If a Parshall Flume becomes submerged, what is the impact on the flow through the flume?

When a Parshall flume becomes submerged, the flow measurement becomes compromised, leading to a decrease in the effective flow rate through the flume. A Parshall flume is designed to provide accurate flow measurements based on the height of the water above the flume's throat. When the flume is submerged, the dynamic relationship between the flow rate and the head increases the resistance to flow and alters the hydraulic conditions.

Submergence occurs when the tailwater level rises to a point that it interferes with the free flow of water exiting the flume, thus increasing the back pressure. This causes a reduction in the effective flow area and disrupts the calibration relationships used to determine flow from head measurements, which can lead to an underestimation of actual flow. Consequently, the overall flow rate through the flume decreases, making it vital for inspectors to recognize this condition in order to maintain accurate measurements and assess potential system inefficiencies effectively.