A theoretical background is given about the attenuation of waves by seagrasses, starting from the pioneering works that treated the vegetation shoots like rigid cylinders, moving like a cantilever. Gradually, the movement of the vegetation is taken into account, and the most recent studies focussing on the posture and flexibility of the vegetation are reviewed.
Physical tests are conducted on a patch of mimics, representing Posidonia oceanica seagrass, in the small wave flume of the Department of Civil Engineering in Ghent. The effects of wave conditions (wave height and wave period) and meadow characteristics (flexural rigidity and submergence ratio) on wave attenuation are analysed by measuring the drag force on the blades, the wave height decay and the bending angle of the blades. Three mimics are constructed with different flexural rigidity, with three submergence ratios under regular waves with five different wave heights and wave periods, leading to a total of 200 tests.
The results show that the wave-attenuating ability of the meadow depends on the wave conditions to which it is subjected. Under larger wave heights and/or wave periods (within a restricted range), the drag force increases and an enhanced wave attenuation is obtained. The wave attenuation can further be assessed from the design characteristics of the meadow. The drag force decreases by either decreasing the submergence ratio of the stiffer mimic, by reducing the flexural rigidity of the leaves or by increasing the submergence ratio of the flexible mimic and in such a way enhancing the reconfiguration of the blades. The lower the drag force per frontal surface area, the lower the capacity of the meadow the attenuate waves.