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180 Fundamentals of Ocean Renewable Energy
spectra. As mentioned earlier, such directional information is useful for wave
energy devices (e.g. attenuators) that must be aligned with the direction of wave
propagation.
7.2.2 Pressure Transducers
Wave buoys, as described in the previous section, are relatively expensive, for
example, in the region of $60,000 for a directional wave buoy, in addition to
operation and maintenance costs. Therefore, under many circumstances, it may
be desirable to seek a more economical solution, particularly at the earlier
stages of project development. One such alternative is to install a sea-bed
pressure sensor. These instruments measure the total pressure, and analysis of
instantaneous pressure at high frequencies (e.g. > 1 Hz) provides a measure
of wave height and period. However, the accuracy of wave measurements by
pressure sensor is strongly influenced by attenuation effects, and so their use is
limited to water depths of less than 20 m [20]. Such a pressure sensor can also
be used to concurrently determine tidal elevations, but care should be taken to
account for changes in the atmospheric pressure when attempting to distinguish
the astronomical component [18]. Note that by installing an array of sea-bed
pressure sensors, information on the directional characteristics of waves can
also be obtained.
7.2.3 Acoustic Waves and Currents
Some ADCPs include an additional vertical-orientated transducer that can mea-
sure the vertical velocity (Fig. 7.5), and can measure wave properties or estimate
turbulence. Such an instrument is known as an Acoustic Waves and Currents
(AWAC). Waves are traditionally measured by wave buoys (Section 7.2.1), but
by using an AWAC, currents and waves can be measured concurrently at a
location of interest. Because this device is deployed on the sea bed, it does not
suffer from constraints such as severe weather impacts or the influence of marine
traffic. Such an instrument is particularly beneficial for studies of wave-current
interaction. Some researchers have assessed the accuracy of wave measurements
by ADCPs compared with wave buoys. Bouferrouk et al. [21] compared the
wave data collected by a five-beam ADCP and four wave buoys at the WaveHub
(UK)—a site that is dedicated to the testing of wave energy devices. Their results
show, in general, good agreement between ADCP and wave buoys. However,
unlike wave buoys, the five-beam ADCP could also detect long-period swell
waves (0.07–0.08 Hz), for small wave heights, and can only operate in relatively
shallow waters.
7.3 REMOTE SENSING
Although, in some ways, an ADCP could be considered a remote-sensing
platform, remote sensing really refers to observations made at some distance
from the medium that is being observed. Remote sensing most often refers to