A long-throated flume is class of flume in which the throat is considered to be long enough to cause the flow to form nearly parallel flow lines.
Parallel flow allows long-throated flumes to be rated by fluid flow analysis - without the need to develop the level-to-flow relationship in a laboratory. Long-throated flumes can have nearly any desired cross-section and can be developed to fit into most canals.
Long-throated flumes are site specific, without standardized dimensions / flow equations. As a result, they can present problems for the casual user and are not commonly used outside of irrigation / water rights applications.
Image: Water Measurement Manual