The complex vortical flow structure arises from the interaction between the flow and a pipeline on seabed, causing erosion scour around the pipeline, which is a threat to the stability of the structure. The viscous flow past a circular cylinder laid on a horizontal bed is analysed experimentally using the particle image velocimetry image technique to understand the vortical flow characteristics downstream of the cylinder. The experiments were conducted in a closed-loop water channel with Reynolds numbers, in the range of 1000 <= Re-D <= 7000 based on the cylinder diameter. To provide a detailed evaluation of the properties of the flow structure behind the cylinder, instantaneous and time-averaged patterns of velocity vector field, corresponding vorticity contours, streamline topology, and Reynolds stress concentrations were presented. The point of the boundary layer detachment from the cylinder surface and the lengths of the primary and secondary downstream separation regions were determined as a function of Reynolds numbers. Instantaneous velocity fields clearly indicate the scour mechanism downstream of the horizontal cylinder.