One of the instruments that was used on our recent student cruise was the so-called MSS (“MicroStructure Sonde”, sometimes also called VMP, “Vertical Microstructure Profiler”) — an instrument that is used to measure how much mixing is going on in the ocean. Those measurements can help us figure out e.g. renewal rates of bottom water in fjords, which are interesting because of the very low oxygen concentrations found there, and their impact on biogeochemistry. And of course it’s also interesting from a purely physical oceanographic curiosity :-)
In the picture below, you see the MSS being deployed: It’s a slim instrument, maybe 1.5m in length, that is attached to an orange cable that runs on a small winch.
And a big THANK YOU to cruise leader Elin (observing from the upper deck) for bringing me along on this cruise! :-)
At the end of the instrument that sticks over the railing in the picture above, you can make out little pins, protected by a metal cage. Those are the sensors for both temperature and velocity shear, both measuring at very high frequency, many many times per second. They are also very sensitve, so in the picture below you see the wooden crate that is used for storing the instrument in between stations.
Once the instrument is deployed into the water, it is not just lowered down in the way a CTD is, but it has to be free-falling through the water. In order to achive that, the person running the winch has to constantly watch the cable going into the water to make sure there is some slack on the cable.
Algot is running the MSS winch
A second way to make sure the instrument is free-falling is to constantly monitor the incoming data on a PC onboard the ship.
Elina is working the MSS “inside job” (Picture by Sonja Wahl)
While the data is being monitored, also the depth the instrument is at is being monitored, or rather its pressure. Since the instrument is free falling, it is not a simple feat to make sure it gets fairly close (approximately 10m) to the bottom, but does not hit the bottom and destroy the sondes. One way we’ve done that on the student cruise is by stopping the outgoing cable when the instrument was at 75% of the water depth and let it fall, and then once the instrument is within 20ish meters of the bottom to start hauling the cable back in (“panic” in the list below ;-))
Snapshot of the piece of paper used to keep track of what’s going on at the current station
Looking at the picture of Algot below, you know that the instrument must be on its way up. Why? Because there is clearly no slack on the cable!
Algot is bringing the MSS back to the surface
In the picture below, do you see the green fringe on the instrument, as well as the rope slung around the metal protection cage thingy for the sondes? Those are there to make sure that no eddies (and especially no trains of eddies) develop while the instrument is falling, because if the instrument was vibrating or moving in some way other than just falling freely, that would influence the data we measure.
The instrument is then brought back on board, and we are ready for the next station!
Algot and Arnt Petter bring the MSS back on board
And which spots did we measure turbulence in? In many, but especially on either side of the fjord’s sill, because that’s where we expect mixing due to tides going in and out (which we also saw in the fjord circulation tank experiment!).
We now have an experiment that shows how a fresh, yellow inflow (representing the freshwater input into fjords close to their heads by rivers) flows over a initially stagnant pool of salt water. As the freshwater plume flows out of the fjord, it entrains more and more salt water from below, thus thickening and setting up a return flow that brings in more salt water from the reservoir (representing the open ocean) on the right.
We drop dye crystals to visualize the surface current going out of the fjord and the return flow going in, and draw the profiles on the tank to be able to discuss them later.
Here is a movie of the whole thing:
But there is more to see!
When tipping the tank to empty it, a lot of turbulence was created at the sill (see movie below). While a fjord typically isn’t tipped very often, what we see here is basically what tides do on the sill (see the waves that keep going back and forth over the sill after the tank is initially lifted? Those are exactly like tides). This could purposefully integrated in teaching rather than only happen by accident, those waves could be created just by surface forcing rather than by tipping the tank. That’s a very nice demonstration to explain high mixing rates in the vicinity of steep topography!
And then there is also the issue of very low oxygen concentrations in some Norwegian fjords, and one proposed solution is to bring the river inflow deep down into the fjord. The idea is that the less dense river water will move up to the surface again, thereby creating mixing and oxygenating the stagnant deep water that, in some cases, hasn’t been renewed in many years.
