Hierarchical attributes and a unifying model of bed forms composed of cohesionless material and produced by shearing flow

Abstract

From those bed forms generated by the shearing flow of a fluid and composed of cohesionless granular material, five hierarchical attributes are recognized: bed-form size, time span of existence of individual bed forms or bed configurations, superpositions, flow regime of bed forms in open-channel flows, and the Russian theory of channel process. The first two attributes permit the definition of three fundamental groups of bed forms common to all sedimentary environments dominated by shearing flow. Each group responds to different formative processes, themselves time hierarchical. The largest bed forms (macroforms), such as point bars, respond to the geomorphological regime of the environment and are relatively insensitive to changes in fluid-dynamic regime during an individual dynamic event (for example, a flood in a river). A two-zone structural model of turbulent boundary layers provides a genetic framework for the two smaller classes of bed forms. Mesoforms, such as dunes in rivers, respond to flow conditions in the outer zone of the turbulent boundary layer as the flow varies through a dynamic event; their lives scale correspondingly with the duration of that event. The smallest bed forms (microforms), for example, current lineations, are governed by the flow structure in the inner zone; their lives are much shorter than the periodicity of dynamic events. The above considerations constitute a unifying model of the origin of bed forms. Three essential components of the model (bed-form size, time, and structure of the turbulent boundary layer) are readily measurable by present experimental techniques.