| Autor: |
Domokos G; Department of Mechanics, Materials and Structures, Budapest University of Technology and Economics, Budapest, Hungary., Jerolmack DJ; Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America., Sipos AÁ; Department of Mechanics, Materials and Structures, Budapest University of Technology and Economics, Budapest, Hungary., Török A; Department of Construction Materials and Engineering Geology, Budapest University of Technology and Economics, Budapest, Hungary. |
| Abstrakt: |
River-bed sediments display two universal downstream trends: fining, in which particle size decreases; and rounding, where pebble shapes evolve toward ellipsoids. Rounding is known to result from transport-induced abrasion; however many researchers argue that the contribution of abrasion to downstream fining is negligible. This presents a paradox: downstream shape change indicates substantial abrasion, while size change apparently rules it out. Here we use laboratory experiments and numerical modeling to show quantitatively that pebble abrasion is a curvature-driven flow problem. As a consequence, abrasion occurs in two well-separated phases: first, pebble edges rapidly round without any change in axis dimensions until the shape becomes entirely convex; and second, axis dimensions are then slowly reduced while the particle remains convex. Explicit study of pebble shape evolution helps resolve the shape-size paradox by reconciling discrepancies between laboratory and field studies, and enhances our ability to decipher the transport history of a river rock. |
