Experiments to Systematically Evaluate the Role of Cohesion in River Morphodynamics

Nacere M Samassi, Fernando David Cúñez, Rachel Glade

While cohesion is thought to be an important control on sediment transport, few studies have
systematically examined the role of cohesion in river morphodynamics. In this study we use
simplified, small-scale laboratory experiments to investigate how increasing sediment cohesion
affects the morphometrics of fluvial channels. Experiments were conducted in a laminar flume
with a mixture of angular, sand-sized plastic particles and different amounts of xanthan gum—a
proxy for cohesive biofilms—to tune cohesion among grains. With increasing cohesion, we
observe a transition from highly mobile, braided to single thread meandering channels, then to
straight, gully-like channels with headcuts that exhibit decreasing distance of retreat with
increasing cohesion. Particle image velocimetry (PIV) shows that cohesion decreases sediment
transport rate even when discharge is increased, suggesting an increase in critical shear stress.
Bank width measurements show that this leads to narrowing of the channel with increasing
cohesion. However, strong qualitative differences in observed channel morphodynamics suggest
that beyond changing critical shear stress, cohesion alters the fundamental processes that govern
channel erosion, including bank strength and the formation of aggregates. Our work suggests a
novel approach of using sediment cohesion to explore the transition between transport limited
and detachment limited channels.