Field investigations of time scales and mechanisms of sand suspending by irregular waves

On the basis of field experiment carried out in 1993 in the Black Sea the time scales and mechanisms of sand suspending by irregular waves are discussed. Synchronous recording of suspended sediment concentration in the water layer of 4-8 cm above the bottom and two horizontal velocity components at the depth of 25 cm were performed near the top of the sybmerged bar for different wave regimes. Three sand suspension mechanisms can be distinguished in field conditions: a) lee vortex outburst from the sand ripples crests during the change of direction of the cross-shore velocity; b) eddy formation in the boundary layer and its outburst to the water column owing to hydrodynamic instability at the decelerating phase of wave flow. Time scales correspond to the mechanisms of sand suspending and are determined by a consequence of suspending events due to irregularity and group structure of waves.