Using a Velocimeter 33
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Since these estimates are based on the wave energy distribution, and not a direct measure of the
free surface, they are considered inferred estimates. Fourier transforms are used to separate the
signals into different frequency bands so that it can determine the direction separately for each band.
This means that if you have a long-period swell coming from one direction, and a shorter period
coming from another, you can tell the direction for each of them separately. The main assumption for
standard PUV wave measurements is that waves at a given frequency come from one primary
direction.
Limitations
The most important thing to understand about the PUV method is that it is limited to
1. Deployment depths that are shallow (less than 10 meters) and
2. waves that are long (approximately periods of 4 seconds or longer).
The limitation of only measuring long waves (swell) is the one that should raise a warning flag for
those who are interested in the complete description of the wave environment.
Example: Investigating a structure’s response only to swell in shallow water. The accuracy of the
solution requires measurements over the entire wind wave band (waves with periods of 0.5-30
seconds). Incomplete coverage of the wind wave band can result in underestimation of wave height
and missing peaks in the spectrum. The only way to improve the coverage of the wind wave band
is to deploy the instrument in relatively shallow water (i.e. 3 meters depth).
2.1.8.2 Wave Parameters
The most common non-directional wave parameters describe height and period of the wave field and
are single values representative of the time series. In many practical situations, the directions of the
waves are also of great interest and these are based on the first pair of Fourier coefficients and
describe the mean direction at a given frequency. The Post-processing of raw wave data will lead to
classic wave estimates for the following wave parameters:
Known as Significant Wave Height, defined as the mean of the
highest 1/3 of all waves in the record’s ranking.
Peak period of the waves corresponding to the peak frequency.
Peak direction is the wave direction at the frequency at which a
wave energy spectrum reaches its maximum
Measure of the directional variance at peak frequency.
Mean direction. Weighted average of all the directions in the
wave spectrum.
Measure of how much of the wave energy over the full spectrum
is from a single direction. Value of 1.0 indicates the energy is
from one primary direction
2.1.8.3 Deployment Considerations and Cautionary Notes
The PUV approach is based on linear wave theory; occasionally situations arise where linear theory
is no longer valid and second order effects become increasingly significant. Such situations occur
when one attempts to improve the instrument’s response by placing it in increasingly shallower
water. Long waves in shallow water become solitary, and hence have a nonlinear profile that is not
accounted for by the PUV transformation. The result is that waves are properly sensed but the wave
height is slightly underestimated.
Perhaps the most difficult aspect of classifying sea state via spectral based estimation is the
realization of wave energy at high frequencies. These types of spectra prove to be quite tricky.
Amongst the problems are the errors in the calculation of moments and cutting off peaks at higher
frequency. One scenario is that an instrument placed in 15 meters of water may never detect the
energy up above 0.25 Hz.
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