The role of attached kelp (seaweed) in lowering threshold of coarse gravel entrainment in tidal flows

Lag deposits of cobble-sized clasts found within the troughs of fine-gravel dunes on a rock-bed intertidal zone are explained by the process of enhanced fluid transport due to attached seaweed. Field determinations of the competence of the tidal currents demonstrated that the currents were not able to transport weed-free clasts on the tidal flat. Controlled flume experiments were used to demonstrate that the presence of the fucoid alga (Fucus vesiculosus (L.)) attached to clasts reduces the critical velocities and shear stresses to levels that allow the tidal currents to entrain the clasts and move them over the backs of dunes to the lee sides. Subsequent burial of the weedy-clasts in the dune troughs by dune progression kills the weed, leaving the latterly exposed clasts as an accumulating weed-free lag. The weight of attached weed typically equals the submerged weight of the clast and the critical velocity and critical shear stress for initial motion are both roughly halved by the presence of the weed. The drag induced by weed is four-times that experienced by weed-free clasts and mobile clasts exhibit near-equal mobility.