5.9 Vertical Non-Uniform Current Effect
225
Seaward to the midstream, the relative height h/ho is decreased to a lesser
extent with a ten-percent wave height excess, occurring at y = -70 km. But
the reversed case (i.e., h < ho) is even observed at IYI > 90 km.
Later on, our theoretical estimation of wave height enlargement in the
Agulhas Current was proved by satellite observations, obtained by Grundligh
and Rossouw (1995).
Thus, the spatial distribution of the wave height in the Agulhas Current
permits us to come to the following conclusion. The probability of observing
abnormal waves is much less in going towards the shore at some distance from
the line of maximum velocity with the 200 m isobath, than going the same
distance from the maximum velocity current in the direction of the open sea.
Conclusion. The obtained results indicate that the peculiar velocity distribution of the Agulhas Current induces refraction of the south-west swell
propagating over a wide area of the south-western Indian Ocean. This leads
to the fact that the swell turns towards the current velocity maximum. The
swell is trapped, intensified by the countercurrent and localized in the vicinity of the maximum velocity, propagating along the south-east coast of South
Africa. As a result, there is a significant concentration of wave energy density, i.e. a focusing takes place in the given area resulting in abnormal wave
generation.
Navigators sailing along the south-east coast of South Africa should be
recommended to remember that freak waves can suddenly appear in that
area. It may happen in the case of the south-west swell propagating against
the current. If the atmospheric pressure decreases while the wind increases,
changing its direction from north-east to south-west, the ship should leave
the 200-m depth region with the strongest current, turning either towards
the coast, with the depth being less than 100m or seaward to deeper than
the 200-m isobath areas.
5.9 Influence of Vertical Non-Uniform Current
on Wind Wave Transformation
Till now we have considered the transformation of wind waves in a current
that do not varying vertically. But this assumption is not always fulfilled in
natural conditions as long as the currents are usually vertically non-uniform.
Thus, in most cases the wave estimation error appears due to ignoring the
current's vertical non-uniformity in the bottom and near-surface layers. That
is why it is important to estimate the flow kinematics not only in the nearbottom layer in sediment transportation problems, but also in wind wave
generation in the surface layer.
In order to describe the wave evolution in a current in general, the adiabatic invariant conservation law should be applied (see Chap. 1). This is valid
for a current that is non-uniform both along the horizontal and the vertical
225
Seaward to the midstream, the relative height h/ho is decreased to a lesser
extent with a ten-percent wave height excess, occurring at y = -70 km. But
the reversed case (i.e., h < ho) is even observed at IYI > 90 km.
Later on, our theoretical estimation of wave height enlargement in the
Agulhas Current was proved by satellite observations, obtained by Grundligh
and Rossouw (1995).
Thus, the spatial distribution of the wave height in the Agulhas Current
permits us to come to the following conclusion. The probability of observing
abnormal waves is much less in going towards the shore at some distance from
the line of maximum velocity with the 200 m isobath, than going the same
distance from the maximum velocity current in the direction of the open sea.
Conclusion. The obtained results indicate that the peculiar velocity distribution of the Agulhas Current induces refraction of the south-west swell
propagating over a wide area of the south-western Indian Ocean. This leads
to the fact that the swell turns towards the current velocity maximum. The
swell is trapped, intensified by the countercurrent and localized in the vicinity of the maximum velocity, propagating along the south-east coast of South
Africa. As a result, there is a significant concentration of wave energy density, i.e. a focusing takes place in the given area resulting in abnormal wave
generation.
Navigators sailing along the south-east coast of South Africa should be
recommended to remember that freak waves can suddenly appear in that
area. It may happen in the case of the south-west swell propagating against
the current. If the atmospheric pressure decreases while the wind increases,
changing its direction from north-east to south-west, the ship should leave
the 200-m depth region with the strongest current, turning either towards
the coast, with the depth being less than 100m or seaward to deeper than
the 200-m isobath areas.
5.9 Influence of Vertical Non-Uniform Current
on Wind Wave Transformation
Till now we have considered the transformation of wind waves in a current
that do not varying vertically. But this assumption is not always fulfilled in
natural conditions as long as the currents are usually vertically non-uniform.
Thus, in most cases the wave estimation error appears due to ignoring the
current's vertical non-uniformity in the bottom and near-surface layers. That
is why it is important to estimate the flow kinematics not only in the nearbottom layer in sediment transportation problems, but also in wind wave
generation in the surface layer.
In order to describe the wave evolution in a current in general, the adiabatic invariant conservation law should be applied (see Chap. 1). This is valid
for a current that is non-uniform both along the horizontal and the vertical