How Parshall Flumes Work
Under normal conditions, the Parshall flume works by accelerating slowly flowing water in such a way that:
at one specific point (called the point of measurement or Ha if you want to be technical) the depth of the water corresponds to the flow rate through the flume
The point of measurement in a Parshall flume is located in the converging section of the flume - at a distance 2/3 of the the converging section’s length (as measured along the wall of the flume) UPSTREAM of the throat. It is definitely NOT in the throat of the flume!
So now that you know where the point of measurement is, simply measure the depth and look up the flow on your chart and you are done!
It’s that simple!
Normally the discharge out of a Parshall Flume is such that it flows freely out of the flume. However conditions in the downstream channel can be such that they restrict the water flowing out of the flume. As the amount of restriction increases, the flow out of the flume slows - ultimately to the point where the flume over indicates how much flow is passing through the flume.
Keep in mind that just because the flume was originally installed with free discharge, downstream conditions can change over time so that what was free discharge is now submerged. Channel conditions changing over time are of particular concern in water rights / irrigation applications.
Submerged flows are a little more difficult to measure:
You need to measure the water level both at the normal (primary / Ha) point of measurement AND another location in the throat (narrowest part) of the flume (Hb).
Keep in mind that the flow at this second location is usually turbulent - and thus difficult to measure. To help get an accurate level reading, a chamber, or well, is usually created off to the side of the flume throat. A tap and piping from the flume to the well allows the water level in the well to equal that in the throat while at the same time dampening much of the turbulence out.
Once you have both levels, you then need to determine the submergence ratio.
The submergence ratio is simply the level in the throat divided by the level at the point of measurement. The higher the ratio, the greater the flume is submerged.
With the submergence ratio calculated, you then need to compare the calculated ratio to the flume’s submergence transition ratio (St). St differs among Parshall Flume sizes (smaller flumes have lower ratios - larger flumes have higher ones).
If the calculated ratio is equal to or greater than the flume’s St, then the flume is submerged and relying on only the normal depth reading (and subsequent flow rate) will over estimate the amount of flow passing through the flume.
Now if the flume is submerged, you’ll need to use the more complex Parshall Flume submerged flow equation. We won’t go into that equation here, but suffice to say, you don’t want to have to use that equation unless you absolutely have to!