The impact of cohesive material on gravel entrainment

Predicting when riverbed sediment is mobile is essential for managing the morphology and ecology of gravel-bed river channels. However, our ability to predict critical shear stress (τ_c) is still such that predictions are only accurate to an order of magnitude at best. One aspect which is often overlooked when predicting grain entrainment, and which likely contributes to our poor predictions of τ_c, is the role of any cohesive material surrounding the gravel grains. This material could be clay, as is commonly found in gravel-bed rivers draining agricultural catchments, and/or biological, such as produced by caddisfly larvae, mussels and biofilms. To assess the potential impact of non-biological cohesion we parameterise a force-balance grain entrainment model to demonstrate that adding plausible values of cohesion can produce an order of magnitude increase in τ_c. We compare our results to two sets of field measurements of grain entrainment forces. The first set are from Bury Green Brook, UK, where there is local variation in the amount of clay matrix in the gravel bed and we assess differences in entrainment forces between individual grains. The second set comprises data from multiple sites with varying amounts of fines in the bed and we compare average entrainment forces. Our field data are consistent with the model results, demonstrating the potential importance of accounting for cohesion when predicting τ_c. Finally, we demonstrate that cohesive forces from clay are also sensitive to water content, and so may be most important in ephemeral channels.