General Flumes are shaped, open-channel flow sections that force flow to accelerate.
Acceleration is produced by converging the sidewalls, raising the bottom, or a combination of both.
When only the bottom is raised with no side contractions, the flume is commonly called a broad-crested weir. When the downstream depth is shallow and enough convergence exists between the upstream and downstream channels, the flow passes through critical depth. Therefore, flumes are sometimes called critical-depth flumes. When flow passes through critical depth, a unique water surface profile occurs within the flume or broad-crested weir for each discharge. This condition is known as free flow.