7.3.1. Importance of Regimes of Flow In case of rigid boundary channels, the resistance to flow is on account of the surface roughness (i.e., grain roughness) only except at very high Froude numbers when wave resistance may also be present. But, in the case of alluvial channels, the total resistance to flow comprises the form resistance (due to bed forms) and the grain resistance. In the ripple and dune regime, the form resistance may be an appreciable fraction of the total resistance. Because of the varying conditions of the bed of an alluvial channel, the form resistance is a highly varying quantity. Any meaningful resistance relation for alluvial channels shall, therefore, be regime-dependent. It is also evident that the stage-discharge relationship for an alluvial channel will also be affected by regimes of flow.
The form resistance, which is on account of the difference in pressures on the upstream and downstream side of the undulations, acts normal to the surface of the undulations. As such, the form resistance is rather ineffective in the transport of sediment. Only grain shear (i.e., the shear stress corresponding to grain resistance) affects the movement of sediment.
