Contact erosion appears at the interface of two soil layers submitted to a groundwater flow. Particles of the finer soil are eroded by the flow and transported through the pores of the coarser layer. Fluvial dykes are often exposed to this phenomenon. Small-scale experi-ment combining Refractive Index Matching medium, and Particle Image Velocimetry were realized to measure the flow characteristics close to the interface between a porous medium and a sandy layer. Distributions of velocities and shear-stresses were obtained. They underline the spatial variability of stress exerted by the flow onto the fine soil which is in direct relation with the variability of the pores geometry. These distributions can be helpful to better model contact erosion by going beyond the simple use of global mean values, like Darcy velocity, as usually proposed in the litterature.