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1.1 General description of flow in open channels Open channel flow (OCF) occurs in canals, rivers and various types of waterways. Open channel flow is mainly driven by gravity unlike flows in closed conduits where the flow is mainly driven by a pressure gradient. An important feature of an open channel flow is the presence of the free surface (zero-shear state). The friction at the bed and the side walls acts as resistance to the flow which is mostly turbulent. The type of bottom and side walls are dependent on the material in which the flow is taking place. Lined canals and canals in rocky strata can be classified as fixed bed channels, and canals in erodible medium are termed as movable bed channels. The shear stress distribution and roughness characteristics of the boundary do not vary with time for a given flow rate in a fixed bed open channel. A movable bed generally accompanies the flow in an alluvium soil. The side slopes and bottom slope of erodible channels vary with time. The flow in open channels can be classified depending on the velocity changes in space and time. The flow is steady if dU/dt = 0 at any point in the flow field and it is unsteady if dU/dt ≠ 0. Though the turbulent flow accompanied by the formation of ripples and dunes on the bed is strictly unsteady, in most practical situations, the time-averaged velocity acquired over long time intervals result in a quasi-steady flow. Flood flows in rivers and surges in power canals are examples of unsteady flows in open channels. Unsteady flows are more difficult to solve because the governing equations are hyperbolic (continuity and momentum). The flow is uniform if dU/dx = 0 and non-uniform if dU/dx ≠ 0. Uniform flow can only occur in long prismatic channels or generated in laboratory flumes. Non-uniform flow can be classified into gradually varied flow (dy/dx is very small) and rapidly varied flow (dy/dx >> 0) depending on the amount of change in the velocity over space. Furthermore, most practical open channel flows are turbulent (Re ≥ 2000) and accompanied by rapid changes in velocities and pressures, in space and time. Eddies and swirls are present in turbulent flow

Bed Forms and Flow Mechanisms Associated with Dunes