Page 68
Aanderaa Data Instruments AS – TD302
Significant Wave Height:
Originally, Significant Wave Height,
1/ 3
H
, was defined as the average height of the 1/3 highest waves in a wave
record.
1/ 3
H
should roughly approximate to visually observed wave heights.
The wave energy for a superposition of sinusoidal waves is given as:
2
2
0
0
8
2
N
N
i
i
i
i
a
g
E
H
g
ρ
ρ
=
=
=
=
∑
∑
Comparing this equation with the expression for the moment of zero order,
m
0
, it is easily seen that an expression
for the wave height can be estimated.
For historical reasons a parameter that corresponds close to
1/ 3
H
has been defined:
0
0
4
m
H
m
=
Note that both
1/ 3
H
and
0
m
H
are called Significant Wave Height. The parameter provided by the WTS is
0
m
H
.
Maximum Wave Height:
The expected Maximum Wave Height is estimated based on the statistical distribution of the wave height parameter.
Assuming that the sea surface can be modelled as a narrow-banded Gaussian Process, the standard model is a wave
amplitude distribution based on the Rayleigh‘s distribution. This yields the following estimate of the Maximum Wave
Height:
(
)
(
)
MD
MD
Max
R
R
C
H
ln
5
.
0
ln
1
⋅
=
=
where
C
1
is a scaling parameter and
R
MD
is equal to the ratio of the
Mean Wave Period
and the
Duration of wave
record (
the number of waves in the wave record). The WTS uses the above expression for calculation of the
maximum wave height.
NOTE! The assumptions regarding the scale parameter, C
1
, and the statistical distribution will influence the estimate
of the maximum wave height.
