Collisions of counter-propagating solitary waves are studied experimentally. First, I spend first few minutes to show our laboratory validations of the existing theoretical predictions. Those are a) the maximum collision amplitude exceeds twice of the incident waves; b) the collision causes attenuation in wave amplitude; c) the collision leaves imprints on the interacting waves with phase shifts and small dispersive trailing waves. Then, I analyze the flow field in terms of accelerations, vorticities, and velocity-gradient tensors in addition to the velocity field. Visualized velocity-gradient tensor fields show that fluid parcels are stretched vertically prior to reaching the state of maximum wave amplitude. After the collision peak, fluid particles are stretched in the horizontal direction. Boundary layer transition is also discussed including the flow separation at the bed. While a solitary wave itself can cause flow attachment to the bed (but not separation), a collision of counter-propagating solitary waves induces flow separation.