These boundaries are rarely used, as dynamic links with 1D models are preferred with the flow boundary applied to the connecting 1D node.
Tip: It is strongly recommended to use a 1D node linked to a 2D HX boundary (see above) in preference to using a flow boundary, especially in flood models where there is major wetting and drying. This arrangement is far more practical, stable and flexible (the boundary can wet and dry, can lie oblique to the grid, and the velocity distribution and flow direction across the boundary is automatically determined).
Combinations
Any number of flow and velocity boundaries can be assigned to the same cell(s). The final velocity is the sum of the velocities assigned.