An antidune is a bedform found in

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

and other channeled environments. Antidunes occur in supercritical flow, meaning that 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 ...

is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flow regime. In antidunes, sediment is deposited on the upstream (stoss) side and eroded from the downstream (lee) side, opposite lower flow regime bedforms. As a result, antidunes migrate in an upstream direction, counter to the current flow. Antidunes are called in-phase bedforms, meaning that the water surface elevation mimics the bed elevation; this is due to the supercritical flow regime. Antidune bedforms evolve rapidly, growing in amplitude as they migrate upstream. The resultant wave at the water's surface also increases in amplitude. When that wave becomes unstable, breaks and washes downstream, much of the antidune bedform may be destroyed.

Formation

Antidunes are typically found in fluvial environments in shallow areas with a high flow rate. Unlike ripples and dunes in lower flow regime, antidunes are generally symmetric and migrate counter to the flow direction. Antidunes evolve rapidly, growing in amplitude as they migrate against the current. When the surface wave above them becomes unstable and breaks (when the surface wave amplitude reaches 1/7 its wavelength) most of the antidune bedform is destroyed and its sediment carried down stream. Antidunes are commonly observed in small streams that flow across beaches into the ocean.

Flume A flume is a human-made channel for water, in the form of an open declined gravity chute whose walls are raised above the surrounding terrain, in contrast to a trench or ditch. Flumes are not to be confused with aqueducts, which are built to t ...

studies have shown that they can also occur in submarine environments beneath density flows like

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

s. Antidunes produce sedimentary structures characteristic of their flow regime, which allow sedimentary

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

s to understand past flow conditions. Unlike low flow regime bedforms like

dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...

and

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 ...

which generally produce downstream dipping cross stratification, antidunes produce a mixture of low-angle downstream and upstream dipping strata. While antidunes migrate upstream, upstream dipping cross-stratification is not indicative of antidunes or upper flow regime conditions. If the Froude number is high enough, cyclic steps may form instead of antidunes. Antidunes migrate upstream because the stream flow is shallow and fast in the trough and slows and deepens over the crest. As a result, the

shear stress Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...

on the bed decreases from trough to crest, allowing sedimentation, and increases from crest to tough, causing erosion. The inertia of the flow moves the shear stress maximum and minimum slightly downstream of the trough and crest. This allows the bedform to amplify with time as erosion occurs in the trough and deposition occurs at the crest. Christopher R. Fielding observed a link between their formation and the climate. Climates that have extreme rainy seasons resulting in runoff create a higher flow velocity within their streams and rivers, thus increasing the ability of upper flow regime structures to form.
Here
is a video showing the formation and destruction of a modern antidune.

History

In 1899 the first description of antidunes was presented by Vaughan Cornish to the

Royal Geographical Society The Royal Geographical Society (with the Institute of British Geographers), often shortened to RGS, is a learned society and professional body for geography based in the United Kingdom. Founded in 1830 for the advancement of geographical scien ...

. He observed that while water was flowing down stream waves occurred that traveled up stream depositing sand and other material. This observation was later validated by John S. Owens in 1908. The term antidune was coined by G.K. Gilbert in a 1914

US Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...

Professional Paper entitled “Transportation of debris by running water”. He wrote this report in conjunction with E. C. Murphy, their description of antidunes and stationary waves that expanded on Cornish and Owens' previous report. Their information was gathered during a laboratory investigation sponsored by the U.S. Geological Survey. The first person to attempt an analytical description of antidunes was Walter B. Langbein in 1942. He applied

dimensional analysis In engineering and science, dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base quantities (such as length, mass, time, and electric current) and units of measure (such as mi ...

to Gilberts' results and came up with transition points using Froude numbers versus

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

and hydraulic radius.Kennedy, John F. (1961) Stationary waves and antidunes in alluvial channels. Technical Report. California Institute of Technology, Pasadena, CA, USA.ch 1. p. 1-8

References

* *Middleton, G. V.
Antidune cross-bedding in a large flume
* AILSA ALLABY and MICHAEL ALLABY. "antidune." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 7 Oct. 2010

* Journal of Geophysical Research, 2009, Vol. 114, Issue F4 * https://web.archive.org/web/20110710201741/http://fluidflow.es-designs.com/?p=219

External links

* https://web.archive.org/web/20081011003303/http://radio.weblogs.com/0100021/stories/2002/03/06/sedimentTransportInAntidun.html * https://web.archive.org/web/20110710201132/http://fluidflow.es-designs.com/?p=150 * https://web.archive.org/web/20110710201620/http://fluidflow.es-designs.com/?p=155 * https://web.archive.org/web/20110710201702/http://fluidflow.es-designs.com/?p=159 * https://web.archive.org/web/20080908004604/http://radio.weblogs.com/0100021/categories/antidunes/2002/06/13.html * https://web.archive.org/web/20110710201741/http://fluidflow.es-designs.com/?p=219 {{river morphology Sedimentology