Experiments were designed to address the role of horizontal pressure gradients on bed dynamics for shoaling, breaking, surf and swash waves. A small-scale laboratory flume with low density sediment is used. The flow in the bottom boundary layer and sediment bed motion is tracked by video measurements. Occurrence of plug flows is evidenced. We also question the role of the vertical pressure gradients on bed destabilization, and the dubious relevance of acoustic measurements to track deep layers of moving sediment. The chosen experimental set-up is favoring the occurrence of the plug flow regime, scaling arguments nevertheless suggest that plug flow is likely to occur in natural coastal zones.