Erosion of biofilm-bound fluvial sediments

The movement of fluvial sediment shapes our rivers. Understanding sediment entrainment has been a goal of hydraulic engineers for almost a century1,2. Previous sediment entrainment models have been informed by laboratory experiments using grains that were free from biological material3. In natural river settings, however, sediments are invariably covered by bacteria, often forming visible biofilms, which comprise diverse consortia of species housed in sticky extracellular polysaccharides. Here we report experiments in a laboratory flume with cyanobacteria grown over sediment. We show that the prevailing model, where grains roll over one another at some critical threshold in shear velocity, does not hold for biofilm-bound sediments. Instead, biostabilized sediment behaves more like an elastic membrane. Fluid flow produces oscillations in the membrane, which can become unstable. Beyond a particular threshold in velocity, the membrane fails catastrophically by ripping and clumps of biofilm-bound sediment become entrained. We use a mathematical model of an oscillating membrane in incompressible flow to show that unstable oscillations will occur over a wide range of elastic material properties at realistic river flow velocities. We find that the horizontal length scale over which oscillations occur is a controlling factor for incipient sediment entrainment of biostabilized sediments. Sediments have a huge impact on natural fluvial processes and on human exploitation of rivers. Scour, transport and deposition of sediment can act to render expensive infrastructure, such as bridges and dams, useless and transport pollutants large distances from their source. However, sediments also harbour rich biological communities that recycle nutrients and remove contaminants. Therefore, the ability to predict the transport of fluvial sediments has far-reaching applications from conservation to river basin management and civil engineering design. Thus conceptual and ultimately mathematical models of the energy dissipation and entrainment mechanisms when river flows and sediment interact are critical in predicting sediment movement. It has been shown that it can take up to five times higher shear stress to entrain biofilm-bound marine sediments than clean ones4; the phenomenon had been called biostablization5,6. It seems to be widespread and potentially reflects a stratagem that has evolved independently in different clades of bacteria to convey an advantage on early colonizers7. The ubiquity of biostabilization has had little impact on practical prediction of sediment transport where empirical models, parameterized using clean sediments in laboratories, prevail despite being foundwanting in real-world rivers8. We inoculated an incubation flumewith cyanobacteriaPhormidium sp. where the bacteria colonized trays of sand with 1.2mm median diameter, 1mm glass beads or 2.2mm gravel on the bed