Freak wave kinematics

Abstract

Prototype records of extreme single waves recorded on the Danish Continental Shelf have been analysed in detail. Statistical calculations show that these waves do not belong to the traditional short term statistical distributions used for ocean waves. The waves are too high, too asymmetric and too steep. Emphasis has been placed on deriving a kinematic model for the extreme wave phenomenon in order to predict forces on offshore structures. A series of model tests aiming at reproducing the recorded waves from nature has been carried out to support the kinematic investigations. It has been found that the horizontal velocities above MWL are higher than predicted by generally accepted kinematic theories, whereas the velocities in the lower part of the water column are somewhat lower than expected. A directional (crossing seas) model is therefore proposed. It includes a spilling breaking velocity profile above the MWL and a modified Wheeler profile below the MWL in order to describe the extreme wave kinematics. (A)