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24 . 2.4.2 Bed Configuration, Flow Phenomena, and Resistance to Flow The bed configuration is the array of bed forms, or absence thereof, . generated on the bed of an alluvial channel by the flow. The bed configu- rations that may form in an alluvial channel are plane bed without sediment movement, ripples, dunes, plane bed with sediment movement, antidunes, and . chutes and pools. These bed configurations are listed in their order of occurrence with increasing values of stream power (, V, as defined in o Equation 2-6) or similar parameters for bed materials having D50 less than 0.6 mm. Sketches of typical bed configurations are shown in Figure 2-7. . 2.4.2.1 Plane Bed Without Sediment Movement . If the bed material of a stream moves at one discharge but not at a smaller discharge, the bed configuration at the smaller discharge will exist as remnant of the bed configuration formed when sediment was moving. For . bed configuration without sediment movement, the problem of resistance to flow is one of rigid-boundary hydraulics. Plane bed without sediment movement has been studied to determine the flow conditions for the beginning of motion . and the bed profiles that would form after beginning of motion. For a plane bed without bed material transport, the Manning's roughness coefficient varies from 0.012 to 0.016. . 2.4.2.2 Ripples In fine sand, ripples will usually occur as soon as particles show ap- preciab1e movement. The separation zone downstream from a ripple causes very . little disturbance on the water surface. The concentration of sediment is small ranging from lO to 200 ppm. Resistance to flow is large with the Manning's n varying from 0.018 to 0.05. As the depth of the flow increases, . resistance to flow due to bed roughness decreases. .