Current scour estimation methods typically over-predict scour, resulting in uneconomical design. This tendency is partly due to the complexity of the scouring process, which indicates that some of its aspects are still not well understood, and can also be attributed to scale effects. Here, experiments are conducted to isolate the influence of relative coarseness (D/d(50)) and flow shallowness (h/D) on scour depth. For the range of D/d(50) in the present study, equilibrium scour depth (d(se)/D) decreases with increasing D/d(50) until a limiting value of D/d(50) = 175, after which d(se)/D approximate to 0.75. Furthermore, d(se)/D is found to depend on h/D when all other scour influencing parameters are held constant. A revised definition of the densimetric Froude number using the velocity along the separating streamline is shown to have an influential role in scour. An improved scour estimation method employing these parameters is presented and compared with current methods.