We model this by putting the inflow (the hose) down into the tank and see the expected behaviour. What we also see: Since the water has a quite strong downward component as it enters the fjord, it stirs up a lot of old dye from the bottom. So possibly something to be aware of since there might be stuff dumped into fjords that you might not necessarily want to stir up…
And last, not least, a bonus picture: This is how we measure temperature at GFI. You would think it should be possible to find a smaller thermometer that isn’t an old reversing mercury one? But in any case, this worked very well, too :-)
Stupid as it sounds, one of my favourite wave watching spots in Bergen is a busy bridge with a view onto another busy bridge. But the bridge goes across a very narrow opening which connects Storelungegårdsvannet, which you see in the pictures, with a fjord and ultimately the open ocean. As the opening is a lot narrower than both the fjord and the lake at the fjord’s head, there are pretty much always strong tidal currents going one way or the other, sometimes leading to a substantial difference in sea surface height on either side of the bridge. I found that quite scary the first couple of times I had to paddle through!
But what I found most fascinating today is how many different colors you see on the water, and how they are all explained by different physical processes.
If we look at the bottom end of the picture above, we see that we can look fairly well into the water and see the sandy/rocky bottom with some algae growing on it. You can look into the water so well because several things came together at the time when I took the picture:
There weren’t a lot of waves to disrupt the view
I’m looking into the water at quite a steep angle, so even though the light going in and out of the water is refracted at the interface, the angle is too steep for total reflection to happen
This lower area of the image is reflecting the dark underside of the bridge rather than the bright sky, making it easier to see the muted colors in the water because there isn’t too much interference with bright colors from elsewhere
Moving on a little up in the picture above: Here we see the reflection of the sky (see the clouds reaching down the slopes of the mountain? (If you don’t see what I mean, check out the image at the top of this post where you can see it both reflected and “for real”)
We see the reflection here because the angle is a lot shallower and we don’t have the bridge’s shade making it possible to look inside.
Notice how the bridge’s reflection doesn’t have a sharp edge but shows up all the turbulence in the water? You can also notice more turbulence on the right side slightly above the middle of the image.
And then there is this grey stripe going all across the reflection of the mountain and houses. That’s where a little breeze is going over the surface, creating ripples. Since the surface is rougher now, we get a lot of bright sky reflected towards us.
Below another image, where you see both sides of the bridge’s reflection. Isn’t it fascinating how turbulence is distorting the reflection? And there is a lot more turbulence on the left than on the right at the bottom end of the bridge’s reflection, where you can still make out the railing of the bridge in the reflection…
In the uppermost image, you also see that it becomes more and more difficult to see the bottom as water depth increases – the water seems to be getting greener and greener, darker and darker.
What else did you spot that I didn’t mention? And do you think you’ll look at Storelungeren the same way as before next time you cross that bridge? ;-)
While student cruises usually have a lot of desired learning outcomes related to being able to use oceanographic instrumentation and knowledge of regional oceanography, ultimately one of their purposes is to equip students to function well as sea-going oceanographers, should they choose to take that direction. So in my opinion, it is very important that they don’t just learn about the science-y side of things, but that they also learn how to work with the research ship’s crew in a constructive way.
Etiquette on a research ship: A sailor’s perspective
I asked my favourite sailor what he thinks we should teach our students about how to behave on a research ship. Here are his top 3:
Always be yourself. If you pretend to be someone you are not, people will find out soon enough anyway.
Just ask. There are no stupid questions and sometimes having asked about something you are not sure about on a ship might end up being crucial for your safety.
Be friendly. ’nuff said.
He says that’s all people need to know about how to behave at sea. While I kind of agree, those three rules are kind of … vague. So here are a couple of things that I have either noticed at sea myself, or heard my favourite sailor & his colleagues complain about during our recent student cruise, so this is stuff that I would explicitly address at some point during the course leading up to the next student cruise, so students go onboard feeling more confident that they know what to expect and how to behave.
Etiquette on a research ship: My compilation
While meal times are often given as a one-hour time slot and you might think that means you can drop in at any time during that one-hour window, that’s not how things work on a ship. Usually, this one-hour window is meant as two 30min windows for people working on different watches. In between those two windows, the first group of people has to get out of the mess (not the mess mess, the room where food is served on a ship is called the mess), the tables have to be cleared completely, and food refilled. So to be polite towards the people making sure you get fed, it’s good advice to arrive on time for your feeding window and don’t linger too long after you are done eating, so they can get the room ready for the next group or finish off that meal to move on to other tasks. If people start wiping the tables, it’s a clear signal that you should find some other spot to lounge in. If, however, you have to be late for a meal due to work reasons, everybody will be happily accommodate you and make sure you leave happy and satisfied. Just don’t push it without a good reason.
