WEBVTT 00:00:00.000 --> 00:00:01.970 align:middle line:90% [SQUEAKING] 00:00:01.970 --> 00:00:03.930 align:middle line:90% [RUSTLING] 00:00:03.930 --> 00:00:05.400 align:middle line:90% [CLICKING] 00:00:05.400 --> 00:00:19.800 align:middle line:90% 00:00:19.800 --> 00:00:21.800 align:middle line:84% JOHN SOUTHARD: This is where we ended last time. 00:00:21.800 --> 00:00:25.480 align:middle line:84% Vito Vanoni, a senior colleague of mine at Caltech, 00:00:25.480 --> 00:00:28.640 align:middle line:90% did this interesting experiment. 00:00:28.640 --> 00:00:30.760 align:middle line:90% He had a flume running. 00:00:30.760 --> 00:00:36.080 align:middle line:84% And he had a microscopic, down-looking view of an area 00:00:36.080 --> 00:00:38.880 align:middle line:84% on the bed, right about at critical conditions. 00:00:38.880 --> 00:00:40.702 align:middle line:90% He used two sediments. 00:00:40.702 --> 00:00:43.160 align:middle line:84% I don't know whether you can read this-- glass beads, about 00:00:43.160 --> 00:00:47.600 align:middle line:84% 0.4 millimeters, and quartz, about a tenth of a millimeter. 00:00:47.600 --> 00:00:55.760 align:middle line:84% And he counted the number of grain movements per unit time. 00:00:55.760 --> 00:00:59.400 align:middle line:84% And he thought that this would be 00:00:59.400 --> 00:01:02.280 align:middle line:84% negligible transport way down-- this is the Shields curve way 00:01:02.280 --> 00:01:04.599 align:middle line:84% down below here, small transport, what 00:01:04.599 --> 00:01:07.200 align:middle line:84% he thought was critical transport, and then general. 00:01:07.200 --> 00:01:10.440 align:middle line:90% So this shows you how well-- 00:01:10.440 --> 00:01:15.800 align:middle line:84% arbitrary, guess you'd say, it is to try to pin it down. 00:01:15.800 --> 00:01:18.960 align:middle line:84% There he is, Vito Vanoni, one of all-time greats 00:01:18.960 --> 00:01:21.380 align:middle line:84% in sedimentation engineering, really nice person. 00:01:21.380 --> 00:01:22.620 align:middle line:90% He'd already retired. 00:01:22.620 --> 00:01:26.960 align:middle line:84% He was the head of the hydraulic lab at Caltech. 00:01:26.960 --> 00:01:28.600 align:middle line:84% Now, the sediment load-- what I mean 00:01:28.600 --> 00:01:32.120 align:middle line:84% by the load is all the sediment particles being transported 00:01:32.120 --> 00:01:34.560 align:middle line:84% at a given time-- is called the sediment load. 00:01:34.560 --> 00:01:39.632 align:middle line:84% And now I'm going to do some of my thought experiments here. 00:01:39.632 --> 00:01:41.340 align:middle line:84% Did I tell you about thought experiments? 00:01:41.340 --> 00:01:44.040 align:middle line:90% They're really important. 00:01:44.040 --> 00:01:46.440 align:middle line:84% You figure out in your mind how things 00:01:46.440 --> 00:01:48.920 align:middle line:84% are going to work in some system or experiment. 00:01:48.920 --> 00:01:50.320 align:middle line:90% And then you do it. 00:01:50.320 --> 00:01:52.722 align:middle line:84% And if it's the same as what you thought, you say, 00:01:52.722 --> 00:01:53.680 align:middle line:90% I've learned something. 00:01:53.680 --> 00:01:54.460 align:middle line:90% That's nice. 00:01:54.460 --> 00:01:59.040 align:middle line:84% And if it's something totally different, you say, oh, shit, 00:01:59.040 --> 00:02:00.820 align:middle line:84% I need to go back and think about it. 00:02:00.820 --> 00:02:03.620 align:middle line:84% So it's nice to have thought experiments. 00:02:03.620 --> 00:02:07.720 align:middle line:84% So you can think about instantaneously freeze a block, 00:02:07.720 --> 00:02:09.060 align:middle line:90% take it out, melt it. 00:02:09.060 --> 00:02:10.460 align:middle line:90% And that sediment's the load. 00:02:10.460 --> 00:02:14.200 align:middle line:84% That's one way you can think about the sediment load. 00:02:14.200 --> 00:02:16.860 align:middle line:84% Now, you probably know there's bed load and suspended load. 00:02:16.860 --> 00:02:19.120 align:middle line:84% And I'll tell you a little bit about it. 00:02:19.120 --> 00:02:20.060 align:middle line:90% They integrate. 00:02:20.060 --> 00:02:22.080 align:middle line:90% It's not a sharp division. 00:02:22.080 --> 00:02:24.900 align:middle line:84% But all the particles moving on or near the bed-- 00:02:24.900 --> 00:02:27.280 align:middle line:84% and by near, I mean pretty close to the bed 00:02:27.280 --> 00:02:29.120 align:middle line:84% relative to the overlying turbulent flow-- 00:02:29.120 --> 00:02:29.870 align:middle line:90% is called bedload. 00:02:29.870 --> 00:02:32.200 align:middle line:90% 00:02:32.200 --> 00:02:34.360 align:middle line:84% And then suspended load is everything moving well 00:02:34.360 --> 00:02:37.520 align:middle line:90% above the bed. 00:02:37.520 --> 00:02:40.240 align:middle line:84% Now, remember, we had the little thought experiment 00:02:40.240 --> 00:02:43.280 align:middle line:84% in which you magically shrink yourself to micro size. 00:02:43.280 --> 00:02:44.960 align:middle line:84% And you're down there among the grains. 00:02:44.960 --> 00:02:48.320 align:middle line:84% Well, if you were down here among some of the bed grains 00:02:48.320 --> 00:02:50.000 align:middle line:84% and you're watching what was happening, 00:02:50.000 --> 00:02:52.000 align:middle line:84% you could imagine things landing and about 00:02:52.000 --> 00:02:55.660 align:middle line:84% to take off, just landed, sitting there, 00:02:55.660 --> 00:02:56.960 align:middle line:90% and part of the bed. 00:02:56.960 --> 00:02:59.070 align:middle line:84% You've got to make sure that you're not 00:02:59.070 --> 00:03:02.130 align:middle line:84% getting flattened between some grain hitting another grain. 00:03:02.130 --> 00:03:05.430 align:middle line:84% But this gives you a picture of what's 00:03:05.430 --> 00:03:08.610 align:middle line:84% happening with the grains moving near the bed. 00:03:08.610 --> 00:03:13.110 align:middle line:90% 00:03:13.110 --> 00:03:14.170 align:middle line:90% This is important. 00:03:14.170 --> 00:03:16.510 align:middle line:84% Most particles travel more slowly in the flow, 00:03:16.510 --> 00:03:18.410 align:middle line:84% or they can get up to speed of the flow. 00:03:18.410 --> 00:03:20.330 align:middle line:84% But they tend to lag behind the flow. 00:03:20.330 --> 00:03:25.190 align:middle line:90% 00:03:25.190 --> 00:03:27.710 align:middle line:84% Now, you've probably heard of saltation. 00:03:27.710 --> 00:03:31.550 align:middle line:84% Saltation is the big thing in aeolian environments. 00:03:31.550 --> 00:03:34.150 align:middle line:84% But a lot of people think that there's 00:03:34.150 --> 00:03:38.790 align:middle line:84% a saltation layer of particles taking off, moving 00:03:38.790 --> 00:03:41.310 align:middle line:84% fairly close to the bed, and landing again. 00:03:41.310 --> 00:03:42.890 align:middle line:90% And it's controversial. 00:03:42.890 --> 00:03:45.310 align:middle line:84% And I've never thought much about it. 00:03:45.310 --> 00:03:47.570 align:middle line:84% It doesn't seem to make that much sense to me. 00:03:47.570 --> 00:03:51.210 align:middle line:84% But a lot of people think that there is saltation near the bed. 00:03:51.210 --> 00:03:53.750 align:middle line:90% 00:03:53.750 --> 00:03:56.950 align:middle line:84% Wash load is the part of the suspended load that 00:03:56.950 --> 00:04:01.090 align:middle line:84% gets washed through the reach of a channel or a flow 00:04:01.090 --> 00:04:03.990 align:middle line:90% or a river or a tidal current. 00:04:03.990 --> 00:04:08.070 align:middle line:84% And it's not represented in the bed. 00:04:08.070 --> 00:04:11.310 align:middle line:84% Bed material load is stuff that's in the bed. 00:04:11.310 --> 00:04:14.390 align:middle line:90% And it can go into suspension. 00:04:14.390 --> 00:04:16.190 align:middle line:84% But it's not washed right through. 00:04:16.190 --> 00:04:18.130 align:middle line:84% Grains can be put up into suspension. 00:04:18.130 --> 00:04:19.510 align:middle line:90% They travel some distance. 00:04:19.510 --> 00:04:23.390 align:middle line:84% And they come down to the bed at some distance downstream. 00:04:23.390 --> 00:04:24.350 align:middle line:90% And this may help you. 00:04:24.350 --> 00:04:25.630 align:middle line:90% I don't know. 00:04:25.630 --> 00:04:27.970 align:middle line:84% Wash load is part of the suspended load. 00:04:27.970 --> 00:04:30.730 align:middle line:84% But bed material load can also be in the suspended load. 00:04:30.730 --> 00:04:34.250 align:middle line:84% And of course, the bedload is clearly bed material load. 00:04:34.250 --> 00:04:37.990 align:middle line:84% So-- in case that helps you a little bit. 00:04:37.990 --> 00:04:40.310 align:middle line:84% By transport stages, I mean, how is it working? 00:04:40.310 --> 00:04:43.070 align:middle line:84% Once you get past the initiation, 00:04:43.070 --> 00:04:45.352 align:middle line:84% the Shields curve, where there's no movement, 00:04:45.352 --> 00:04:46.310 align:middle line:90% first you have rolling. 00:04:46.310 --> 00:04:48.470 align:middle line:90% And then the person who-- 00:04:48.470 --> 00:04:50.990 align:middle line:90% I copied this from somebody-- 00:04:50.990 --> 00:04:52.490 align:middle line:84% thinks that saltation is important-- 00:04:52.490 --> 00:04:54.230 align:middle line:90% so that's saltation and rolling. 00:04:54.230 --> 00:04:58.270 align:middle line:84% And then when you get up to a certain criterion, 00:04:58.270 --> 00:04:59.550 align:middle line:90% it goes into suspension. 00:04:59.550 --> 00:05:04.390 align:middle line:84% And you have suspension, saltation, unrolling, 00:05:04.390 --> 00:05:06.230 align:middle line:90% all at the same time. 00:05:06.230 --> 00:05:09.310 align:middle line:84% If you want to know more about transport modes, 00:05:09.310 --> 00:05:12.630 align:middle line:84% there's a thing in the notes you can look at. 00:05:12.630 --> 00:05:16.830 align:middle line:84% Now, this is a big deal in engineering practice, sediment 00:05:16.830 --> 00:05:17.970 align:middle line:90% transport rates. 00:05:17.970 --> 00:05:21.350 align:middle line:84% How much sediment is going to go through the system? 00:05:21.350 --> 00:05:23.710 align:middle line:90% You think of the-- 00:05:23.710 --> 00:05:26.790 align:middle line:84% a unit sediment transport rate in any given slice, 00:05:26.790 --> 00:05:31.270 align:middle line:84% or the whole flow could be a total sediment transport rate. 00:05:31.270 --> 00:05:34.110 align:middle line:84% And many formulas have been developed 00:05:34.110 --> 00:05:37.550 align:middle line:84% to try to predict sediment load for that-- sediment transport 00:05:37.550 --> 00:05:43.110 align:middle line:84% rates because of its importance in hydraulic engineering. 00:05:43.110 --> 00:05:44.810 align:middle line:90% There's no analytical solution. 00:05:44.810 --> 00:05:48.430 align:middle line:84% So you have to work semi-empirically and have-- 00:05:48.430 --> 00:05:50.670 align:middle line:84% make some assumptions and then work from there. 00:05:50.670 --> 00:05:52.610 align:middle line:84% And I'm not going to do anything with it here. 00:05:52.610 --> 00:05:55.350 align:middle line:84% But you should recognize that sediment transport 00:05:55.350 --> 00:05:59.110 align:middle line:84% rate is an important consideration in sedimentation. 00:05:59.110 --> 00:06:02.270 align:middle line:84% And here's how you could conceptualize it. 00:06:02.270 --> 00:06:04.310 align:middle line:84% There's my magic screen stretching right 00:06:04.310 --> 00:06:05.450 align:middle line:90% across the flow. 00:06:05.450 --> 00:06:07.190 align:middle line:84% And you somehow magically pick out 00:06:07.190 --> 00:06:10.470 align:middle line:84% all the particles that are going past into that screen 00:06:10.470 --> 00:06:13.210 align:middle line:84% or through that screen at-- per unit time. 00:06:13.210 --> 00:06:15.130 align:middle line:84% And that's the sediment transport rate. 00:06:15.130 --> 00:06:18.710 align:middle line:90% 00:06:18.710 --> 00:06:20.250 align:middle line:90% Here's another way of doing it. 00:06:20.250 --> 00:06:24.150 align:middle line:84% Here's my magic vacuum suction trap. 00:06:24.150 --> 00:06:25.250 align:middle line:90% There's no such thing. 00:06:25.250 --> 00:06:26.690 align:middle line:90% But it's nice to think about. 00:06:26.690 --> 00:06:29.230 align:middle line:84% And so it catches everything that's going by, 00:06:29.230 --> 00:06:30.790 align:middle line:90% coming along in the flow. 00:06:30.790 --> 00:06:33.910 align:middle line:84% And all the sediment transport and that pile of sediment, 00:06:33.910 --> 00:06:37.970 align:middle line:84% if you normalize per unit time, is the sediment transport rate. 00:06:37.970 --> 00:06:41.310 align:middle line:84% So now you know something about the sediment transport rate. 00:06:41.310 --> 00:06:44.990 align:middle line:84% How do you measure the sediment being transported? 00:06:44.990 --> 00:06:46.750 align:middle line:90% And that's a tricky business. 