Computational Fluid Dynamics modelling in fish pass design

To coincide with World Fish Migration Day on 24 October 2020, our fisheries specialist David Mould has been looking at the use of Computational Fluid Dynamics (CFD) modelling in fish pass design.

Fish pass design usually has to balance the ecological requirements with constraints of other users, particularly when the weir requiring passage has a functional use. For example a gauging weir or for abstraction upstream.

Whilst the design of a new fish pass can often be based on standardised methods, for example the Larinier fish pass design, each installation site will have its own specificities that must be accounted for.

Computational Fluid Dynamics (CFD) modelling provides a cost-effective alternative to physical modelling which can provide designers with access to all the hydraulic parameters required to make performance assessments and design improvements in consideration of multiple competing design criteria. The visualisations are also accessible and easily understood by non-specialists.

Here we look at three studies in which CFD has been used in our fish passage improvement projects:

Modelling scour risk and channel morphology change following weir removal in a high risk location.

Our client owns a weir under a bridge and would like to remove it to reduce maintenance costs, and improve fish passage and sediment connectivity. Therefore we needed to look at the potential impact of weir removal on stability at a bridge. CFD modelling was used after an initial modelling study which showed a significant increase in shear stress under the bridge. The CFD model was set up with a mobile bed, related to observed sediment size, to allow the model to scour and deposit as appropriate.

The plots below show net sediment height change through the simulation:

  • Red = Deposition
  • Blue = Scour.

The simulations showed significant undermining of abutments in a 2-year flood event, and greater scour during a 100-year return period event. CDF modelling image: 2 year and 100 year return periods - scour

The results showed that without significant mitigation, removing the weir will lead to scour of the bridge foundations and sediment release downstream, at even moderate flood events.

Thermal Plume modelling

Our client wanted to locate a warm water discharge downstream of a recently constructed fish pass. There was concern that the discharge could spread across the river to form a thermal barrier and reduce the attraction efficiency of the pass. Traditional thermal modelling techniques average across the whole section, and so are not suitable for understanding the spatial extent of any plume.

3D CFD modelling was used as it has excellent representation of mixing, and allows fully spatial modelling of any plume. Using collected topographic survey data, we made an idealised representation of the river.

The model was set up for the worst case: low river flow, full thermal discharge. Results showed that the area of higher temperatures is confined to the left hand bank. Temperatures return to river temp around 40m downstream of discharge. This is likely to have a minimal impact on the attraction efficiency of the pass.

Thermal modelling image

Changes to sediment and leaf deposition as a result of fish pass installation

Our client was concerned that locating a fish pass downstream of a pumping station would increase the amount of sand and leaves deposited in front of the screen. A CFD model was set up, which includes the pumps abstracting and sand and leaves seeded in the water column. The location of leaves is represented by the green dots and sand by the red dots.

Firstly the model was run for the existing situation to give a baseline. A range of flows and abstraction scenarios were modelled but only one is presented here for clarity. The graph shows the cumulative number of sand and leaf particles reaching the screen.Abstraction model and graph pre-change

We then ran the model with the fish pass in place, combined with a variety of mitigation measures. The preferred solution was to re-profile the river bank between the pass and the pumping station. It showed a reduction in the amount of and silt reaching the screens. This enabled works on the fish pass to proceed.Abstraction model and graph, post-change

Want to know more?

Email David Mould for more information about our work on fish pass design, computational fluid dynamic modelling and any of these projects.

You can also find out more on our Fisheries webpage.

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