In this paper, the flow and local scour variation around two submerged and unsubmerged tandem piers are studied using 3D flow model where the upstream pier is submerged while the downstream pier is emergent. The model uses a finite-volume method to solve the non-transient Navier-Stokes equations for three dimensions on a general non-orthogonal grid. The k turbulence model is used to solve the Reynolds-stress term. The numerical model solves the sediment continuity equation in conjunction with van-Rijn's bed-load sediment transport formula to simulate the bed evolution. The 3D flow model is verified through experimental study in a non cohesive bed material in an experimental flume. The different causes of local scour around two submerged and unsubmerged piers are simulated well, such as bow flow, down flow, horseshoe vortex, pressure variation and lee-wake vortex. It is found from this study that the maximum local scour depth by interaction between two tandem submerged unsubmerged piers depends on submersion ratio of upstream pier, the densimetric Froude number, the longitudinal distance between piers and the ratio of pier diameter to channel bed width. The maximum scour depth decreases by increasing the submerged pier height then begin to increase by increasing the submerged pier to a height larger than half the water depth and in general the maximum scour depth is less than that of two unsubmerged piers. The results show good agreement between simulation and experimental results. Also, empirical equations are developed for computing the maximum scour depth due to the interaction between two submerged unsubmerged piers with circular shapes as a function of submergence ratio, piers spacing, densimetric Froude number and channel width to pier diameter ratio.