For a Flume:  submergence is the effect of downstream flow conditions on the free discharge of flow out of the flume.  

• Submergence can occur because of a change in the downstream slope, reduction in the downstream channel, bends, elbows, or even vegetative growth
• The submergence transition is defined as the ratio of the downstream water level to the water level at the flume's point of measurement at which the free flow out of the flume must be corrected
• Uncorrected, the indicated flow rate will overstate the actual flow rate
• As submergence increases the greater the correction
• Some flumes - Parshall, Cutthroat, and H Type - have had equations developed to correct for submerged flow
• Other flumes - Palmer-Bowlus, Trapezoidal, and RBC - have not had equations developed to correct for submerged flow (primarily because their submergence ratios are so high)
• In general it is advisable to design for free flow conditions - although occasionally channel configurations may be such that an installation has to be designed from the outset for submerged flow
• Submerged flow equations can be complex and most flow meters are not preprogrammed for them