Thank the cook & galley personnel
This should go without saying, but if someone puts a nice meal on the table in front of you, say thank you. If the food was delicious, let the cook know. “Takk for maten” is something that comes pretty much automatic out of every Norwegian’s mouth, but whatever your background, I think everyone should adopt it on a ship (and maybe also at home ;-)).
“No work clothes” means “no work clothes”
On ships, there are usually areas that you are supposed to not walk through, or hang out in, wearing work clothes. That’s because the ship is the crew’s home for long periods at a time (and also yours while you are at sea), and keeping a home nice and tidy is a big part of making it feel like home. And also it’s just mean to make the cleaning crews do extra work just because you couldn’t be bothered to change out of your fishy boots.
When you leave your cabin, leave the door open
Leaving the door to your cabin open when you are not in it makes it a lot easier for the crew to get their work done. They won’t knock on your door when it’s closed because they are respecting your privacy and your sleep, but they want to empty your trash, put new towels in your cabin, clean, etc.. The larger you make the time window for them to do that by just leaving your cabin door open, the less they have to organize their work day around catering towards you.
Be quiet on corridors, people are sleeping
You are not the only one going on watches (and even worse — just because you don’t go on watch doesn’t mean that other people are not), so be considerate of other people’s sleep. While it sucks to be tired as a scientist on a ship, other people have safety-relevant work to do (and also just live on the ship for many weeks at a time) so they should definitely be able to get the sleep they need.
Also consider whether you really have to go to your own comfy cabin and your own comfy toilet during your watch if you know people are sleeping in the cabins next to yours. Cabin doors are loud, vacuum toilets are really loud, but walls between cabins are more like paper than like actual walls. If you can avoid making unnecessary noises that might wake up other people by just going to a common restroom, you should probably consider doing that.
Respect people’s privacy
There is not a lot of spaces where you can hide on a ship to get your alone time when you need it. So do not enter other people’s cabins unless invited, and don’t go knocking on their doors unless there is a good reason. People will leave their doors open if they are open to communications, if the doors are closed it means you should leave people alone unless you really have a good reason.
Also the cabins are the only private spaces people get. If you wouldn’t go into someone’s bedroom in their house without explicit permission, why would you do it on a ship?
No matter how funny it is: don’t invade people’s privacy by entering their private space without being invited unless you know them very well and know that they are fine with it!
Access to all areas?
Usually, you are free to go pretty much wherever you like on a research ship (except, as I said above, into other people’s private spaces). If areas are off limit (like for example the engine room or spaces where food is stored and prepared), you will be told that. But it’s still good practice to ask whether it’s ok to hang out. For example, in heavy weather or very tight straights, people on the bridge might prefer to not having you hanging around and possibly obstructing their work. And while they will tell you that, just asking whether it’s ok to be there makes it less awkward for everybody involved. Same if you visit other scientists in their labs, or the crew in the trawl mess — sometimes it might not be immediately obvious to you that people are concentrating on their work, even though they might look like they are just chilling, and that you are getting in the way of that. Or even just getting in the way of people chilling when they need to do that.
Be on time for handover between watches
Even if you are told that your watch runs from midnight to six in the morning and from noon to six in the evening, that doesn’t mean you show up at midnight and noon sharp. It means that the other watch wants to be able to leave at midnight and noon sharp, so handover should have happened before that time. It’s good practice to show up at least 5 minutes before watch changes.
Be on time for stations
People not being ready to start working when the ship is on station is a pet peeve of mine. Ship time is very expensive, so spending it on waiting for someone who wanted to get a hot chocolate right when the ship is ready to take measurements (instead of looking at the screen that shows you the navigation data of the ship, including ETAs of stations etc and getting it while there still is plenty of time) is a very bad use of taxpayers’ money.
Also be aware that there are a lot of people waiting for you once the ship is in position to start measuring: The officers on the bridge, the deck crew possibly standing outside in cold, windy, rainy weather, your other scientist colleagues. Not very good for the general mood if they unnecessarily have to wait for you.
It’s cold and in the middle of the night for the crew, too
Just because they might not let you see it doesn’t mean you are the only one that is tired and cold and feels cranky. I guess this goes back to rule no 3: Always be friendly and considerate of the people around you…
Radio communication is safety relevant
Having fun with a radio is fun, but there are a lot of people working on the bridge or the deck that have to listen to everything you say on the radio. So if you try to be overly funny, you might end up annoying people, and worse, making it more difficult for them to do their job and keep you safe.
