When a river mouth has a flat converging shape with a tidal range exceeding 6 to 9 m, the river may experience a tidal bore. A tidal bore is a positive surge, and the aim of the study is to detail the turbulent field beneath an undular bore. New unsteady experiments were conducted in a large channel. Turbulence measurements were performed with high-temporal resolution using side-looking acoustic Doppler veloci-metry. Detailed turbulent measurements were conducted along a vertical profile for a Froude number of 1.4. Instantaneous velocity measurements showed a marked flow deceleration at all vertical elevations. Turbulent stresses were deduced from high-pass filtered data. Maximum Reynolds stresses were observed beneath the whelps, under and just before each crest. The findings suggest that preferential bed erosion takes place be-neath each wave crest, and the eroded material is advected in the undular wave motion behind the bore front
