Near Bed Turbulent Coherent Structures and Sea Bed Evolution Due to Long and Short Waves

Abstract:

The influence of long and short waves on the generation and evolution of near bed turbulent coherent structures and the sea floor geometry has remained an important but often poorly resolved parameterization within tsunami and wave propagation models. A laboratory study to examine the near bed turbulent evolution and sediment bed response was conducted at near field scale. Two-dimensional observations of the flow field were obtained with a submerged Particle Imaging Velocimetry system looking at a 9 cm by 17 cm region just above a movable ripple sand bed subjected to forcing caused by free-surface gravity waves with 30 cm wave height and a 4 second period. Robust particle tracking techniques and high resolution cameras allowed for millimeter scale resolution of the velocity field and sea floor evolution. Periods of high suspension were concomitant with high near-bed velocities as observed with a high resolution acoustic Doppler profiler. The growth of the boundary layer was particularly observable during the longer duration offshore directed flow. The vortex is created during flow reversal in the ripple trough, growing to roughly the height of the ripple. The coherent structure is ejected during the subsequent half wave cycle and sheared apart at the peak of the onshore directed flow. The high shear associated with the vortices are correlated to sediment suspension and subsequent sediment transport resulting in an onshore migration rate of 1.5 mm/min.