Mixing properties of a shallow basin due to wind-induced chaotic flow

As important environmental features, mixing properties of inland water bodies in unsteady flow conditions are investigated. Time-dependent motion, often resulting in chaotic behavior, requires the Lagrangian description of the transport. As a simple example, unsteady hydrodynamics driven by periodical wind forcing in a simplified shallow lake geometry is considered to explore the main chaotic properties. In the modelled flow field methods identifying strong and weak shearing sub-regions are proposed and applied as mixing indicators. These include the determination and inter-comparison of the finite size Lyapunov exponents (FSLE), the residence time, and the implementation of the so-called leaking method. Coherent structures as stable and unstable manifolds are also identified, playing the role of Lagrangian barriers that hinder local transversal material transfer, and avenues that significantly channel transport. The primary effect of turbulent diffusion on the FSLE fields is also demonstrated.