00:06:46.750 --> 00:06:50.710 align:middle line:84% Suspended load is not too difficult to do. 00:06:50.710 --> 00:06:55.060 align:middle line:84% You can have an array of little flip-top bottles 00:06:55.060 --> 00:06:57.300 align:middle line:90% that you can remotely flip. 00:06:57.300 --> 00:07:00.120 align:middle line:84% You take a sample here, a sample here, a sample here. 00:07:00.120 --> 00:07:02.980 align:middle line:84% And then you integrate it, and you get a suspended load 00:07:02.980 --> 00:07:04.620 align:middle line:90% transport rate. 00:07:04.620 --> 00:07:07.260 align:middle line:90% Bedload is tougher. 00:07:07.260 --> 00:07:09.863 align:middle line:84% Various bedload traps have been devised. 00:07:09.863 --> 00:07:10.780 align:middle line:90% You put it on the bed. 00:07:10.780 --> 00:07:14.760 align:middle line:84% And you somehow let the sediment get into the trap. 00:07:14.760 --> 00:07:16.720 align:middle line:84% It's almost like catching an animal in a trap. 00:07:16.720 --> 00:07:18.660 align:middle line:84% And you close the trap and bring it up. 00:07:18.660 --> 00:07:21.277 align:middle line:84% And they work sort of well, but not really that great. 00:07:21.277 --> 00:07:22.860 align:middle line:84% So you just keep in mind that it's not 00:07:22.860 --> 00:07:25.620 align:middle line:84% easy to measure sediment transport rate. 00:07:25.620 --> 00:07:30.200 align:middle line:84% So that's my pitch on sediment transport in water. 00:07:30.200 --> 00:07:32.260 align:middle line:84% So now let's think about the wind. 00:07:32.260 --> 00:07:36.380 align:middle line:84% That's an important consideration as well. 00:07:36.380 --> 00:07:37.620 align:middle line:90% Why is it so great? 00:07:37.620 --> 00:07:39.160 align:middle line:90% Here's a mistake. 00:07:39.160 --> 00:07:43.740 align:middle line:84% I have discovered a few serious mistakes, thoughtless mistakes, 00:07:43.740 --> 00:07:44.760 align:middle line:90% in my notes. 00:07:44.760 --> 00:07:49.900 align:middle line:84% And I'm making an addendum before the next presentation 00:07:49.900 --> 00:07:52.380 align:middle line:90% to show them to you. 00:07:52.380 --> 00:07:54.580 align:middle line:90% This should be rho s over rho. 00:07:54.580 --> 00:07:56.020 align:middle line:90% How did I do that? 00:07:56.020 --> 00:08:00.180 align:middle line:84% So the density ratio, sediment density over fluid density, 00:08:00.180 --> 00:08:02.680 align:middle line:84% is far greater in wind than in water. 00:08:02.680 --> 00:08:06.100 align:middle line:84% And that has implications for the independence of movement 00:08:06.100 --> 00:08:07.940 align:middle line:90% of sediment particles. 00:08:07.940 --> 00:08:10.360 align:middle line:84% If the density difference isn't great, 00:08:10.360 --> 00:08:14.940 align:middle line:84% they can do their inertial thing more or less unaffected 00:08:14.940 --> 00:08:18.040 align:middle line:84% by the turbulent flow that they're in, the wind flow. 00:08:18.040 --> 00:08:22.020 align:middle line:90% 00:08:22.020 --> 00:08:28.900 align:middle line:84% Ralph Alger Bagnold, the biggest figure in aeolian sand transport 00:08:28.900 --> 00:08:30.900 align:middle line:90% that's ever lived-- 00:08:30.900 --> 00:08:34.539 align:middle line:84% I almost had a chance to meet him because he was visiting MIT 00:08:34.539 --> 00:08:37.980 align:middle line:84% here in the late '50s, when I was an undergrad. 00:08:37.980 --> 00:08:40.720 align:middle line:84% And my advisor had him over for dinner one night. 00:08:40.720 --> 00:08:42.080 align:middle line:90% But of course, I wasn't invited. 00:08:42.080 --> 00:08:44.580 align:middle line:84% So I never got a chance to meet Bagnold. 00:08:44.580 --> 00:08:49.980 align:middle line:84% He did his work in the years before the First World 00:08:49.980 --> 00:08:52.440 align:middle line:84% War and the Second World War in the Libyan desert. 00:08:52.440 --> 00:08:55.460 align:middle line:84% And he went out on expeditions and did all kinds of things. 00:08:55.460 --> 00:08:59.360 align:middle line:84% This book-- you can look at it because I parked-- 00:08:59.360 --> 00:09:00.140 align:middle line:90% I bought a copy. 00:09:00.140 --> 00:09:03.180 align:middle line:84% And I parked it in Kristin's lab. 00:09:03.180 --> 00:09:04.660 align:middle line:90% It's a classic. 00:09:04.660 --> 00:09:06.980 align:middle line:90% And it's worth looking through. 00:09:06.980 --> 00:09:09.280 align:middle line:84% So I figured I'd like to get one. 00:09:09.280 --> 00:09:11.820 align:middle line:84% So I went to AbeBooks, which is a-- 00:09:11.820 --> 00:09:13.700 align:middle line:84% I don't know what you know about AbeBooks. 00:09:13.700 --> 00:09:15.740 align:middle line:84% But you can buy books, used books, 00:09:15.740 --> 00:09:17.620 align:middle line:84% from hundreds and hundreds of booksellers 00:09:17.620 --> 00:09:20.273 align:middle line:90% all around the world. 00:09:20.273 --> 00:09:20.940 align:middle line:90% I typed this in. 00:09:20.940 --> 00:09:23.740 align:middle line:90% And I copied-- $5, a good copy. 00:09:23.740 --> 00:09:24.560 align:middle line:90% And I bought it. 00:09:24.560 --> 00:09:26.140 align:middle line:90% And I got it. 00:09:26.140 --> 00:09:28.360 align:middle line:84% It's one of these Dover reprints. 00:09:28.360 --> 00:09:30.100 align:middle line:90% It was brand new. 00:09:30.100 --> 00:09:34.180 align:middle line:84% The next most expensive cost $50. 00:09:34.180 --> 00:09:38.260 align:middle line:84% So fortunately, I was able to get this thing for $5. 00:09:38.260 --> 00:09:40.400 align:middle line:84% So how do you study sediment moved by the wind? 00:09:40.400 --> 00:09:41.740 align:middle line:90% Well, you build a wind tunnel. 00:09:41.740 --> 00:09:44.440 align:middle line:84% And we've built two wind tunnels, 00:09:44.440 --> 00:09:45.960 align:middle line:90% one small and one pretty big. 00:09:45.960 --> 00:09:48.580 align:middle line:90% 00:09:48.580 --> 00:09:51.720 align:middle line:84% The one we built, the person would have been like this tall, 00:09:51.720 --> 00:09:52.740 align:middle line:90% not that tall. 00:09:52.740 --> 00:09:55.400 align:middle line:84% That's the extent of my artistic ability, stick figure. 00:09:55.400 --> 00:09:57.460 align:middle line:84% I never got beyond stick figures. 00:09:57.460 --> 00:10:00.220 align:middle line:84% So you have a flared entrance, a rectangular duct 00:10:00.220 --> 00:10:03.460 align:middle line:84% with a sand bed, and a collector that keeps the sand in. 00:10:03.460 --> 00:10:06.820 align:middle line:84% And the air is sucked out by a fan. 00:10:06.820 --> 00:10:10.340 align:middle line:90% And it's very effective. 00:10:10.340 --> 00:10:14.420 align:middle line:84% But if you want to really see the grains moving, 00:10:14.420 --> 00:10:18.340 align:middle line:84% here's the channel, the side view of the channel of the duct. 00:10:18.340 --> 00:10:20.820 align:middle line:84% You have a strobe light-- blink, blink, blink-- 00:10:20.820 --> 00:10:22.620 align:middle line:90% and a collimator and a slit. 00:10:22.620 --> 00:10:25.260 align:middle line:84% So you can look from the side and watch 00:10:25.260 --> 00:10:27.980 align:middle line:84% the saltating particles-- bloop, bloop, bloop, bloop, bloop. 00:10:27.980 --> 00:10:29.680 align:middle line:84% It's really a nice way of seeing-- 00:10:29.680 --> 00:10:32.420 align:middle line:84% or you go out into the wind and look 00:10:32.420 --> 00:10:35.580 align:middle line:90% at a sand bed on a windy day. 00:10:35.580 --> 00:10:37.060 align:middle line:90% That's a lot of fun. 00:10:37.060 --> 00:10:39.060 align:middle line:84% Grotzinger and I and a couple other people 00:10:39.060 --> 00:10:44.340 align:middle line:84% were out on the White Sands in New Mexico, gypsum sands. 00:10:44.340 --> 00:10:45.920 align:middle line:84% It was a windy day, really windy day. 00:10:45.920 --> 00:10:46.980 align:middle line:90% It was a hot day. 00:10:46.980 --> 00:10:49.450 align:middle line:90% And we were sweaty. 00:10:49.450 --> 00:10:51.910 align:middle line:84% And we ended up looking like snowmen at the end of the day. 00:10:51.910 --> 00:10:53.290 align:middle line:90% It stuck to us. 00:10:53.290 --> 00:10:54.810 align:middle line:90% It was hilarious. 00:10:54.810 --> 00:10:56.610 align:middle line:90% [LAUGHING] 00:10:56.610 --> 00:11:00.810 align:middle line:84% So saltation is a big deal in aeolian transport. 00:11:00.810 --> 00:11:05.730 align:middle line:84% Saltation trajectories-- they take a pretty steep angle 00:11:05.730 --> 00:11:06.910 align:middle line:90% up into the wind flow. 00:11:06.910 --> 00:11:11.330 align:middle line:84% And then they arch over and splash down later on. 00:11:11.330 --> 00:11:12.750 align:middle line:90% It's very, very characteristic. 00:11:12.750 --> 00:11:15.450 align:middle line:84% And that's what you can see if you did your strobe experiment 00:11:15.450 --> 00:11:18.250 align:middle line:90% that I just mentioned. 00:11:18.250 --> 00:11:20.770 align:middle line:84% And the amazing thing is that the spin rates 00:11:20.770 --> 00:11:22.410 align:middle line:90% are fantastically high. 00:11:22.410 --> 00:11:25.250 align:middle line:84% And I've never really understood why. 00:11:25.250 --> 00:11:27.030 align:middle line:84% I must-- must be theories about it. 00:11:27.030 --> 00:11:31.070 align:middle line:84% But I haven't managed to get into it. 00:11:31.070 --> 00:11:34.450 align:middle line:84% But they're spinning at hundreds of revolutions per second, which 00:11:34.450 --> 00:11:35.870 align:middle line:90% seems incredible. 00:11:35.870 --> 00:11:37.890 align:middle line:90% But it's true. 00:11:37.890 --> 00:11:40.570 align:middle line:84% And one more point I want to make-- 00:11:40.570 --> 00:11:42.650 align:middle line:84% there's an aerodynamic threshold. 00:11:42.650 --> 00:11:47.330 align:middle line:84% You have a flat, planar sediment surface in the wind tunnel. 00:11:47.330 --> 00:11:50.050 align:middle line:84% Crank up the wind velocity till some particles finally 00:11:50.050 --> 00:11:51.370 align:middle line:90% starts to move. 00:11:51.370 --> 00:11:53.430 align:middle line:84% And that's called the aerodynamic threshold. 00:11:53.430 --> 00:11:57.490 align:middle line:84% But suppose you already have a saltating system 00:11:57.490 --> 00:11:59.570 align:middle line:84% and you gradually decrease the velocity. 00:11:59.570 --> 00:12:02.010 align:middle line:84% Because of the effect of the saltating grains hitting 00:12:02.010 --> 00:12:04.330 align:middle line:84% other grains when they land, you have 00:12:04.330 --> 00:12:08.130 align:middle line:84% to go to a much lower velocity of wind velocity 00:12:08.130 --> 00:12:10.530 align:middle line:90% before the saltation stops. 00:12:10.530 --> 00:12:12.770 align:middle line:84% There's a-- I don't know what you 00:12:12.770 --> 00:12:14.230 align:middle line:90% call it-- a hysteresis effect. 00:12:14.230 --> 00:12:15.150 align:middle line:90% You can call it that. 00:12:15.150 --> 00:12:19.130 align:middle line:90% 00:12:19.130 --> 00:12:22.330 align:middle line:84% What causes the particle to be launched at the high angle? 00:12:22.330 --> 00:12:25.290 align:middle line:84% That's always been a somewhat controversial issue. 00:12:25.290 --> 00:12:28.110 align:middle line:84% And it has to be some combination of lift forces. 00:12:28.110 --> 00:12:30.903 align:middle line:84% Remember I told you about lift forces on an airfoil? 00:12:30.903 --> 00:12:32.570 align:middle line:84% It's because of the pressure differences 00:12:32.570 --> 00:12:33.970 align:middle line:90% in the bottom and the top. 00:12:33.970 --> 00:12:38.010 align:middle line:84% Well, clearly, there are lift forces on these particles. 00:12:38.010 --> 00:12:42.530 align:middle line:84% And of course, the splashing down particles 00:12:42.530 --> 00:12:44.010 align:middle line:84% can kick up more particles and make 00:12:44.010 --> 00:12:50.610 align:middle line:84% them vulnerable to high angles of launching. 00:12:50.610 --> 00:12:53.610 align:middle line:84% And we're talking about a saltation 00:12:53.610 --> 00:12:54.850 align:middle line:90% carpet about this high. 00:12:54.850 --> 00:12:59.290 align:middle line:84% It's down around your legs and up to your waistline. 00:12:59.290 --> 00:13:02.590 align:middle line:84% And seldom in-- even in the very strongest winds, 00:13:02.590 --> 00:13:04.350 align:middle line:84% it wouldn't be more than about 2 meters. 00:13:04.350 --> 00:13:06.270 align:middle line:90% So you hear about sandstorms. 00:13:06.270 --> 00:13:07.870 align:middle line:84% You often read about big sandstorms. 00:13:07.870 --> 00:13:10.710 align:middle line:84% You're choking and the-- because you can't see anything. 00:13:10.710 --> 00:13:11.932 align:middle line:90% Those are dust storms. 00:13:11.932 --> 00:13:12.890 align:middle line:90% They're not sandstorms. 00:13:12.890 --> 00:13:13.970 align:middle line:90% They're dust storms. 00:13:13.970 --> 00:13:15.910 align:middle line:90% Keep in mind they're sandstorms. 00:13:15.910 --> 00:13:18.493 align:middle line:90% 00:13:18.493 --> 00:13:20.910 align:middle line:84% Now, I don't know whether any of you are planetary people. 00:13:20.910 --> 00:13:25.330 align:middle line:84% But you can think about the threshold, shear stress needed 00:13:25.330 --> 00:13:27.450 align:middle line:90% to get saltation going. 