Don’t discuss safety issues
If the crew tells you to wear a life vest on top of your floatation suite (that is certified as being sufficient in itself) when going on a small boat trip, or a helmet when taking water samples, just wear it. In the end they are the ones that know better, and they are the ones responsible for your safety so even if they are, in your opinion, unnecessarily cautious, they are just doing your job making sure you are safe. So even if it seems unnecessary to you, if they tell you to do something, just do it.
If plans change, let people know early on (and maybe explain why)
Changing your plans might require a lot of work on the crew‘s part — putting together different instrumentation, rearranging equipment on deck, changing out winches, all kinds of stuff that you might not be aware of. So if you happen to change your plans, let them know as soon as possible so it creates the least amount of stress for them.
Also offer to explain the scientific reasons why you now think the new plan is better than the old one. In my experience, in general the crew is really curious about what they are helping you achieve (and what you really could not achieve on your own if they weren’t there to help!), and really appreciate if you let them in on what you are doing for what purpose. And also what the outcomes are!
Don’t make a cruise longer than it has to be
Even though it might be fun for you to extend your cruise for a couple of extra hours just because it’s so nice to be at sea and you feel like you payed for that day of ship time anyway, don’t change arrival times back in port on a short notice without a really good reason. The crew might have made plans with their family and friends whom they don’t see very often, that they will have to cancel. This is going to make a lot of people not very happy!
And this goes without saying: Don’t extend a cruise just to get the extra pay you get for every day you spend at sea. While I find it hard to imagine people actually do that, I have heard from so many different crew that they think a lot of scientists do that, that it’s hard to ignore the possibility that it actually happens, and quite often at that.
Etiquette on a research ship: Your take?
What do you think? Do you agree with the “rules” I put up above? Are there any more things students should be told about? What do you wish you had known about life onboard a research ship before you first went to sea?
Edit to include Twitter wisdom on etiquette at sea (08.02.2019):
On Elin’s student cruise (read more about that here) very nice wave watching was to be had, both on the water as well as in the sky.
In the picture below, if you look slightly left of the mountain top in the right of the picture, you see five parallel cloud stripes — evidence of the air moving in a wave motion after going over that mountain top! This motion results in clouds being there for certain phases of the waves and then no clouds for others, and since the movement is periodic, this results in cloud stripes. Now if I knew more about cloud formation I could probably tell you what changes with height except for pressure, and how that results in cloud formation or no cloud formation, and hence whether the cloud stripes indicate wave crests or wave troughs. My gut says troughs. Does anyone know?
Another very nice wave pattern is seen below: Kelvin-Helmholz instabilities! Those are shear instabilities that will eventually start breaking. Unfortunately I went back to work and next time I looked I didn’t find them again.
My friend Alice runs a really interesting Instagram account that I love following. She posts about being a PhD student in physics didactics, does #experimentalfriday (which you might remember from her recent guest post on my blog), gives helpful advice for mental health topics and takes beautiful pictures. Check it out — @scied_alice. A couple of days ago she posted about having found some research on how proximity to the ocean and a person’s state of mind are connected. So obviously I had to ask whether she would write about it for my blog, and I am super stoked she did! Here is what she writes:
Scientific reasons why the ocean boosts mental health
When was the last time you were at the sea and just took in everything it offered? The smell of salty water, the light breeze on your skin, the sound of rolling waves. Do you remember the feeling it gave you? That sense of calmness and relaxation, the inner peace and quiet, ultimately setting you in a state of easy meditation, giving you that break from everyday life you just needed? The impact of the ocean on physical, mental and emotional well-being seems so obvious and intuitive but there is actual scientific research on that topic. Who wouldn’t want to understand what exactly is going on in the humans’ mind and body when encountered with the ocean or any great body of water? So let me tell you about some of the findings I stumbled upon, researching a short post on why I myself like to live at the coast of the Baltic Sea.
