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Chapter 9 - Tides — Transcript

by Jeffrey Ashley · 5,851 words · 860 segments · language en · Watch on YouTube

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0:00Hello, my name is Dr. Jeff Ashley and
0:02welcome to chapter 9. Chapter 9 deals
0:05with tides and most of us have seen
0:07tides or interacted with them before.
0:10So, what causes tides? Tides are
0:13periodic raising and lowering of the
0:15ocean sea level. They occur daily and
0:19they can really be traced back to a
0:21phenomenon that involves gravitational
0:24laws that were explained by Isaac
0:26Newton. There's also some other things
0:28driving that by pra primarily it's a
0:31gravitational influence. Tides are
0:34caused by a combination of gravity and
0:36also motion between the earth, moon and
0:38the sun system. Let's first introduce
0:41this idea of a barisenter. A beer center
0:44between specifically the moon and the
0:46earth system. It's a common center of
0:49mass or a balance point and it's beneath
0:51the earth's surface. Let's take a look
0:53at this diagram to get a better idea of
0:55what bareris center
0:57is. Okay, so you have the moon and the
1:00earth and it's a system that is rotating
1:03around each other. However, because of
1:07the earth's larger mass versus the moon,
1:10we end up with a rotation system here
1:13that is centered around this beer
1:15center. And as we said previously that
1:18it is existing this point in time or
1:21this point on earth is existing beneath
1:24the earth's surface. So there it is
1:27uh it is the balance point of these two
1:30entities the earth and the moon. So the
1:33center isn't a true center here. The
1:36center is right here and that's barac
1:38center. And in your book it says it's
1:40akin to a sledgehammer where you have
1:43the weighty earth over here as that
1:46leaded head of the sledgehammer and the
1:48moon is lighter and over here. When you
1:51throw that sledgehammer into the air and
1:54start rotating it, it actually rotates a
1:57little weird because of that bear
1:59center. It's not a rotation that's like
2:01this. It's a rotation that is a little
2:04offkilter, which means, and I like this
2:08diagram better, we have the Earth Moon
2:11system that is not moving in a straight
2:14path around the Sun, but it's orbiting
2:17in a way that because of that bear
2:19center, you get this little hoppy motion
2:23that is based on that throwing of the
2:25sledgehammer like movement. So, let's
2:28play out this short video on that
2:30movement around the sun of the Earth and
2:35the Moon system. Well, to do this
2:37activity, you only need four simple
2:38things. You need a lamp with a shade
2:40removed, a white foam ball, a pencil,
2:44and a dark room. The darker the room,
2:47the better. Now, in this model, the lamp
2:50is going to represent the sun. And of
2:52course, the white foam ball is going to
2:53represent the moon. And your head is
2:56going to represent the earth. So, let me
2:58show you how this
3:02works. We know that the moon orbits the
3:05earth. So, in this model, I'm going to
3:07get the moon, the foam ball, to orbit
3:09the earth, my head.
3:13Notice that no matter where the moon is
3:15in its orbit, half of it is always lit
3:17up by the
3:20sun. But when the moon is in this
3:23position, I can't see any of the lighted
3:26side of the moon from
3:27Earth. My head
3:32from this angle, you can see what I am
3:34seeing from Earth. We call this new
3:36moon. When it's new moon, we look up at
3:39the sky and we can't see the moon. It's
3:41there, but the side that's reflecting
3:43sunlight is away from us. Now, as I
3:46slowly make the moon orbit to my left, I
3:48can begin to see a tiny sliver of that
3:50lighted side. We call that a crescent
3:53moon. As the moon keeps orbiting, we can
3:56see more and more of that lighted side.
3:58We see half of the lighted half, which
4:01is a quarter moon. In this case, a first
4:03quarter
4:05moon. As we keep orbiting, we see almost
4:08the whole lighted side. We call that a
4:10gibbus
4:12moon. Then when we can see the entire
4:14lighted half of the moon, that's a full
4:17moon. As the moon continues in its
4:20orbit, we begin to see less and less of
4:22that lighted side. We're back to a
4:24gibbous moon, but this time the light is
4:27on the opposite
4:33side. Then another quarter moon, last
4:37quarter moon to be exact.
