Particle over-passing on depth-limited gravel bars
Article first published online: 14 JUN 2006
Volume 37, Issue 2, pages 345–355, April 1990
How to Cite
CARLING, P. A. (1990), Particle over-passing on depth-limited gravel bars. Sedimentology, 37: 345–355. doi: 10.1111/j.1365-3091.1990.tb00963.x
- Issue published online: 14 JUN 2006
- Article first published online: 14 JUN 2006
- (Manuscript received 21 February 1989; revision received 25 September 1989)
An experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of ‘hump-back’ bars and along the top of planar gravel sheets. Hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like-sized and finer particles. For prescribed conditions, flat upper-stage plane sand-beds develop over the crestal location with pebbles rolling easily over the sandy bed. At the brinkpoint, flow separation ensures effective segregation of the gravel from the sand. Over the slip-face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed-form. Consequently, distinctive vertical assemblages of open-work and closed contact framework gravels could be generated as another bar migrates over, and preserves, the initial structure.
In respect to the observed mechanisms of sorting over the bars, a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper-stage plane-bed conditions on the crestal platform. The model then is used to ascertain whether the depositional environment ascribed to certain facies in the Bunter Pebble Beds, described in a recent publication, is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle-segregation process.