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32 . slope, velocity, and fall diameter of bed materials are known, one can predict the form of bed roughness by using this relationship. . 2.4.3.1 Lower Flow Re~imes Lower flow regime begins with the beginning of motion. The resistance to flow is large and sediment transport small. The bed form is either ripples or dunes or some combination of the two. The water surface undulations, if they exist, are out of phase with the bed surface and there is a relatively large separation zone downstream from the crest of each ripple or dune. Resistance to flow is caused mainly by form roughness. . . 2.4.3.2 Upper Flow Regimes In the upper flow regime, resistance to flow is relatively small and sediment transport is large. The usual bed forms are plane bed or anti dunes , The water surface is in phase with the bed surface except when an antidune breaks and normally the fluid does not separate from the boundary. A small separation zone may exist downstream from the crest of an antidune prior to its breaking. Resistance to flow is the result of grain roughness with the grains moving, sand waves and their subsidence, and energy dissipation when the antidune breaks. . . . . 2.4.3.3 Transitions The transition zone encompasses the bed forms that occur during the passage from lower regime to upper regime. The bed configuration in the transition zone is erratic. It may range from that typical of the lower flow regime to that typical of the upper flow regime, depending mainly on antecedent conditions. The bed configuration may also oscillate between dunes and plane bed due to changes in resistance to flow and consequently, the changes in depth and slope as the bed form changes. . . .