4:41Another
4:45crescent and finally back to new
4:48moon. Kind of cool. You can do this at
4:51home if you want. Maybe you already did
4:52this in elementary school or high
4:54school, but it actually shows the Earth
4:57moon system in its rotation around the
5:00sun. So, let's go back to that diagram.
5:03So, what we were seeing there was
5:05actually a full lunar cycle. So we're
5:08actually just looking at this cycle
5:10right here. And a lunar cycle takes
5:13around 29 12
5:15days. Okay. So we said what's driving
5:18the tides and that was largely
5:20gravitational forces. So let's just
5:22refresh our memory on gravity.
5:24Gravitational force is derived from
5:26Newton's law of universal gravitation.
5:28And it says that every object that has a
5:31mass in the universe is attracted to
5:33every other object that has a mass. And
5:35in fact you can remember maybe from
5:37physics class that that force of
5:40attraction is proportional to the
5:42product of the two masses of those
5:44entities and it's inversely proportional
5:48to the square of the distance. So larger
5:51the mass the greater the force of
5:53attraction or gravitational forces.
5:56However the distance is important as
5:58well. As you go to greater greater diff
6:00distances you decrease that
6:02gravitational force. Okay. So let's just
6:05take the Earth Moon system again. Here's
6:07the Earth. Here's the moon. We said that
6:10there's a gravitational force that's
6:12happening here because the distance over
6:16here, let's just say we're on this side
6:17of the Earth. Look at the distance
6:20between the Earth's surface and the
6:22center of the
6:23moon. It's the shortest distance
6:26possible. Okay, here take a look. That
6:30distance is much larger. It is the
6:33larger distance or the largest distance
6:35away from the moon at that point in
6:37time. We've got two names, new names
6:40here. The zenith is called the point on
6:42earth closest to the moon at that
6:44particular point in time. It's not
6:46always there. The earth is rotating. And
6:49the nater or nadir is the point on earth
6:52furthest from the moon. So the greatest
6:55force gravitational force will be acting
6:58at the zenith. The least gravitational
7:00force on earth will be acting at the
7:04nadir. It's not all gravitational force.
7:06We got to throw in another force and
7:09that other force is called centropedal.
7:11Not centrifugal but centripedal. And
7:14this is said to be the center seeking
7:16force. If you have a object with a mass
7:19and you're swinging it along, this is
7:21what centripedal force is. It keeps our
7:23planets and that little ball in orbit
7:26via gravitational attraction.
7:28Essentially, it's tethering the Earth
7:30and the Moon
7:33together. If you look at centripedal
7:35forces, here's what you come up with.
7:38They're all in the same direction. These
7:40vectors are showing them at various
7:43locations on Earth. All in the same
7:45direction, pointing horizontally to that
7:48entity called the moon. And actually,
7:51they are all of the same length. So,
7:53those forces are exactly the same around
7:57the Earth. take you back to this one
7:59because look at this was gravitational
8:01forces and these are the vectors
8:03represented them representing them. The
8:06longest vector is here. So that is the
8:09strongest gravitational force at that
8:11point in time. And look at the
8:13directionality. It's directed right to
8:15the center of the moon. You get a little
8:17less gravitational pull as you get
8:19further away from it. That makes sense.
8:21So a shorter vector here. But look at
8:23the directionality. It's not horizontal
8:26like the centripedal forces. It is
8:30directed towards the center of the mass.
8:32And again, right here, furthest away
8:34from the moon, you've got the shortest
8:36vector, but again, it's pointed to the
8:38center of mass. So, let's bring these
8:41two things together, centripedal forces
8:44and gravitational forces, and we get
8:47resultant forces. So, this is the
8:49mathematical difference between the
8:51gravitational and the centripedal
8:53forces. Essentially, we're looking at
8:55the resultant force in in blue here. How
8:58did you get that or how did they get it?