I Golden Bay Beach, Malta (Picture by Alice Langhans)
It seems like yesterday, that the professor giving a speech at my graduation ceremony talked about the happiness level in several states in Germany and how lucky we are if we get a job in Schleswig-Holstein, which scored the happiest state several years in a row now (Schlinkert & Raffelhüschen, 2018). I had to smile because at that moment I had already taken a job in Kiel, the state capitol of Schleswig-Holstein. Apparently, the proximity to the coast and the access to blue (water) space has shown to have a positive effect on well-being. This could be one factor explaining the happiness level in Schleswig-Holstein, because the state is enclosed between the North and the Baltic Sea on each side and everyone I know enjoys that advantage to its fullest by spending lots of time at the waterfront. According to a British study, citizens living at the coast report better physical and mental health (White, Alcock, Wheeler, & Depledge, 2013)and Japanese colleagues, Peng and Yamashita(2016), concluded that people with ocean view from their homes were calmer than their inland neighbors. And if you are thinking of choosing a retirement home at the coast, they have good news as well: positive psychological effects were highest for elderly people.
IIBay of Kiel, Laboe, Germany (picture by Alice Langhans)
There seems to be something happening to people when visiting the beach or waterfront and scientific research finds answers to that in our senses. The feeling of calmness, relaxation and peace is stimulated by the view of the blue space. A study conducted in Wellington, NZ showed lower psychological distress in people with visibility of what they called blue space, images of the Pacific Ocean and the Tasman Sea (Nutsford, Pearson, Kingham, & Reitsma, 2016). Neuroscientist Michael Merzenich claims that being in the clear and simple environment of the ocean, humans have a sense of security and safety because it’s a stable and predictable environment (Yeoman, 2013). Makes sense, doesn’t it? Because everything unusual is instantly outstanding against the calm and flat horizon.
III Dingli Cliffs, Malta (Picture by Alice Langhans)
And there is even more to a visit to the beach than just the view of water. It’s the sheer sound of incoming waves that soothes the mind and relaxes the spirit. Sounds with wave patterns were found to be the most relaxing because they lower the cortisol level, a stress hormone and activate the parasympathetic nervous system, slowing us down and promoting relaxation (Heiser, 2017). This has the same effect as meditation.
IV Hargen an Zee, Netherlands (Picture by Alice Langhans)
So the next time you’re at the beach: Take everything in, take a deep breath, concentrate on your body and the calmness the ocean triggers in you and enhance those positive effects the ocean has on you! Enjoy!
Nutsford, D., Pearson, A. L., Kingham, S., & Reitsma, F. (2016). Residential exposure to visible blue space (but not green space) associated with lower psychological distress in a capital city. Health & Place, 39, 70–78. https://doi.org/10.1016/j.healthplace.2016.03.002
I love how powerful Powerpoint is, at the same time there surely is a way out there to create these kind of animations with a little less copy & pasting, and especially without manually moving tons of stuff by juuust a tiny little bit from frame to frame?
How would you build these kinds of pictures? I’m even considering Matlab at this point (which I really don’t think would be such a stupid idea after all)
This is an animated gif. If it isn’t playing, I have no idea why not… It is playing on Twitter (link here)
Still not inside an elevator, but now that I have my elevator pitch down to short and sweet (it’s really only 30 seconds if you don’t watch the contact stuff in the end), maybe I will be able to manage to film it without being interrupted like I was the previous dozen attempts…
The whole #friendlywave thing (where I explain your wave picture) is starting off great! Here is one that reached my via my Twitter; link to thread here.
What’s going on in the north east of Île d’Yeu, France? Here are four pictures from the Twitter thread that got me intrigued: Because of the awesome waves they were displaying, but also because they introduced me to ESA’s EO browser which is so amazing that I don’t think I will be able to stop playing anytime soon!
First, a true color image of the Île d’Yeu and, more importantly, the wave field around it (Click on all pictures to enlarge).
And this is what the topography in that area looks like:
Zooming in on the area north of the eastern tip where something interesting is happening……this checkerboard pattern of waves! Now the question is what causes those waves. Well, let’s find out, shall we?
I couldn’t figure out exactly where the image above was from, but I am seeing a very similar pattern in the pictures that I saved off the EO browser myself.
First, here is a true color image again (click to enlarge, or click the link to see it on the browser to play yourself)
True color image of Île d’Yeu and surrounding ocean, acquired with EO browser, January 28th, 2019.
Here is the same image, except with my annotations on it. I have marked a couple of wave crests to show what I think is going on. What I see here (and please let’s discuss this! I’m super curious to hear what you think!) is a wave field coming in from west northwest-ish (see straight-ish fronts on the top left). When this wave field encounters an obstacle in its path (the island), it gets diffracted, kind of as if there were two very wide slits on either side of the island (a very simple example of that here). It’s difficult to follow the wave crests that pass the island on its north side, but the ones that go round the south side are clearly visible as they turn around the eastern tip of the island.
