The Area Velocity (AV) method has several advantages when compared with those that rely on weirs and flumes. The first is that, in addition to measuring flow under free flow conditions, it can also be used to measure flow under submerged, full pipe, surcharged, and reverse flow conditions. The latter conditions can occur due to a variety of circumstances, such as undersized sewers, inflow and infiltration, and tidal effects.

The AV technique can be used in two ways. First, the depth and velocity can be measured manually, and then used to determine the wetted area and flow rate at a particular time. Alternatively, an AV flow meter can be used to measure the liquid level and velocity in the channel, and automatically calculate the flow rate. These two alternatives are further discussed in the following sections.

Devices traditionally used to measure velocity in open channels are commonly known as current meters. There are a number of different types of current meters available. Some of the more common types of velocity measuring techniques include floats, tracers, pitot tubes, eddy-shedding vortex meters, turbine meters, rotating element meters, Doppler ultrasonic meters, Doppler laser meters, and electromagnetic probe meters. ​

AV flow meters are commonly used in sewer flow monitoring, inflow and infiltration studies, combined sewer overflow studies, wastewater treatment plant operations, and regulatory compliance.   Doppler technologies used for flow velocity measurement include several types of acoustic, optical, and electronic Doppler measurement. The following sections discuss each of these technologies, their operating principles, and their advantages and limitations  

Weighted Average Continuous W​​ave Doppler

With this type of measurement device, a sensor mounted in the flow stream transmits a continuous signal of a given frequency into the flow. Reflections off of suspended particles and entrained air in the flow are continuously returned to the sensor. These reflections are representative of a sizeable cross-sectional wetted area. This return signal is then analyzed and weighted proportionally. The result is that the reported velocity is a true mean velocity based upon cross-sectional measurement of the flow rather than a single point run through a conversion factor.   Some continuous wave sensors, such as the TIENet sensors from Teledyne Isco, are solid-state and completely sealed, which prevents the possibility of internal fouling of the sensor. 

Peak Measurement Continuous Wav​e Doppler

Peak Doppler devices identify the peak velocity in the flow stream by determining the highest frequency present in the return signal. Peak Doppler devices have an advantage in that they do not depend on a particular weighting function.   For non-free-flow conditions, the accuracy of this measurement remains dependent on the accuracy of manual or automatic calibration measurements of the velocity profile during installation of the meter. Above and beyond normal measurement errors, an additional error can arise from both the calibration measurement and the variations in the velocity profile.  

Pulse Doppler with Range G​​ating

A pulse Doppler device with range gating is capable of measuring velocity in multiple, discrete volumes throughout the water column. These velocity data are used to calculate a flow velocity distribution throughout the flow, which is then integrated over the flow's cross-sectional area to determine the discharge.   However, unlike a continuous wave Doppler device, where the receive circuit actively listens for all returned echoes to estimate an average or peak velocity, a range-gating system divides the return signal of each beam into discrete regular intervals, often referred to as “depth cells," that correspond to different depths in the flow along each beam. Velocity is calculated from the frequency shift measured in each interval. The result is a profile, or linear distribution of velocities, along the direction of each beam. The number of intervals in which velocity is measured depends on the depth of flow.  

Non-Contact​ Laser Doppler

Velocity Measurement Below the Surface A laser velocimeter, as the name suggests, uses a laser beam to measure velocity remotely. Mounted above the water, the sensor transmits a laser beam that penetrates the water and focuses at single or multiple points below the surface of the flow stream. The light is scattered back to the sensor. The returned light is frequency shifted due to the Doppler effect and the motion of the flow. The frequencies of the transmitted light and received light are compared to determine a Doppler shift, which is then used to calculate flow stream velocity.  

Electrom​agnetic

Electromagnetic probes measure velocity using the Faraday principle. The Faraday principle states that a conductor moving through a magnetic field produces a voltage that is proportional to the velocity of the conductor.   Electromagnetic probes measure the local velocity of the flow at the location of the electrodes at the bottom of the channel. Because the electrodes on the probe are exposed to the flow, they are subject to fouling and often require frequent cleaning. The accuracy of an electromagnetic probe is questionable during the time that its electrodes are fouled.  

Surface Rad​ar

Traditionally, surface velocity measurement involved observing an object floating on the surface and measuring the amount of time it took that object to travel a specific distance. Radar Doppler systems remotely measure a surface velocity using the Doppler principle with electromagnetic radiation as the measurement medium.   A radar Doppler device transmits an electromagnetic signal to the liquid surface at a known frequency and defined angle. Turbulence on the water surface reflects the signal back to the transmitter. This returned signal is analyzed for a Doppler shift in frequency that gives the velocity of that portion of the surface illuminated by the electromagnetic radiation. The main advantage offered by such a remote-sensing device is that it is installed outside of the flow stream.

​Frequently Asked Que​stions

What is a water velocity ​met​er?

Devices traditionally used to measure velocity in open channels are commonly known as current meters. There are a number of different types of current meters available. 

How ​​do you m​​easure water velocity?

In the simplest method, a current meter turns with the flow of the river or stream. The current meter is used to measure water velocity at predetermined points (subsections) along a marked line, suspended cableway, or bridge across a river or stream. The depth of the water is also measured at each point.

What instrument is used to measure veloci​ty i​​n water?

Velocity can be measured directly, using a flowmeter (essentially a speedometer for water) or inferred by timing the movement of a float in the water.