ANALYSIS OF MIXING AT RIVER MOUTHS: THE ROLE OF MACROVORTICES AND SUB-GRID TURBULENCE

INTRODUCTION A good understanding of flow mixing is important for properly describing the morphodynamics of the nearshore and river mouths. In recent years the modeling of flow mixing in natural shallow water regions has seen improvement thanks to the increasing computational power and the development of reliable and feasible models for large eddies and sub-grid turbulence. We here investigate the interplay between macrovortices, i.e. large-scale vortices with vertical axes, typical of shallow water flows, and the sub-grid turbulence characterizing river mouth flows, where a river current interacts with incoming sea waves. Generation of macrovortices is ensured by differential breaking at an estuarine shoal, whose presence also allows for inspection of the role of complex bathymetries (Olabarrieta et al., 2014). To this end, numerical simulations of wave-current interaction are performed by means of a solver for wave propagation in shallow water by Brocchini et al. (2001), integrated with a simple Horizontal Large Eddy Simulation (HLES) model proposed by Grosso et al. (2007). THE NUMERICAL MODEL The numerical model used for the tests (Brocchini et al., 2001) is based on the Nonlinear Shallow Water Equations (NSWE) and implements the finite-volume Weighted Averaged Flux method. The model is modified to give account of the dynamics induced by a river discharging at sea, and allows for computation of the morphological evolution of the bottom through the weak hydro-morphodynamic coupling proposed by Postacchini et al. (2012). The model also implements a simple HLES model to consider, in an approximate fashion, the dissipative terms due to sub-grid turbulence, which would, otherwise, be neglected by a classic approach of the NSWE (Grosso et al., 2007). THE TESTS Similarly to the wave-current interaction tests performed in the work of Olabarrieta et al. (2014), we reproduce the interaction of a steady river jet with weak-to-strong waves over a bathymetry characterized by a large river mouth shoal, and the generation, organization and migration of macrovortices as function of the sub-grid turbulence (HLES eddy viscosity varied between 0 and 0.1). THE RESULTS The numerical simulations reveal a significant generation and offshore advection of macrovortices at the seaward edge of the mouth shoal. Important turbulent structures are also observed, during the initial stages of the motion, at the corners of the inlet mouth, where the canalized, intense river current spreads over the shoal and interacts with the quiescent sea waters; these transient vortices dissipate fairly rapidly. The activation of the HLES model does not seem to have significant effects on the transient inlet vortices, causing only a limited decrease in their intensity as the sub-grid turbulence increases. Conversely, the shoal macrovortices are more sensible to changes in sub-grid turbulence: their intensity sensibly decreasing as the eddy viscosity increases. Notably, use of eddy viscosity parameters of the order 10-1 suppresses the generation of macrovortices in the shoal zone. Further considerations on these mechanisms will be illustrated at the conference.