Zooming in to look at it more closely:
True color image of Île d’Yeu and surrounding ocean, acquired with EO browser, January 28th, 2019.
And here is my annotated version of the wave field. You recognise the wave crests that were propagating along the southern side of the island, then turned around the eastern tip and are now spreading northward. And you see the wave crests of the waves that travelled along the north coast all along. Notice how they are crossing in a crisscross pattern?The area with the really dense red checkered pattern is the one I think was shown on the original picture on Twitter. So my interpretation is that it’s an interference pattern of waves, all originating in the same wave field, being diffracted l’Île de Yeu. What do you think? Do you agree?
What I find quite interesting is that it’s very easy to follow the crests that propagate northward around the eastern tip, but a lot more difficult to do the same for the ones propagating southward. I could imagine that the explanation is the topography: The waves propagating in the north of the island were in shallower water for pretty much the length of the island, so they might have lost a lot of their energy already, whereas the ones from the south only run into shallower water once they’ve turned around the eastern tip of the island.
Thanks, Rémi, for pointing me to ESA’s awesome EO browser and to your super interesting Twitter!
P.S.: Speaking of topography: Of course the change in water depth could also have an effect on the wave field by refracting the waves towards the slower medium, i.e. the shallower water. But I don’t think that’s the case here. Do you?
#friendlywave is the new hashtag I am currently establishing. Send me your picture of waves, I will do my best to explain what’s going on there!
When it rains, it pours, especially in LA. So much so that they have flood control channels running throughout the city even though they are only needed a couple of days every year. But when they are needed, they should be a tourist attraction because of the awesome wave watching to be done there! As you see below, there are waves — with fronts perpendicular to the direction of flow and a jump in surface height — coming down the channel at pretty regular intervals.
Even though this looks very familiar from how rain flows in gutters or even down window panes, having this #friendlywave sent to me was the first time I actually looked into these kinds of flows. Because what’s happening here is nothing like what happens in the open ocean, so many of the theories I am used to don’t actually apply here.
Looks like tidal bores traveling up a river
The waves in the picture above almost look like the tidal bores one might now from rivers like the Severn in the UK (I really want to go there bore watching some day!). Except that bores travel upstream and thus against the current, and in the picture both the flow and the waves are coming at us. But let’s look at tidal bores for a minute first anyway, because they are a good way to get into some of the concepts we’ll need later to understand roll waves, like for example the Froude number.
Froude number: Who’s faster, current or waves?
If you have a wave running up a river (as in: running against the current), there are several different scenarios, and the “Froude number” is often used to characterize them. The Froude number Fr=u/c compares how fast a current is flowing (u) with how fast a wave can propagate (c).
Side note: How do we know how fast the waves should be propagating?
The “c” that is usually used in calculating the Froude number is the phase velocity of shallow water waves c=sqrt(gH), which only depends on water depth H (and, as Mike would point out, on the gravitational constant g, which I don’t actually see as variable since I am used to working on Earth). (There is, btw, a fun experiment we did with students to learn about the phase speed of shallow water waves.) This is, however, a problem in our case since we are operating in very shallow water and the equation above assumes a sinusoidal surface, small amplitude and a lot of other stuff that is clearly not given in the see-saw waves we observe. And then this stuff quickly gets very non-linear… So using this Froude number definition is … questionable. Therefore the literature I’ve seen on the topic sometimes uses a different dispersion relation. But I like this one because it’s easy and works kinda well enough for my purposes (which is just to get a general idea of what’s going on).
Back to the Froude number.
If Fr<1 it means that the waves propagate faster than the river is flowing, so if you are standing next to the river wave watching, you will see the waves propagating upstream.
Find that hard to imagine? Imagine you are walking on an elevator, the wrong way round. The elevator is moving downward, you are trying to get upstairs anyway. But if you run faster than the elevator, you will eventually get up that way, too! This is what that looks like:
If Fr>1 however, the river is flowing faster than waves can propagate, so even though the waves are technically moving upstream when the water is used as a reference, an observer will see them moving downstream, albeit more slowly than the water itself, or a stick one might have thrown in.
On an escalator, this is what Fr>1 looks like:
But then there is a special case, in which Fr=1.
