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A bedform is a geological feature that develops at the interface of fluid and a moveable
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
, the result of bed material being moved by fluid flow. Examples include
ripples Ripple may refer to: Science and technology * Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid ** Ripple, more generally a disturbance, for example of spacetime in gravitational waves * Ripple (electri ...
and dunes on the
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
of a river. Bedforms are often preserved in the
rock record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, san ...
as a result of being present in a depositional setting. Bedforms are often characteristic to the flow parameters, and may be used to infer flow depth and velocity, and therefore the
Froude number In continuum mechanics, the Froude number (, after William Froude, ) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). The Froude number is based on ...
.


Bedforms Initiation

Bedforms are omnipresent in many environments (e.g., fluvial, eolian, glaciofluvial, deltaic and deep sea), although there is still some debate on how they develop. There are two separate, though not mutually exclusive, models of bedform initiation: defect initiation and instantaneous initiation.


Defect Initiation

The defect theory proposes that the turbulent sweeps that are generated in turbulent flows entrain sediment that upon deposition generates defects in a non-cohesive material. These deposits then propagate downstream via a flow separation process, thus developing bedform fields. The origin of the defects is thought to be linked to packets of hairpin vortex structures. These coherent turbulent structures give rise to entrainment corridors on the mobile bed, forming grain lineations that interact with the low-speed streaks generating an agglomeration of grains. Once a critical height of grains is reached, flow separation occurs over the new structure. Sediment will be eroded close from the reattachment point and deposited downstream creating a new defect. This new defect will thus induce formation of another defect and the process will continue, propagating downstream while the accumulations of grains quickly evolve into small bedforms.


Instantaneous Initiation

In general, the defect propagation theory plays a bigger role at low
sediment transport Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems ...
rates since for high rates defects maybe washed away and bedforms generally initiated across the entire bed spontaneously. Venditti et al. (2005) report that instantaneous initiation begins with the formation of a cross-hatch pattern, which leads to chevron-shaped forms that migrate independently of the pattern structure. This chevron-like structure reorganizes to form the future crest lines of the bedforms. Venditti et al. (2006), based on the earlier model by Liu (1957), proposed that instantaneous initiation is a manifestation of an interfacial hydrodynamic instability of Kelvin-Helmholtz type between a highly active pseudofluid sediment layer and the fluid above it. In addition, Venditti et al. (2005) imply that there is no linkage between the instantaneous initiation and coherent turbulent flow structures, since spatially- and temporally-random events should lock in place to generate the cross-hatch pattern. Moreover, there is no clear explanation of the effect of turbulence in the formation of bedforms since bedforms may also occur under laminar flows . It is important to note, that laminar-generated bedform studies used the temporally-averaged flow conditions to determine the degree of
turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
, indicating
Reynolds number In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domin ...
in the laminar regime. However, instantaneous process, such as burst and sweeps, which are infrequent at low Reynolds number but still present, can be the driving mechanisms to generate the bedforms. The generation of bedforms in laminar flows is still a topic of debate within the scientific community, since if true, it suggests that there should be other processes for defect development other than the one suggested by Best (1992). This alternative model for bedform development at low
sediment transport Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems ...
rates should explain the generation of defects and bedforms for cases where the flow is not turbulent.


Bedform phase diagrams

Phase or stability diagrams are defined as graphs that show the regimes of existence of one or more stable bed states. The stability of the bed can be defined when the bedform is in equilibrium and does not change in time for the same flow condition. This invariance over time must not be confused with a static morphology or frozen equilibrium; on the contrary, the bed moves and adjusts in a dynamic equilibrium with the flow and sediment transport for that particular condition. These phase diagrams are used for two main purposes: i) for prediction of bed states in a known flow and sediment transport condition, and, ii) as a tool for the reconstruction of paleoenvironments from a known bed state or sedimentary structure. Despite the great utility of such diagrams, they are very difficult to construct, making them either incomplete or very hard to interpret. This complexity lies in the number of variables needed to quantify the system.


Bedforms vs. flow

Typical unidirectional bedforms represent a specific flow velocity, assuming typical sediments (sands and silts) and water depths, and a chart such as below can be used for interpreting
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
s,Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 45-49, with increasing water velocity going down the chart. This chart is for general use, because changes in grain size and flow depth can change the bedform present and skip bedforms in certain scenarios. Bidirectional environments (e.g. tidal flats) produce similar bedforms, but the reworking the sediments and opposite directions of flow complicates the structures. This bed form sequence can also be illustrated diagrammatically:


Types of Bedforms


Lower Plane Bed

"Lower plane bed" refers to the flat configuration the bed of a river that is produced in via low rates of
sediment transport Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems ...
.


Upper Plane Bed

"Upper plane bed" features are flat and characterized by a unidirectional flow with high rates of
sediment transport Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems ...
as both
bed load The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or saltating (hop ...
and
suspended load The suspended load of a flow of fluid, such as a river, is the portion of its sediment uplifted by the fluid's flow in the process of sediment transportation. It is kept suspended by the fluid's turbulence. The suspended load generally consists ...
. Upper plane bed conditions can produce parting
current lineation Parting lineation (also known as current lineation or primary current lineation) is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone). The ori ...
s, which are typically subtle streaks on the bed surface due to the high energy flow.


See also

*
Churn turbulent flow Churn turbulent flow is a two-phase gas/liquid flow regime characterized by a highly-agitated flow where gas bubbles are sufficient in numbers to both interact with each other and, while interacting, coalesce to form larger distorted bubbles with ...
*
Sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
*
Sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...


References

{{river morphology Sedimentary structures