Surface gravity waves exist in the oceans as multi-directional nonlinear phenomena.
Understanding how these two properties interact is intrinsically important
in itself. Furthermore, an understanding of this relationship may be used to gain
insight into other oceanic phenomena. This thesis first describes an experimental
investigation into the relationship between directionality and non-linearity (Part
I). This relationship was then used as a tool to estimate the directional spreading
of field data (Part II).
Experiments have been conducted in which directionally spread focused wave
groups were created in a wave tank. The relationship between the degree of
directional spreading and the second-order bound harmonics of the wave groups
was examined, in particular the formation of a `set-up'. These measurements were
then compared to predictions from second-order theories, finding good agreement.
The two-dimensional structure of the bound waves was explored giving new insight
into the underlying physics. Experiments were then carried out for directionally
spread crossing wave groups. It is believed that the crossing of two sufficiently
separated wave groups may be the cause of an anomalous set-up in the second-order
bound waves observed for some extreme and potentially freak waves. This
set-up is reproduced experimentally. Again, the results of these test agreed very
well when compared to second-order theory.
The insight gained from the foregoing experiments was then utilised in the analysis
of field data. A method, which requires only a single measurement to estimate the
observed degree of directional spreading, was applied to a large dataset of field
measurements from the North Alwyn platform in the North Sea. This method
was then compared to conventional approaches, which require multiple concurrent
measurements. The method that requires only a single measurement was shown
to be effective, and presents a promising approach to gaining additional insight
about the directional spreading of point observations