This control is a specialized control that allows for the definition of up to five separate circular orifices at different elevations. The control is characterized by a lowest orifice elevation which is BELOW the outlet elevation. The outlet elevation is considered the elevation of the receiving pipe from the pond. The remaining four orifices are located as a distance from the previous orifice.
Figure 13: Multiple Orifice Structure
Note that the orifice diameters are in inches. This implies that the diameters are supposed to be rather small (no 3 ft diameter orifice). However, the program is not very intelligent and will compute flows through orifice up to 5 ft in diameter.
The program does not consider the occurrence of weir flow through any orifice. All flow through any orifice is assumed to be under submerged orifice flow conditions. Driving heads applied to the orifice are computed to the orifice invert. No adjustment is attempted to correct for flow to the centroid of the orifice.
The lowest orifice is simply a placeholder to identify where it physically is located. For hydraulic computations, the elevation that matters is the outlet elevation. Flows through each orifice are computed relative to the outlet elevation.