A flow visualisation and a particle tracking method as well as laser Doppler velocimetry were used as non-intrusive, optical measurement techniques, to investigate wave induced motion in the vicinity of a semi-submerged vertical wall, an important design feature of coastal structures. The tests were carried out with monochromatic waves. Parameters have been defined, which cause non-linear motion in the wave induced flow field. Experimental research indicates that velocity measurement in non-linear velocity fields are essential to get a reliable basis for a theoretical approach to non-linear motion caused by the interaction of regular waves with semi-submerged coastal structures.
Different, non-intrusive optical methods have been increasingly applied to velocity measurements made in coastal engineering laboratories in the recent years (Agrawal et al., 1987 and Oreated et al., 1992). The kinetics of the wave boundary layer the correlation between wind stresses and generated velocity profiles beneath the water surface (Wierzimok, 1990) and the interaction of "wave induced flow fields with wave damping structures (Belorgey et al., 1986 and Lengricht and Oraw, 1995) or with oscillating water column wave energy converters (Morrison and Oreated, 1992) have been investigated. The non-intrusive, optical measurement techniques are the laser Doppler velocimetry, the particle image velocimetry and several other particle tracking techniques. Laser Doppler velocimetry is being continually developed in its components but has problems with non stationary flows (Graw et al., 1990). Nevertheless laser Doppler velocimeter applications have been found to be adequate in wave induced flow field analysis (Liu and Lin, 1987 and Skjelbreia et al., 1989). Contrary to the laser Doppler velocimetry, which is fundamentally a point measurement technique the particle image velocimetry and the known particle tracking techniques give an impression of the flow characteristics over an entire region of the flow field. It has been shown that particle image velocimetry is applicable to a flow field within a breaking wave (Greated et al., 1992).
