Threshold constraints on the size, shape and stability of alluvial rivers

The geometry of alluvial river channels both controls and adjusts to the flow of water and sediment within them. This feedback between flow and form modulates flood risk, and the impacts of climate and land-use change. Considering widely varying hydro-climates, sediment supply, geology and vegetation, it is surprising that rivers follow remarkably consistent hydraulic geometry scaling relations. In this Perspective, we explore the factors governing river channel geometry, specifically how the threshold of sediment motion constrains the size and shape of channels. We highlight the utility of the near-threshold channel model as a suitable framework to explain the average size and stability of river channels, and show how deviations relate to complex higher-order behaviours. Further characterization of the sediment transport threshold and channel adjustment timescales, coupled with probabilistic descriptions of river geometry, promise the development of future models capable of capturing rivers’ natural complexity

Structure-property relationships from universal signatures of plasticity in disordered solids

When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, "softness," designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively