For a Palmer-Bowlus flume to operate as designed, the flow out of the flume must be free and not restricted by downstream conditions. When downstream resistance to flow increases above a certain point and now reduces the upstream velocity through the flume a backwater effect is created. This transitions from free, unrestricted flow, to one of backwater / slowed velocity is known as the submergence transition (St).
It is important to remember that although a Palmer-Bowlus flume may have originally designed for free-flow at an installation, changes in the downstream hydraulics can submerged the flume. For Palmer-Bowlus flume, these changes are typically the result of sediment deposit, added hydraulic structures, or reconfigured channels. Don't assume that a once free-flowing flume will always be a free-flowing flume.
Typical of long-throated flumes, submergence transition (St) values for Palmer-Bowlus flumes range from 85% to 90%.
Submerged Flow Equation
Unlike Parshall or Cutthroat flumes, there are no corrections available when a Palmer-Bowlus flume becomes submerged. Openchannelflow only recommends using Palmer-Bowlus flumes when free-flow conditions will always be present.
Secondary Point of Measurement (Hb)
The secondary point of measurement (Hb) is located downstream of the flume. As with Ha, the zero elevation from which the Hb level is determined is top of the ramp in the throat.