9:01They took an
9:04arrow starting at the tip of the
9:07centripedal force and going to the tip
9:10of the grav gravitational attraction of
9:12the moon. So, you draw that from tip to
9:15tip. Start with the red arrow going to
9:17the black arrow and the resultant forces
9:20there. So, look at that one. It's
9:21directed. It's a little vector but it's
9:23directed up. Let's do the same thing
9:27from the red to the black and the
9:30resultant force is in blue. Ah, okay.
9:32So, it's a horizontal
9:34line that is directed towards the moon
9:37over here. This is the oddball one.
9:39Again, you're going from the tip of the
9:41red to the tip of the black. Whoa.
9:44Notice that the resultant force is away
9:47from the moon. But is it equal in
9:50magnitude or strength? So equal in
9:53magnitude but opposite directions. So
9:56you see that some of these resultant
9:58forces are moving away from the moon.
10:01Some of them are directed towards the
10:03moon. So let's take a look at what this
10:06resultant force is. So it's a resulting
10:09force from the centripedal forces and
10:12the gravitational attraction of the
10:14moon. And it actually is what delivers
10:18the tidal forces. So let's start with a
10:21nonrotating earth here. And how are
10:23tides generated? The resultant force
10:26that we saw there has significant
10:28horizontal component. It pulls water
10:31because water can move and stretch along
10:35both towards the moon and away from the
10:37moon on the other side. So water being
10:40fluid can move and you get these bulges,
10:43these two bulges on one side of the
10:45earth towards the moon and the other
10:48side of the earth away from the moon. So
10:51imagine I like saying it's like pulling
10:53out your shirt like this. You're bulging
10:55that, right? But I'm bulging it towards
10:58the moon and I'm bulging it away from
11:00the
11:01moon. This isn't a non-rotating Earth
11:04right now. We're going to have to start
11:06rotating it to make it more realistic
11:08and see what happens. So, let's put the
11:11Earth in motion because it is rotating.
11:14And let me stand up again and bulge my
11:16shirt out. So, that represent the bulges
11:19of water. But my body is rotating, but
11:22those bulges are staying in the same
11:24position. So you can imagine that as my
11:27body rotates, which represents the
11:29rotation of the earth, but those bulges
11:31are staying the same, you're going to
11:34get this uh issue of tidal bulges and
11:37tidal bulge movement. It results when a
11:39force pushes water into two simultaneous
11:42bulges. We said one towards the moon,
11:44one away from the moon. These are called
11:46lunar bulges. And they arise, as we
11:49said, from centropedal and gravitational
11:51forces. So people start off with a
11:54simple situation where the moon exists
11:57right here over the equator. We're going
11:59to see that that's not true. Uh and we
12:01start off with a uh idealized earth that
12:05is just all water. There's no continents
12:08because continents then influence tides
12:10as well.
12:12So on this side of the earth, water
12:14bulges away. On this side, it bulges
12:17towards the moon. And again, let's put
12:19this spinning Earth into uh motion here.
12:22You can imagine if you're here at one
12:24point in time, you're going to receive
12:27this tidal bulge, aka a higher tide. But
12:31as that Earth spins, right, you're
12:33spinning with it. You're going to
12:36encounter an area that's not close to
12:40that moon side or opposite side. And
12:43that has there's no tidal bulge there,
12:46right? It's actually like that, the
12:48skinny part where there's no bulge, and
12:51that's called low tide. So, let's look
12:53up some terminology on the moon's tidal
12:55bulges. A tidal period is said to be the
12:58time between high tides.
13:01Okay? So, you go from a high tide to a
13:04low tide and back to a high tide. That
13:06tidal period is from high to high. A
13:08lunar day, I mentioned this before, it's
13:10the time between two successive overhead
13:13moons. That's 24 hours, but a little
13:17more. 24 hours and 50 minutes. 24 hours
13:21is our solar
13:23day. But a lunar day is 24 hours and 50
13:27minutes. This explains why you get high
13:29and low tides that change all the time.
