A flume “drowns” or is submerged when the flow out of the flume is restricted by downstream conditions to the point where the free flow discharge equation is no longer valid – over indicating the actual flow rate through the flume. When a flume is drowned, adjustments must be made to the discharge equation for the installation to be accurate.
The transition from free, unrestricted flow, to drowned, or submerged flow, is known as the submergence transition (St). For submergence ratios below the submergence transition, the flow is free, for submergence ratios above the submergence transition, the flow is submerged. The submergence ration is the head at the point of measurement in the flume (Ha) over the head at the secondary (downstream) point of measurement (Hb).
Submergence transitions can be as low as 25% (for HS / H flumes) or as high as 90% (for larger Palmer-Bowlus flumes). For flumes with low submergence ratios, it is normally best to design for free-spilling discharge as the measurement complexity of submerged flow normally outweighs the benefits of a drowned installation.