How are Flows Measured?

Flow measurement in the UK rarely presents the difficulties of access, large velocity ranges, inadequate hydraulic conditions and paucity of hydrometric equipment and trained personnel that are common in less hospitable environments throughout the world. Nonetheless UK rivers – typically short, shallow and subject to substantial artificial disturbance – present a challenge both in relation to accurate flow measurement and in ensuring the ‘fitness-for-purpose’ of the derived flow data.

Indirect Flow Measurement

River flows are normally measured indirectly – relying on the conversion of a record of water level (or stage) to flow using a stage-discharge relation, often referred to as the rating or calibration. At primary gauging stations stage is generally measured and recorded against time by instruments actuated by a float in a stilling well.  Solid state loggers are normally deployed to record water level, having gradually replaced the punched tape and analogue chart recorders that were used as standard 30 years ago. For the great majority of the gauging stations in the UK provision is made for the routine transmission of river levels directly to a regional processing centre, most commonly by telephone line. 

Stage-Discharge Relation

For a substantial proportion of UK gauging station a stable relationship between river level and river flow is achieved by installing a gauging structure, usually a weir or flume with known hydraulic characteristics. In the absence of such structures, the development of the stage-discharge relation is a two stage process. Firstly, stream velocity is measured using propeller-type current meters or other methods – increasingly Acoustic Doppler Current Profilers (ADCPs) are being used, the latter offer significant advantages (e.g. speed of flow assessment and greater safety for the operators) over traditional current meters.  In a few cases, where field conditions permit, ADCPs may be used to provide a continuous flow record.

Thence, the mean velocity is combined with the cross-sectional area of the river to provide a measurement of flow. This procedure is repeated throughout the flow range allowing rating equations to be developed which facilitate the conversion of water levels into river flows. Gauging stations are normally sited in river sections characterised by their ability to maintain a reasonably stable relationship between river level and flow. However, this relationship may be disturbed by changes to the hydraulic characteristics of the gauging reach, for example due to changes in the bed profile following a flood or the seasonal impact of aquatic plant growth.  Such circumstances are relatively common and necessitate an ongoing review, and updating, of the rating. 

Ultrasonic Flow Measurement

For ultrasonic gauging stations, which have found wide application since the late-1970s, a stable relationship between river level and flow is not a necessary requirement. Flows are computed on-site where the times are measured for acoustic pulses to traverse a river section along an oblique path in both directions. The mean river velocity is related to the difference in the two timings and the flow is then assessed using the river's cross-sectional area. 

Accurate computed flows can be expected for stable river sections and within a range in stage that permits good estimates of mean channel velocity to be derived from a velocity traverse set at a series of fixed depths. Accuracy can be compromised by high suspended sediment concentrations or heavy weedgrowth which can impede the acoustic signal, or by thermal stratification in the water column – serving to deflect the acoustic beams. 

Electromagnetic Flow Measurement

Flow data from electromagnetic gauging stations may also be computed on-site. The technique requires the measurement of the electromotive force (emf) induced in flowing water as it cuts a vertical magnetic field generated by means of a large coil buried beneath the river bed or constructed above it. This emf is sensed by electrodes at each side of the river and is directly proportional to the average velocity in the cross-section. 

As a consequence of technical, maintenance and health and safety issues there remain only a very modest number of electromagnetic stations remain operational in the UK