Experimental investigation of the impact of macroalgal mats on the wave and current dynamics

Macroalgal mats of Ulva intestinalis are becoming increasingly common in many coastal and estuarine inter- tidal habitats, thus it is important to determine whether they increase flowresistance, promote bed stability and therefore reduce the risk of erosion favoring tidal flooding or degradation of coastal lagoons. Venier et al. (2012) [6] studied the impact of macroalgal mats of Ulva intestinalis on flow dynamics and sediment stabil- ity for uniform flow. Here we extend their experimental work to the case of vegetation under the combined action ofwaves and currents. These hydrodynamic conditions are very common in many shallowcoastal en- vironments and lagoons. The experimental facility employed in the present study and the series of flowruns are the same as that used by Venier et al. (2012)[6]. However,waves have been superposed to uniform current flowing firstly over amobile sediment bed coveredwith U. intestinalis, then over a bare sediment surface. For the depth, wave and current conditions considered in the experiments, the time-averaged vertical profile of horizontal velocity for the case of coexistingwaves and current turns out to be very close to that observed for a pure current, both with and without vegetation. However, contrary to what was observed in the case of a unidirectional current, in the presence of waves the time averaged velocity profile is only weakly influenced by the vegetation, whose main effect is to attenuate velocity oscillations induced by waves and to slightly increase the overall bed roughness.