13:32And it explains why you need tidal
13:34charts to predict these. They're out of
13:37phase by about 50 minutes. So, I'm at
13:41the beach this week. uh high tide maybe
13:43was at 400 pm today. Tomorrow it will be
13:474:00 p p.m. uh 4:50 p.m. So let's look
13:51at this lunar day and we said that it's
13:54not like a solar day. Um and let's
13:57position ourselves here. Uh this is
14:00looking down. So you're looking down on
14:02the north pole. So let's position oursel
14:04uh somewhere on the equator here and
14:06start the earth rotating. So here we
14:09are. Uh the earth and the moon here have
14:13influence and we see that oh yeah we're
14:15in a tidal bulge area. So that would be
14:18a high tide. But as the earth rotates
14:21and we're going to rotate it 6 hours
14:23later that we have moved over here.
14:25We're in an area that's not a tidal
14:28bulge. It's the skinny part. So we
14:31experiencing not high tides but low
14:33tides. Uh then we come back 12 hours
14:36later and we're opposite side here.
14:37Remember, tidal bulges are pretty much
14:40equal on either side. The moon facing
14:42side and the non-moon facing side. 6
14:45hours later, we're over here. We were in
14:48a high tide situation here and a low
14:50tide
14:50situation. Come back to our starting
14:53point and that would be a solar day, 24
14:55hours, but the moon is now actually over
14:59here. So, the moon has gone over here.
15:02So, that explains this lunar cycle of 24
15:05hours plus 50 minutes.
15:08So tidal bulges, we just saw that
15:10because of centripedal force and
15:12gravitational force with the Earth Moon
15:15system that seems to be developing these
15:18tidal bulges. Well, they are, but
15:20they're lunar tidal bulges caused by the
15:22moon. What about the sun? The sun is a
15:25pretty big mass. Do we have gradu
15:27gravitational attraction towards that
15:29and maybe some bulges because of it?
15:32Yes, they're similar to lunar bulges.
15:35So, they're solar bulges, but they're
15:37about half the size. This is because the
15:40sun, although it's 27 million times more
15:42massive than the moon, it's further
15:45away. It's 390 times further from the
15:48Earth. So, it's got a big mass. You
15:50would think a lot of gravitational pull
15:52and effect, a lot of bulges, but no,
15:55that distance actually negates or
15:57offsets that power of the gravitational
15:59force. So the moon's closer distance to
16:02the earth results in moon exerting more
16:04control over the earth's tides than the
16:07sun. But we got to take into
16:09consideration that the sun does have an
16:11influence and does manage to bulge the
16:14sea as well. Well, here you have a
16:16figure that just explains what I said
16:19about the relative size of the moon, the
16:21earth and the sun. Sun is massive in its
16:26mass. However, the distances come into
16:29play here. So if you look at the force
16:32of
16:33gravitation and that equals a
16:36proportionality to between the two
16:38masses and also divided by the distance
16:42that distance here because it's so huge
16:45is making that solar bulge lesser of an
16:49impact than the uh lunar bulge.
