Continuity
Open channel flow has a liquid surface that is open to the atmosphere. This boundary is not fixed by the physical boundaries of a closed conduit. Water is essentially an incompressible fluid, so it must increase or decrease its velocity and depth to adjust to the channel shape. If no water enters or leaves a stream (a simplification that can be made over short distances) the quantity of the flow will be the same from section to section. Since the flow is incompressible, the product of the velocity and cross-sectional area is a constant. This conservation of mass can be written as the continuity equation as follows:
While the continuity equation can be used with any consistent set of units, it is normally expressed as:
Q = quantity of flow (ft3/s)
A = cross-sectional area (ft2))
V = average velocity (ft/s)