Influence of weir design on mortality of downstream migrating fish
Baumgartner, L., Bettanin, M. and McPherson, J., 2011. Influence of weir design on mortality of downstream migrating fish. Presentation given at the 2011 Annual Australian Society for Fish Biology Conference and 5th Australian Technical Workshop on Fishways. 22 – 24 July 2011, Townsville, North Queensland, Australia.
An estimated 10,000 dams and weirs are currently installed throughout the Murray-Darling Basin. Two major weir designs, undershot and overshot, are constructed on Australian waterways. Each delivers water in a substantially different way and fish attempting to pass either design could be subjected to a range of different hydraulic forces. A controlled field study sought to determine the impacts of downstream passage of each weir type into low and high tailwater conditions under a range of simulated discharges. Experiments were replicated over different life history stages of seven native fish species commonly recorded in the Murray-Darling Basin. Passage through overshot weirs was associated with substantially greater survival in all species. Few fish died and the main welfare issues arose when overshot weirs discharged into shallow water. Under these conditions fish were physically injured when impacting with the downstream weir apron. Large proportions of golden perch (>90%) and silver perch (> 90%) larvae died during undershot weir passage. Murray cod larvae (> 50% mortality) were also substantially impacted. Computational Fluid Dynamics determined that undershot weirs were characterised by higher values of shear, turbulence and pressure changes which are likely impacting on fish. Options to design and retrofit undershot weirs with ‘fish friendly’ hydraulic modifiers should be explored to minimise these threats to native fish. Alternatively, assessing the applicability of stationary screens may represent a useful mechanism to prevent fish entrainment and subsequent injuries.