00:13:27.450 --> 00:13:28.968 align:middle line:90% Mars is much greater than Earth. 00:13:28.968 --> 00:13:30.510 align:middle line:84% And Earth is much greater than Venus. 00:13:30.510 --> 00:13:34.790 align:middle line:84% And it mainly has to do with differences in the air density, 00:13:34.790 --> 00:13:36.690 align:middle line:90% the atmospheric density. 00:13:36.690 --> 00:13:40.510 align:middle line:84% Is anybody in planetary stuff here? 00:13:40.510 --> 00:13:41.230 align:middle line:90% There you go. 00:13:41.230 --> 00:13:44.930 align:middle line:90% 00:13:44.930 --> 00:13:46.750 align:middle line:90% I just have to mention this. 00:13:46.750 --> 00:13:48.710 align:middle line:84% We're dealing with stuff on Earth. 00:13:48.710 --> 00:13:50.710 align:middle line:84% And now they're doing great things on Mars. 00:13:50.710 --> 00:13:53.870 align:middle line:84% But there are zillions-- not literally, but figuratively-- 00:13:53.870 --> 00:13:55.530 align:middle line:90% zillions of planets out-- 00:13:55.530 --> 00:13:57.833 align:middle line:84% Earth-like planets with saltation going on. 00:13:57.833 --> 00:13:59.250 align:middle line:84% And I don't think we're ever going 00:13:59.250 --> 00:14:00.150 align:middle line:90% to know anything about them. 00:14:00.150 --> 00:14:01.010 align:middle line:90% But there they are. 00:14:01.010 --> 00:14:04.650 align:middle line:84% I figure that there are probably 10 to the 20 something stars 00:14:04.650 --> 00:14:06.210 align:middle line:90% in the observable universe. 00:14:06.210 --> 00:14:08.690 align:middle line:84% And some non-negligible percentage of them 00:14:08.690 --> 00:14:10.270 align:middle line:90% would have Earth-like planets. 00:14:10.270 --> 00:14:14.170 align:middle line:84% Now, it might be 10 to the 17th, 10 to the 18th. 00:14:14.170 --> 00:14:16.277 align:middle line:84% And so there are these people like us out there 00:14:16.277 --> 00:14:17.110 align:middle line:90% watching this stuff. 00:14:17.110 --> 00:14:19.170 align:middle line:84% And we're never going to know about it. 00:14:19.170 --> 00:14:20.770 align:middle line:90% That's what I think, anyway. 00:14:20.770 --> 00:14:21.270 align:middle line:90% All right. 00:14:21.270 --> 00:14:22.770 align:middle line:90% So even on Earth-- 00:14:22.770 --> 00:14:24.330 align:middle line:90% I made this up once. 00:14:24.330 --> 00:14:26.990 align:middle line:84% And we looked at the density ratios. 00:14:26.990 --> 00:14:29.250 align:middle line:84% This time, I did it right, rho s over rho. 00:14:29.250 --> 00:14:30.910 align:middle line:90% And you start at 1. 00:14:30.910 --> 00:14:33.830 align:middle line:84% And anything on the left is just floating sediment. 00:14:33.830 --> 00:14:37.450 align:middle line:84% But when the sediments are denser than the water-- here's 00:14:37.450 --> 00:14:38.890 align:middle line:90% sand and water. 00:14:38.890 --> 00:14:40.750 align:middle line:84% Golden water-- gold is very dense. 00:14:40.750 --> 00:14:44.660 align:middle line:84% We did some experiments once with tungsten particles. 00:14:44.660 --> 00:14:47.040 align:middle line:84% Tungsten is almost as dense as gold. 00:14:47.040 --> 00:14:50.440 align:middle line:84% And we had some interesting-- the ripples are much smaller. 00:14:50.440 --> 00:14:53.360 align:middle line:84% Snow in air-- and I have a surprise slide for you later 00:14:53.360 --> 00:14:54.360 align:middle line:90% about that-- 00:14:54.360 --> 00:14:56.920 align:middle line:84% sand in air, and sand on Mars would be way 00:14:56.920 --> 00:14:58.220 align:middle line:90% over there, 10 to the fifth. 00:14:58.220 --> 00:15:01.440 align:middle line:90% 00:15:01.440 --> 00:15:05.840 align:middle line:84% This is a big topic for us, for sedimentary geologists. 00:15:05.840 --> 00:15:10.400 align:middle line:84% And I've been dealing with it for decades and decades. 00:15:10.400 --> 00:15:13.080 align:middle line:90% So there's a strong coupling. 00:15:13.080 --> 00:15:15.840 align:middle line:90% Flow makes the sediment move. 00:15:15.840 --> 00:15:19.560 align:middle line:84% And the sediment movement develops a geometrical bed 00:15:19.560 --> 00:15:21.640 align:middle line:90% configuration. 00:15:21.640 --> 00:15:24.320 align:middle line:84% But there's an inverse effect here 00:15:24.320 --> 00:15:28.600 align:middle line:84% because the bed configuration, once it's formed, 00:15:28.600 --> 00:15:33.155 align:middle line:84% greatly influences the sediment transport, and also the flow. 00:15:33.155 --> 00:15:35.280 align:middle line:84% So there's a forward problem and an inverse problem 00:15:35.280 --> 00:15:36.460 align:middle line:90% here you have to deal with. 00:15:36.460 --> 00:15:39.640 align:middle line:90% 00:15:39.640 --> 00:15:43.280 align:middle line:84% This is just a terminological thing. 00:15:43.280 --> 00:15:44.800 align:middle line:84% A bed configuration for me-- it's 00:15:44.800 --> 00:15:48.460 align:middle line:84% a distinctive pattern of geometrical features on the bed 00:15:48.460 --> 00:15:49.680 align:middle line:90% caused by a flow. 00:15:49.680 --> 00:15:52.600 align:middle line:84% And a bedform is any one of those individual things 00:15:52.600 --> 00:15:55.280 align:middle line:90% that are in the configuration. 00:15:55.280 --> 00:15:58.480 align:middle line:84% Now, most people nowadays lump these together 00:15:58.480 --> 00:16:00.840 align:middle line:90% and call it bedform, one word. 00:16:00.840 --> 00:16:02.820 align:middle line:84% I'm fighting a losing battle on this. 00:16:02.820 --> 00:16:05.880 align:middle line:84% Well, I'm not even in the fight anymore. 00:16:05.880 --> 00:16:07.840 align:middle line:84% And I think of a bed state as a kind 00:16:07.840 --> 00:16:11.020 align:middle line:84% of existence field for some kind of bed configuration. 00:16:11.020 --> 00:16:15.840 align:middle line:84% And we'll be dealing with that considerably pretty soon here. 00:16:15.840 --> 00:16:17.360 align:middle line:90% Why are we doing this? 00:16:17.360 --> 00:16:21.240 align:middle line:84% Well, there's a great variety of bed configurations 00:16:21.240 --> 00:16:24.440 align:middle line:84% that can be produced by flows, given the flow 00:16:24.440 --> 00:16:27.560 align:middle line:90% velocity and the sediment size. 00:16:27.560 --> 00:16:30.420 align:middle line:84% And so we have a lot of sedimentary structures, 00:16:30.420 --> 00:16:33.000 align:middle line:84% current-generated sedimentary structures, 00:16:33.000 --> 00:16:35.380 align:middle line:84% that we would like to be able to interpret. 00:16:35.380 --> 00:16:39.100 align:middle line:84% So if there's only one kind of bed configuration, 00:16:39.100 --> 00:16:41.040 align:middle line:84% it would be a dull world for sedimentologists 00:16:41.040 --> 00:16:43.120 align:middle line:84% because there wouldn't be this great variety of structures 00:16:43.120 --> 00:16:43.880 align:middle line:90% to see. 00:16:43.880 --> 00:16:45.920 align:middle line:84% But yet we have an enormous variety 00:16:45.920 --> 00:16:47.500 align:middle line:90% of structures-- is possible. 00:16:47.500 --> 00:16:48.380 align:middle line:90% Some we understand. 00:16:48.380 --> 00:16:49.760 align:middle line:90% Some we don't understand. 00:16:49.760 --> 00:16:51.800 align:middle line:90% And I'll get to that, also. 00:16:51.800 --> 00:16:53.940 align:middle line:84% So we'll start with unidirectional flows. 00:16:53.940 --> 00:16:58.400 align:middle line:90% 00:16:58.400 --> 00:17:00.880 align:middle line:90% There's my flume. 00:17:00.880 --> 00:17:04.940 align:middle line:84% And it's about the right scale, given the person standing there. 00:17:04.940 --> 00:17:07.319 align:middle line:84% It can be small or it can be even larger. 00:17:07.319 --> 00:17:09.920 align:middle line:84% But what you do is have a channel 00:17:09.920 --> 00:17:15.040 align:middle line:84% that you can tilt by means of a screw jack. 00:17:15.040 --> 00:17:18.560 align:middle line:84% And there's a pump of various kinds 00:17:18.560 --> 00:17:20.579 align:middle line:84% that circulate the water in the sediment. 00:17:20.579 --> 00:17:23.760 align:middle line:84% So the flow starts at the head box, 00:17:23.760 --> 00:17:25.579 align:middle line:90% flows down, moves the sediment. 00:17:25.579 --> 00:17:28.680 align:middle line:84% The water and sediment go into the tail box and is pumped out. 00:17:28.680 --> 00:17:33.600 align:middle line:84% So it's a closed-circuit recirculating flume. 00:17:33.600 --> 00:17:37.600 align:middle line:84% You're recirculating both the sediment and the water. 00:17:37.600 --> 00:17:40.560 align:middle line:84% There's another kind, similar, but not quite the same. 00:17:40.560 --> 00:17:44.160 align:middle line:84% You can have a freefall into the tail box. 00:17:44.160 --> 00:17:45.680 align:middle line:90% The sediment collects. 00:17:45.680 --> 00:17:47.300 align:middle line:90% The water gets recirculated. 00:17:47.300 --> 00:17:49.740 align:middle line:84% And you have to keep adding sediment at the upstream end. 00:17:49.740 --> 00:17:51.157 align:middle line:90% They both work. 00:17:51.157 --> 00:17:52.740 align:middle line:84% They're useful for different purposes. 00:17:52.740 --> 00:17:54.680 align:middle line:90% But that's basically what-- 00:17:54.680 --> 00:17:59.720 align:middle line:84% this was my living, basically, for decades. 00:17:59.720 --> 00:18:02.060 align:middle line:84% So in a very general way, this is what you would see. 00:18:02.060 --> 00:18:04.600 align:middle line:90% This is the first cut. 00:18:04.600 --> 00:18:07.440 align:middle line:84% With increasing flow velocity, we finally get to the point 00:18:07.440 --> 00:18:09.020 align:middle line:90% where sediments move. 00:18:09.020 --> 00:18:10.020 align:middle line:90% Ripples develop. 00:18:10.020 --> 00:18:13.960 align:middle line:84% This is the right medium-- fine to medium sand. 00:18:13.960 --> 00:18:16.480 align:middle line:84% And then eventually they disappear 00:18:16.480 --> 00:18:18.920 align:middle line:90% and dunes form, larger features. 00:18:18.920 --> 00:18:21.180 align:middle line:84% Then it all washes out to a plain bed. 00:18:21.180 --> 00:18:23.400 align:middle line:84% And eventually, if the water is shallow enough 00:18:23.400 --> 00:18:25.763 align:middle line:84% and the velocity is high enough, you get antidunes. 00:18:25.763 --> 00:18:27.180 align:middle line:84% You may have heard of these terms. 00:18:27.180 --> 00:18:27.722 align:middle line:90% I don't know. 00:18:27.722 --> 00:18:30.320 align:middle line:90% But we'll deal with them. 00:18:30.320 --> 00:18:33.060 align:middle line:84% And I want to introduce the concept of flow regimes. 00:18:33.060 --> 00:18:35.400 align:middle line:84% This is something that developed mainly 00:18:35.400 --> 00:18:37.200 align:middle line:90% among hydraulic engineers. 00:18:37.200 --> 00:18:40.990 align:middle line:84% There's a lower flow regime and an upper flow regime. 00:18:40.990 --> 00:18:45.830 align:middle line:84% The lower flow regime are examples 00:18:45.830 --> 00:18:51.230 align:middle line:84% of a rugged bed, a rugged bed of ripples and dunes. 00:18:51.230 --> 00:18:53.090 align:middle line:84% And they wash out to have a plain bed. 00:18:53.090 --> 00:18:55.090 align:middle line:84% And that's called the upper flow regime. 00:18:55.090 --> 00:18:56.928 align:middle line:84% Just it's a terminological matter. 00:18:56.928 --> 00:18:57.970 align:middle line:90% You should know about it. 00:18:57.970 --> 00:19:00.510 align:middle line:90% 00:19:00.510 --> 00:19:04.750 align:middle line:84% So ripples-- it's pretty clear, I guess. 00:19:04.750 --> 00:19:06.870 align:middle line:90% I do have a pointer here. 00:19:06.870 --> 00:19:11.790 align:middle line:84% That's the crest, trough, trough, et cetera. 00:19:11.790 --> 00:19:13.632 align:middle line:84% This is called the stoss surface. 00:19:13.632 --> 00:19:15.090 align:middle line:84% And this is called the lee surface. 00:19:15.090 --> 00:19:16.250 align:middle line:90% It's just terminology. 00:19:16.250 --> 00:19:18.470 align:middle line:90% But we'll be using that. 00:19:18.470 --> 00:19:21.430 align:middle line:84% And typically, what happens you'll see in a moment. 00:19:21.430 --> 00:19:24.230 align:middle line:84% Sediment comes up to the crest, gets dumped, 00:19:24.230 --> 00:19:30.190 align:middle line:84% slides down in little miniature gravity flows, grain flows, 00:19:30.190 --> 00:19:34.110 align:middle line:84% to cause the slip face to advance. 00:19:34.110 --> 00:19:35.910 align:middle line:90% That's very cartoonish. 00:19:35.910 --> 00:19:38.510 align:middle line:84% But now this tells you a little bit more about what's going-- 00:19:38.510 --> 00:19:40.270 align:middle line:90% and this is sort of complicated. 00:19:40.270 --> 00:19:41.770 align:middle line:90% The flow is from left to right. 00:19:41.770 --> 00:19:43.470 align:middle line:90% There's a ripple profile. 00:19:43.470 --> 00:19:47.270 align:middle line:84% These little arrows going up the stoss side 00:19:47.270 --> 00:19:50.830 align:middle line:84% are bedload transport, basically. 