The mechanisms and time scales of sand suspension by irregular waves are considered on the basis of field experiments "Novomichailovka'93" (Black Sea), "Norderney'94" (North Sea) and "Ebrodelta'96" (Mediterranean Sea). The synchronous data for the suspended sediment concentration and the fluid velocity components were recorded by optical and electromagnetic sensors with a high frequency response. The sensors had been installed in a distance above the bottom of four to fifteen centimeters. The mechanism of the vortex ejection due to the shear instability of the bottom boundary layer is the most probable reason for sand suspension from the bed in area which are located seaward of the wave breaking point. In those areas the waves are greatly deformed and the bottom is nearly flat. The suspension events coincide in time with the first half part of decelerating phase or with the flow reversal after the passing of the wave crest. For such conditions the statistically significant coherence between sediment concentration and cross-shore velocity is confirmed for the peak frequency of the wave spectrum and its first harmonic. The concentration follows the velocity fluctuations with phase lag of -π/4 at the frequency of wave spectrum peak. At the low frequency band (f<0.08 Hz) the sediment concentration poor correlate both with the velocity and its envelope.
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