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Categorizing the types of roughness in environmental flows is important to establish the scaling laws for the roughness and bottom friction parameterization. The dependence of roughness height on roughness element geometry is discussed to clarify the difference between the two distinct d-type and k-type roughness under oscillatory flows. Direct numerical simulations (DNS) of oscillatory flow over transverse square and triangle bars were conducted, by varying the height-to-space ratio
w
/
k
and the shape of bars. DNS results show that the major difference between d-type and k-type roughness is the contribution from the frictional drag and pressure drag to the total stress. For d-type of roughness, relatively large frictional drag may play important roles in larvae recruitment and sediment transport. In addition to the ratio
w
/
k
, the shape of the roughness element also affect the transition from d-type to k-type roughness. At intermediate range of Keulegan-Carpenter number (
KC
), the inertial force and drag force are of equal importance. Simulation results of wave friction factor were compared with theoretical results, and the results are very sensitive of the choice of the characteristic roughness length scale.