00:19:50.830 --> 00:19:54.350 align:middle line:84% This can be stuff in suspension as well. 00:19:54.350 --> 00:19:57.490 align:middle line:84% And for reasons I'll tell you in a minute, a lot of sediment-- 00:19:57.490 --> 00:20:00.790 align:middle line:84% some sediment can get suspended in the trough 00:20:00.790 --> 00:20:03.470 align:middle line:84% and continues on as suspended load. 00:20:03.470 --> 00:20:08.070 align:middle line:84% The grains reach the crest, flow down. 00:20:08.070 --> 00:20:09.190 align:middle line:90% So that helps. 00:20:09.190 --> 00:20:15.670 align:middle line:84% But this is going to be better to understand. 00:20:15.670 --> 00:20:18.550 align:middle line:90% Here's a crest, a trough. 00:20:18.550 --> 00:20:20.150 align:middle line:90% The flow is this way. 00:20:20.150 --> 00:20:23.750 align:middle line:90% So the flow separates. 00:20:23.750 --> 00:20:26.150 align:middle line:84% I told you about that in the last one. 00:20:26.150 --> 00:20:27.290 align:middle line:90% They separate. 00:20:27.290 --> 00:20:30.110 align:middle line:84% The flow separates as the turbulent shear layer develops 00:20:30.110 --> 00:20:32.550 align:middle line:84% a wake, an expanding wake, which, incidentally, has 00:20:32.550 --> 00:20:34.050 align:middle line:90% to go far, far down the flow-- 00:20:34.050 --> 00:20:38.350 align:middle line:84% I may have mentioned it-- before it comes back to invisibility. 00:20:38.350 --> 00:20:41.770 align:middle line:84% And you get a rotating vortex in there, 00:20:41.770 --> 00:20:44.510 align:middle line:84% which tends to carry sediment up onto at least the lower 00:20:44.510 --> 00:20:46.070 align:middle line:90% part of the slip face. 00:20:46.070 --> 00:20:47.490 align:middle line:90% And keep that in mind. 00:20:47.490 --> 00:20:51.410 align:middle line:84% And then the wake reattaches at this point. 00:20:51.410 --> 00:20:56.750 align:middle line:84% And then it's-- flow up the stoss side of the next ripple. 00:20:56.750 --> 00:20:58.450 align:middle line:84% That's typically how ripples work. 00:20:58.450 --> 00:21:02.310 align:middle line:90% 00:21:02.310 --> 00:21:04.170 align:middle line:90% But ripples are very changeable. 00:21:04.170 --> 00:21:05.910 align:middle line:90% They're fickle things. 00:21:05.910 --> 00:21:09.510 align:middle line:84% They move along and sometimes disappear. 00:21:09.510 --> 00:21:11.230 align:middle line:90% A new ripple forms. 00:21:11.230 --> 00:21:14.190 align:middle line:84% Ripples can divide into two, or they can merge together. 00:21:14.190 --> 00:21:16.590 align:middle line:90% And so we did this once. 00:21:16.590 --> 00:21:18.010 align:middle line:90% This is a cartoon. 00:21:18.010 --> 00:21:20.350 align:middle line:84% But we did this once, stationed a whole lot of students 00:21:20.350 --> 00:21:21.570 align:middle line:90% down along the flume. 00:21:21.570 --> 00:21:23.870 align:middle line:84% And each one of them keeps track of a ripple 00:21:23.870 --> 00:21:26.730 align:middle line:84% as a function of time, along with a scale along the-- 00:21:26.730 --> 00:21:28.210 align:middle line:90% and then we plot it all up. 00:21:28.210 --> 00:21:31.530 align:middle line:84% And these are the-- what do you want to call them-- 00:21:31.530 --> 00:21:34.430 align:middle line:90% the trajectories of ripples. 00:21:34.430 --> 00:21:36.410 align:middle line:84% And you can see how they come and go. 00:21:36.410 --> 00:21:37.370 align:middle line:90% They divide. 00:21:37.370 --> 00:21:37.890 align:middle line:90% They merge. 00:21:37.890 --> 00:21:38.610 align:middle line:90% They disappear. 00:21:38.610 --> 00:21:39.910 align:middle line:90% They reappear. 00:21:39.910 --> 00:21:42.890 align:middle line:84% So they're very changeable things. 00:21:42.890 --> 00:21:47.030 align:middle line:90% 00:21:47.030 --> 00:21:48.770 align:middle line:84% This is just a non-climbing ripple. 00:21:48.770 --> 00:21:53.950 align:middle line:84% There's a train of ripples along a surface. 00:21:53.950 --> 00:21:56.930 align:middle line:84% But if the sediment is not much available, 00:21:56.930 --> 00:21:59.870 align:middle line:84% if not enough sediment is available to maintain 00:21:59.870 --> 00:22:03.030 align:middle line:84% the ripples, you get what's called starved ripples. 00:22:03.030 --> 00:22:06.230 align:middle line:84% And I'll show you a nice slide of starved ripples. 00:22:06.230 --> 00:22:11.270 align:middle line:84% But if the bed is aggrading while the ripples are moving, 00:22:11.270 --> 00:22:13.830 align:middle line:84% then we build up the bed in the form of what 00:22:13.830 --> 00:22:15.530 align:middle line:90% are called climbing ripples. 00:22:15.530 --> 00:22:18.750 align:middle line:84% Each ripple climbs up the back of the last one. 00:22:18.750 --> 00:22:21.270 align:middle line:84% This is a little fakey because they don't throw 00:22:21.270 --> 00:22:22.850 align:middle line:90% right up on top, typically. 00:22:22.850 --> 00:22:25.550 align:middle line:84% But they can shear off the top of the ripple 00:22:25.550 --> 00:22:26.810 align:middle line:90% and be deposited on that. 00:22:26.810 --> 00:22:28.850 align:middle line:90% I'll get to that very soon. 00:22:28.850 --> 00:22:32.350 align:middle line:90% 00:22:32.350 --> 00:22:34.660 align:middle line:84% I'm going to show you some photos of how ripples 00:22:34.660 --> 00:22:36.120 align:middle line:90% develop from a planar bed. 00:22:36.120 --> 00:22:39.175 align:middle line:90% It's an interesting problem. 00:22:39.175 --> 00:22:41.800 align:middle line:84% You're going to see that they-- first, they're two-dimensional. 00:22:41.800 --> 00:22:43.258 align:middle line:84% And then they're three-dimensional. 00:22:43.258 --> 00:22:44.340 align:middle line:90% Now, this is tricky. 00:22:44.340 --> 00:22:46.220 align:middle line:90% This is a hydraulics term. 00:22:46.220 --> 00:22:51.140 align:middle line:84% Two-dimensional features are those that have straight crests. 00:22:51.140 --> 00:22:55.500 align:middle line:84% And so they look the same at every flow-parallel 00:22:55.500 --> 00:22:56.860 align:middle line:90% cross-section. 00:22:56.860 --> 00:22:59.400 align:middle line:84% Three-dimensional features, that I'm going to talk, 00:22:59.400 --> 00:23:01.920 align:middle line:84% but every section is going to look different. 00:23:01.920 --> 00:23:05.100 align:middle line:84% So there's 2D bedforms, and there's 3D bedforms. 00:23:05.100 --> 00:23:07.700 align:middle line:84% That can confuse people if they don't-- if they're thinking 00:23:07.700 --> 00:23:08.720 align:middle line:90% in a different way. 00:23:08.720 --> 00:23:11.980 align:middle line:90% 00:23:11.980 --> 00:23:15.540 align:middle line:84% And this is sort of like what I said about saltation and wind. 00:23:15.540 --> 00:23:19.460 align:middle line:84% The flow velocity needed for-- to keep pre-existing ripples 00:23:19.460 --> 00:23:22.420 align:middle line:84% going is a lot less than the need for initiation 00:23:22.420 --> 00:23:23.320 align:middle line:90% of plane bed. 00:23:23.320 --> 00:23:25.800 align:middle line:84% And again, because the sediment is already in motion-- 00:23:25.800 --> 00:23:27.660 align:middle line:84% has already created these features. 00:23:27.660 --> 00:23:30.320 align:middle line:84% And so they keep operating until finally, 00:23:30.320 --> 00:23:32.900 align:middle line:84% the current gets so low that the ripples shut off. 00:23:32.900 --> 00:23:37.100 align:middle line:84% And you get a static bed configuration. 00:23:37.100 --> 00:23:39.400 align:middle line:84% And you may have sediment coming along, draping it. 00:23:39.400 --> 00:23:41.560 align:middle line:84% There's a lot of draping involved, sometimes. 00:23:41.560 --> 00:23:44.380 align:middle line:90% 00:23:44.380 --> 00:23:47.880 align:middle line:84% This would be a not quite starved ripple. 00:23:47.880 --> 00:23:51.220 align:middle line:84% There's a nice ripple moving from left to right. 00:23:51.220 --> 00:23:56.080 align:middle line:84% And you can see that the trough is almost bare of sediment. 00:23:56.080 --> 00:23:58.520 align:middle line:84% And the next ripple over here, which isn't in the picture, 00:23:58.520 --> 00:24:01.140 align:middle line:84% looks a lot like the one you were just looking at-- 00:24:01.140 --> 00:24:03.420 align:middle line:84% so almost starved, but not quite. 00:24:03.420 --> 00:24:04.680 align:middle line:90% It's from northern Maine. 00:24:04.680 --> 00:24:07.220 align:middle line:90% 00:24:07.220 --> 00:24:09.220 align:middle line:90% This is from my flume. 00:24:09.220 --> 00:24:12.580 align:middle line:84% It's about a meter wide flow from top to bottom. 00:24:12.580 --> 00:24:14.340 align:middle line:90% That's a foot rule there. 00:24:14.340 --> 00:24:19.620 align:middle line:84% And you can see that the ripples have very irregular shapes. 00:24:19.620 --> 00:24:22.540 align:middle line:84% But they're dominantly flow transverse. 00:24:22.540 --> 00:24:25.840 align:middle line:84% And I want to especially point out that there are deep troughs. 00:24:25.840 --> 00:24:27.255 align:middle line:84% Here's a trough with water in it. 00:24:27.255 --> 00:24:28.380 align:middle line:90% Here's a trough with water. 00:24:28.380 --> 00:24:29.940 align:middle line:90% And it's reflecting a light. 00:24:29.940 --> 00:24:32.760 align:middle line:84% And that's going to figure importantly 00:24:32.760 --> 00:24:35.500 align:middle line:84% when we talk about sedimentary structures produced 00:24:35.500 --> 00:24:39.540 align:middle line:84% by movement of ripples during aggradation, during bed 00:24:39.540 --> 00:24:41.380 align:middle line:90% aggradation-- 00:24:41.380 --> 00:24:43.383 align:middle line:90% nice picture of ripples. 00:24:43.383 --> 00:24:44.800 align:middle line:84% Here's a side view of the ripples. 00:24:44.800 --> 00:24:47.680 align:middle line:84% And you can see this is in very fine sand. 00:24:47.680 --> 00:24:50.560 align:middle line:84% And it had some fine organic matter in it. 00:24:50.560 --> 00:24:55.460 align:middle line:84% And so that accentuates the foreset. 00:24:55.460 --> 00:24:57.220 align:middle line:90% Those are foreset laminae. 00:24:57.220 --> 00:24:58.980 align:middle line:90% The foreset is toward the flow. 00:24:58.980 --> 00:25:01.180 align:middle line:84% And the backset is on the back of the ripple. 00:25:01.180 --> 00:25:05.000 align:middle line:84% And they're dominantly flow transverse. 00:25:05.000 --> 00:25:07.780 align:middle line:84% But you can see they're rather irregular-- 00:25:07.780 --> 00:25:11.060 align:middle line:90% classic ripple picture. 00:25:11.060 --> 00:25:13.580 align:middle line:84% I have some pictures from the ancient. 00:25:13.580 --> 00:25:16.300 align:middle line:84% Sometimes, you see bedding surfaces that are rippled, 00:25:16.300 --> 00:25:19.100 align:middle line:90% which are nice to see. 00:25:19.100 --> 00:25:22.260 align:middle line:90% I wish I'd put a scale in this. 00:25:22.260 --> 00:25:23.420 align:middle line:90% It's a closeup of ripples. 00:25:23.420 --> 00:25:29.160 align:middle line:84% And you can see the sharp crests, the irregular shapes, 00:25:29.160 --> 00:25:32.940 align:middle line:84% planform shapes, the deep troughs here and there. 00:25:32.940 --> 00:25:36.380 align:middle line:84% That gives you the essence of these ripples. 00:25:36.380 --> 00:25:39.240 align:middle line:84% There's another bedding plane from the Dakota sandstone. 00:25:39.240 --> 00:25:42.460 align:middle line:90% 00:25:42.460 --> 00:25:45.320 align:middle line:84% A closeup-- I can't remember where I took that. 00:25:45.320 --> 00:25:48.540 align:middle line:84% But it's a nice closeup of a curved crest 00:25:48.540 --> 00:25:50.000 align:middle line:90% of a ripple and a trough. 00:25:50.000 --> 00:25:54.180 align:middle line:90% 00:25:54.180 --> 00:25:56.060 align:middle line:84% Sometimes, you see an interesting thing 00:25:56.060 --> 00:25:58.260 align:middle line:90% called linguoid ripples. 00:25:58.260 --> 00:25:59.840 align:middle line:90% I've seen this very seldom. 00:25:59.840 --> 00:26:05.080 align:middle line:84% I saw this once in Utah, near the Green River. 00:26:05.080 --> 00:26:07.660 align:middle line:90% 00:26:07.660 --> 00:26:12.580 align:middle line:84% They're shaped convex downstream. 00:26:12.580 --> 00:26:15.800 align:middle line:84% But they come together and be just-- 00:26:15.800 --> 00:26:22.640 align:middle line:84% the crest is just downstream of the next trough upstream. 00:26:22.640 --> 00:26:24.400 align:middle line:84% So they have a very regular pattern. 00:26:24.400 --> 00:26:26.180 align:middle line:90% I don't understand it at all. 00:26:26.180 --> 00:26:27.515 align:middle line:90% But it's not very common. 00:26:27.515 --> 00:26:29.640 align:middle line:84% There's something going on that I don't understand. 00:26:29.640 --> 00:26:32.280 align:middle line:84% And I don't think anybody else understands, either. 00:26:32.280 --> 00:26:36.620 align:middle line:90% 00:26:36.620 --> 00:26:41.300 align:middle line:84% Spacing ripples is almost always in the range 00:26:41.300 --> 00:26:43.740 align:middle line:90% of 10 to 15 centimeters. 00:26:43.740 --> 00:26:46.580 align:middle line:90% It depends on the grain size. 00:26:46.580 --> 00:26:49.780 align:middle line:84% The spacing is a weakly increasing function 00:26:49.780 --> 00:26:53.460 align:middle line:84% of the sediment size within the range of flow 00:26:53.460 --> 00:26:54.920 align:middle line:90% for which ripples are formed. 