Trapezoidal flumes have relatively high (75 to 85%) 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 Trapezoidal flumes is determined by comparing the level at the secondary point of measurement to that of the primary point of measurement. The secondary point of measurement (Hb) is located in the discharge section of a Trapezoidal 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 the flow out of the flume; so the free-flow discharge equation should be used.
Submerged Flow Equation
Submerged flow equations have not been published for Trapezoidal flumes. 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.
Secondary Point of Measurement (Hb)
The secondary point of measurement (Hb) for a Trapezoidal flume is in the last section of the flume (J in the Trapezoidal Flume Master Dimensions Drawing).
As the floor of the Trapezoidal flume is flat, the zero elevation is the same for both measurement points.