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The Submergence Ratio (S) of a Parshall flume is a comparison of upstream and downstream water levels to determine if downstream conditions are affecting (slowing) the discharge out the flume.  

Since the Parshall flume is an empirical device, its important that the level readings used for the submergence ratio calculations are only taken at the primary (Ha) and secondary (Hb) points of measurement for the flume.

submergence ratio equation for parshall and other flume types 

The point at which downstream flow conditions affect the discharge out of the flume is termed the Submergence Transition (St).

For Parshall flumes, the Submergence Transitions are (per ASTM D1941):

1-3 inch Parshall

50%

6 inch Parshall

60%

1-8 foot Parshall

70%

10-50 foot Parshall

80%