Fluid Flow

Antidunes on the Toutle River, 1984

Geologists find a way to simulate the great Missoula floods: “Scientists use detailed computer simulations to get a play-by-play look at how the great floods unfolded thousands of years ago.”

(Via Oregon Local News.)

Supercritical flows seem likely across the scablands, at least until water backed up at the entrance of the Gorge. Similar conditions probably occurred at the Gorge outlet near Portland. There you had expanding flow that could have produced and preserved some very large antidunes. It all kind of depends on the sediment load of the flows.

I am trying to imagine my little beach experiments blown up a few thousand times. Impressive, to say the least.

24 years ago right now. At 8:22 a.m. on May 18, 1980 the landscape in the photo above was shattered and transformed into something new. [Mac Net Journal]

I am pleased that someone else recognized this event. I was in California on May 18th, but experienced one ash fall that summer while out at Willipa Bay. Five years later, I was working in the blast zone and on the Toutle River, looking at the river deposits produced by the subsequent floods. During this work, I found sedimentary structures produced by antidunes, which ultimately led to my dissertation.

While reviewing the referer logs for this weblog, I came across a MSN search for “antidune experiments” that generated two hits. I went to the search to see if there were any new data on antidunes.

Fluid Flow was the third listing. The second listing was the antidune reference page that I maintain. The top listing pointed to a DOE site that I have never heard about. More importantly the title described antidune structures exactly. So off I went, hoping that someone had made similar observations to mine.

What I found, however, was a citation for an abstract that I wrote in 1987. I didn’t know that it was available on the web.

The abstract is important because it provides a model of how antidunes produce internal sedimentary structures. 17 years later, it still works, though I think upstream dipping strata are more common than I did in 1987.

NASA offers free aero/fluid dynamics software for OS X. NASA Langley Research Center has released Tetrahedral Unstructured Software System, or TetrUSS, for Mac OS X. TetrUSS is used in aerodynamics and fluid dynamics analysis, and has been used on major projects including High Speed Research / High Speed Civil Transport, Hyper-X, Abrupt Wing Stall, Mars Scout, Joint Strike Fighter and more. What’s more, the software has been used in the civilian aerospace industry, academy, automotive, biomedical and civil engineering fields. [MacCentral]

Using this to work on antidune flow might be interesting.

DeltaGraph coming to Mac OS X [MacCentral]

This is great news. I used DeltaGraph 1.5 to general most of the graphs in my dissertation. To date, DeltaGraph remains the best program for generating scientific graphs that I have found. It is the only application I have that regularly forces me to boot into Classic.

DeltaGraph has changed hands multiple times in the past 13 years. The fact that it is still viable and still being upgraded attests to its value.

Last year I combined some of the thoughts and observations on sediment transport in antidune flow that I posted here into as single story. Unfortunately, I never provided a link to that story. So I am doing it now.

The discussion provides an overview of sediment transport in antidune flow and provides a basis for understanding the upstream migration and amplification of antidunes.

The following abstract is from my dissertation, “Sedimentology of Antidune Flow: controls on sediment transport and stratification”

Antidunes are bed configurations that form in sediment under fast, shallow flows. These wave-like features are highly unstable and contrast with more common bedforms, like ripples and dunes, by migrating upstream and changing dramatically in morphology over very short periods of time. Antidunes often amplify rapidly, deforming the flow above them until the water surface becomes unstable and collapses, partially or completely destroying the bedform. Because antidunes display such a dynamic behavior, it is difficult to observe interactions between the flow and the bed and collect data from the flows. As a result, our understanding of how antidunes produce preservable sedimentary structures is limited. Without this understanding, the identification of antidune structures in ancient deposits is difficult and often suspect.

The present study aims to improve our understanding of antidunes and their sedimentary structures. It relies on field observations and descriptions of small streams that contain antidunes, the sedimentary structures produced by these streams, and similar structures in ancient deposits. Flow data from streams were collected by video taping antidunes and their associated flows. Experiments where stream channels were altered to form antidunes in rapidly aggrading settings augmented the observations and provided a direct link between antidunes and their internal structures.

Antidunes, when migrating in rapidly aggrading settings, produce an intricate pattern of stratification consisting of two distinct types of laminae. The most common type of laminae typically dip in a downstream direction at variable angles and are a type of translatent strata that forms as the antidune trough migrates on an aggrading bed. These thin laminae truncate underlying structures and form the bounding surfaces around inversely graded, lenticular packets of sediment. A second type of laminae mark the instantaneous position of an antidune’s upstream face and occur within the lenticular packets. These laminae dip upstream, downlap onto the translatent strata and may occur sporadically. The appearance of antidune structures varies dramatically with aggradation rate and degree of stability displayed by the antidunes. As a result, these structures may be useful in interpreting paleoflow conditions.

In order to publish my dissertation online, I needed to get all the equations converted to gif images. So I spent some time this afternoon back in Word 3, taking screen shots of the equations. These are the equations for Chapter 4 on sediment transport in antidune flow.

Antidune References

I recently posted the reference list from my dissertation to the ES Designs site. Feel free to link to them if you ever have a need to.

Each reference has a named anchor consisting of first authors last name and the publication year (<a name=”clifton1973″>), so you can point directly to a reference.

Multiples are differentiated by sequential letters at the end of the entries (for example if there were multiple entries from 1973 with Clifton as the first author, the second reference would be named clifton1973a, the third would be named clifton1973b, etc).

You can see examples of how you can link the the list in the discussion below.

The distribution of shear stress along the bed drives sediment transport , with the sediment in the flow being a function of the bed shear stress. Sediment discharge is greater where bed shear stress is greater, because the higher bed shear stresses can remove more sediment from the bed.

indicate that changes in bed elevation with time (whether the sediment is being eroded from or deposited to the bed) is a function of sediment discharge along the bed in a two-dimensional flow. Which in turn is directly related to the bed shear stress. If you move along the bed from a zone of high sediment discharge to low sediment discharge, sediment will accumulate with time. Conversely if you move from a zone of low sediment discharge to high sediment discharge, sediment will be removed from the bed with time.

Because sediment discharge is directly related to bed shear stress, the points of maximum and minimum bed shear stress on the bed define boundary points between zones of increasing and decreasing sediment discharge. Erosion (removal of sediment from the bed over time) occurs where bed shear stress is increasing along the bed. Deposition (addition of sediment to the bed over time) occurs where bed shear is decreasing along the bed.

Now if you look at the patterns of bed shear stress along the bed and the resulting zones of deposition and erosion, you can see why antidunes migrate upstream and why they typically build in amplitude.

Starting the crest of the upstream antidune, there is a shear stress minimum just downstream of the crest. From this minimum, bed shear stress increases along the downstream side of the bedform to a maximum just downstream of the trough. Erosion occurs along this interval.

Moving on from this bed shear stress maximum, bed shear stress drops along the upstream side of the bedform, as the flow slows down and gets deeper, to the bed shear stress minimum just downstream from the crest. Depostion occurs in this interval of decreasing bed shear stress.

The result is that sediment is added to the upstream side of the bedform and removed from the downstream side. As this happens, the bedform moves upstream. Similarly, since the trough is in a zone of erosion and the crest is in zone of deposition. The bedform will build in amplitude over time.