Abstracts Physics

Wave damping characteristics of dynamic breakwaters:

by S.B. Smit

New developments in the design of off-shore harbours, involving land reclamation and the use of large vessels (draughts larger than 10 meter) have made it necessary for breakwaters to be constructed in deeper water. In deep water, vertical caisson breakwaters have been found to provide better economical solutions than rubble mound breakwaters, because they use less material per cross-section. These vertical caisson breakwaters lead to a high level of wave reflection, a phenomenon that causes unfavourable disturbance and which creates a threat to safe navigation of vessels. Because ofthis high wave reflection factor the wave loads on a vertical breakwater are also become high. To reduce the high reflection of waves and the wave loads on the vertical breakwater the search for innovative breakwater designs, which dissipate the wave energy effectively, is necessary. New existing innovative breakwaters like perforated and slit-type caissons are effective in reducing wave reflection and wave loads, but only in a narrow range of wave frequencies. The main question of this study is whether it is possible to dissipate wave energy in a dynamical way by transferring the wave energy to a movable element, which is able to dissipate wave energy effectively in a wide range of wave frequencies? New innovative dynamic breakwaters with movable elements are presented in this report. The wave damping characteristics of the dynamic breakwater with a horizontally translating flap are analysed. A simple mass-spring-dashpot model has been derived and this can be used to describe the dynamic behaviour of the movable flap. The damping characteristics of a dynamic breakwater with movable flap depend entirely on the hydraulic boundaries (wave height, wave period and water depth) and the characteristics of the movable flap (mass, spring stiffness and damping). The hydraulic performance of the dynamic breakwater is described by the reflection of the waves in front of the movable flap. To study the hydraulic performance of the dynamic breakwater with movable flap the analytic solution is obtained and a computer model is used. The analytical solution is derived to describe the wave reflection for waves that are assumed to be long waves, so vertical accelerations are not simulated in the analytical solution. From the analytical solution it can be concluded that, in theory, the dynamic breakwater with movable flap can damp waves totally for one specific wave period if the characteristics of the flap are tuned to this wave period. The main conclusion drawn from this study is that a dynamic breakwater with a movable flap is able to reduce the wave heights in a wider range of frequencies than a static breakwater. For a dynamic breakwater with movable flap with characteristics that are tuned to certain target wave conditions it is possible to reduce the wave reflection by 50 % for a wide range of wave frequencies.