Antidunes
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An antidune is a
bedform A bedform is a geological feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the ...
found in
fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...
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 force field (the latter in many applications simply due to gravity). The Froude number is ba ...
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 studies have shown that they can also occur in submarine environments beneath density flows like
turbidity current A turbidity current is most typically an Ocean current, 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 ...
s. Antidunes produce
sedimentary structure 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 parti ...
s characteristic of their flow regime, which allow sedimentary
geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...
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, flat ...
and ripples 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 alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...
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), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March ...
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 United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March ...
. The first person to attempt an analytical description of antidunes was Walter B. Langbein in 1942. He applied dimensional analysis to Gilberts' results and came up with transition points using Froude numbers versus
velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...
and
hydraulic radius The Manning formula or Manning's equation is an empirical formula estimating the average velocity of a liquid in an open channel flow (flowing in a conduit that does not completely enclose the liquid). However, this equation is also used for calc ...
.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
Dissertation Abstract
– Fluid Flow by Tom Clifton


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