General instrument characteristics

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.

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