In ocean engineering projects, submerged semi-cylindrical structures are used as sea crossing tunnels. Previous studies have extensively examined the flow field around cylindrical structures, but research on flow around semi-cylinders is relatively limited. This paper presents a physical experiment on flow features around smooth and coarse semi-cylindrical structures under current conditions. The experiment results demonstrate that the surface roughnesses of the semi-cylinders significantly influences the features of the flow field around the structure, including turbulence intensities and the turbulence kinetic energy of the wake flow. Specifically, a coarser surface on the semi-cylinder could decrease the turbulence intensity of the wake, which could effectively minimize potential local scour downstream of it.