00:26:54.920 --> 00:26:59.480 align:middle line:84% But they're very stable in this size range. 00:26:59.480 --> 00:27:00.980 align:middle line:84% So if you see something much bigger, 00:27:00.980 --> 00:27:02.300 align:middle line:90% you know it's not a ripple. 00:27:02.300 --> 00:27:04.840 align:middle line:84% Now, on other planets, this may be different. 00:27:04.840 --> 00:27:08.140 align:middle line:84% And I'll make a brief mention of that later. 00:27:08.140 --> 00:27:08.640 align:middle line:90% All right. 00:27:08.640 --> 00:27:10.580 align:middle line:90% So we did this experiment. 00:27:10.580 --> 00:27:12.580 align:middle line:84% This was, I think, the second paper 00:27:12.580 --> 00:27:15.560 align:middle line:84% I ever published with one of my graduate students. 00:27:15.560 --> 00:27:17.540 align:middle line:90% A guy did a master's thesis. 00:27:17.540 --> 00:27:21.500 align:middle line:84% And we're looking down on a planar sediment bed in the flume 00:27:21.500 --> 00:27:23.280 align:middle line:90% and make a little indentation. 00:27:23.280 --> 00:27:26.860 align:middle line:84% And that indentation produces erosion deposition 00:27:26.860 --> 00:27:29.150 align:middle line:90% to produce an incipient ripple. 00:27:29.150 --> 00:27:30.965 align:middle line:84% And that one triggers two others. 00:27:30.965 --> 00:27:32.590 align:middle line:84% And you can see that it keeps on going. 00:27:32.590 --> 00:27:34.570 align:middle line:84% And eventually, it fills the whole channel. 00:27:34.570 --> 00:27:37.070 align:middle line:84% And I'll show you that in a minute. 00:27:37.070 --> 00:27:37.570 align:middle line:90% Bob? 00:27:37.570 --> 00:27:39.810 align:middle line:84% AUDIENCE: So that sort of looks like your linguoid ripples 00:27:39.810 --> 00:27:40.930 align:middle line:90% that you showed before. 00:27:40.930 --> 00:27:42.555 align:middle line:84% JOHN SOUTHARD: Well, sort of like, yes. 00:27:42.555 --> 00:27:45.710 align:middle line:84% But this gallops off into total irregularity. 00:27:45.710 --> 00:27:49.370 align:middle line:84% Why it stays regular in that pattern 00:27:49.370 --> 00:27:53.730 align:middle line:84% when the ripples are fully established, I don't know. 00:27:53.730 --> 00:27:56.230 align:middle line:84% I think it's an interesting question. 00:27:56.230 --> 00:28:02.810 align:middle line:84% So my colleague, Venditti, and Mike Church, his advisor, 00:28:02.810 --> 00:28:05.290 align:middle line:84% did an interesting study in which they 00:28:05.290 --> 00:28:06.470 align:middle line:90% did the same kind of thing. 00:28:06.470 --> 00:28:07.450 align:middle line:90% They triggered it. 00:28:07.450 --> 00:28:09.010 align:middle line:84% And it expanded somewhat differently 00:28:09.010 --> 00:28:12.290 align:middle line:90% from what I showed you. 00:28:12.290 --> 00:28:15.485 align:middle line:84% The initial ripples are almost two-dimensional, 00:28:15.485 --> 00:28:17.610 align:middle line:84% although there are some irregularly developing down 00:28:17.610 --> 00:28:17.870 align:middle line:90% here. 00:28:17.870 --> 00:28:19.970 align:middle line:84% But you could see if you went far enough down, 00:28:19.970 --> 00:28:23.085 align:middle line:84% it would develop into three-dimensionality. 00:28:23.085 --> 00:28:23.710 align:middle line:90% Here's another. 00:28:23.710 --> 00:28:25.730 align:middle line:84% Here's a closeup-- we did this ourselves-- 00:28:25.730 --> 00:28:29.490 align:middle line:84% in which you can see that the ripples are just-- 00:28:29.490 --> 00:28:31.390 align:middle line:84% flow from right to-- from left to right. 00:28:31.390 --> 00:28:33.650 align:middle line:84% Ripples are just forming each other. 00:28:33.650 --> 00:28:35.390 align:middle line:84% And at first, as they're growing, 00:28:35.390 --> 00:28:36.930 align:middle line:90% they're two-dimensional. 00:28:36.930 --> 00:28:39.210 align:middle line:84% They're already getting three-dimensional. 00:28:39.210 --> 00:28:41.210 align:middle line:84% The ones that are older are already 00:28:41.210 --> 00:28:43.970 align:middle line:90% getting three-dimensional. 00:28:43.970 --> 00:28:45.090 align:middle line:90% Yeah, so we-- 00:28:45.090 --> 00:28:47.170 align:middle line:84% AUDIENCE: --might be a naive question, but-- 00:28:47.170 --> 00:28:47.830 align:middle line:90% JOHN SOUTHARD: I'm sorry? 00:28:47.830 --> 00:28:49.570 align:middle line:84% AUDIENCE: This might be a naive question. 00:28:49.570 --> 00:28:50.950 align:middle line:84% JOHN SOUTHARD: There are no naive questions. 00:28:50.950 --> 00:28:52.783 align:middle line:84% AUDIENCE: How do you tell the flow direction 00:28:52.783 --> 00:28:54.310 align:middle line:84% from the configuration of the bed? 00:28:54.310 --> 00:28:55.890 align:middle line:84% And can you always tell what the-- 00:28:55.890 --> 00:28:57.015 align:middle line:90% JOHN SOUTHARD: Yes, always. 00:28:57.015 --> 00:28:58.770 align:middle line:84% Remember, go back to the cross-section 00:28:58.770 --> 00:29:00.290 align:middle line:90% of a typical ripple. 00:29:00.290 --> 00:29:04.610 align:middle line:84% So the upstream, stoss side is always fairly gentle. 00:29:04.610 --> 00:29:09.070 align:middle line:84% The downstream side typically is at or near the angle of repose, 00:29:09.070 --> 00:29:11.550 align:middle line:84% although it can be smoothed out a little bit. 00:29:11.550 --> 00:29:19.970 align:middle line:84% So that's a good word for it, the definitive parallel current 00:29:19.970 --> 00:29:21.610 align:middle line:90% indicator. 00:29:21.610 --> 00:29:23.030 align:middle line:90% I'm glad you pointed that out. 00:29:23.030 --> 00:29:26.130 align:middle line:90% 00:29:26.130 --> 00:29:31.303 align:middle line:84% Here's another example in which the ripples down here-- they're 00:29:31.303 --> 00:29:31.970 align:middle line:90% already forming. 00:29:31.970 --> 00:29:34.050 align:middle line:84% And they're two-dimensional, gradually 00:29:34.050 --> 00:29:36.250 align:middle line:84% passing into pretty-- three-dimensional. 00:29:36.250 --> 00:29:37.310 align:middle line:90% It's pretty typical. 00:29:37.310 --> 00:29:39.930 align:middle line:90% 00:29:39.930 --> 00:29:41.050 align:middle line:90% I love this photo. 00:29:41.050 --> 00:29:46.250 align:middle line:84% I took this photo on a tidal sandbar at low tide 00:29:46.250 --> 00:29:47.670 align:middle line:90% up there at Plum Island. 00:29:47.670 --> 00:29:49.990 align:middle line:84% And you can see this is a lens cap. 00:29:49.990 --> 00:29:51.930 align:middle line:90% And those are bird tracks. 00:29:51.930 --> 00:29:53.810 align:middle line:84% And the same kind of thing was happening 00:29:53.810 --> 00:29:56.390 align:middle line:90% on this planar surface. 00:29:56.390 --> 00:29:59.730 align:middle line:84% As the tide changed and it came across the bar 00:29:59.730 --> 00:30:03.850 align:middle line:84% and increased in velocity, it produced these growing patterns 00:30:03.850 --> 00:30:05.890 align:middle line:84% of ripples, which eventually would 00:30:05.890 --> 00:30:10.890 align:middle line:84% have merged and become a total three-dimensional field. 00:30:10.890 --> 00:30:15.970 align:middle line:84% What was always-- mystified me is, 00:30:15.970 --> 00:30:18.890 align:middle line:84% why did we have a nice totally planar bed 00:30:18.890 --> 00:30:21.250 align:middle line:90% surface in the first place? 00:30:21.250 --> 00:30:23.450 align:middle line:84% Somehow, during the falling tide, 00:30:23.450 --> 00:30:26.610 align:middle line:84% it went from high-velocity flow plane bed 00:30:26.610 --> 00:30:30.210 align:middle line:84% to being exposed without developing ripples 00:30:30.210 --> 00:30:31.350 align:middle line:90% during that time. 00:30:31.350 --> 00:30:31.870 align:middle line:90% It happened. 00:30:31.870 --> 00:30:33.350 align:middle line:84% But I'm not sure how it happened. 00:30:33.350 --> 00:30:36.290 align:middle line:84% But once we had the plane bed, once the plane bed was there 00:30:36.290 --> 00:30:38.830 align:middle line:84% and the tide came in and velocity increased again, 00:30:38.830 --> 00:30:41.650 align:middle line:84% it produced new ripples just like that. 00:30:41.650 --> 00:30:43.630 align:middle line:84% It looks like dinosaur tracks, doesn't it? 00:30:43.630 --> 00:30:44.610 align:middle line:90% [LAUGHING] 00:30:44.610 --> 00:30:46.050 align:middle line:90% 00:30:46.050 --> 00:30:50.250 align:middle line:84% Now, here's a mystery slide for you. 00:30:50.250 --> 00:30:52.783 align:middle line:84% This is a parting surface in a sandstone. 00:30:52.783 --> 00:30:54.450 align:middle line:84% You can see the parting lineation, which 00:30:54.450 --> 00:30:57.410 align:middle line:84% I'll mention later, gives you the flow direction-- or the flow 00:30:57.410 --> 00:30:58.690 align:middle line:90% orientation. 00:30:58.690 --> 00:31:03.450 align:middle line:84% And here are these depressions with little ridge marks 00:31:03.450 --> 00:31:05.770 align:middle line:90% that are-- 00:31:05.770 --> 00:31:09.450 align:middle line:84% if this is a ripple trough that just was starting to form, 00:31:09.450 --> 00:31:10.930 align:middle line:84% you can see the curvature is such 00:31:10.930 --> 00:31:12.682 align:middle line:84% that the flow is in that direction. 00:31:12.682 --> 00:31:13.890 align:middle line:90% I think I'm right about that. 00:31:13.890 --> 00:31:16.330 align:middle line:84% And I've never seen anything like that before. 00:31:16.330 --> 00:31:18.193 align:middle line:84% Just this one time I've seen this. 00:31:18.193 --> 00:31:19.610 align:middle line:84% It gives you an idea what could be 00:31:19.610 --> 00:31:22.150 align:middle line:84% happening when the ripples are just getting going. 00:31:22.150 --> 00:31:24.840 align:middle line:90% 00:31:24.840 --> 00:31:30.680 align:middle line:84% There's a really good paper in 2014 by my close colleague, 00:31:30.680 --> 00:31:35.220 align:middle line:84% Mauricio Perillo, on genesis of ripples, 00:31:35.220 --> 00:31:37.553 align:middle line:84% both unidirectional flow, oscillatory flow, and combined 00:31:37.553 --> 00:31:38.053 align:middle line:90% flow. 00:31:38.053 --> 00:31:39.140 align:middle line:90% It's a really nice paper. 00:31:39.140 --> 00:31:41.760 align:middle line:90% 00:31:41.760 --> 00:31:44.080 align:middle line:84% Now let's talk about barchan dunes. 00:31:44.080 --> 00:31:46.800 align:middle line:84% It looks like I'm changing the subject, but not entirely. 00:31:46.800 --> 00:31:48.620 align:middle line:84% You probably know what a barchan dune is. 00:31:48.620 --> 00:31:51.160 align:middle line:84% In the desert, you find these masses of sediment 00:31:51.160 --> 00:31:56.480 align:middle line:84% with horns tailing off downstream. 00:31:56.480 --> 00:31:57.500 align:middle line:90% That's just a diagram. 00:31:57.500 --> 00:32:00.560 align:middle line:90% You've seen that before. 00:32:00.560 --> 00:32:05.600 align:middle line:84% We made them in a flume in very coarse silt underwater. 00:32:05.600 --> 00:32:09.340 align:middle line:84% Now, this channel is only 17 centimeters wide. 00:32:09.340 --> 00:32:11.340 align:middle line:90% So these are very small dunes. 00:32:11.340 --> 00:32:14.520 align:middle line:84% And my feeling is we had a sand surface. 00:32:14.520 --> 00:32:19.600 align:middle line:84% It was actually coarse silt. It was even finer than sand size. 00:32:19.600 --> 00:32:21.340 align:middle line:84% And it was just about a threshold. 00:32:21.340 --> 00:32:22.980 align:middle line:90% And some grains started to move. 00:32:22.980 --> 00:32:24.720 align:middle line:90% But most grains weren't moving. 00:32:24.720 --> 00:32:28.590 align:middle line:84% And I think that the origin of the barchans 00:32:28.590 --> 00:32:30.080 align:middle line:84% is there's some place where there's 00:32:30.080 --> 00:32:33.960 align:middle line:84% enough irregularity in the bed to catch some of the moving 00:32:33.960 --> 00:32:34.720 align:middle line:90% grains. 00:32:34.720 --> 00:32:38.760 align:middle line:84% And the moving grains then build a structure 00:32:38.760 --> 00:32:44.180 align:middle line:84% with a stoss slope and a lee slope downstream. 00:32:44.180 --> 00:32:47.280 align:middle line:84% And I think the horns are there because the flow starts 00:32:47.280 --> 00:32:51.000 align:middle line:84% to diverge and carries sand around both sides 00:32:51.000 --> 00:32:52.960 align:middle line:90% to make the horns. 00:32:52.960 --> 00:32:55.585 align:middle line:84% And I'd have to go back and read Bagnold's book 00:32:55.585 --> 00:32:56.460 align:middle line:90% to make sure of that. 00:32:56.460 --> 00:32:58.440 align:middle line:90% But I think that's how it works. 00:32:58.440 --> 00:32:59.900 align:middle line:90% And these are very small. 00:32:59.900 --> 00:33:01.020 align:middle line:90% It's like this size. 00:33:01.020 --> 00:33:04.440 align:middle line:90% 00:33:04.440 --> 00:33:07.380 align:middle line:84% This is what I just told you about how they might form. 00:33:07.380 --> 00:33:10.060 align:middle line:84% But that's my picture of it, anyway. 