Developing effective fishways: Advances in fish passage modelling

Home Insights EnvironmentDeveloping effective fishways: Advances in fish p...

For centuries, human-kind has constructed dams and weirs on rivers to control flooding, provide irrigation, storage and hydropower, with little account for fish passage. The design of these structures has a major impact on the abundance and diversity of fish because movement along waterways is essential for the survival and productivity of many species. Designing fish passage structures, or fishways, to enable all fish to pass through dams and weirs is a complex exercise. A modelling approach can be beneficial to help ensure a variety of species can handle the water velocities and turbulence levels in passage structures.

Of the world’s large rivers, only a third remain free flowing, and this is expected to continue to decline as human population and energy need increases. Since European settlement, native fish populations in Australia have been heavily impacted by these man-made structures. Whilst consideration and design of fishway engineering projects has improved in recent years, a large legacy of older structures still exists across the world which continue to hamper fish migration.

Fish survival is dependent on successful and safe movement in order to facilitate a number of biological imperatives including:

Local and daily movement – to access food, avoid predators, shelter and habitats
Seasonal migration – breeding cycle in response to rising water level or temperatures
Upstream migration – to access new habitats or established reproducing areas
Downstream migration – post spawning and to avoid predators

To characterise the scale of the issue, in Australia, in New South Wales Murray Darling basin alone, there are more than 4000 man-made waterway barriers that impede fish passage. And as a result, the quantity of native fish alive in the region today is estimated to be less than 10% of pre-European settlement numbers.

Fishways typically help fish bypass barriers by dividing large energy differentials, such as highly concentrated velocity streams, into smaller steps that the fish can tackle in a more incremental manner.

Fishway hydrodynamics (the flow of water within and around a fishway), is a crucial element here. Fishway hydrodynamics can be difficult to predict as every fishway is unique, but Computational Fluid Dynamics (CFD) modelling has emerged as a powerful tool for prediction and optimisation. CFD modelling uses a computer to solve the equations of fluid flow and can provide a prediction of water velocities and turbulence in and around any fishway structure.

The results can be used to confirm if the fishway will provide the water velocities and turbulence levels that are needed to allow for fish to move up and downstream, and can be integrated into the design process.

In Australia, legislation in most states requires that new projects consider fish passage impacts and requires developers to provide fishways for any new structures or to retrofit them to existing structures if modifications are being undertaken. These policies reflect community concerns around fish populations as an environmental and economic problem.

CFD was once considered a high-cost process, but recent advances in computing power have meant the accessibility and affordability of this method has become more viable. It is now considered best practice by many in terms of fishway design in all but the simplest of scenarios.

Case study: Dights Falls with Melbourne Water

Situation:

Dights Falls, first built in 1895 is an iconic and historic weir, located on the Yarra River in Collingwood, Melbourne. It plays an important role in controlling water levels in the Yarra River. It is also an obstacle to upstream fish passage, which is problematic as the Yarra is a major coastal river and home to many native fish species. The weir has an existing fishway which is effective at low river flow rates but water becomes too turbulent at high flow. Melbourne Water engaged GHD to investigate modifications to the existing fishway to ensure fish can still migrate during high flows in the river.

Solution:

GHD used CFD and field observations to identify the hydrodynamic limitations of the existing fishway. This CFD model was then used to evaluate a range of modification options for Melbourne Water and to select a preferred strategy for fishway improvement. CFD was used to further refine the preferred fishway upgrade option and to develop the detailed design for construction.

Result:

GHD has designed a major modification to the fishway which was adopted by Melbourne Water and has now finished being constructed. Over time, long-term fish monitoring will determine the benefits of the modified fishway to fish population and health.

The design of fishways calls for professionals from a broad range of backgrounds including ecologists, biologists, fish passage specialists, hydrologists, civil and structural designers, geotechnical engineers, and hydraulics specialists. And while fishways are often discussed in regards to large structures, any piece of water infrastructure that impedes the natural flow of a stream including culverts, trash racks or low-level road crossings, can act as barriers to fish movement.

CFD can help bridge the gap between traditional modelling techniques and the challenge of creating fishways that are effective and safe for multiple fish species. Integrating CFD with effective fishway design software, as well as strong professional relationships with researchers and regulating authorities, forms a robust approach to promoting long-term fish population recovery.