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26 . 2.4.2.3 Dunes When the shear stress and the stream power are gradually increased over a bed of ripples, or if the bed material is coarser than 0.6 mm over a plane bed, a new flow condition will be achieved that causes dunes to form. Dunes cause large separation zones in the flow. With dunes, as with tranquil flow over an obstruction, the water surface is always out of phase with the bed surface. The flow accelerates over the crest of the dunes and decelerates over the troughs. The sediment transport rate is relatively small. The concentration of bed materials is approximately 100 to 200 ppm. Resistance to flow caused by dunes is large, but not as large as that caused by ripples formed of finer sand and at shallow depth. For dunes, the Manning's n varies from 0.018 to 0.035. Resistance to flow increases with an increase in depth for coarser sands (D50 > 0.3 mm) and decreases with an increase in depth for finer sands (D50 < 0.3 mm). This is because the amplitude of dunes can increase with the increasing depth so that the relative roughness can remain essentially constant or even increase with increasing depth of flow. However, a decrease in angularity of the dunes in finer sands as they in- crease in size reduces their form roughness and their resistance to flow as well. e . . . . . . 2.4.2.4 Plane Bed With Sediment Movement If the shear stress or the stream power is continous1y increased, the size of dunes increases until the dunes reach a maximum height at a certain stream power. Thereafter, a transition regime is reached when the dunes start to diminish in amplitude with the further increase of the stream power. Finally, the dunes completely disappear and a flatbed is formed. In this case, the concentration of bed material ranges from about 1500 to 3000 ppm. . . .