Measurement of velocity

The simplest method for determining a velocity of flow is by timing the movement of a float over a known distance (sometimes called float gauging). Surface floats comprising any available floating object are often used in rough preliminary surveys; these measurements give only the surface velocity and a correction factor must be applied to give the average velocity over a depth. A factor of 0.7 is recommended for a river of 1 m depth with a factor of 0.8 for 6 m or greater (BS EN ISO 748, 2007). Specially designed floats can be made to travel at the mean velocity of the stream (Fig. 7.11). The individual timing of a series of floats placed across a stream to determine the cross-sectional mean velocity pattern could become a complex procedure with no control of the float movements. Therefore, this method is recommended only for reconnaissance discharge estimates.

The determination of discharge at a permanent river gauging station is best made by measuring the flow velocities with a current meter. This is a reasonably accurate

Fig. 7.11 Floats: (a) surface float; (b) canister float for mean velocity; (c) rod float by mean velocity.

(Reproduced with permission from R. W. Herschy (ed.) (2009) Streamflow Measurement, 3rd edn, © 2009, by permission of Taylor & Francis, Oxford.)

Fig. 7.11 Floats: (a) surface float; (b) canister float for mean velocity; (c) rod float by mean velocity.

(Reproduced with permission from R. W. Herschy (ed.) (2009) Streamflow Measurement, 3rd edn, © 2009, by permission of Taylor & Francis, Oxford.)

Fig. 7.12 A Valeport 'Braystoke' 001 current meter, showing the location of the magnet and reed switch. (Reproduced with permission of Valeport Ltd.)

instrument that can give a nearly instantaneous and consistent response to velocity changes. There are two main types of meter in current use: the impeller type, which has a single impeller rotating on a horizontal axis, and the electromagnetic type.

The impeller current meter (BS ISO 2537, 2007; Fig. 7.12) records the true normal velocity component with actual velocities up to 15° from the normal direction. Following use of a calibration of impellor revolutions to river velocity, this method gives an attainable accuracy of ± 1.5 per cent that can be obtained with the impeller-type current meter in the range of velocities between 0.3 and 10ms-1 (125-mm diameter impellor).

The operation of an electromagnetic current meter (Fig. 7.13) utilises the Faraday principle, where water flow cuts lines of magnetic flux, inducing an electromagnetic force (emf) that is sensed by two electrodes. These current meters can be used to

Dimensions in mm

Fig. 7.13 A Valeport electromagnetic (EM) current meter showing the location of two electrodes and magnet. (Reproduced with permission of Valeport Ltd.)

Dimensions in mm

Fig. 7.13 A Valeport electromagnetic (EM) current meter showing the location of two electrodes and magnet. (Reproduced with permission of Valeport Ltd.)

Fig. 7.14 An acoustic Doppler current profiler (ADCP) showing the transducers. (Reproduced with permission of Teledyne RD Instruments.)

measure river velocities as slow as 0.03 ms-1 (and up to 4 ms-1). They also have the advantage of not having moving parts that can be caught in weeds or damaged against rocks.

In addition to the use of impellor-based and electromagnetic current meters, ultrasonic (Doppler) flow meters (BS EN ISO 6416, 2005) are used increasingly within the UK. These devices use transducers ('loud speakers'), and measure the sound returned (echo) by scatters (e.g. fine sediment) within the water. The Unidata Starflow unit can measure a river velocity of range of 0.02-4.5 m s-1 with an accuracy of ± 2 per cent.

The acoustic Doppler current profiler (ADCP; Fig. 7.14) uses the same principle, but can give a very detailed distribution of river velocity at many locations over the river cross-section when mounted on a boat or float (Fig. 7.15).

To ensure that the mean velocity in a cross-section is estimated with good accuracy, it is recommended that the velocities are averaged over at least five transects across the river. This technique has recently become very popular within agencies such as the United States Geological Survey and the Environment Agency of England and Wales for discharge measurement.

Ultrasonic gauges are permanently installed at over 150 gauging stations within the network of the Environment Agency of England and Wales. These installations comprise pairs of ultrasonic transducers mounted at different depths on both river banks and determine the average velocity at each depth across the channel width (Fig. 7.16). These devices are particularly suitable for rivers subject to tidal flows and where gauging structures are impractical (see Herschy, 2009).

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