e. range of flows over which the fishway will operate) allowing for changes to bed and water levels downstream (i.e. tailwater)
• easy and continuous access[2].

Entrance to structure

The location of the entrance to the fishway is crucial. Fish need to be able to detect flows from the structure to be attracted to the opening. Only then will they have an opportunity to ascend. Key considerations in placing the entrance include:

• locating it at the upstream limit of migration for upstream migrants or downstream limit of migration for downstream migrants
• locating it outside any recirculation eddy or reverse flow area
• locating it on the bank, because fish tend to migrate along banks, and in very wide rivers, a fishway on both banks can be considered
• physical or computational modelling (e.g. to predict the location of recirculation eddies) may assist in determining the location
• removal of hydraulic barriers downstream of the fishway entrance
• accessibility, and a continuous and effective fishway flows (e.g. fish are not attracted to flow over the spillway away from the fishway) under all flow conditions
• auxiliary water can be useful for increasing fish attraction towards the entrance (i.e. attraction flow)
• flows that are appropriate for all sizes of fish and sourced from high-quality surface water[2].

Exit from structure

At the exit (i.e. where fish exit the structure, typically at the upstream end), it is essential to consider the orientation of the exit (with respect to the flow pathways) to ensure that fish are not swept back downstream (or upstream in the case of downstream migrators)[3].

Fish may also benefit from exiting the structure into some form of cover (e.g. large woody debris or rock structures). For example, cover can protect fauna from predators as they enter a large impoundment.

The exit may have a trash rack (also called exit screen) installed to minimise the amount of floating debris entering the structure. Debris can block components of the structure such as the slots, ridges or pools in vertical slots or rock ramps, or chambers in mechanical fishways, thereby altering the hydraulic conditions. The trash rack also needs to be designed and maintained so that it does not impede fish passage, with spacing in the rack allowing for the largest species to physically pass through.[3]

All fishways should have a maintenance and monitoring programs.

Sizing

Key size considerations for fish passage structures include:

• total head loss caused by the barrier (i.e. height of the barrier in comparison to surrounding natural channel)
• slot widths or widths between the rocks or ridges in rock ramps
• the dimensions (depths and widths) of the pools, channels and/or chambers
• drops between pools.

These factors influence the velocity, turbulence and slope. Velocity and turbulence need to be assessed at varying upstream and downstream water levels to ensure the fishway is operating to specification.

Fishways need to allow for fish swimming movements and resting in terms of individual fish size and mass movements. Migrating populations may be composed of small or large fish and may be adults or juveniles, and the fishway must be able to accommodate the size-range and the maximum number/biomass of fish that will need to utilise it on any given day[3]. Mass migrations may consist of billions of fish[1].

Biopassage structures

Most biopassage structures in Queensland are currently designed for fish and include the following types:

References

1. ^ Marsden, T, Berghuis, A & Stuart, I (2017), Fitzroy Barrage Cone Fishway Upgrade and Monitoring Report. [online], Report to the Fitzroy Basin Association, Fisheries Collective. Available at: https://www.researchgate.net/publication/321709683_Fitzroy_Barrage_Cone_Fishway_Upgrade_and_Monitoring_Report.
2. ^ a b O'Connor, J, Stuart, I, Jones, M & Arthur Rylah Institute for Environmental Research (2017), Guidelines for the design, approval and construction of fishways. [online] Available at: http://nla.gov.au/nla.obj-664261559 [Accessed 25 February 2021].
3. ^ a b c O’Connor, J, Stuart, I & Mallen-Cooper, M (2015), 'Performance, operation and maintenance guidelines for fishways and fish passage works', Trove. [online] Available at: https://nla.gov.au/nla.obj-480267623 [Accessed 10 November 2020].

Last updated: 10 May 2021