The electronic equipment to measure fluid velocity, pressure and wave height commonly consists of:
electromagnetic velocity sensors,
acoustic velocity sensors (point sensors and profilers),
optical sediment concentration point sensors,
acoustic sediment concentration point sensors and profilers,
optical particle tracking sensors (size and fall velocity),
acoustic bed level sensors (altimeters, single/multi beam echo sounders; bed profilers, side scan sonar),
data storage discs.
This section describes different instruments to measure velocity. For additional background information, see also light fields and optics in coastal waters. For information about the instruments, see also Acoustic point sensors (ASTM, UHCM, ADV).
Velocities and bed-shear stresses, instrument characteristics and accuracies
Electro-magnetic velocimeters (EMV) are among the best instrumentation available for studying the structure of the bottom boundary layer where sediment transport takes place.
The EMV’s are robust, resistant to fouling, moderately intrusive, and reasonably inexpensive, but they also suffer from severe limitations including offset drift, limited frequency response and relatively large sampling volume.
Within the last few years, acoustic instruments have become increasingly available for coastal conditions. These instruments are also reasonably robust, resistant to fouling, and increasingly affordable. In addition, acoustic instruments are less intrusive, have better frequency responses and smaller sampling volumes. Examples are, the Acoustic Doppler Current profiler (ADCP), Ultrasonic Velocity Profiler (UVP) and the Acoustic Doppler Velocitymeter (ADV).
Electro-Magnetic Velocitymeter (EMV)
These instruments are based on the principle that a conducting fluid will generate a voltage proportional to the flow velocity as it passes through the magnetic field created by the sensor.
Acoustic Doppler Velocitymeter (ADV)
Basically, the ADV measures the velocity of particles (sediments) at a point in the water column from the Doppler shift in frequency of the emitted and received acoustic signals (without calibration) in 2 or 3-directions, depending on the sensor arrangement. The system includes three modules: sensor, signal conditioning module and signal processing module. The measurement probe consists of four ultrasonic transducers: a transmit transducer located at the bottom end of the stem and three receive transducers, slanted about 30o from the axis of the transmit transducer and pointed at the sampling volume, which is located about 0.1 m below the probed tip. Hence, the flow velocity in the sampling volume is not disturbed by the presence of the probe. The acoustic frequency is of the order of 10 MHz. The accuracy is of the order of approx.1% of the reading.
Acoustic Doppler Current Profiler (ADCP, UVP)
ADCP instruments are being used as:
bottom-mounted (big-size upward-looking for velocity profiles over the water column; or small-size downward-looking for near-bed velocity profiles),
ship-mounted (big-size downward-looking).
The ADCP profiler measures the current profile in water using Acoustic Doppler technology. It is designed for stationary and non-stationary (ship’s hull mounted) applications. It can be deployed on the bottom, on a mooring rig, on a buoy or on any other fixed structure. It is a complete instrument and includes all the parts required for a self-contained deployment with data stored to an internal data logger. Typical applications include coastal studies, online monitoring and scientific studies in rivers, lakes, estuaries and tidal channels.
Phased Array Doppler Sonar (PADS)
An acoustic signal from a bottom-mounted sonar is projected in a wide horizontal fan, radiating outward in the water from the instrument package and filling the water column in shallow water. The sound scatters off particles in the water (especially bubbles) and off the bottom. Some backscattered sound returns to the sonar, where the signal is received on an array, beamformed into returns from various directions, and analyzed for frequency shift versus direction and elapsed time since transmission. For direct-path transmission and return, the time-delay since transmission translates to distance from the sonar. The frequency shift of the backscattered signal (Doppler shift) is proportional to the radial component of the velocity of scatters at the sample volume.
Coherent Doppler Velocity Profiler (CDVP)and Cross-Correlation Velocity Profiler (CCVP)
The backscattered signal from suspended particles in the flow can be utilised to determine the velocities of the particles. The two techniques are: coherent Doppler method, and cross-correlation method.
The coherent Doppler method is based upon pulse-to-pulse phase coherence between consecutive transmissions to measure the radial component of the velocity along the beam axis. This instrument uses Doppler shift to obtain the Doppler flow velocity. The Doppler frequency is obtained from the pulse-to pulse coherence (phase coherence). Averages over pulse pairs are taken.
The correlation method employs a pair of horizontal separated transducers directed vertically downward and cross-correlation of the backscattered signals from the transducer pairs is used to obtain the velocity.
Unlike the coherent Doppler system, the correlation method is incoherent, as it is the signal intensity that is used. The basic requirement is that there are fluctuations in the suspension field, which have spatial scales greater than the distance between the transducers and that these fluctuations can be cross-correlated. Small-scale turbulent fluctuations cannot be measured.
Comparison of measured velocities
Results of various instrument comparisons are given in the manual:
Electro-Magnetic Velocitymeter (EMV) and Laser Doppler Velocitymeter (LDV)
Acoustic Doppler Velocitymeter (ASTM) and Electro-Magnetic Velocitymeter (EMV)
Acoustic Doppler Velocitymeters (ADV)
Ultra-sonic Velocity Profiler (UPV) and Particle Image Velocitymeter (PIV)
Fluid pressure and wave height instruments
This section describes instruments to measure fluid pressure and wave height. For more information, see also Currents and turbulence by acoustic methods.
General instrument characteristics, accuracies and selection criteria
Water level fluctuations in deeper water generally are measured by pressure sensors or by wave buoys. Water level fluctuations can also be measured by capacitance wires/rods attached to poles jetted into the bed.
Bottom-mounted applications can, in principle, also be used to determine the instantaneous wave height by using the horizontal velocity measured in the near-surface region and linear wave theory.
The wave rider buoy (hull diameters up to 1 m) is a spherical buoy, which measures wave height and direction. The wave height measurement is based on the principle of measuring vertical accelerations. The direction measurement is based on the translational principle which means that horizontal motions instead of wave slopes are measured.
Various types of pressure sensors are commercially available. Generally, piezo-electric transducers are used. Piezo-electric materials such as quartz crystals produce an electric field under deformation by pressure forces. The instrument offset can be determined in the laboratory prior to deployment and taken into account by the calibration curve.
The ADCP instruments can also be used for wave field measurements. The basic principle behind wave measurement is that the wave orbital velocities below the surface can be measured by the highly accurate ADCP. The ADCP (with 2 Hz data recording and upgrade of software for waves) measures the subsurface orbital velocities created by the wave field. This raw data is averaged to create a mean current profile, and is accumulated into time series for waves processing. Each time series of data is called a burst. From this burst, velocity power spectra, directional spectra, and mean water levels are calculated. The ADCP should be bottom mounted, upward facing (within 5 degrees of the vertical) with a pressure sensor for measuring tide and mean water depth.
Comparison of measured wave heights
Results of various instrument comparisons are given in the manual:
Pressure sensor and capacity wire
Pressure sensor and surface following wave gauge
Velocity sensor, fluid pressure sensor and capacity wires
Pressure sensor and resistance wave staff
Accelerometer and DGPS on Waverider buoy