16:54some other terms we have to deal with
16:56here um as the earth's rotation and the
16:59tides are influenced here. So we have a
17:02high tide um when we say it's a flood
17:05tide or a flooding tide water moved
17:08towards the shore. So if you're standing
17:09on the shore like again I'm at the beach
17:11here high tide that tide is rising. Okay
17:15so that's a flooding tide. Imagine like
17:17a flood would come in it is like moving
17:19water up and up and up and increasing
17:21your sea level
17:23momentarily. When something is said to
17:25be ebbing it's moving away for the shore
17:28shore or going away. So an ebbing tide
17:31is one that is going from high tide back
17:34to a low tide. A flooding tide is going
17:36from a low tide to a high tide. The
17:39tidal bulges are fixed relative to the
17:42sun and the moose moon's positions and
17:45the earth's rotation moves differently
17:48according to geographic locations into
17:50those
17:52bulges. Some other terminology here you
17:54might have heard about tidal range or I
17:57call it tidal excursion. It's the
17:59difference in height between the high
18:01and the low tides. So tidal excursion
18:04around Philadelphia area. If you go down
18:06to the Delaware River or even parts of
18:09the Skooko River before the Museum of
18:11Art because there's a dam there, you
18:13will see title ranges or excursions on
18:16the order of four or five feet in
18:19certain
18:20places. We'll see this and it will come
18:22into play in a moment that you can have
18:24the moon, Earth, and Sun align just like
18:27in that little video we watched. That
18:30term is called sissyagy. It's a really
18:33cool word um and difficult to say sisy
18:37when the moon is in its first or the
18:41third quarter phase here. So it's not
18:43aligned. The earth, moon, sun system is
18:46not aligned on one plane. It's offset.
18:49It's perpendicular. We call that
18:51quadriure. We're going to look at some
18:54diagrams to make
18:57that more clear. So here we have the
19:01monthly title cycle and it explains some
19:04of our tide
19:06terminology. When we have that
19:09means the sun and the moon either the
19:12new moon or the full moon align on the
19:15same plane as the earth. So the earth
19:17moon sun system are along that same
19:19plane. We said that with What
19:22happens here? We get the exertion of
19:24both the lunar bulges and the solar
19:28bulges. So here we have overlaid both
19:31the lunar tidal bulges and then a little
19:34extra here from the solar bulges. So
19:36it's like an extra extra bulge thanks to
19:39now both the moon we learned that before
19:41and now we have to bring in the bulge
19:42from the sun. So under this condition
19:46when you have alignment or sisy we have
19:49a new or full moon we've got the
19:51greatest tidal ranges or the greatest
19:54height of our high tides. This is called
19:57a spring tide. And you might have heard
19:59about this certainly influences when we
20:01have a hurricane coming through because
20:03if that hurricane does happen on a
20:06spring tide and we have and we may have
20:09a storm surge that'll heighten the sea
20:12level even more. Not good. Totally
20:16opposite when you're in quadriure. So
20:18the sun, the earth, and the moon are
20:21perpendicular here. Either the first
20:24quarter moon or the third quarter moon.
20:26So there's not two moons here. It's just
20:28that moon moving through in those
20:30positions. This is where we have the
20:32least or lowest tidal range. We call
20:34this quadriure as I said before because
20:36it looks like a quadrant. And we have
20:39quarter moons in both of these phases.
20:41These are the lowest ranges, lowest
20:43tides. And we call these lowest tides
20:45neep tides. So think spring tides, you
20:48got your highest high tide. Neep tides
20:50you'll have your lowest high tide. So we
20:53just saw the new moon, the full moon,
20:56and the quarter moon. And from that
20:58video, we see how they appear to us on
21:01Earth. We also have, and we can go back
21:03to that diagram, a crescent moon and a
21:08gibbus moon. But if the crescent moon is
21:11the moon moving from the new to the
21:13first quarter, we call it waxing or
21:15coming. We also have a gibbus moon. We
21:19can say that it's the moon moving from
21:21the first quarter to the full. And we
21:23call that again waxing gibbus.
21:26Waning is now used when you're in that
21:29last
21:31quarter. And the waning gibbus is the
21:33moon moving from the full to the last
21:35quarter. And the waning crescent is that
21:38last quarter, the moon moving from the
21:40last quarter back to the new
21:43moon. So you can see it depicted here.
21:46Words are great. Listening to me is
21:47great. But here we have it. Right? So
21:49these terms you can impress your
21:50friends, right? If you went from a new
21:53moon where you're not seeing anything
21:54and you start to see that crescent, ah,
21:57we're beginning the lunar cycle again
21:59and that is a waxing or coming crescent.
22:03Right? If you're over here and you see
22:05that crescent, but it was a full moon
22:09maybe two weeks
22:10ago. Um, that means that it's waning.
