River currents, wind, and waves drive bed-load transport, in which sediment particles
collide with each other and Earth’s surface. A generic consequence is impact attrition
and rounding of particles as a result of chipping, often referred to in geological
literature as abrasion. Recent studies have shown that the rounding of river pebbles
can be modeled as diffusion of surface curvature, indicating that geometric aspects
of impact attrition are insensitive to details of collisions and material properties.
We present data from fluvial, aeolian, and coastal environments and laboratory experiments
that suggest a common relation between circularity and mass attrition for particles
transported as bed load. Theory and simulations demonstrate that universal characteristics
of shape evolution arise because of three constraints: (i) Initial particles are mildly
elongated fragments, (ii) particles collide with similarly-sized particles or the
bed, and (iii) collision energy is small enough that chipping dominates over fragmentation
but large enough that sliding friction is negligible. We show that bed-load transport
selects these constraints, providing the foundation to estimate a particle’s attrition
rate from its shape alone in most sedimentary environments. These findings may be
used to determine the contribution of attrition to downstream fining in rivers and
deserts and to infer transport conditions using only images of sediment grains.