Study of The Mass Exchange in Lateral Bank Cavities with Different Aspect Ratios Using Large-Eddy Simulations

Mass exchange between lateral cavities and the main channel in river flow is of special relevance due to its impact in relevant hydrodynamic processes such as sediment deposition or transport of nutrients. The hydrodynamics developed inside the cavities depend on its geometrical characteristics especially its longitudinal extension over which the mixing between the embayments and main channel takes place. The modification of the cavity shape directly impacts the shear layer formed between the high-momentum channel flow and the recirculating flow within the cavity which is key in the exchange of mass and momentum. In those cavities with a square-like shape, the flow is characterised by a large-scale recirculating eddy dominating that occupies the entire enclosed volume, while the cavities featuring a rectangular-like shape increases the outer flow capability of entraining within the cavity promoting the mass exchange.

The configurations adopted in this research comprise a straight channel flow featuring symmetric bank cavities on both sides with different geometries, and the bulk Reynolds and Froude numbers are approximately 50,000 and 0.421. Large-eddy simulations using the in-house code Hydro3D are performed to analyse the flow developed within each of the different cavity configurations. The computational domain comprises two cavities on each side of the channel and periodic streamwise conditions are applied, which is equivalent to an ideal scenario of infinite series of lateral cavities.

The computed first-order statistics are well validated with experimental measurements and results show that the flow is highly three-dimensional despite the relatively shallow flow conditions. The principal momentum transport mechanisms, such as advection or due to pressure gradient, are analysed across the interface between the cavities and the main channel. This is accomplished with the analysis of the transport of time-averaged transversal velocities by means of the Reynolds Averaged Navier-Stokes equation for all the considered geometries, and the relative contribution of each the transport terms will be determined. These results will be linked with the sediment trapping efficiency of the different geometries obtained from experimental campaigns in order to assess the design of the geometry of lateral bank cavities with the goal to minimise unfavourable phenomena such as the excessive sediment deposition within the cavities.