Lacking an extended throat or discharge section, HS / H / HL flumes have little resistance to the effects of submergence. For a H type 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 H flume installation may have originally designed for free-flow, changes in the downstream hydraulics can submerged the flume. Typically, these changes are to do with downstream vegetation, sediment deposit, added hydraulic structures, or channels reconfigured. Don't assume that a once free-flowing flume will always be a free-flowing flume.
Designed for free-spilling discharge, the submergence transition - a ratio of the secondary point of measurement depth (Hb – located in the downstream channel) to the depth at the primary point of measurement (Ha) - is quite low (HS / H 25%, HL 30%).
Submerged Flow Equation
As the flume passes from free to submerged flow (through the submergence transition), the free-flow equation for the flume can no longer be used uncorrected. A new equation relating the submerged head to the free-flow head must be used. If the free-flow equation is used without correcting the head, it will increasing overstate the flow as the degree of submergence rises. Also, a secondary level measurement must be taken flume to determine the level of submergence – adding to the cost and complexity of determining flow.
Secondary Point of Measurement (Hb)
The secondary point of measurement (Hb) is measured in the downstream channel. The zero elevation from which the two levels are references is the floor of the HS / H / HL flume.