22:14It's disappearing. And that crescent
22:15will get smaller and smaller. This way
22:17that crescent will get bigger and
22:19bigger. So complicating factors of
22:21tides. So right now we know that tides
22:24are thanks to the l lunar bulges and the
22:26solar bulges. But we keep showing these
22:30diagrams where the moon is right over
22:32the
22:33equator. Declination the angular
22:36distance of the moon or sun above or
22:38below the earth's surface. So the sun to
22:41earth is actually at a declination of
22:43around 23 and a half degrees north and
22:45south of the equator. So it's not over
22:48the equator. Moon and Earth, we've got
22:5028.5 degrees north and south of the
22:52equator. So these lunar and solar bulges
22:55shift from the
22:56equator. So before we were showing it
22:59right around the equator with a rotating
23:01Earth. So you thought, oh, the maximum
23:02tides would always be around the
23:04equator. Not true. This declination now
23:07we're showing the moon over here. We see
23:10that again the tidal bulge here. We
23:12don't see the solar bulge but the tidal
23:13bulge which is responsible for most of
23:15the tide influencing force here. now at
23:18a declination year. So not highest high
23:22tide here at the equator, but you're
23:24going to have the highest high tide here
23:25because of that declination. So
23:27complicating factor declination. Other
23:30complicated factors are there's slightly
23:33elliptical orbits here in both the
23:35Earth's orbit around the sun and the uh
23:39the cycle from the Earth Moon system. So
23:42let's just start with the Earth's orbit
23:44around the Sun. We've got the greatest
23:46tidal ranges, the greatest high high
23:48tides when you are in perihelion when
23:51the earth is closest to the sun. That
23:54happens in
23:55January. The title range is least in
23:59aphithelon when the earth is furthest
24:01from the sun and that happens in July.
24:03Let's take a look at this. So, it's an
24:06elliptical system here, but there is a
24:09point in time when the earth is closest
24:11to the sun.
24:13I said that's perihelion January
24:18and in July you're furthest away from
24:21the sun. Same thing happens with the
24:23moon. We've got the moon's orbit around
24:25the earth. We've got title ranges
24:28greatest when the moon is closest to the
24:30earth and we call that perigee. And we
24:32get the tidal ranges are least in
24:35apogee. That's when the moon is furthest
24:37from the earth. And this perigee ape
24:40cycle is 27 1/2
24:43days. So we see that here again the
24:46elliptical orbit. But we also see at one
24:50point in time the earth is its closest
24:52to the moon and that's called parige. In
24:55apogee the moon is furthest away. So
24:57it's going to influence our tides as
24:59well. Again going back to the
25:01gravitational force related to the
25:03distance. So one weird effect of these
25:06orbits is sometimes every 1 and a half
25:08years or so we get these exceptionally
25:11high tidal ranges and this happens when
25:14the spring tide coincides with perigee.
25:16We call these proxyian tides. Let's just
25:19take a look at how the moon appears both
25:21in perigee and apogee. So in apogee we
25:25said the moon appears smaller than
25:26normal because that distance is the
25:29largest. In perigee, we are closest to
25:31the moon. So the moon appears larger
25:34around 14% larger. So tidal predictions
25:37are really difficult because we have to
25:40go from an idealized non-rotating earth
25:42that only has water. Throw in the
25:45continents, throw in it rotation, throw
25:47in a declination, it becomes highly
25:49complicated very quickly. The moon's
25:52declination determines possible
25:54positions of these tidal bulges. Moon
25:57directly overhead yields a high tide. We
26:00generally have two high tides and two
26:02low tides per lunar day. We have six
26:05lunar hours between high and low tides.