00:33:10.060 --> 00:33:13.000 align:middle line:90% 00:33:13.000 --> 00:33:14.660 align:middle line:84% This is my favorite current ripple. 00:33:14.660 --> 00:33:15.840 align:middle line:90% I love this ripple. 00:33:15.840 --> 00:33:16.880 align:middle line:90% We made it in a flume. 00:33:16.880 --> 00:33:18.440 align:middle line:90% And I took a nice photo of it. 00:33:18.440 --> 00:33:20.120 align:middle line:90% These are centimeters down here. 00:33:20.120 --> 00:33:22.780 align:middle line:84% So here are all the features that I was telling you about. 00:33:22.780 --> 00:33:28.280 align:middle line:84% We have a stoss surface, a lee surface. 00:33:28.280 --> 00:33:31.700 align:middle line:84% The sand is transported as bedload up to the crest. 00:33:31.700 --> 00:33:35.280 align:middle line:90% 00:33:35.280 --> 00:33:39.000 align:middle line:84% It moves down by way of little miniature grain flows 00:33:39.000 --> 00:33:41.040 align:middle line:90% out into the trough. 00:33:41.040 --> 00:33:44.160 align:middle line:84% But you can see that there is some sediment being carried 00:33:44.160 --> 00:33:48.520 align:middle line:84% back up onto the lee side by the reverse flow in the wake 00:33:48.520 --> 00:33:49.660 align:middle line:90% that I told you about. 00:33:49.660 --> 00:33:52.880 align:middle line:84% And you can see how if you follow laminae down, 00:33:52.880 --> 00:33:55.620 align:middle line:90% they tail out and get planar. 00:33:55.620 --> 00:33:57.280 align:middle line:90% And you can see that-- 00:33:57.280 --> 00:34:03.200 align:middle line:84% the dark material being worked up onto the lee slope. 00:34:03.200 --> 00:34:05.640 align:middle line:84% But there are other things I can talk about. 00:34:05.640 --> 00:34:10.760 align:middle line:84% Earlier on, the ripple had a stoss side which was erosional. 00:34:10.760 --> 00:34:13.520 align:middle line:84% And in total equilibrium, the stoss side 00:34:13.520 --> 00:34:15.199 align:middle line:90% would always be erosional. 00:34:15.199 --> 00:34:18.420 align:middle line:84% But things happen upstream somewhere. 00:34:18.420 --> 00:34:19.560 align:middle line:90% Ripples change. 00:34:19.560 --> 00:34:22.280 align:middle line:84% The supply of-- local supply of sediment coming along 00:34:22.280 --> 00:34:24.840 align:middle line:84% to feed this ripple can be great enough so 00:34:24.840 --> 00:34:30.880 align:middle line:84% that now we have a stoss surface with aggradation. 00:34:30.880 --> 00:34:34.880 align:middle line:84% Those would be called foreset stratifications. 00:34:34.880 --> 00:34:36.719 align:middle line:90% And it goes down on that way. 00:34:36.719 --> 00:34:40.020 align:middle line:84% The other thing is this ripple is riding over another one. 00:34:40.020 --> 00:34:42.199 align:middle line:90% There was an earlier ripple. 00:34:42.199 --> 00:34:43.400 align:middle line:90% This one came along. 00:34:43.400 --> 00:34:47.340 align:middle line:84% And its trough eroded down into the earlier ripple, 00:34:47.340 --> 00:34:49.639 align:middle line:84% but left some of the ripple foreset 00:34:49.639 --> 00:34:51.280 align:middle line:90% from the previous ripple. 00:34:51.280 --> 00:34:53.620 align:middle line:84% You get a lot out of this picture, I think. 00:34:53.620 --> 00:34:56.600 align:middle line:90% 00:34:56.600 --> 00:34:58.100 align:middle line:90% Now let's talk about dunes. 00:34:58.100 --> 00:35:01.040 align:middle line:90% 00:35:01.040 --> 00:35:02.880 align:middle line:90% Too bad. 00:35:02.880 --> 00:35:05.550 align:middle line:84% At flow velocity-- no, dunes are much larger than ripples. 00:35:05.550 --> 00:35:07.820 align:middle line:90% And I'll get to that. 00:35:07.820 --> 00:35:10.160 align:middle line:84% Low flow velocities are two-dimensional 00:35:10.160 --> 00:35:12.300 align:middle line:84% and-- at higher velocities are three-dimensional. 00:35:12.300 --> 00:35:14.040 align:middle line:90% I have some pictures for you. 00:35:14.040 --> 00:35:16.500 align:middle line:84% Here are 2D dunes in that same channel that I showed you, 00:35:16.500 --> 00:35:18.520 align:middle line:90% medium sand. 00:35:18.520 --> 00:35:21.030 align:middle line:84% They're not perfectly two-dimensional. 00:35:21.030 --> 00:35:22.310 align:middle line:90% They're somewhat sinuous. 00:35:22.310 --> 00:35:26.070 align:middle line:84% But they're close to being regular, 00:35:26.070 --> 00:35:28.730 align:middle line:84% whereas at a higher flow velocity in that same sand, 00:35:28.730 --> 00:35:30.350 align:middle line:84% in the same channel, you can see they 00:35:30.350 --> 00:35:31.870 align:middle line:90% get highly three-dimensional. 00:35:31.870 --> 00:35:35.790 align:middle line:84% And what I think is important is to see these deep scour 00:35:35.790 --> 00:35:36.965 align:middle line:90% pits here and there. 00:35:36.965 --> 00:35:38.590 align:middle line:84% And they're going to figure prominently 00:35:38.590 --> 00:35:41.350 align:middle line:84% in the stratification that develops when these ripples are 00:35:41.350 --> 00:35:44.350 align:middle line:90% moving on the bed. 00:35:44.350 --> 00:35:48.810 align:middle line:84% This is actually a big, big dune because, first of all, it's big. 00:35:48.810 --> 00:35:51.790 align:middle line:84% And second, we built this enormous duct. 00:35:51.790 --> 00:35:54.590 align:middle line:84% And we ran 80-degree seawater through it. 00:35:54.590 --> 00:35:56.870 align:middle line:84% Now, I didn't tell you about scale modeling. 00:35:56.870 --> 00:35:59.630 align:middle line:84% And if you want to know, I can tell you about scale modeling. 00:35:59.630 --> 00:36:03.590 align:middle line:84% And using the lower viscosity water gives you a scale ratio 00:36:03.590 --> 00:36:04.990 align:middle line:90% of about 2 and 1/2. 00:36:04.990 --> 00:36:10.630 align:middle line:84% So what's going on in here is 2 and 1/2 times as big as it would 00:36:10.630 --> 00:36:13.270 align:middle line:90% be if we didn't have the water. 00:36:13.270 --> 00:36:15.850 align:middle line:84% This is about 4 or 5 meters along that distance. 00:36:15.850 --> 00:36:16.770 align:middle line:90% It's a big dune. 00:36:16.770 --> 00:36:19.510 align:middle line:84% And you can see the same way the Dune at first was-- 00:36:19.510 --> 00:36:24.470 align:middle line:84% it had a lot of curvature, it finally 00:36:24.470 --> 00:36:30.910 align:middle line:84% built up to a good, strong stoss surface. 00:36:30.910 --> 00:36:33.190 align:middle line:84% And you can see the sediment being 00:36:33.190 --> 00:36:37.010 align:middle line:84% moved in bedload or low temporary suspension. 00:36:37.010 --> 00:36:39.390 align:middle line:84% And when it gets to the separation point, 00:36:39.390 --> 00:36:42.430 align:middle line:84% the sediment which was being carried along to the crest 00:36:42.430 --> 00:36:44.810 align:middle line:84% is raining out onto that surface. 00:36:44.810 --> 00:36:46.910 align:middle line:84% So it's not just a matter of grain flows 00:36:46.910 --> 00:36:48.050 align:middle line:90% going on the surface. 00:36:48.050 --> 00:36:50.110 align:middle line:84% That stuff can be settling out onto the surface 00:36:50.110 --> 00:36:53.510 align:middle line:90% at the same time. 00:36:53.510 --> 00:36:54.730 align:middle line:90% That's not the same dune. 00:36:54.730 --> 00:36:56.950 align:middle line:90% But it's the same duct. 00:36:56.950 --> 00:37:00.050 align:middle line:84% In this case, the ripple was building upward. 00:37:00.050 --> 00:37:02.950 align:middle line:84% So if you picked the crest point where 00:37:02.950 --> 00:37:04.630 align:middle line:84% the-- changes from this to this, you 00:37:04.630 --> 00:37:07.150 align:middle line:84% can see it moving upward as the ripple is growing. 00:37:07.150 --> 00:37:08.670 align:middle line:90% And you get the same-- 00:37:08.670 --> 00:37:09.490 align:middle line:90% you have same view. 00:37:09.490 --> 00:37:13.290 align:middle line:84% I love the way this is falling out in little sheets of sand. 00:37:13.290 --> 00:37:16.390 align:middle line:90% 00:37:16.390 --> 00:37:20.750 align:middle line:84% Same duct-- this is an even bigger dune. 00:37:20.750 --> 00:37:22.510 align:middle line:90% This is all one big dune. 00:37:22.510 --> 00:37:24.950 align:middle line:84% But there are dunes, smaller dunes, 00:37:24.950 --> 00:37:28.030 align:middle line:84% on the big ones, superimposed upon the big ones. 00:37:28.030 --> 00:37:32.230 align:middle line:84% And so whenever you have a stretch of surface, 00:37:32.230 --> 00:37:35.990 align:middle line:84% whether it's a planar bed or the back of a very large dune, 00:37:35.990 --> 00:37:38.630 align:middle line:84% smaller dunes can develop if they have enough distance 00:37:38.630 --> 00:37:39.250 align:middle line:90% to develop. 00:37:39.250 --> 00:37:42.210 align:middle line:84% In this case, you have superimposed dunes. 00:37:42.210 --> 00:37:45.030 align:middle line:90% 00:37:45.030 --> 00:37:47.410 align:middle line:84% So 2D dunes-- I don't remember where I took that. 00:37:47.410 --> 00:37:49.930 align:middle line:84% It may be up in the North Shore of Massachusetts. 00:37:49.930 --> 00:37:52.150 align:middle line:90% But I don't remember for sure. 00:37:52.150 --> 00:37:54.990 align:middle line:84% This is the Arkansas River near Tulsa, 00:37:54.990 --> 00:37:57.590 align:middle line:84% nice two-dimensional dunes, not very big. 00:37:57.590 --> 00:37:58.610 align:middle line:90% There's no scale here. 00:37:58.610 --> 00:38:00.390 align:middle line:84% But you can imagine from the trees 00:38:00.390 --> 00:38:04.470 align:middle line:84% what it looked like, with lots of superimposed ripples. 00:38:04.470 --> 00:38:06.870 align:middle line:90% Now, that leads to a question. 00:38:06.870 --> 00:38:10.150 align:middle line:84% Were those ripples in equilibrium with the dunes 00:38:10.150 --> 00:38:17.150 align:middle line:84% as the river stage fell and became exposed subaerially, 00:38:17.150 --> 00:38:22.270 align:middle line:84% or were they produced when the flow decreased to the point 00:38:22.270 --> 00:38:25.470 align:middle line:84% where it couldn't maintain the dunes, but could make ripples? 00:38:25.470 --> 00:38:27.510 align:middle line:84% And I don't know how you tell the difference. 00:38:27.510 --> 00:38:30.310 align:middle line:84% I know from our experiments in the lab 00:38:30.310 --> 00:38:33.830 align:middle line:84% that you can have ripples superimposed on dunes. 00:38:33.830 --> 00:38:36.430 align:middle line:84% You can even have ripples superimposed 00:38:36.430 --> 00:38:39.190 align:middle line:84% upon dunes, which themselves are superimposed 00:38:39.190 --> 00:38:40.418 align:middle line:90% on even larger dunes. 00:38:40.418 --> 00:38:42.210 align:middle line:84% I don't think you can go farther than that. 00:38:42.210 --> 00:38:43.090 align:middle line:90% But you can do that. 00:38:43.090 --> 00:38:44.210 align:middle line:90% I've seen it happen. 00:38:44.210 --> 00:38:47.170 align:middle line:90% 00:38:47.170 --> 00:38:48.850 align:middle line:84% So "opportunistic" is a nice word. 00:38:48.850 --> 00:38:51.490 align:middle line:84% Current ripples and even dunes are opportunistic. 00:38:51.490 --> 00:38:53.990 align:middle line:84% Whenever there's a stretch of temporarily planar bed, 00:38:53.990 --> 00:38:56.890 align:middle line:84% there's enough distance, and enough-- strong enough flow, 00:38:56.890 --> 00:38:58.630 align:middle line:84% you can make dunes-- you can make 00:38:58.630 --> 00:39:01.870 align:middle line:84% ripples of dunes that are superimposed on other dunes. 00:39:01.870 --> 00:39:05.310 align:middle line:84% I think "opportunistic" is a good adjective to use for that. 00:39:05.310 --> 00:39:08.610 align:middle line:84% Parker River estuary, covered with ripples-- and again, 00:39:08.610 --> 00:39:11.990 align:middle line:84% there's the same problem, whether those ripples 00:39:11.990 --> 00:39:13.990 align:middle line:84% were in equilibrium with the dunes 00:39:13.990 --> 00:39:17.460 align:middle line:84% when the dunes were moving or superimposed during waning flow. 00:39:17.460 --> 00:39:18.800 align:middle line:90% I would vote for the latter. 00:39:18.800 --> 00:39:19.740 align:middle line:90% But I'm not sure. 00:39:19.740 --> 00:39:21.320 align:middle line:90% That's a meter stick for scale. 00:39:21.320 --> 00:39:24.980 align:middle line:90% 00:39:24.980 --> 00:39:28.820 align:middle line:84% Another view-- this is from Bay of Fundy, 00:39:28.820 --> 00:39:32.980 align:middle line:84% Cobequid Bay, part of the Bay of Fundy, largely straight-crested. 00:39:32.980 --> 00:39:36.140 align:middle line:90% You can see superimposed dunes-- 00:39:36.140 --> 00:39:39.380 align:middle line:84% not too clearly, but they're there-- on the large dune. 00:39:39.380 --> 00:39:41.100 align:middle line:90% This is an island. 00:39:41.100 --> 00:39:42.820 align:middle line:84% Isn't that a strange-looking island? 00:39:42.820 --> 00:39:46.260 align:middle line:84% The spring tidal range up in this end of the Bay of Fundy 00:39:46.260 --> 00:39:48.100 align:middle line:90% is 50 feet. 00:39:48.100 --> 00:39:49.340 align:middle line:90% Isn't that amazing? 