Balances of forces operating on a grain, and channel geometries. (a) Force balancing explaining the entrainment of sediment particles. A sediment particle with diameter D will be transported if the ratio between the fluid’s shear stress τ and the particle’s inertia force F ranges between 0.03 and 0.06, which is the Shields27 variable ϕ. These relationships can then be used to compute slope angles S at equilibrium conditions. The variable d denotes the water depth, g is gravitational acceleration, and ρ
correspond to water and sediment densities, respectively. (b) Section through a braidplain illustrating water depths during low (dark blue) and bankfull discharge (pale blue), when all gravel bars bordering a channel are flooded. In this case, heights of foresets and longitudinal bars define the water depths during bankfull discharge conditions. (c) Illustration of how water depths representing bankfull discharge situations are registered in the sedimentologic archive. Accordingly, we measured the difference between the top of a conglomerate bank and the underlying erosional surface (α), and thicknesses of foresets and longitudinal bars (α, β). (d) Photo showing sediments of unit B. We used thicknesses of forests as proxy for channel depth, and we measured the largest clasts (D
) at the base of the foresets. Please see person and meter stick for scale.