Exploring Variabilities in Gravel Mobility Using Force-Balance Models

Understanding the variabilities of bedload mobility is fundamental in predicting the likelihood of erosion and deposition patterns in gravel bedded rivers, which subsequently assist towards modelling geomorphic adjustment and flood risk change over reach to catchment scales. In most applications, the shear stress required to initiate sediment transport (τ*_c) is typically assumed from relations with channel slope or the median bed material grain size, and is generally assumed temporally constant. However, flume investigations identify important relations between grain arrangement (for example, grain protrusion and imbrication) and sediment flux, which vary in response to flood history. Given the complexities of river systems, grain-scale linkages between water-working history, bedload characteristics, and grain mobility remain largely unexplored in the field.

 

We use a combination of gravel bed microtopography data, collected via structure-from-motion photogrammetry, and in-situ grain resistance tests to resolve a grain force balance model for 45 upland gravel surfaces across England and Wales. Grain resistance forces (F_R), and subsequent estimates of τ*_c, are used to explore grain scale drivers of particle mobility, as well as their spatial and temporal variabilities. We interpret flow histories of sampled surfaces using typical water-working indicators (including bed surface roughness, imbrication extent and grain size sorting). Water-working metrics are compared against resistance force distributions, to address the hypothesis that conditioned surfaces exhibit systematically higher mobility thresholds. We also consider the relative role of grain shape on bed topography and stability trends. In practical application, our findings can offer more targeted, process-based, estimates of τ*_c for a given channel reach, even when grain surface characteristics are only known qualitatively. Such improvements in τ*_c estimates are critical in furthering our ability to predict sediment fluxes and geomorphic change in gravel dominated channels, particularly in response to climate change, where the temporal sensitivity of τ*_c is likely to be important.