Wave Groupiness in the Nearshore Area by Hilbert Spectrum

Abstract

The Empirical Mode Decomposition (EMD) method for nonlinear and non-stationary time series analysis, also known as Huang Hilbert Transform (HHT), is applied to wave field data from the nearshore area. The frequency-time distribution of the energy, designed as a Hilbert spectrum is utilized for examination of the group structure of the sea waves. The necessary condition for existence of wave grouping is determined on the base of the results of ernpirical mode decomposition of the data. The HHT method provides a unique basis for expansion of the data, derived from and based on the data. The variations of the group characteristics across the shore are examined and their relationship with the local wave characteristics is investigated outside and inside the surf zone. The Hilbert spectrum provided new insights on the wave and group transformation in the cross-shore direction. INTRODUCTION Wave groups are considered as a forcing mechanism for various types of oscillations in the coastal area. The transformation of the wave group structure in the process of wave breaking is closely related with the generation of low frequency waves in the surf zone Longuett-Higgins and Stewart (1962), Symonds et al. (1982) and Schaffer (1993). The wave groups are identified as a source of edge waves by Bowen and Guza (1978), shear instability waves Hailer et at (1999) and lead to fluctuation of rip currents Kennedy and Dalrymple (2001). The importance of the group structure of the sea waves is well-recognized, however, the question of appropriate method for wave group analysis is still open. Wave groups are time localized transient events and their appearance in the sea surface elevation records shows that the wave process is not stationary as usually considered in the conventional wave data analysis. As far as real sea waves are also non-linear and this non-linearity increases in the *Dept. Dynamics of Coastal Zone, Institute of Oceanology, Bulgarian Academy of Sciences, Vama, Bulgaria Present address: Port and Airport Research Institute, Nagase 3-1-1, Yokosuka 239-0826, Japan velcheva@ipc.pari.go.j p