15 September 2010
Posted by Callan Bentley
This video was produced by my friend Pete Berquist. It shows rapidly moving “sheets” of sand saltating down Grandview Beach in Hampton, Virginia, during high winds associated with Hurricane Earl.
What do you notice here? A couple of things jump out at me, but I’d be curious to hear what this video makes Mountain Beltway readers think about…
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.
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I’m curious if saltation is the dominant transport process here, or whether it could be a modified form of transport via suspended load (rather than bed load). Thoughts?
Wouldn’t it pretty much have to be saltation, given the location of all the sand grains only a few cm above the surface? Correct me if I’m wrong, Mr. Videographer, but that’s what it looks like to me. Were your knees getting stung by sand as well as your ankles?
Another thought — during the scene where the camera is pointed downwind, there is a low-lying area (I want to call it a swale, but I think that terminology isn’t accurate) filled with water. As the blown sand moves along, it appears that none crosses this watery barrier. If the grains were suspended, the water wouldn’t be an obstacle — but if they’re saltating, then they’re going to touch down on that water and abruptly sink rather than bounce up again. Since I can’t see any sand making it across the water, I’m inferring that saltation is the mode of transport…
You’re absolutely right – I think the “pond” in which sand is deposited demonstrates that saltation is taking place and that it is a significant process, however, the wind speed was sufficient to suspend particles a few centimeters above the surface (and yes, there was quite a bit of sand on my knees when I got back to the truck, but no where near as much as feet/ankles) and hold them in suspension for quite a distance. Grains were not rolling (velocity too high) and the distance that grains jumped were quite long (didn’t measure). These observations made me wonder where we make the distinction between bed load and suspended load transport. How far does a grain have to travel in suspension before call it “suspended load”? If a grain touches the bed just once in it’s transport life, what is it considered? It would appear that saltation is a process that may apply to both bed and suspended load, no?
I was reading in Boggs’ Principles of Sed/Strat this morning about “intermittent suspension” and wondered if that could be a viable mechanism here.
Thanks for the post, Callan. Curious to see any other discussion your post conjures up.
Well, I just had to weigh in – for what it’s worth!
I’ve seen this on beaches in high winds on several occasions and, in part, I think that the contrast, both visually and mechanically, between the wet sand substrate and the blowing sand is probably a factor. But this kind of phenomenon is also common in desert environments.
These are the “bursts” of sand that make headway on quantification and modelling of sand transport so challenging: non-linear, spatially and temporally rapidly fluctuating, fluxes of sand. The primary mechanism must still be saltation, but individual grain “leaps” are very long indeed. I guess that this may look like suspension, but in wind transport, the traditional definition of suspension is that it’s indefinite – which this clearly isn’t (and the observation about crossing the water is important). But it is interesting that the height that the sand grains attain seems to be quite limited – there must be a complex vertical velocity gradient, but exactly why this is I really don’t know.
Mositure is certainly a key factor in all this – a wet surface, unsurprisingly, has a much higher threshold wind velocity for sand movement than a dry one. Some relevant research (and possibly useful references) can be found in a paper by Stout at http://www.lbk.ars.usda.gov/wewc/biblio/jstout/Stout_2004_ESPL_Threshold.pdf
That’s about all I can think of for the moment…..
Thanks, Michael. I knew we could count on you to offer some wise arenaceous counsel. Thanks for the point about how MOST of the sand on the beach is wet, and therefore not as likely to get picked up by the wind. I hadn’t picked up on that, but now that I re-watch the video, I see that is not only true, but “color-coded” to boot!
So it looks to me like what we’re seeing here, morphologically, are little miniature longitudinal dunes (parallel to wind direction), but I associate the “loose sediment moving over a non-participatory substrate” style with barchans, barchanoids, and transverse dunes. So, what’s up with that?
Is it incorrect to call these wind-parallel stringers of sand “mini-transverse-dunes” — Is scale paramount over geometric analogy?
Its definitely interesting to think about sediment transport processes here. However, I have been spending a lot of time separating zircon from sands lately, and it made me wonder if significant differentiation of light and heavy minerals occurs during these storms. I can see a bunch of heavies in the video, and I bet they are much more difficult to transport in an aeolian fashion.
I think scale is paramount over geometric analogy! These things are very transient features. As for the heavies, these conditions would be great for winnowing – but also for activating (“entraining”) grains that normally wouldn’t be, and therefore increasing the supply for placer minerals.
I like the “bursts” of sand phrase that Michael put forward. One thing I noticed was that the sand seems to move in waves sideways sometimes, like meanders – maybe due to tiny changes in wind direction? This seems more apparent looking at it coming toward rather than going away from, so maybe it is only an apparent motion.
Callan and Pete,
Thanks for posting this video, it’s always enlightening to actually see the full dynamics of these processes.
This made me break out my copy of ‘Sedimentology and Sedimentary Basins’ text by Mike Leeder (which I highly recommend). I’m just going to quote him here (from p. 132):
“In saltating motion, grains rise steeply (>45 deg) from the bed, ascending to a height of a few grain diameters, then undergo a smooth motion like ballistic trajectory leading into a shallow-angled (~10 deg) descent back to bed. … Saltation may occur in both laminar and turbulent flows.”
He never mentions distance. When discussing suspension the key point is that the trajectory by which the grain moves up from the bed is highly irregular as compared to saltation. However, saltating grains may have irregular trajectories as a function of increased grain-to-grain collision dynamics (what he calls “collision-interrupted saltating grains”).
It’s all about the fall velocity of the grains w/in the context of the horizontal flow velocity.
As for suspended load, Leeder states:
“Suspended load includes all grains kept aloft by fluid turbulence so that the weight force of the suspended grains is balanced by an upward momentum transfer from fluid eddies.”
So, in this case, even though the distance between the “bounces” might be quite long, not to mention the complexity of grains schooling into those streaks, I would still consider this bed load transport.
Awesome stuff.