Effect of a surface biofilm on sediment transport implemented in a 1D numerical model

The transport of sediment shapes rivers and deltas, and has a huge impact on natural fluvial processes and human interaction within these environments. Conservation and hydraulic engineering applications in river basins crucially depend on understanding the processes of scour, transport and deposition of sediments. The sediment entrainment process in mathematical models are typically based on laboratory experiment using clean (abiotic) sediments. However, natural sediments are rich in biological communities, often forming visible biofilms which include sticky Extracellular Polymeric Substances (EPS). The presence of biological communities has been shown to significantly increase the critical shear stress of sediment entrainment compared with clean sediment, and these communities are recognized as ‘ecosystem engineers’ as they act as bio-stabilizers. Furthermore, biofilms provide stability, such that only the most energetic conditions can remove them in a sudden catastrophic way. In this study, a one-dimensional (1D) morphodynamic model for rivers is implemented to account for the development and growth of a surface biofilm subject to variable hydrodynamic disturbances (e.g. tidal forces) and with a biofilm-dependent erodibility. The 1D form of the shallow water equations are simplified with the aid of the quasi-steady approximation and the Exner equation expressing the conservation of bed material is used to compute the changes in channel bed elevation. The effect of geochemical drivers such as light, temperature and nutrients, which affect the presence or absence and growth of a biofilm, is accounted for in the model. Previous studies have shown that when sediments are covered by biofilms, entrainment occurs via biomat failure and the carpet-like detachment of biofilm-sediment composites. Different hydrodynamic conditions are tested to investigate their role in eroding the biofilm and detaching it from the sediment surface.