NERC-NSFGEO The impact of cohesive material on critical shear stress in gravel bed rivers

Many rivers, including >50% of UK channels, have gravel beds. Managing these channels requires predictions of when, and how much of, the riverbed will move, for applications including designing infrastructure, mitigating flood risk and river restoration. The amount of sediment transport depends on the balance between the shear stress applied to the bed by the flow (t), and the critical shear stress (tc) at which riverbed sediment starts to move. tc is strongly affected by grain size, but large uncertainties in predicting tc remain. These uncertainties in tc are problematic because they mean that we currently cannot predict bedload transport to better than an order-of-magnitude. One important factor that controls tc is how grains are arranged on the riverbed. The project team has led recent advances in understanding these controls, developing new methods to measure and model the impact of grain arrangement, which has produced a paradigm shift in predicting tc. However, a substantial omission so far, and from all bedload transport models, is that they neglect the effects of commonly occurring cohesive material that sticks gravel together. This includes clay (often alongside sand and silt), biological secretions such as Extracellular Polymeric Substances (EPS) produced by bacteria and biofilms, and silk threads produced by larger organisms such as mussels and caddisfly larvae. Previous work has shown that such cohesive material can increase tc by up to ten times, and so understanding its effect is essential for improving bedload predictions. However, the current data are derived from varying methods, and consider different cohesive materials individually rather than in combination. Consequently, they do not provide a comprehensive understanding of cohesive effects, nor are they sufficient for model development. Our aim is to incorporate cohesive effects into our models for tc, improving their predictive capability and making them widely applicable. To achieve this, we will combine field data, laboratory flume modelling, and theoretical model development, and will address both non-biological (clay) and biological (EPS and caddisfly silk) cohesion. Our objectives are: O1) To monitor where and when cohesive material is found in gravel bed rivers, creating the first dataset that includes both biological and non-biological cohesion. This will identify where cohesive material is most prevalent in river channels, and inform conditions for flume experiments in O2. O2) To measure how biological and non-biological cohesive materials, alone and in combination, affect resisting forces, sediment structure, velocity profiles and tc in gravel-bed channels. By measuring all of these effects concurrently our dataset will be the first that is sufficient for model development. O3) To use novel flume experiments with see-through beds to measure how cohesive material affects near- and within-bed turbulence and velocity profiles. These experiments supplement O2 by quantifying near-/within-bed flow that cannot be measured in O2, and will be the first application of this technique to beds with realistic gravel-shaped grains. O4) To use the outputs from O1 to O3 to develop and test a new version of our entrainment model that will incorporate cohesive effects. Our findings will improve predictions of tc and bedload in gravel-bed rivers with cohesive material. Our outputs will range from rule-of-thumb recommendations for situations without field data, to methods for parameterising our entrainment model when it is possible to measure cohesive material. These will be of benefit to anyone involved in managing and studying gravel-bed rivers.