CASE STUDY - CFD MODELLING OF AN INNOVATIVE "NURSERY SLOT" FISHWAY
Fishways, often referred to as fish ladders, serve a pivotal function in facilitating fish migration at waterway barriers. They enable a diverse array of fish species to navigate upstream, over or through obstructions.
Aussie fish, known to be weaker swimmers compared to their international counterparts, introduce additional complications in the design of fishways. When there are moderate to high velocities, turbulence, or air entrainment either at the entrance or within the fishway, these conditions can lead to a failure in facilitating the migration of Australian species.
However, a fishway's efficacy should not be solely measured by its ability to transport adult native species, as this does not guarantee a robust solution. The successful migration of fish at earlier life stages—such as juveniles or larvae—indicates a more effective fishway. This allows the species to colonize a larger segment of the upstream catchment and subsequently boosts biomass, which supports other sensitive receptors.
Initial Coarse modelling
Coarse CFD modelling was initially undertaken to assess the following:
- Flow regimes of the nursery slot across multiple pools
- A comparison of the nursery slot and other weir configurations for a broad range of operating conditions
- The impact of different depths over the nursery slot
To assess the impacts of turbulence without averaging of the Navier-Stokes equations, the Large Eddy Simulation (LES) turbulence model was selected.
The mesh domain was selected with cell sizes varying between 10 to 3mm3.
CFD modelling of the finer mesh shows valuable insights to the rapid change in velocities as depths increase over the shallowest step.
The 10mm depth obviously shows the lowest velocity result, with velocities not exceeding ~ 0.65m/s across the entire section. The 20mm depth shows higher velocities but this doesn't necessarily mean post-larvae fish wont migrate for this event. Fish migrate through fishways with the following characteristics:
- Burst Speed: The maximum speed a fish can achieve in a short burst, generally sustained for only a few seconds due to the high energy expenditure.
- Burst Length: The distance a fish can travel during its burst speed. Essentially, it's a measure of how far a fish can go in one 'burst' of maximum speed before it needs to rest or reduce its speed due to energy expenditure.
- Critical Swim Speed (Ucrit): This is a measure of a fish's sustained swimming speed, which is the highest speed a fish can maintain before fatigue sets in.
- The above can vary widely among different species of fish, depending on their size, body shape, and muscle structure. Larger, more muscular fish or adult fish of the same species generally have higher burst speeds, lengths and critical swim speeds.
Knowing this, the 20mm depth (or possibly the 40mm) may be sufficient for some post-larvae species. Further monitoring and substantial investigatory work would be needed to confirm this.
FINDINGS & OUTCOMES
Initial coarse modelling of the entire fishway found the nursery slot performed significantly better than the alternatives (the stepped weirs). A finer mesh of the nursery slot in isolation was found to be necessary to capture the velocity gradient under shallow (< 20mm depth) conditions.
The finer mesh modelling was conducted on a single step of the nursery slot with varying depths (10mm, 20mm and 40mm). While the 10mm depth performed the best, none of the three scenarios should be discounted for post larvae and early juvenile (life stages) migration at this stage. Further monitoring and investigatory work may provide insight on which of these three (or an alternative) depths is the upper limit for the species (empire gudgeon, tarpon, agassizi's glassfish ect) of interest.
Interestingly, the 10mm depth scenario had a fairly minor increase in velocity as flow plunged over the step into the downstream pool. This may suggest a shallow film / sublayer + buffer layer effect. The surface tension solver was included in the CFD model so this effect, if present, would have been simulated. Us at Forward Hydro have our own homework investigating this further!
Regardless of what the modelling is showing, success is measured in practice, with initial monitoring of the site indicating the nursery slot is providing passage for post-larvae species.
After only 3 days of monitoring so far, the nursery slot rock ramp demonstrated an ability to pass weaker swimming juvenile fish, with the median size of all fish captured during the study equating to just 35 mm, with 15 mm empire gudgeon (Hypseleotris compressa) being the smallest size class recorded.
Significantly, 60% of native fish species recorded successfully ascending the nursery slot rock ramp were juvenile diadromous species undertaking life-cycle dependent migrations, including socio-economic species; barramundi, giant herring, tarpon and mullet and eel sp.
1215 fish successfully ascended the fishway per day, comprising 13 species (12 native, 1 introduced). 7 of these natives were diadromous – so just under 60% of all native fish recorded were undertaking life-cycle dependent migrations, including barramundi, giant herring, tarpon, mullet sp., eel sp., empire gudgeon and banded scat.
Congratulations to Matt and the Catchment Solutions team for this impressive feat!