The effect of kelp in wave damping

It is generally acknowledged that bottom vegetation enhances the damping of surface waves in shallow waters. Thus the harvesting of kelp forests by trawling and associated change of conditions have given rise to conflicts. In order to secure a scientifically good basis for management ofthe marine resources and the environment, the role of the kelp forest as habitat for fish and marine animals and the wave damping effect need to be investigated and quantified. The hydrodynamical conditions in a kelp forest have been investigated with aid of ultrasonic current meters (UCMs) attached to bottom mounted rigs. Data sampling at a rate of 2 Hz of the three velocity components and pressure ensured adequate frequency resolution of the energy containing surface waves of 5-15 sec. periods. Analysis of frequency spectra of wave energy suggested a new main mechanism of wave damping giving a model which has been formulated mathematically and compared with observations. Wave attenuation by kelp forests in shallow waters has been substantiated by measurements at Hustadvika at a site which is strongly exposed to waves from the open ocean. The reduction of wave energy from the outer to inner part of kelp belt over a distance of 258 m was 70-85 % with highest value at low tide. Velocity measurements at two levels, above and below canopy, reveal almost identical results. This remarkable documentation contradicts earlier assumptions and findings concerning sheltering effect of kelp, i.e. Eckman & al. (1989). The working hypothesis of the present analysis is that the viscous drag of the kelp fronds is the dominating factor. The results from the corresponding mathematical model are in good agreement with observations. The wave conditions in a kelp forest have been measured and modelled. The results of the experiment provide a good basis for evaluation of wave damping effects and may be compared with laboratory experiments and field investigations after harvesting.