Present address: Department of Earth and Planetary Sciences, University of California, Berkeley, CA, 94720, USA.
On the transition between 2D and 3D dunes
Article first published online: 17 NOV 2005
Volume 52, Issue 6, pages 1343–1359, December 2005
How to Cite
VENDITTI, J. G., CHURCH, M. and BENNETT, S. J. (2005), On the transition between 2D and 3D dunes. Sedimentology, 52: 1343–1359. doi: 10.1111/j.1365-3091.2005.00748.x
- Issue published online: 17 NOV 2005
- Article first published online: 17 NOV 2005
- Manuscript received 5 August 2004; revision accepted 20 July 2005.
- 3D bedforms;
- fluvial processes;
- sediment transport
Sediment transport in sand-bedded alluvial channels is strongly conditioned by bedforms, the planimetric morphology of which can be either two- or three-dimensional. Experiments were undertaken to examine the processes that transform the bed configuration from two-dimensional (2D) dunes to three-dimensional (3D) dunes. A narrowly graded, 500 μm size sand was subjected to a 0·15 m deep, non-varying mean flow ranging from 0·30 to 0·55 m sec−1 in a 1 m wide flume. Changes in the planimetric configuration of the bed were monitored using a high-resolution video camera that produced a series of 10 sec time-lapsed digital images. Image analysis was used to define a critical value of the non-dimensional span (sinuosity) of the bedform crestlines that divides 2D forms from 3D forms. Significant variation in the non-dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random. Images also reveal that, once 2D bedforms are established, minor, transient excesses or deficiencies of sand are passed from one bedform to another. The bedform field appears capable of absorbing a small number of such defects but, as the number grows with time, the resulting morphological perturbations produce a transition in bed state to 3D forms that continue to evolve, but are pattern-stable. The 3D pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment, which leads to bifurcation. The experiments demonstrate that 2D bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator.