Venturi flume

A venturi flume is a critical-flow open flume with a constricted flow which causes a drop in the hydraulic grade line, creating a critical depth.

It is used in flow measurement of very large flow rates, usually given in millions of cubic units. A venturi meter would normally measure in millimetres, whereas a venturi flume measures in metres.

Measurement of discharge with venturi flumes requires two measurements, one upstream and one at the throat (narrowest cross-section), if the flow passes in a subcritical state through the flume. If the flumes are designed so as to pass the flow from sub critical to supercritical state while passing through the flume, a single measurement at the throat (which in this case becomes a critical section) is sufficient for computation of discharge. To ensure the occurrence of critical depth at the throat, the flumes are usually designed in such way as to form a hydraulic jump on the downstream side of the structure. These flumes are called 'standing wave flumes'

Comparison with weirs

Venturi flumes have two advantages over weirs where the critical depth is created by a vertical constriction. First, the hydraulic head loss is smaller in flumes than in weirs. Second, there is no dead zone in flumes where sediment and debris can accumulate; such a dead zone exists upstream of the weirs.

Parshall flume

One of the most common weirs is the Parshall flume which was developed by Dr. Ralph L. Parshall of the U.S. Bureau of Reclamation in 1922. It consists of three sections, a converging section, a throat section and a diverging section. The critical depth develops in the throat section. The flow upstream and downstream of the throat is subcritical and supercritical respectively. The hydraullic jump forms in the diverging section. The flow in the flume is a 'free flow' if the ratio of the downstream head H_s, measured in the downstream stilling well to the upstream head H_a measured in the upstream stilling well, H_s/H_a is less than about 0.70. The discharge for the free- flow condition is given by

Q=KHⁿ_a

where K and n are functions of the throat width W.

A venturi flume is similar to the Parshall flume, without the contoured base, but the cross section is usually rectangular, the inlet shorter, and there is a general taper on the outlet similar to the venturi meter.. Because of their size, it is usual for these meters to be open to their surroundings just like a river or stream and therefore this type of measurement is referred to as open-channel flow measurement. Parshall flumes are much more efficient than standard flumes and generate a standard wave to effect a measurement.

A good example can be found via google earth: 50°58'41.34"N, 5°51'36.81"E, eye altitude 200 m. This is in the 'Geleenbeek', near Geleen in Holland