The aim of this work is to provide convincing evidence on the turbulent processes induced by three-dimensional (3D) bores, based on physical and computational fluid dynamics (CFD) studies of undular tidal bores, a phenomenon very similar to a tsunami-like bore propagating inland along a river. The numerical study is performed by solving the Navier-Stokes equations with a large eddy simulation method in order to access the turbulent flow evolution during the bore passage. Two and three dimensional simulations are performed with and without turbulence before bore generations to inspect the effect of coherent structures on the bore propagation. A complex three dimensional flow takes place during the bore passage. Beneath the undulation crests, a strong shear is observed near the channel bed. Moreover, ejection of turbulent structures occurs during the propagation of undular bores depending on the initial flow conditions. These simulations provide the first detailed three dimensional data of undular bores intricate flow structure. The results showed that the propagation of the bore front drastically changes the properties of the water column. It is also highlighted that for an upstream current exceeding a threshold value, near-bed eddies are generated and ejected in the water column independently of the free surface characteristics. Our simulations improve the understanding of positive surges which could be extended to tsunami-like bores studies.