In some cases, friction is small so this equation works well. E.g., describes lift forces on grains in a flow. Turbulent Flow Flow is NOT smooth layers. Turbulent eddies can transfer momentum over much of flow depth. High speed fluid can sweep down to bed and increase amount of sediment moved.
Laminar flow occurs at low flow velocities and depths.
Turbulent flow occurs at higher velocities and depths. Froude Number Dimensionless number used to determine whether flow is subcritical or supercritical. Q sed is volume, mass, or weight of sediment moving per unit bed area and time.