Submerged and emerged rigid vegetation impact on bedforms and sediment suspension under wave action

A series of experiments were carried out in a laboratory wave flume to investigate the impact of coastal rigid vegetation on suspended sediment transport and the generation of bedforms for a range of wave conditions for both submerged and emerged vegetation. Rigid arrays of cylindrical wooden dowels were used as vegetation mimics on a sandy bed. Two vegetation densities were selected, representing dense and sparse vegetation meadows. Synchronised flow velocity and suspended sediment concentration measurements were performed using particle image velocimetry and an acoustic backscatter sensor. Seabed ripples were observed in all cases where the near-bed velocity exceeded the threshold of sediment motion. The near-bed velocity governed sediment suspension on both bare and vegetated sediment seabeds. Near-bed sediment concentration on densely vegetated seabeds was lower than that of bare seabeds under the same wave conditions. These observations highlight the importance of considering the role of vegetation in shaping seabed morphology and the resultant suspended sediment concentrations. Then, the near-bed sediment concentration formulae used on bare sediment beds are validated for use on vegetated seabeds. • Near-bed velocities are reduced by rigid vegetation, thus altering sediment suspension. • Ripple formation is disrupted by the stem vortices generated by rigid vegetation, although ripples are present up to the limits of wake fraction to bed area ratio = 60%. • The ripples generated within vegetation are the primary driver of sediment suspension within rigid vegetation canopies.