We all know the usual points of measurement, H_a (primary point of measurement), H_b (point of measurement to determine submergence), but what about H_c?

The Hc point of measurement appears in Dr. Parshall’s earliest works on what would become his namesake flume. The original intent of Hc was to allow another measurement location for determining the degree of submergence in a flume. 

Due to the turbulent nature of flow in the throat, H_b is usually measured in a stilling well.  Likewise, the flow in the discharge portion of the flume is also turbulent (although not as great as in the throat) and Hc also usually measured in a stilling well.

Observations by Skogerboe, Hyatt, England, and Johnson while investigation the submergence of a two-foot Parshall flume, showed that while the water level in the H_b stilling well fluctuated more than in the H_c stilling well, that the accuracy obtained from the submergence plots was essentially equal.  

Consequently, the investigators felt that there was no particular advantage in changing the point of measurement from H_b to H_c for their submerged flow investigations.

This is not to say that the levels were the same at H_b and H_c, merely that the end result was nearly equal. For their investigation, the submergence transition for the flume they studied was found to be 66% at H_b and 74% at H_c – so it is clear that the water levels were not the same (as H_a was constant).

Therefore, if the secondary point of measurement, H_b, is obstructed or otherwise cannot be used, the data from H_c has been shown to be equally useful, however the submergence transition changes due to the change in location.  This new value of submergence transitions using H_c in lieu of H_c have not been widely published (if published at all).

 

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Source:  Submergence in a Two-Foot Parshall Flume

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