00:39:49.340 --> 00:39:52.260 align:middle line:84% So when you go work on this, you have three, four, maybe 00:39:52.260 --> 00:39:53.800 align:middle line:90% five hours at most to work. 00:39:53.800 --> 00:39:55.760 align:middle line:84% You have to get off as the tide comes in. 00:39:55.760 --> 00:39:57.420 align:middle line:84% And the saying around the Bay of Fundy 00:39:57.420 --> 00:39:59.720 align:middle line:84% is, the bay never gives up its dead. 00:39:59.720 --> 00:40:01.500 align:middle line:90% [LAUGHING] 00:40:01.500 --> 00:40:03.035 align:middle line:90% It's tricky. 00:40:03.035 --> 00:40:04.660 align:middle line:84% Otherwise, you lose-- you swim to shore 00:40:04.660 --> 00:40:07.180 align:middle line:90% and you lose all your equipment. 00:40:07.180 --> 00:40:08.592 align:middle line:90% It happens. 00:40:08.592 --> 00:40:09.800 align:middle line:90% Here's a better view of that. 00:40:09.800 --> 00:40:12.800 align:middle line:84% Here's a better view, close to it, from the right to left, 00:40:12.800 --> 00:40:14.900 align:middle line:84% where you can see the typical troughs 00:40:14.900 --> 00:40:17.100 align:middle line:84% of the superimposed dunes and ripples 00:40:17.100 --> 00:40:18.760 align:middle line:90% on the superimposed dunes. 00:40:18.760 --> 00:40:22.060 align:middle line:90% 00:40:22.060 --> 00:40:25.740 align:middle line:84% Another place in the Bay of Fundy where the flow is stronger 00:40:25.740 --> 00:40:28.560 align:middle line:84% and it makes semi-three-dimensional dunes, 00:40:28.560 --> 00:40:30.420 align:middle line:90% not grossly three-dimensional. 00:40:30.420 --> 00:40:33.820 align:middle line:84% And you can see as the water-- the water stands in the troughs 00:40:33.820 --> 00:40:34.660 align:middle line:90% here and there. 00:40:34.660 --> 00:40:39.220 align:middle line:84% That's Gerry Middleton, my older colleague, one of my mentors. 00:40:39.220 --> 00:40:41.080 align:middle line:90% He took me out. 00:40:41.080 --> 00:40:46.020 align:middle line:84% He had three students working on the Bay of Fundy at one point. 00:40:46.020 --> 00:40:48.060 align:middle line:84% Even more three-dimensional-- there's 00:40:48.060 --> 00:40:51.420 align:middle line:84% some small river whose name I don't remember near Atlanta. 00:40:51.420 --> 00:40:53.900 align:middle line:84% I was there doing consultation from some guy who 00:40:53.900 --> 00:40:55.660 align:middle line:90% had a business next to this. 00:40:55.660 --> 00:40:58.100 align:middle line:84% That's a Dr. Pepper scan right there-- 00:40:58.100 --> 00:40:58.660 align:middle line:90% can. 00:40:58.660 --> 00:41:01.580 align:middle line:84% And you can see the prominent scour 00:41:01.580 --> 00:41:05.800 align:middle line:84% pits in this three-dimensional pattern of dunes. 00:41:05.800 --> 00:41:08.500 align:middle line:90% 00:41:08.500 --> 00:41:09.840 align:middle line:90% Dave Mohrig gave me this. 00:41:09.840 --> 00:41:12.340 align:middle line:84% You know who Dave Mohrig is, I guess. 00:41:12.340 --> 00:41:14.400 align:middle line:84% He did his thesis work on the Platte River. 00:41:14.400 --> 00:41:16.380 align:middle line:84% He had a balloon that he parked in 00:41:16.380 --> 00:41:18.520 align:middle line:90% some big, cavernous warehouse. 00:41:18.520 --> 00:41:21.380 align:middle line:84% And every day, he would bring it out. 00:41:21.380 --> 00:41:22.780 align:middle line:90% It was inflated. 00:41:22.780 --> 00:41:26.493 align:middle line:90% And it was positively buoyant. 00:41:26.493 --> 00:41:28.160 align:middle line:84% He and his assistant would bring it out, 00:41:28.160 --> 00:41:31.320 align:middle line:84% park it above the river with a camera, down-looking camera, 00:41:31.320 --> 00:41:32.460 align:middle line:90% and they would-- 00:41:32.460 --> 00:41:33.600 align:middle line:90% these are people. 00:41:33.600 --> 00:41:34.680 align:middle line:90% That's a person. 00:41:34.680 --> 00:41:36.220 align:middle line:90% That's a person. 00:41:36.220 --> 00:41:37.460 align:middle line:90% I think that's a person. 00:41:37.460 --> 00:41:39.660 align:middle line:90% I know that's a person. 00:41:39.660 --> 00:41:42.100 align:middle line:84% It gives you a good idea of what ripples really 00:41:42.100 --> 00:41:43.680 align:middle line:90% look like in a river system. 00:41:43.680 --> 00:41:47.020 align:middle line:90% 00:41:47.020 --> 00:41:48.940 align:middle line:90% Dunes can get enormous. 00:41:48.940 --> 00:41:52.340 align:middle line:84% Here we are on the crest of a big dune 00:41:52.340 --> 00:41:58.060 align:middle line:84% on the left flank of the Mississippi River near Kentucky. 00:41:58.060 --> 00:42:00.820 align:middle line:84% And we're all standing at the crest. 00:42:00.820 --> 00:42:05.780 align:middle line:84% And the lee slope goes way down there. 00:42:05.780 --> 00:42:09.940 align:middle line:84% So dunes can get as big as the flow allows. 00:42:09.940 --> 00:42:12.020 align:middle line:84% You're going to see very soon that ripples 00:42:12.020 --> 00:42:14.020 align:middle line:90% form in sands coarser than a-- 00:42:14.020 --> 00:42:15.820 align:middle line:84% finer than about half a millimeter. 00:42:15.820 --> 00:42:18.080 align:middle line:84% Dunes can form in a wider range of sizes. 00:42:18.080 --> 00:42:20.740 align:middle line:84% We did some experiments at the University of Tsukuba 00:42:20.740 --> 00:42:23.603 align:middle line:84% in Japan, north of Tokyo, using gravels. 00:42:23.603 --> 00:42:25.020 align:middle line:84% And the reason they're all colored 00:42:25.020 --> 00:42:27.640 align:middle line:84% is we colored each size a different color 00:42:27.640 --> 00:42:32.480 align:middle line:84% so we could see sorting during the motion of the dunes. 00:42:32.480 --> 00:42:34.220 align:middle line:84% And you can see some of these particles 00:42:34.220 --> 00:42:37.060 align:middle line:84% are whipping up the stoss side and being 00:42:37.060 --> 00:42:39.320 align:middle line:90% projected over the crest. 00:42:39.320 --> 00:42:41.540 align:middle line:84% And that's the big one right there that came in. 00:42:41.540 --> 00:42:44.580 align:middle line:84% It's going to fall down on the trough. 00:42:44.580 --> 00:42:45.840 align:middle line:90% It's exciting to watch. 00:42:45.840 --> 00:42:48.460 align:middle line:90% 00:42:48.460 --> 00:42:50.860 align:middle line:90% We're back to Cobequid Bay. 00:42:50.860 --> 00:42:55.300 align:middle line:84% We cut a section through one of these dunes. 00:42:55.300 --> 00:42:56.980 align:middle line:84% I don't know how well you can see. 00:42:56.980 --> 00:42:59.900 align:middle line:84% But you see a set of laminae up here. 00:42:59.900 --> 00:43:02.260 align:middle line:84% And you can see a set of laminae down here. 00:43:02.260 --> 00:43:04.740 align:middle line:84% And then there are other things going the other direction. 00:43:04.740 --> 00:43:09.260 align:middle line:84% And my feeling is that because of the tide-- 00:43:09.260 --> 00:43:13.773 align:middle line:84% there's a strong tide going one way. 00:43:13.773 --> 00:43:14.690 align:middle line:90% And then it shuts off. 00:43:14.690 --> 00:43:16.270 align:middle line:84% And there's a weaker tide going the other way, 00:43:16.270 --> 00:43:17.450 align:middle line:90% current going the other way. 00:43:17.450 --> 00:43:19.170 align:middle line:84% And it flips things over and makes 00:43:19.170 --> 00:43:22.590 align:middle line:84% the inner structure of this dune very complicated 00:43:22.590 --> 00:43:26.477 align:middle line:84% rather than being all nicely regular foresets. 00:43:26.477 --> 00:43:28.310 align:middle line:84% And that tells you something about the flow. 00:43:28.310 --> 00:43:29.810 align:middle line:90% The flow is reversing. 00:43:29.810 --> 00:43:31.070 align:middle line:90% There's an asymmetry of flow. 00:43:31.070 --> 00:43:35.032 align:middle line:90% But it's a reversing flow. 00:43:35.032 --> 00:43:36.990 align:middle line:84% That's the same picture-- shows the same thing. 00:43:36.990 --> 00:43:38.150 align:middle line:84% I thought this would look better. 00:43:38.150 --> 00:43:40.030 align:middle line:84% But the other one looks pretty good, too. 00:43:40.030 --> 00:43:44.770 align:middle line:90% 00:43:44.770 --> 00:43:48.450 align:middle line:84% Out offshore, you can take sonar images. 00:43:48.450 --> 00:43:56.210 align:middle line:90% We were doing a survey in the-- 00:43:56.210 --> 00:44:00.250 align:middle line:84% what's the name of the water body between the coast-- 00:44:00.250 --> 00:44:02.850 align:middle line:84% south coast and Martha's Vineyard? 00:44:02.850 --> 00:44:03.650 align:middle line:90% Vineyard Sound. 00:44:03.650 --> 00:44:04.150 align:middle line:90% That's it. 00:44:04.150 --> 00:44:05.510 align:middle line:90% This is in Vineyard Sound. 00:44:05.510 --> 00:44:10.370 align:middle line:84% And they look sort of like the other ones I've shown you. 00:44:10.370 --> 00:44:15.530 align:middle line:84% Snow dunes-- it was a bitter, bitter, cold day, January day. 00:44:15.530 --> 00:44:21.850 align:middle line:84% This was back-- must have been in the early '70s, mid '70s. 00:44:21.850 --> 00:44:23.550 align:middle line:90% And I saw these dunes. 00:44:23.550 --> 00:44:26.530 align:middle line:84% And I couldn't resist going out on the Harvard Bridge 00:44:26.530 --> 00:44:29.890 align:middle line:84% on the west side and taking this photograph of snow dunes 00:44:29.890 --> 00:44:32.650 align:middle line:84% because snow particles are particles. 00:44:32.650 --> 00:44:35.090 align:middle line:84% And they're less dense than quartz sand. 00:44:35.090 --> 00:44:36.490 align:middle line:90% They make dunes. 00:44:36.490 --> 00:44:38.150 align:middle line:90% And they're not all flakes. 00:44:38.150 --> 00:44:43.010 align:middle line:84% There are all kinds of different shapes of snowflakes. 00:44:43.010 --> 00:44:48.330 align:middle line:90% And so I'm glad I did that. 00:44:48.330 --> 00:44:51.210 align:middle line:84% Now, here's a factor that's different from ripples. 00:44:51.210 --> 00:44:53.810 align:middle line:84% The spacing goes as the flow depth. 00:44:53.810 --> 00:44:57.130 align:middle line:84% And with a factor of something like 7 for fully developed 00:44:57.130 --> 00:44:59.850 align:middle line:84% dunes, if you have a flow, an open channel flow, 00:44:59.850 --> 00:45:02.210 align:middle line:84% over a sand bed and you make dunes, the dunes, 00:45:02.210 --> 00:45:06.370 align:middle line:84% when they reach equilibrium, are something like a factor of 1/7 00:45:06.370 --> 00:45:08.330 align:middle line:90% of the depth of the water. 00:45:08.330 --> 00:45:09.870 align:middle line:90% It varies, plus or minus. 00:45:09.870 --> 00:45:10.910 align:middle line:90% But it's-- Lyle? 00:45:10.910 --> 00:45:14.378 align:middle line:84% AUDIENCE: Is that pure empirical-- purely empirical? 00:45:14.378 --> 00:45:15.170 align:middle line:90% JOHN SOUTHARD: Yes. 00:45:15.170 --> 00:45:15.863 align:middle line:90% Yeah. 00:45:15.863 --> 00:45:17.530 align:middle line:84% I don't know whether anybody has tackled 00:45:17.530 --> 00:45:22.010 align:middle line:84% the problem of predicting why they're the size they are. 00:45:22.010 --> 00:45:23.090 align:middle line:90% I don't know. 00:45:23.090 --> 00:45:25.690 align:middle line:84% But that's a general observation. 00:45:25.690 --> 00:45:27.290 align:middle line:84% But of course, they may be smaller 00:45:27.290 --> 00:45:29.550 align:middle line:84% because they haven't fully developed, or may 00:45:29.550 --> 00:45:32.010 align:middle line:84% be-- they may be larger as a function of water depth 00:45:32.010 --> 00:45:34.050 align:middle line:90% because the flow is decreasing. 00:45:34.050 --> 00:45:39.770 align:middle line:84% But in equilibrium, that's a pretty good predictor. 00:45:39.770 --> 00:45:40.270 align:middle line:90% All right. 00:45:40.270 --> 00:45:41.790 align:middle line:90% Now it's almost time to quit. 00:45:41.790 --> 00:45:44.712 align:middle line:90% But I'll get started on this. 00:45:44.712 --> 00:45:46.670 align:middle line:84% Now we're going to do the dimensional analysis. 00:45:46.670 --> 00:45:49.250 align:middle line:84% You probably suspected that I'm going to get into this. 00:45:49.250 --> 00:45:50.890 align:middle line:84% Remember the-- from the first lecture, 00:45:50.890 --> 00:45:52.190 align:middle line:90% you do dimensional analysis? 00:45:52.190 --> 00:45:54.530 align:middle line:84% And it's very useful as a framework. 00:45:54.530 --> 00:45:56.170 align:middle line:90% So what do we have to deal with? 00:45:56.170 --> 00:45:58.330 align:middle line:84% Well, depth and velocity of flow, 00:45:58.330 --> 00:46:03.370 align:middle line:84% obviously, particle size, fluid density. 00:46:03.370 --> 00:46:05.150 align:middle line:84% And you know what I forgot in this slide? 00:46:05.150 --> 00:46:07.730 align:middle line:90% 00:46:07.730 --> 00:46:09.130 align:middle line:90% The viscosity of the water. 00:46:09.130 --> 00:46:13.010 align:middle line:84% Sediment density, specific weight, and gravity, g-- 00:46:13.010 --> 00:46:15.390 align:middle line:84% Now, I don't want to belabor this. 00:46:15.390 --> 00:46:17.858 align:middle line:84% But you clearly need the specific weight, 00:46:17.858 --> 00:46:19.650 align:middle line:84% the weight per unit volume of the sediment, 00:46:19.650 --> 00:46:20.470 align:middle line:90% because it has a weight. 