KeywordsVelocity Magnitude Oscillatory Flow Hydraulic Condition Unidirectional Flow Flow Velocity Increase
The term “bedforms” is used for to describe rhythmic topographic features on the surface of granular beds. A wide variety of such features form in response to specific ranges of hydraulic conditions and grain size. Apart from their intrinsic scientific interest, bedforms are important in both geology and engineering. Large subaqueous bedforms can be obstacles to navigation, and their migration can be a threat to submarine structures. Bedforms also play an important role in determining the resistance to flow. Bedforms are also one of the most useful tools available for interpreting ancient sedimentary environments from outcrops.
When unidirectional flow operates on relatively fine-grained sand (less than 0.7 mm), the following bedforms appear in order as flow “strength” is increased: current ripples, dunes, upper-regime plane bed (absence of bedforms), antidunes, and cyclic steps. If the bed material is coarser than 0.7 mm, the ripple regime is replaced by lower-regime plane bed.
Under purely oscillatory flows over fine-grained sediments, small symmetrical regular straight-crested (2D) ripples, less regular 3D ripples, and large 3D ripples appear with increasing oscillatory velocity magnitude. In the case of long-period oscillatory flows, large dome-like mounds with gentle slopes appear; these are known as “hummocky beds.” In the case of coarser sediment, ripples tend to stay straight-crested. When the velocity magnitude becomes large, ripples are washed out to plane bed.
When unidirectional flow is superimposed on oscillatory flow, i.e., combined wave-current flow, bedforms change corresponding to the sum of unidirectional flow velocity and oscillatory flow velocity magnitude. When the unidirectional flow velocity is smaller than a threshold, bedforms are the same as those under purely oscillatory flow. When unidirectional flow velocity increases beyond this threshold, bedforms show characteristic rounded crests and asymmetrical profiles. The threshold at which unidirectional flow affects bedform shape decreases with increasing oscillatory period. When the unidirectional component dominates, large, asymmetric dune-like bedforms appear. On the other hand, when the oscillatory component dominates, hummocky beds appear. These bedforms wash out to plane bed at sufficiently high velocities.
Various bed phase diagrams have been proposed to categorize the relationship between bedforms and hydraulic conditions. In order to construct such diagrams, the relevant parameters which govern the relationships must be specified. In general, bed state is a function of seven parameters. Five parameters are common to all cases: grain size, sediment density, fluid density, fluid viscosity, and sediment submerged specific weight. For unidirectional flow, two more parameters, such as flow velocity and flow depth, are needed. For oscillatory flow, two of the following three parameters must be specified: oscillation period, orbital diameter, and maximum orbital velocity. Dimensional analysis allows the seven variables to be grouped into four dimensionless variables. A large body of experimental research on bedforms under unidirectional flows and short-period oscillatory flows is available. Our knowledge of long-period oscillatory flows and multidirectional combinations of combined waves and currents, however, remains limited.
- Bridge, J., and Demicco, R., 2008. Earth Surface Processes Landforms and Sediment Deposits. Cambridge, UK/New York: Cambridge University Press, pp. 121–254. Chap. 5, 6, 7.Google Scholar
- Garcia, M. H., 2008. Chapter 2 Sediment transport and morphodynamics. In: Garcia, M. H. (ed.), Sedimentation Engineering. ASCE (American Society of Civil Engineers), pp. 21–163.Google Scholar
- Leeder, M., 2011. Chapter 7 Bedforms and sedimentary structures in flows and under waves. In Leeder, M. (ed.), Sedimentology and Sedimentary Basins, 2nd edn. Willey-Blackwell, pp. 132–170.Google Scholar
- Southard, J.B., Chapter 12 Bed configurations. In Southard, J. B., 12.090 Introduction to Fluid Motions, Sediment Transport, and Current-Generated Sedimentary Structures, Fall 2006. (MIT OpenCourseWare: Massachusetts Institute of Technology), http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-090-introduction-to-fluid-motions-sediment-transport-and-current-generated-sedimentary-structures-fall-2006, 350-443.Google Scholar