Methods of discharge measurement

The two most common methods of determining the discharge of a flume are flow-measuring flumes and measuring weirs. In both methods, there is a fixed relationship between discharge depth h and discharge Q.

Flow-measuring flumes

Venturi flumes are specially shaped flumes with defined lateral contraction, sometimes also with a shaped bottom. The constriction dams up the discharge Q. The backed-up water ensures that subcritical discharge occurs in the flume. The constriction is where acceleration (including flow transition) from subcritical to supercritical discharge takes place. Critical discharge is present at the narrowest cross-section. This results in a hydraulic jump in the expansion section of the venturi flume. The discharge Q is calculated from the discharge depth hu in the upstream water.

The GUNT venturi flumes have a flat bottom.

To prevent distortions to the measurement in the venturi flume, it is essential that discharge is free. The discharge depth hu in the upstream water should not be affected by the downstream water.

Parshall flumes are venturi flumes with a profi led bottom. The ratios of constriction and enlargement are defined. Parshall flumes are commercially available as a complete component including a discharge curve (discharge Q as a function of the discharge depth hu in the upstream water). They are widely used in North America.

A plan view of venturi or Parshall flume, B side view of a Parshall flume; 1 narrowest cross-section, 2 hydraulic jump; hu upstream water discharge depth, Q discharge

Trapezoidal flumes are another type of flow-measuring flumes. The flow cross-section is triangular or trapezoidal with smooth walls. In contrast to Parshall flumes, they often have a smaller pressure head loss for the same discharge and are more suitable for small discharges. Flow-measuring flumes are mainly used in wastewater treatment plants because they are well suited for contaminated water. They can be easily maintained.

Trapezoidal flume HM 162.63

Measuring weirs

Measuring weirs are usually sharp-crested weirs. They have a simple design, require little space and are relatively easy to construct.

Measuring weirs are used in order to determine the discharge Q. The quantity is measured by detecting the weir head ho upstream of the weir. There must be a minimum distance of 3ho between the measuring point and the weir. To convert the weir head ho into the discharge Q, there are approximation formulae that take into account the geometry of the measuring weir and the discharge coefficient according to Poleni.

Measuring weirs always have free overfall.

Sharp-crested weirs in the form of plate weirs exist with different geometries, such as:

  • rectangular weir according to Rehbock

Use at relatively uniform discharge of more than 50m³/h, but reduced accuracy in the lower part of the measuring range. The rectangular weir requires guaranteed aeration.

  • v-notch weir according to Thomson

Use with varying discharges (0,75...240m³/h); high measuring accuracy for smaller discharges.

  • trapezoidal weir according to Cipoletti

Use at relatively uniform discharges greater than 125m³/h.

Aerated free overfall at the sharp-crested weir

vu velocity in the upstream water, ho weir head, W height of weir

Flow over typical measuring weirs in side and plan view

rectangular weir without contraction

v-notch weir according to Thomson

trapezoidal weir according to Cipoletti