00:46:20.470 --> 00:46:21.670 align:middle line:90% That's what makes it fall. 00:46:21.670 --> 00:46:25.610 align:middle line:84% But you also need the fluid density in itself 00:46:25.610 --> 00:46:26.830 align:middle line:90% because of inertia. 00:46:26.830 --> 00:46:29.330 align:middle line:84% And I'll talk about that in a minute. 00:46:29.330 --> 00:46:30.430 align:middle line:90% But I left off-- 00:46:30.430 --> 00:46:33.610 align:middle line:90% I knocked off the viscosity. 00:46:33.610 --> 00:46:37.010 align:middle line:84% So I got to think about that for next time. 00:46:37.010 --> 00:46:40.330 align:middle line:84% So why do we need specific weight rather than just density 00:46:40.330 --> 00:46:41.770 align:middle line:90% and viscosity? 00:46:41.770 --> 00:46:43.990 align:middle line:84% Well, it has to do with buoyancy. 00:46:43.990 --> 00:46:46.030 align:middle line:84% You know about Archimedes' principle, 00:46:46.030 --> 00:46:48.850 align:middle line:84% I guess, about buoyancy, why things pop up 00:46:48.850 --> 00:46:50.910 align:middle line:84% when they're less dense in the water, 00:46:50.910 --> 00:46:53.650 align:middle line:84% because the water pressure forces above 00:46:53.650 --> 00:46:56.590 align:middle line:84% are less than the water pressure forces below. 00:46:56.590 --> 00:46:58.490 align:middle line:90% And they go up to the surface. 00:46:58.490 --> 00:47:01.610 align:middle line:84% He's one of my all-time heroes, Archimedes, one of the greatest 00:47:01.610 --> 00:47:03.050 align:middle line:90% minds in human history. 00:47:03.050 --> 00:47:07.360 align:middle line:84% Anyway, so given that, you need sediment density as well 00:47:07.360 --> 00:47:08.780 align:middle line:90% as the weight per unit volume. 00:47:08.780 --> 00:47:09.320 align:middle line:90% Why? 00:47:09.320 --> 00:47:13.000 align:middle line:84% Because solid particles moving through the flow to the fluid 00:47:13.000 --> 00:47:14.040 align:middle line:90% have inertia. 00:47:14.040 --> 00:47:19.760 align:middle line:84% They want to travel in straight lines without any interruption. 00:47:19.760 --> 00:47:24.160 align:middle line:84% And so this inertial effect is really separate 00:47:24.160 --> 00:47:26.200 align:middle line:84% and had to be in addition to the-- and so 00:47:26.200 --> 00:47:29.080 align:middle line:90% I did a thought experiment. 00:47:29.080 --> 00:47:31.040 align:middle line:90% We're in a spaceship-- 00:47:31.040 --> 00:47:34.360 align:middle line:90% no gravity, a big tank of water. 00:47:34.360 --> 00:47:35.600 align:middle line:90% You can do that. 00:47:35.600 --> 00:47:39.640 align:middle line:84% And then you have a little port-- gunport 00:47:39.640 --> 00:47:44.440 align:middle line:84% that you shoot a particle, a dense particle, into the water. 00:47:44.440 --> 00:47:45.740 align:middle line:90% Now, it has no weight at all. 00:47:45.740 --> 00:47:46.407 align:middle line:90% It's weightless. 00:47:46.407 --> 00:47:47.320 align:middle line:90% It's no weight. 00:47:47.320 --> 00:47:50.600 align:middle line:84% But it still interacts with the fluid in the tank. 00:47:50.600 --> 00:47:52.198 align:middle line:90% It has inertia. 00:47:52.198 --> 00:47:53.740 align:middle line:84% If the flow is turbulent in the tank, 00:47:53.740 --> 00:47:55.020 align:middle line:90% it would want to make it move. 00:47:55.020 --> 00:47:56.820 align:middle line:84% But it has this inertia-- does this thing. 00:47:56.820 --> 00:47:59.560 align:middle line:84% So you need density as well as specific weight. 00:47:59.560 --> 00:48:00.940 align:middle line:90% That's a side point, I guess. 00:48:00.940 --> 00:48:03.440 align:middle line:90% But it's important to deal with. 00:48:03.440 --> 00:48:08.920 align:middle line:84% So there are various ways of non-dimensionalizing. 00:48:08.920 --> 00:48:11.600 align:middle line:84% Remember Buckingham's theorem that I mentioned 00:48:11.600 --> 00:48:15.160 align:middle line:90% in the first presentation? 00:48:15.160 --> 00:48:17.640 align:middle line:84% And there are various ways of doing it. 00:48:17.640 --> 00:48:20.880 align:middle line:84% But the way that's most useful for us as sedimentologists 00:48:20.880 --> 00:48:23.640 align:middle line:84% is to non-dimensionalize the flow depth and the flow 00:48:23.640 --> 00:48:25.500 align:middle line:90% velocity and the sediment size. 00:48:25.500 --> 00:48:29.000 align:middle line:84% And of course, you have the density ratio, too. 00:48:29.000 --> 00:48:34.440 align:middle line:84% And so the natural thing would be, and we're going to do it-- 00:48:34.440 --> 00:48:37.600 align:middle line:84% oh, incidentally, I don't know whether you-- 00:48:37.600 --> 00:48:40.160 align:middle line:90% I said that you can do this. 00:48:40.160 --> 00:48:42.120 align:middle line:84% And there's a little section in the notes, 00:48:42.120 --> 00:48:45.160 align:middle line:84% if you want to look at my notes, on how you do it. 00:48:45.160 --> 00:48:46.260 align:middle line:90% It's a simple procedure. 00:48:46.260 --> 00:48:48.520 align:middle line:84% You pick out three repeating variables. 00:48:48.520 --> 00:48:51.947 align:middle line:84% And the rest fall into place to make these dimensionless. 00:48:51.947 --> 00:48:53.780 align:middle line:84% And it's just basically high school algebra. 00:48:53.780 --> 00:48:57.520 align:middle line:84% It's easy to do, just so you know. 00:48:57.520 --> 00:48:59.240 align:middle line:84% So we can draw three-dimensional-- 00:48:59.240 --> 00:49:02.240 align:middle line:84% dimensionless diagram-- dimensionless flow 00:49:02.240 --> 00:49:04.100 align:middle line:84% velocity, dimensionless flow depth, 00:49:04.100 --> 00:49:05.580 align:middle line:90% dimensionless sediment size. 00:49:05.580 --> 00:49:08.720 align:middle line:84% And we can make sections through it in various ways. 00:49:08.720 --> 00:49:11.295 align:middle line:84% I'm going to be almost out of time here. 00:49:11.295 --> 00:49:12.420 align:middle line:90% And I'm going to show you-- 00:49:12.420 --> 00:49:14.795 align:middle line:84% I think this is probably where I'm going to have to quit. 00:49:14.795 --> 00:49:17.640 align:middle line:84% I want to show you some sections, some depth velocity 00:49:17.640 --> 00:49:22.080 align:middle line:84% sections for a given grain size and some velocity size sections 00:49:22.080 --> 00:49:23.720 align:middle line:90% for a given flow depth. 00:49:23.720 --> 00:49:25.500 align:middle line:90% I did this long ago. 00:49:25.500 --> 00:49:28.760 align:middle line:84% And I got all the literature data I could find 00:49:28.760 --> 00:49:30.778 align:middle line:84% and plotted up some of these section graphs. 00:49:30.778 --> 00:49:32.820 align:middle line:84% And I'm going to say quite a bit more about them. 00:49:32.820 --> 00:49:33.880 align:middle line:90% But I have to quit. 00:49:33.880 --> 00:49:36.800 align:middle line:90% So this is a good place to stop. 00:49:36.800 --> 00:49:41.000 align:middle line:90% And so any further questions? 00:49:41.000 --> 00:49:43.530 align:middle line:84% AUDIENCE: I was wondering if I could ask a quick question 00:49:43.530 --> 00:49:44.280 align:middle line:90% about [INAUDIBLE]? 00:49:44.280 --> 00:49:46.738 align:middle line:84% JOHN SOUTHARD: I wish people would ask more questions here. 00:49:46.738 --> 00:49:50.520 align:middle line:84% AUDIENCE: So is it well understood why you jump from 00:49:50.520 --> 00:49:54.323 align:middle line:84% ripple spacing to dune spacing without some gradual-- 00:49:54.323 --> 00:49:55.240 align:middle line:90% are you going to get-- 00:49:55.240 --> 00:49:58.160 align:middle line:84% JOHN SOUTHARD: I think I have a slide in here. 00:49:58.160 --> 00:50:03.280 align:middle line:84% There's a spectral gap between the standard ripple size 00:50:03.280 --> 00:50:07.280 align:middle line:84% and dune size, which is always larger than the ripple size. 00:50:07.280 --> 00:50:10.100 align:middle line:84% But they vary with flow conditions. 00:50:10.100 --> 00:50:12.160 align:middle line:90% And that's the way it is. 00:50:12.160 --> 00:50:13.820 align:middle line:90% And I'm getting ahead of myself. 00:50:13.820 --> 00:50:16.480 align:middle line:84% But I'll say that there have been various attempts 00:50:16.480 --> 00:50:23.000 align:middle line:84% to do theoretical solutions to figure out 00:50:23.000 --> 00:50:26.680 align:middle line:84% the mechanics of why there are ripples and why there are dunes. 00:50:26.680 --> 00:50:29.160 align:middle line:84% And I've read through the literature. 00:50:29.160 --> 00:50:31.780 align:middle line:84% I can't tell you anything really intelligent about it. 00:50:31.780 --> 00:50:33.740 align:middle line:90% But you do a stability analysis. 00:50:33.740 --> 00:50:36.340 align:middle line:90% You start out with a planar bed. 00:50:36.340 --> 00:50:38.260 align:middle line:90% You introduce a disturbance. 00:50:38.260 --> 00:50:40.080 align:middle line:84% Is the disturbance-- is wiped out 00:50:40.080 --> 00:50:44.000 align:middle line:84% as for when we're dealing with high-speed plane bed flow, 00:50:44.000 --> 00:50:50.320 align:middle line:84% or does it magnify itself until it reaches an equilibrium? 00:50:50.320 --> 00:50:53.080 align:middle line:84% And there have been several attempts at stability analyses 00:50:53.080 --> 00:50:53.813 align:middle line:90% like that. 00:50:53.813 --> 00:50:55.480 align:middle line:84% And we just have to accept that it works 00:50:55.480 --> 00:50:57.460 align:middle line:90% that way for our purposes. 00:50:57.460 --> 00:50:58.740 align:middle line:90% That's all I can say about it. 00:50:58.740 --> 00:50:59.907 align:middle line:90% Does that make sense to you? 00:50:59.907 --> 00:51:00.688 align:middle line:90% AUDIENCE: Yeah. 00:51:00.688 --> 00:51:01.730 align:middle line:90% JOHN SOUTHARD: All right. 00:51:01.730 --> 00:51:03.910 align:middle line:84% So I will hope to see you-- oh, I'm sorry. 00:51:03.910 --> 00:51:04.467 align:middle line:90% Kristen? 00:51:04.467 --> 00:51:05.550 align:middle line:90% AUDIENCE: I have one more. 00:51:05.550 --> 00:51:07.950 align:middle line:84% And so I can understand in a flume 00:51:07.950 --> 00:51:10.430 align:middle line:84% that you're most often starting from a lower plane 00:51:10.430 --> 00:51:12.210 align:middle line:90% bed that gets disturbed-- 00:51:12.210 --> 00:51:15.110 align:middle line:84% JOHN SOUTHARD: Well, typically, you screed the bed. 00:51:15.110 --> 00:51:16.430 align:middle line:90% You use a screed. 00:51:16.430 --> 00:51:21.110 align:middle line:84% You mount a blade that goes a little bit deeper in the sand. 00:51:21.110 --> 00:51:27.010 align:middle line:84% And you run it down some guide bars parallel to the bottom. 00:51:27.010 --> 00:51:29.330 align:middle line:84% And you screed it off very nicely, and you start. 00:51:29.330 --> 00:51:31.230 align:middle line:84% But of course, that's unrealistic. 00:51:31.230 --> 00:51:32.210 align:middle line:90% It seldom works. 00:51:32.210 --> 00:51:34.170 align:middle line:84% And I showed you the dinosaur footprints. 00:51:34.170 --> 00:51:35.370 align:middle line:90% Sometimes, it works. 00:51:35.370 --> 00:51:36.330 align:middle line:90% Yes? 00:51:36.330 --> 00:51:39.550 align:middle line:84% AUDIENCE: So the beach example-- how often is it an upper plane 00:51:39.550 --> 00:51:43.710 align:middle line:84% bed that then you initiate ripples from as opposed 00:51:43.710 --> 00:51:47.560 align:middle line:84% to a lower plane bed, like in a storm to-- 00:51:47.560 --> 00:51:49.310 align:middle line:84% JOHN SOUTHARD: Oh, well, if you're dealing 00:51:49.310 --> 00:51:51.930 align:middle line:90% with pretty fine sands-- 00:51:51.930 --> 00:51:54.590 align:middle line:84% you will see soon, in the next few slides, 00:51:54.590 --> 00:51:58.290 align:middle line:84% that there's a range of flow for which there's no movement, 00:51:58.290 --> 00:51:59.910 align:middle line:90% and then ripples set in. 00:51:59.910 --> 00:52:03.510 align:middle line:84% And then there's plain bed and no dunes. 00:52:03.510 --> 00:52:05.730 align:middle line:84% Dunes don't develop in very fine sands. 00:52:05.730 --> 00:52:08.990 align:middle line:84% And so then you could wash the ripples out to plain bed. 00:52:08.990 --> 00:52:11.730 align:middle line:84% And then if the flow velocity decreases again, 00:52:11.730 --> 00:52:13.090 align:middle line:90% you develop ripples on it. 00:52:13.090 --> 00:52:16.010 align:middle line:84% I have a slide that shows that that we did in the lab. 00:52:16.010 --> 00:52:17.970 align:middle line:84% So does that answer your question? 00:52:17.970 --> 00:52:19.013 align:middle line:90% AUDIENCE: Yeah. 00:52:19.013 --> 00:52:20.930 align:middle line:84% JOHN SOUTHARD: So I hope to see you next week. 00:52:20.930 --> 00:52:24.770 align:middle line:84% But if not, it'll be maybe the week after, or maybe even later. 00:52:24.770 --> 00:52:28.790 align:middle line:84% So we're about halfway through this PowerPoint number 3. 00:52:28.790 --> 00:52:31.150 align:middle line:84% And that'll leave us, I think, enough time 00:52:31.150 --> 00:52:35.260 align:middle line:84% to do all of number 4, which is pretty long and heavy. 00:52:35.260 --> 00:52:59.000 align:middle line:90%