Fr=1 means that the current and the waves are moving at exactly the same velocity, so a wave is trapped in place. We see that a lot on weirs, for example, and there are plenty of posts on this blog where I’ve shown different examples of the so-called hydraulic jumps.
See? In all these pictures above there is one spot where the current is exactly as fast as the waves propagating against it, and in that spot the flow regime changes dramatically, and there is literally a jump in surface height, for example from shooting away from where the jet from the hose hits the bottom of the tank to flowing more slowly and in a thicker layer further out. However, all these hydraulic jumps stay in pretty much the same position over pretty long times. This is not what we observe with tidal bores.
On an escalator, you would be walking up and up and up, yet staying in place. Like so:
Tidal bores, and the hydraulic jumps associated with their leading edges, propagate upstream. But they are not waves the way we usually think about waves with particles moving in elliptical orbits. Instead, they are waves that are constantly breaking. And this is how they are able to move upstream: At their base, the wave is moving as fast as the river is flowing, i.e. Fr=1, so the base would stay put. As the base is constantly being pushed back downstream while running upstream at full force, the top of the wave is trying to move forward, too, moving over the base into the space where there is no base underneath it any more, hence collapsing forward. The top of the wave is able to move faster because it’s in “deeper” water and c is a function of depth. This is the breaking, the rolling of those waves. The front rolls up the rivers, entraining a lot of air, causing a lot of turbulent mixing as it is moving forward. And all in all, the whole thing looks fairly similar to what we saw in the picture above from Verdugo Wash.
But the waves are actually traveling DOWN the river
However there is a small issue that’s different. While tidal bores travel UP a river, the roll waves on Verdugo Wash actually travel DOWN. If the current and the waves are traveling in the same direction, what makes the waves break instead of just ride along on the current?
What’s tripping up these roll waves?
Any literature on the topic says that roll waves can occur for Fr>2, so any current that is twice as fast as the speed of waves at that water depth, or faster, will have those periodic surges coming downstream. But why? It doesn’t have the current pulling the base away from underneath it as it has in case of a wave traveling against the current, so what’s going on here? One thing is that roll waves occur on a slope rather than on a more or less level surface. Therefore the Froude number definitions for roll waves include the steepness of the slope — the steeper, the easier it is to trip up the waves.
Shock waves: Faster than the speed of sound
Usually shock waves are defined as disturbances that move faster than the local speed of sound in a medium, which means that it moves faster than information about its impending arrival can travel and thus there isn’t any interaction with a shock wave until it’s there and things change dramatically. This definition also works for waves traveling on the free surface of the water (rather than as a pressure wave inside the water), and describe what we see with those roll waves. Everything looks like business as usual until all of a sudden there is a jump in the surface elevation and a different flow regime surging past.
If you look at such a current (for example in the video below), you can clearly see that there are two different types of waves: The ones that behave the way you would expect (propagating with their normal wave speed [i.e. the “speed of sound”, c] while being washed downstream by the current) and then roll waves [i.e. “shock waves” with a breaking, rolling front] that surge down much faster and swallow up all the small waves in their large jump in surface elevation.
In the escalator example, it would look something like this: People walking down with speed c, then someone tumbling down with speed 2c, collecting more and more people as he tumbles past. People upstream of the tumbling move more slowly (better be safe than sorry? No happy blue people were hurt in the production of the video below!).
Looking at that escalator clip, it’s also easy to imagine that wave lengths of roll waves become longer and longer the further downstream you go, because as they bump into “ordinary” waves when they are about to swallow them, they push them forward, thus extending their crest just a little more forward. And as the jump in surface height gets more pronounced over time and they collect more and more water in their crests, the bottom drag is losing more and more of its importance. Which means that the roll waves get faster and faster, the further they propagate downstream.
Speaking of bottom drag: When calculating the speed of roll waves, another variable that needs to be considered is the roughness of the ground. It’s easy to see that that would have an influence on shallow water. Explaining that is beyond this blog post, but there are examples in the videos Mike sent me, so I’ll write a blogpost on that soon.
So. This is what’s going on in LA when it is raining. Make sense so far? Great! Then we can move on to more posts on a couple of details that Mike noticed when observing the roll waves, like for example what happens to roll waves when two overflow channels run into each other and combine, or what happens when they hit an obstacle and get reflected.
Thanks for sharing your observations and getting me hooked on exploring this cool phenomenon, Mike!