Bed Configurations

Abstract

Abstract A striking characteristic of sediment transport on noncohesive beds is the development of geometrical forms or elements on a scale much larger than the grains themselves. These forms always show a strong element of regularity in arrangement, although usually they are irregular in detail. Most are elongated either parallel or transverse to the flow. Parallel-to-flow bed forms seem to be a manifestation of spiraling fluid motion, called secondary flow, that is parallel to the main flow and superimposed upon it. The convergent and divergent transverse components of near-bed flow may mold the bed into a series of ridges and troughs; the bed geometry in turn affects the structure of the flow, but not fundamentally. Transverse-to-flow bed forms, which are more commonly preserved in the sedimentary record, are dynamically more complicated in that they arise from a strong interaction between the flow and the bed. The nonuniformity of flow over transverse bed forms is not antecedent to the bed forms but comes about by the interaction between the flow and the bed; we'll see that these bed forms can be produced by uniform flow over a bed that is planar on the scale of grains. In this chapter we'll consider only transverse bed forms under unidirectional flows, but you should bear in mind that transverse bed forms under oscillatory flows are also interesting and important. See Clifton (1976), Harms et al. (1982), and Allen (1982, Vol. 1) for more information on oscillatory-flow bed forms* Transverse bed forms move upstream or downstream at velocities much smaller than the flow velocity, by erosion of sediment at some points and deposition at other points. Most are at least crudely wave-shaped and are therefore often likened to waves. They are not waves in the dynamic sense, as water waves or sound waves are, although you'll see that they have certain wave like properties in a kinematic as well as a geometric sense. Geologists and engineers alike have marveled at bed forms. Apart from their intrinsic fascination as a physical phenomenon, bed forms are of practical importance: (i) they constitute obstacles to navigation and a threat to submarine structures; (ii) when present in a channel they are the most important determinant of flow resistance; (iii) they have an important and complicating effect on sediment transport rate; (iv) their existence leads to the development of some of the most common stratification features in sedimentary deposits, and these structures are one of the most useful tools available in the interpretation of ancient sedimentary environments.