Measurements of near bottom wave-induced pressures and orthogonal, horizontal velocity components, at several locations around Australia, have demonstrated that directional wave spectra and wave kinematics may be routinely monitored
This presentation briefly addresses the measurement system, discusses various analysis techniques which have been applied to the data, and illustrate subsequent application of the information in numerical wave modelling and determination of marine engineering criteria
Particular applications include studies on shallow water pipeline stability, near shore wave penetration, dredge spoil dispersion and scour
During recent years, recognition of the importance of directional wave information in many facets of coastal and ocean engineering design has been increasing Goda et al (1978) have illustrated the importance of wave directionality for studies of wave diffraction and refraction in harbours, Fornstall et al (1978) have demonstrated the inadequacies of unidirectional wave theories in describing wave kinematics, and Sand et al (1981) have discussed the applications of directional wave information to the hydrodynamics and design of offshore structures and pipelines
Buchan et al (1984) describes the development of a shallow water directional wave recording _system, based on the Sea Data 635-12s Directional Wave Recorder The instrument measures fluctuations in absolute pressure and horizontal current, and records the information internally on magnetic cassette It is usually deployed in a sea bed frame
Following Fornstall et al (1978), data processing involves the modelling of wave kinematics using linear wave theory, allowing the application of spectral analysis techniques The directional distribution of the sea state is described by a "spreading function" If the sea state is composed of a few resolvable, predominant wave trains, the spreading function may be satisfactory approximated by the widely accepted cosine 2s model of Longuet-Higgins et al (1963) Such a description of the sea state is based on only three frequency dependent parameters These are E(f), the omni directional energy density, 8(f), the principal direction of propagation of waves of particular frequency f, and s(f), a measure of the sharpness of the spread of wave directions about the pnncipal direction
Because of the directionality or "short-crestedness" of most sea states, application of unidirectional wave theory is known to cause over-estimates of wave-induced currents from surface wave measurements Simultaneous surface wave height and near-bottom wave-induced current measurements have allowed calibration of unidirectional linear wave theory for specific oceanographic environments Consequently surface wave data may be used to compute characteristic near-bottom wave-induced velocities
Since then, the findings and recommendations of this study have been incorporated in a new generation of Wave Radar, and this paper will describe the rationale for the changes, and will describe how they have been implemented Finally, data obtained from the Esso Odin Field using the new version of the Wave Radar will be presented
The statistics of individual wave heights are known to approximate the Rayleigh distribution Analysis of near-bottom wave-induced currents shows that wave-induced currents follow a similar distribution As a result, individual current speeds may be statistically related to characteristic wave-induced velocities
