As a long-throated flume, RBC flumes have high (90%) submergence transitions (St). Submergence is where downstream conditions control the flow out of a flume. The reduction of flow rate and discharge velocity out of the flume, meant that the indicated flow rate as derived from level measurement at the primary point of measurement (Ha) overstates the actual flow rate through the flume.
The submergence transition for RBC flumes is determined by comparing the level at the secondary point of measurement to that of the primary point of measurement. Like the H flume, the secondary point of measurement (Hb) is located downstream of the flume, while the primary point of measurement (Ha) is located upstream of the flume’s throat. Above this ratio (Hb/Ha), and the flow through the flume is being controlled (slowed) by downstream conditions. Below the submergence transition and downstream conditions do not control; so the free-flow discharge equation should be used.
Submerged Flow Equation
As the submergence in the flume increases, small measurement errors determining the level become critical. This, coupled with the already high submergence transition of 90%, means that submerged flow equations have not been developed.
As a result, if submergence is suspected, the flume must be raised in the channel or the downstream conditions modified so that submerged flow no longer occurs.