The capturing of flocs by migrating subaqueous dunes

Flocculated particles, formed by the aggregation of clay particles, are common in rivers. These flocs exhibit a different behaviour than primary particles: they can deform and break apart, and they have greater settling velocities than the particles of which they are composed. The latter allows flocs, unlike primary clay particles, to deposit on the river bed in mildly turbulent conditions, potentially leading to interactions with the bed. Particularly in sand-bedded rivers, where bedforms shape the riverbed, there exists a potential interaction between flocs and the riverbed.

Physical experiments were carried out in an annular flume, using a flocculant to induce flocculation. Different amounts of flocculant and various shear stress conditions were applied, and the resulting floc characteristics and bedform geometry were measured.

Under lower shear conditions, the flocs were larger and transport rates were lower than under high shear conditions. However, under both shear conditions, flocs were transported via saltation and in suspension, and they became integrated within the sediment bed either as individual flocs, clusters, or sheets. Deposition occurred predominantly on the leeward side of the dune, revealing distinct stratigraphy patterns. The presence of flocs had a negligible impact on the actual geometry of the bedforms.

This investigation highlights the active role of flocculated clay particles in sediment transport in riverine systems, contrary to the general assumption that clay particles behave passively as wash load. This finding has the potential to affect sediment transport rates of fines and contaminants and could have far-reaching impacts on the interpretation of mud deposits in the sedimentary rock record. For modelling and predicting the sediment dynamics in river systems a comprehensive understanding of the transport mechanisms of clay flocs is essential and should be taken into account.