GUIDE WAVE ANALYSIS AND FORECASTING - WMO

matter and timing the intervals between successive crests
passing by. It is necessary to do this for a number of
waves, as many as is convenient and preferably at least
ten, to form a good average value.
8.3.1.2 Observations from coastal stations
HEIGHT AND PERIOD
At coastal stations, it is important to observe the height
and period of waves at a spot where they are not
deformed either by the wave being in very shallow water
(i.e. of a depth only a low multiple of the wave height),
or by the phenomenon of reflection. This means that the
spot chosen for observations should be well outside the
breaker zone; not on a shoal or in an area where there is
a steep bottom gradient, nor in the immediate vicinity of
a jetty or steep rocks which could reflect waves back on
to the observation point. The observation point should be
fully exposed to seaward, i.e. not sheltered by headlands
or shoals.
Observations are generally more accurate if a fixed
vertical staff with some form of graduated markings can
be used to judge the height of passing waves. Observing
an object on the surface, such as a floating buoy, may
also improve estimation.
If the observations are to be of use for wave
research it is important that:
(a) They should always be taken at the same place, so
that correction for refraction, etc. can be applied
later;
(b) The exact mean depth of water at the place and
time of observation should be known, so that
corrections for change of height with depth can be
applied later.
It is worth repeating here that, as for observations
from ships, only the well-developed waves in the centres
of the groups should be chosen for observation. The flat
and badly formed waves between the groups should be
entirely omitted, both from height and period observations.
The mean height and period of at least 20 waves,
chosen as above and hence not necessarily consecutive,
should be noted.
DIRECTION
Coastal observations of wave direction are only meaningful
at that particular location. If the user does not
realize that the data were obtained at a site where
shallow water has a major effect, i.e. refraction, his interpretation
of the report could be erroneous. This problem
is described more extensively in Chapters 1 and 7 (see
also Figures 1.6, 1.7 and 1.8 which illustrate some
effects in the coastal zone).
8.4 Instruments for wave measurements
Many different techniques are used for the measurement
of sea waves. There is no universal instrument suitable
for all wave measurements. An instrument used at one
location may be entirely inappropriate at another posi-
WAVE DATA: OBSERVED, MEASURED AND HINDCAST 91
tion. Furthermore, the type of instrument to be deployed
depends on the application for which the wave data is
required. For example, the design of breakwaters at the
coast depends on the properties of the longer waves or
swell while the motions of small ships are influenced by
short, steep seas. Care must therefore be taken in the
choice of instrument. Finally, the chosen instrument
must be capable of being readily deployed and have a
good prospect of giving a high return of wave data for
climatological purposes over at least one year.
8.4.1 Types of instrument
Wave measurement techniques can be grouped into three
categories:
(a) Measurements from below the sea surface;
(b) Measurements at the sea surface;
(c) Measurements from above the sea surface.
8.4.1.1 Wave measurements from below the surface
Systems to measure waves from below the surface have
an advantage in that they are not as vulnerable to
damage as systems on the surface. However, there are
problems in bringing the data ashore as cable is expensive
and can be damaged. An alternative technique is to
transmit the information by radio from a buoy moored
nearby.
Pressure transducers are most often used in shallow
water (< 15 m) but have also been mounted below the
surface on offshore platforms in deep water. Here the
change in pressure at the sensor is a measure of wave
height. A pressure spectrum at the measurement depth
can be derived from the pressure signal using spectral
analysis. The measured pressures must be corrected for
hydrodynamic attenuation with depth. For this purpose
linear wave theory (see Section 1.2) is used (although
there is evidence that the correction it gives is too small).
One-dimensional wave spectra and associated parameters
such as significant wave height can then be
calculated. The attenuation has the effect of filtering out
the shorter wavelengths, but for most practical applications
the loss of high frequency wave information is not
a disadvantage. However, if the water depth is greater
than about 10–15 m, the attenuation affects too much of
the frequency range and the correction factors become
very large, diminishing the value of the data.
Inverted echo sounders on the sea-bed can also be
used in shallow water. The travel time of the narrow
beam of sound is directly related to wave elevation and
gives a measurement without depth attenuation. In
severe seas, however, the sound beam is scattered by
bubbles from breaking waves making the measurements
unreliable.
8.4.1.2 Wave measurements at the sea surface
In shallow water, where a platform or structure is available,
it is possible to obtain wave measurements at the
sea surface by using resistance or capacitance wave
staffs. The wave elevation is then directly related to the