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The AAPG/Datapages Combined Publications Database

CSPG Special Publications


Sedimentology of Gravels and Conglomerates — Memoir 10, 1984
Pages 61-68
Fluvial Processes

Particle Interaction and Its Effect on the Thresholds of Initial and Final Bedload Motion in Coarse Alluvial Channels

Ian Reid, Lynne E. Frostick


The data-base for initial motion of bedload is substantial, though it comes almost exclusively from laboratory studies in which practical difficulties generally preclude the use of coarse sediment. Estimates of the conditions at initial motion on coarse-grained stream beds rely on field observations, and these are rare. The lack of field data is attributed to the difficulties of registering the initial and final movement of bedload. This has been remedied by the development of two new instruments: a continuously recording bedload pit sampler, and an electro-magnetic bedload sensor which automatically registers the entrainment of individual tagged particles. A 4-year monitoring programme illustrates the short-comings of extrapolating laboratory-based predictions of thresholds of motion to larger particle sizes and applying them in the field. Extensions of the familiar curves such as those of Shields and Lane seriously underestimate the force required to initiate motion. Field values reflect the importance of microscale bedforms such as pebble clusters in delaying bedload transport. Particle interlock and mutual protection confer a stability in excess of that which would be possessed by the constituent clasts isolated on a plane bed. Perhaps more surprising is that the tractive force associated with the finish of bedload transport is shown to be one-third of that at the point of initial motion; shear stress at the bed, averaged for several floods, is 0.79 kg.m−2 and compares with 2.20 kg.m−2 for the start of bedload. This is important because bedload transport continues longer than might be predicted using standard bedload equations and because palaeohydraulic inference usually assumes the levels of tractive force involved with entrainment and not the much lower levels associated with deposition.

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