Australia’s lowland rivers are particularly prone to thermal stratification due to its hot, dry, and sunny climate, with propensity for prolonged drought. This can result in periods of stable stratification which inhibits turbulent mixing and can lead to low levels of dissolved oxygen (hypoxia) in the bottom layers. Stratified conditions are also favourable for the growth of toxic blue-green algae (cyanobacteria). These conditions can result in fish mortality, such as the “Menindee fish kill” in the Darling River at Menindee, NSW late 2018 early 2019. This extreme event was caused by hypoxic water in the hypolimnion, a strong cyanobacteria bloom, terminated by a cold front which triggered full mixing and depletion of oxygen throughout the water column.
Geometry and bathymetry, along with meteorological conditions, play crucial roles for the existence of persistent thermal stratification in natural rivers. Fluid flows through a bend lead to strong lateral secondary circulations, resulting in asymmetric spatial distributions of turbulent mixing, and affect the distribution of mean velocity and turbulent shear stress within the river. Those factors co-determine the strength of turbulent mixing and thus the breakdown or inhibition of the build-up of stratification in a river section. While a critical flow velocity might be sufficient to generally suppress persistent stratification, it is the geometrical aspect which determines longitudinal change in stratification in a river. The effects of geometrical differences in meanders on mean flows, temperature distributions, flow separations, turbulent characteristics, and boundary shear stresses, were investigated through Direct Numerical Simulations of idealized open-channel meanders with and without radiative heating. Results comparing neutral and stratified flows will be presented. The results will improve the capability of advanced hydrodynamic models, which, combined with satellite remote sensing, can determine bloom formation locations and assess risk of potential fish kills in slow flowing rivers.