Antidune
Encyclopedia
An antidune is a bedform
found in fluvial
environments. It occurs where there is supercritical flow
, meaning that the Froude number
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 stoss (upstream) side and eroded from the lee (downstream) 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 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, most of the antidune bedform is destroyed.
Antidunes are commonly observed in small streams that flow across beaches into the ocean. Flume
studies have show that they can also occur in submarine environments beneath density flows like turbidity current
s. Antidunes produce sedimentary structures characteristic of their flow regime, which allow sedimentary geologist
s to understand past flow conditions. Unlike low flow regime bedforms like dunes 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.
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
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.
. 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 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
to Gilberts' results and came up with transition points using Froude numbers versus velocity
and hydraulic radius.
Bedform
A bedform is a depositional feature on the bed of a river or other body of flowing water that is formed by the movement of the bed material due to the flow. Bedforms are characteristic to the flow parameters, and are particularly to flow depth and velocity, and therefore the Froude...
found in fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
environments. It occurs where there is supercritical flow
Supercritical flow
A supercritical flow is when the flow velocity is larger than the wave velocity. The analogous condition in gas dynamics is supersonic....
, meaning that the Froude number
Froude number
The Froude number is a dimensionless number defined as the ratio of a characteristic velocity to a gravitational wave velocity. It may equivalently be defined as the ratio of a body's inertia to gravitational forces. In fluid mechanics, the Froude number is used to determine the resistance of an...
is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flow regime
Sedimentary structures
Sedimentary structures are those structures formed during sediment deposition.Sedimentary structures such as cross bedding, graded bedding and ripple marks are utilized in stratigraphic studies to indicate original position of strata in geologically complex terrains and understand the depositional...
. In antidunes, sediment is deposited on the stoss (upstream) side and eroded from the lee (downstream) 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 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, most of the antidune bedform is destroyed.
Formation
Antidunes are typically found in fluvial environments in shallow areas with a high flow rate. Antidunes look similar to ripples that go in the direction of the flow, but are easily identified by their rapid change and moving in opposition to the flow of the water. 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
Flume
A flume is an open artificial water channel, in the form of a gravity chute, that leads water from a diversion dam or weir completely aside a natural flow. Often, the flume is an elevated box structure that follows the natural contours of the land. These have been extensively used in hydraulic...
studies have show that they can also occur in submarine environments beneath density flows like turbidity current
Turbidity current
A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows...
s. Antidunes produce sedimentary structures characteristic of their flow regime, which allow sedimentary geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s to understand past flow conditions. Unlike low flow regime bedforms like dunes 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.
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
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
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 SocietyRoyal Geographical Society
The Royal Geographical Society is a British learned society founded in 1830 for the advancement of geographical sciences...
. 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 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
Dimensional analysis
In physics and all science, dimensional analysis is a tool to find or check relations among physical quantities by using their dimensions. The dimension of a physical quantity is the combination of the basic physical dimensions which describe it; for example, speed has the dimension length per...
to Gilberts' results and came up with transition points using Froude numbers versus velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
and hydraulic radius.
External links
- http://radio.weblogs.com/0100021/stories/2002/03/06/sedimentTransportInAntidun.html
- http://fluidflow.es-designs.com/?p=150
- http://fluidflow.es-designs.com/?p=155
- http://fluidflow.es-designs.com/?p=159
- http://radio.weblogs.com/0100021/categories/antidunes/2002/06/13.html
- http://fluidflow.es-designs.com/?p=219