Turbulent Mixing beneath an Undular Bore Front

Abstract

In macrotidal estuaries, a tidal bore may form during spring tide conditions when the flood tide is confined to a narrow channel. Most field occurrences showed well-defined undulations behind the leading wave, that is, an undular bore process. Herein, detailed free-surface and turbulence measurements were performed beneath undular bore fronts using side-looking acoustic Doppler velocimetry and nonintrusive free-surface measurement devices in a laboratory channel. Undular bores were observed for Froude numbers less than 1.7, which compared favourably with past studies. Velocity measurements with a temporal resolution of 50 Hz showed a marked effect of the bore passage. Longitudinal velocities were characterised by rapid flow deceleration at all vertical elevations, while large fluctuations of transverse velocities were recorded beneath the front. Turbulent Reynolds stress data highlighted high levels in the lower flow region, including next to the bed. Maximum normal and tangential turbulent stresses were observed immediately upstream of and at wave crests.