The roles of geometry and viscosity in the mobilization of coarse sediment by finer sediment

• 成果类型: Article
• 署名作者: Hassan, Marwan A.; Parker, Gary; Hassan, Yarra; An, Chenge; Fu, Xudong; Venditti, Jeremy G.
• 署名单位: University of British Columbia; University of Illinois System; University of Illinois Urbana-Champaign; University of Illinois System; University of Illinois Urbana-Champaign; University of Washington; University of Washington Seattle; China Institute of Water Resources & Hydropower Research; Tsinghua University; Simon Fraser University
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-10647
• DOI: 10.1073/pnas.2409436121
• 发表日期: 2024-09-17
• 关键词: transport rates; friction angle; sand; TRANSITION; bedload; motion; river; MODEL

摘要

In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near- bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect.