  Shallow flows are common in natural and human-made environments. Even for
simple rectangular shallow reservoirs, recent laboratory experiments show that
the developing flow fields are particularly complex, involving large-scale
turbulent structures. For specific combinations of reservoir size and hydraulic
conditions, a meandering jet can be observed. While some aspects of this
pseudo-2D flow pattern can be reproduced using a 2D numerical model, new 3D
simulations, based on the unsteady Reynolds-Averaged Navier-Stokes equations,
show consistent advantages as presented herein. A Proper Orthogonal
Decomposition was used to characterize the four most energetic modes of the
meandering jet at the free surface level, allowing comparison against
experimental data and 2D (depth-averaged) numerical results. Three different
isotropic eddy viscosity models (RNG k-${\epsilon}$, k-${\epsilon}$,
k-${\omega}$) were tested. The 3D models accurately predicted the frequency of
the modes, whereas the amplitudes of the modes and associated energy were
damped for the friction-dominant cases and augmented for non-frictional ones.
The performance of the three turbulence models remained essentially similar,
with slightly better predictions by RNG k-${\epsilon}$ model in the case with
the highest Reynolds number. Finally, the Q-criterion was used to identify
vortices and study their dynamics, assisting on the identification of the
differences between: i) the three-dimensional phenomenon (here reproduced), ii)
its two-dimensional footprint in the free surface (experimental observations)
and iii) the depth-averaged case (represented by 2D models).
