The Department of Civil Engineering and Geomatics' Hydraulics Laboratory at Universidad Nacional Autónoma de México has an interesting video posted showing the hydraulic profile of a Parshall flume under both free and submerged flow conditions.
The Laboratory’s Parshall flume is constructed of a clear polycarbonate and, as such, the flow is shown from the side where it is easier to understand what is happening in the flume under differing flow conditions.
The video opens with canaleta Parshall operating under free-flow conditions with several inches of flow going through it.
As the video progresses, the discharge from the flume is dammed - resulting in flow backing up into the flume, submeging it.
At 6:50, there is a close up of the water level through the converging and throat sections of the flume. Of particular note here is the draw down in the water surface that is visibile. As the flow approaches the throat and is accelerated, the water level decrease. Short-throated flumes like the Parshall have only one primary point of meaurement where the flow equations hold true. This is in contract to long-throated flume, like the Palmer-Bowlus and RBC, where the point of measurement can be taken anywhere upstream of a certain distance from throat with equal accuracy.
Source: YouTube
