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Submerged Flow

flow in a submerged galvanized steel parshall flume measuring water rights flowsFor a Parshall flume to operate as designed, the flow out of the flume must be free and not restricted by downstream conditions.  When downstream resistance to flow increases above a certain point and now reduces the upstream velocity through the flume a backwater effect is created.   This transitions from free, unrestricted flow, to one of backwater / slowed velocity is known as the submergence transition (St).

It is important to remember that although a Parshall flume may have originally designed for free-flow at an installation, changes in the downstream hydraulics can submerged the flume.  Typically, these changes are due to downstream vegetation, sediment deposit, added hydraulic structures, or channel reconfiguration.  Don't assume that a once free-flowing Parshall will always be a free-flowing Parshall.

Submergence Transition

The submergence transition is the ratio of the secondary point of measurement depth (Hb – located in the throat of the Parshall flume) to the depth at the primary point of measurement (Ha – located in the converging section of the flume).  

For Parshall flumes, the submergence transition increases as the flume size increases.

The chart below (per ASTM D1941) provides the submergence transition for Parshall flumes as:

1-3 inch Parshall


6 inch Parshall


1-8 foot Parshall


10-50 foot Parshall



Submerged Flow Equation

As the flume passes from free to submerged flow (through the submergence transition), the free-flow equation for the flume can no longer be used.  A new and considerably more complex submerged flow equation must be used, as the free-flow equation will increasing overstate the flow as the degree of submergence increases.  

Also, a secondary level measurement must be taken in the flume to determine the level of submergence – adding to the cost and complexity of determining flow.

Secondary Point of Measurement (Hb)

measuring Ha and Hb for submergence in a Parshall flumeThe secondary point of measurement (Hb) is located in the throat of the Parshall, just upstream of the junction of the throat and the diverging (outlet) sections of the flume.  The zero elevation from which the Hb level is determined is the same as the floor of the converging section (in which Ha is measured).

As the submergence in the flume increases, small measurement errors determining the level become critical.  Submergence corrections are therefore capped at 95%, although the difficult of measuring Hb make a submergence ratio of 90% the upper end of correctable flow.

Aid to Measuring Hb

dual stilling wells on a fiberglass Parshall flume used to measure submerged flow

Flow through the throat of a Parshall flume can be quite turbulent – making an accurate determination of Hb difficult.  To make the measurement of Hb easier, a stilling well is typically used.  The stilling well helps to reduce surface turbulence and dampen sudden changes in water level so that more accurate depth readings can be taken. 

Keep in mind that stilling wells are not suitable for use on sanitary flows and may create a maintenance issue where sediments / solids are present as they may precipitate out of the flow and collect in the bottom of the stilling well (and potentially clog the inlet port).

Benefits of Submerged Flow

While Openchannelflow normally recommends that Parshall flume installations be designed for free-flow, that's not to say that submerged flow installations don't have certain benefits.  

The Installation / retrofit costs are usually less for submerged flow installations as there isn't a need to elevate the flume in the channel, build up the upstream channel banks, or modify the downstream channel floor.