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Genesis of Fluvial Bedforms: Abstract
Bedform genesis is doubly germane to fluvial sedimentology. Not only do streams display a complex variety of bedforms, despite simpler gross-flow conditions than in other environments, but bedforms are also an important component of existing facies models. This paper presents some recent findings on the origin of fluvial bedforms.
Three classes of bedforms are defined on the basis of formative factors and physical scales. The class of largest bedforms, macroforms, includes those bedforms which do not depend upon local flow conditions but which instead respond to long-term hydrological and geomorphological factors. Fluvial macroforms include alternate bars, point bars, scroll bars, and pool-and-riffle sequences. Their wavelength scales with stream width. Mesoforms include those bedforms that respond directly to flow conditions of the outer zone of the local boundary layer of the flow system. Their wavelength scales with boundary-layer thickness (= flow depth in streams), and their lifetimes are commensurate with the duration of individual hydrological events. Fluvial mesoforms include dunes, Coleman’s large-scale lineation, and antidunes. The smallest bedforms, microforms, respond to flow conditions of the inner zone of the turbulent boundary layer and thus scale with wall variables. Fluvial microforms include current lineation and small scale ripples.
The conceptual simplicity of this classification is realized incompletely in fluvial streams, owing to the poorly understood bedforms variously referred to as braid bars, linguoid bars, spool bars, unit bars, etc. Theoretical models of the braiding mechanism suggest that these “braid” bars are macroforms, but observations of their behavior in natural streams show mesoformal and macroformal traits.
A new approach to bedform genesis involves the concept of flow structures. Recent laboratory experiments indicate that turbulent shear flows contain one or more inherent structures, each consisting of a more-or-less deterministic secondary flow superimposed upon the prevailing unidirectional mean flow. The main microscale structure is the wall streaks of the turbulent boundary layer, which govern the formation of microforms. Mesoscale structures include the bursting phenomenon, longitudinal roll vortices, in-phase waves, and possibly transverse roll vortices. Wavelength of mesoscale flow structures is proportional to boundary-layer thickness. A speculative explanation of the well documented sequence of bedforms with increasing flow regime involves the successive domination of microscale structure (producing ripples), bursting (dunes), longitudinal roll vortices (upper flat bed and large-scale lineation), and in-phase waves (antidunes).
Other flow structures, possibly width dependent, are associated with macroforms. Examples include spiral motion in bends, which maintains point bars, and obscure long-period oscillations in flow velocity, which may be associated with alternate bars and initiation of meandering.
There remains a critical need to test further these proposals.
Acknowledgments and Associated Footnotes
1 Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201
Copyright © 2009 by the Canadian Society of Petroleum Geologists