Non-linear and non-stationary sea waves

The work presents the results of recent investigations of the spectral properties of wind waves measured at sea. A special attention is paid on freak waves as well as on the waves in the coastal zone. Higher order stationary spectra and especially bispectra of these waves is estimated and studied. The temporal evolution of the lower spectra of the abnormal waves is presented using different methods. For the coastal zone waves the development of the bispectra and the time-frequency spectra in space is presented too. Time-frequency second-, third-and fourth-order moment spectrum is used to investigate short-time properties of storm sea state with freak wave like the "New Year Wave".