Open channel flows involve the existence of a free surface between the flowing fluid and a gas above it. In such flows, the liquid and the gas are clearly separated, not interpenetrating (mixing), and the density ratio between them is large. The force of gravity and inertia governs the flow.
Due to a low density and negligible viscosity, both the inertia and the viscous force of the gas are negligible. The only influence of the gas is the pressure acted on the interface. There is no need to analyze the region of gas. The free surface is calculated as a boundary with constant pressure (for example, zero pressure by totally ignoring the air effect).
Open channel flow is computed by using the volume of fluid method. Surface tension and droplets or fluid separating into separate regions are not included in the calculations. Also, include only a single fluid in the model. (The regular fluid flow analysis can handle different fluids as long as the parts with different fluids are not connected together.)
Open Channel Flow can be performed on 3D models only. The analysis is unsteady (transient). In general, everything relevant to unsteady fluid flow in the documentation is relevant to open channel flow. Some loads that are available in steady or unsteady fluid flow are not available in open channel flow. The page for each load clarifies when that load is not available for open channel flow.