Using Physical Models to Predict Fish Mortality at Water Infrastructure

The operation of large-scale river development projects is leading to worldwide declines in fisheries productivity. The role of smaller barriers is less-understood but is largely thought to simply restrict movement rather than facilitate any fish welfare issues. This study sought to investigate welfare issues associated with downstream fish movement through two different types of low head barrier. A physical model was constructed, under field conditions, to replicate a 2.68m structure which simulated an undershot gate or a fixed crest overshot weir. Seven species of fish from a range of life history stages (larvae, juvenile, adult) were passed through the structure and recaptured downstream to ascertain any passage related impacts. Adults of large-bodies species (> 500mm FL) passed relatively unharmed, but juvenile fish experienced increased mortality when passing through undershot gates. Larval (< 20 mm) and small-bodied species (< 100 mm) experienced substantial mortality at undershot gates, especially when discharged into low tailwater conditions. Computational Fluid Dynamic modelling of the experimental confirmed fish experienced rapid decompression and increased fluid shear during passage which could potentially lead to the observed injuries. Both mortality data and CFD modelling confirmed that passage through overshot weirs was far safer for all life history stages and contained more stable hydraulic conditions.