Stratification in the presence of an axial convergent front: Causes and implications

•Flood tide stratification in presence of an axial convergent front observed.•Along-channel velocity found to deviate from log profile.•Axial convergent front hypothesised to cause along-channel velocity deviation.•1D model used to show how stratification formed on flood tide.•Turbulence found to be constricted on flood tide due to presence of stratification.

Flood tide salinity stratification in the presence of an axial convergent front is revealed with new data. The data were collected during sampling campaigns in the Conwy estuary, North Wales, the location of a previous study for axial convergent fronts. The stratification, with a maximum observed vertical density difference of 5 kg m−3, begins as soon as the saline intrusion arrives and lasts throughout the flood tide. The flood stratification is shown to be caused by a tidal straining-type process. The along-channel shear is modified by cross-channel currents developed during the formation of the convergent front such that surface currents are smaller than those at mid-depth. The modified shear interacts with the horizontal density gradient to form the stratification on the flood tide. Implications for the turbulence and sediment transport regime in the estuary are discussed.