Bottom and Side Outlet Orifices in SWMM 5

Note: The main difference between an Bottom and Side Outlet orifice at the same offset elevation and the same diameter is the depth at which the flow in the orifice will switch between weir flow and orifice flow. The Side Outlet orifice has Weir flow until the Orifice is full but the Bottom Orifice has Weir flow until the Critical Height which is usually shorter than the maximum depth of the orifice.

For a circular orifice the Critical Height is:

Critical Height = Orifice Discharge Coefficient / 0.414 * Orifice Opening / 4

For a rectangular orifice the Critical Height is:

Critical Height = Orifice Discharge Coefficient / 0.414 * (Orifice Opening*Width) / (2.0*(Orifice Opening+Width))