26:08So based on this and based on that
26:10declination of the moon here, we get
26:13pretty complicated charts like this. So
26:16this goes over a tidal cycle. We can
26:19have a high higher tide, low tide, and
26:23then a lower high tide, and then a low
26:25tide, and back to a higher high tide. It
26:28gets complicated. You can look at these
26:31videos uh online and see this in action
26:34animation ones. And I'll probably point
26:36you to mastering oceanography to
26:38solidify this comp uh concept as well,
26:40but it gets complicated. So, those are
26:42idealized tides, real tides. Okay, as I
26:47mentioned that continents and the
26:49friction with the seafloor could modify
26:51these tidal bulges. Idealized tides do
26:55not account for continents or friction.
26:57So everything we've been talking about
26:58has been an idealized tide model. Tides
27:02are forced waves not freely propagating
27:05due to being pulled by an astronomical
27:08force. So these idealized tidal bulges
27:11really cannot form, cannot keep up with
27:14Earth's rotation. Crests and troughs of
27:16tides rotate around an amphodroic point.
27:20This is where it gets really
27:21complicated. I just want you to
27:23understand that real tides are
27:25complicated and it makes tidal charts
27:28very very difficult to forecast. Um
27:33certain areas we can call co-tidle lines
27:36and they radio out or radiate out from
27:39these amphodomic points. They connect
27:42simultaneous high tide
27:45points. Just going to show this for the
27:47fun of it. Not going to mention any more
27:49of it because uh this would be an
27:51advanced uh oceanography course to go
27:54into all of this. So the effect of
27:56continents, continents interrupt that
27:59free movement of the tidal bulges. Tidal
28:02ranges amplified over shallow
28:04continental shelves. So this is amazing.
28:07Tides are dictated by almost 400
28:10factors. So when you're sitting on a
28:12coast and you're looking at the tide,
28:14400 factors are probably coming in to
28:17make those predictions of how high it is
28:20or how low it is and even the
28:23timing. Thus, it's really difficult to
28:26have a good mathematical model to use
28:30for tides. Some other terminology here.
28:34In certain places, you can have one high
28:36tide and one low tide per day. It
28:38doesn't have to be two and two. This is
28:40called
28:41dural. Semidural means that you have two
28:44high tides and two low tides per day,
28:47just like we do along the east coast of
28:48the United States. And the title range
28:50is about the
28:51same. You can have areas of mixed title
28:54patterns. You can have two high tides,
28:56two low tides per day, but the title
28:59range can be different.
29:02So here we have a nice worldwide map
29:04here. And if you want to experience
29:06semidural patterns, as I say, go to the
29:09east coast. If you want to look at one
29:11high tide, one low tide, the Gulf of
29:13Mexico region here and parts of the
29:15Caribbean. And if you want to look at
29:18certainly the prevalence of these mixed
29:20tidal patterns, you can see them in
29:22blue. If you look on a monthly basis
29:25here, we know that spring tides are the
29:27highest tides. So these are plotting
29:31this is every little wave here is again
29:35a day. So two highs, two lows. So in
29:40certain areas Boston is a semidal. We're
29:43going to see two high tides, two low
29:45tides. But you can then also know that
29:48in areas where you have in areas of the
29:51calendar where you have that
29:53you're going to have spring tides. These
29:56are higher than normal, right? you're
29:58going to in quadriure have neep tides.
30:01So that's why how come these are high
30:04tides that are lower here and higher
30:06here because of that position and the
30:09alignment with the earth moon sun
30:12system. So it gets complicated. You can
30:14go through these semidal is the most
30:16common on the east coast as I said. So
30:19that's typical but we can also see uh
30:22mix patterns here or dural patterns um
30:25that get a little more complicated.
30:27really cool. I suggest that you uh
30:30Google and take a look at some of the
30:33video the time lapse imagery of this. I
30:36think this image suffices, but one of
30:38the areas of interest for extreme title
30:41ranges is the Bay of Fundy in Nova
30:43Scotia. It's the world's largest title
30:45range. And it's a unique inbaitment
30:48here, the Bay of Fundy. Because of the
30:50morphology and again 400 factors driving
30:54this in forming these tides, you get a
30:57super high tide and a super low tide. A
30:59tidal excursion that is
31:01incredible. Another cool thing it
31:03happens in coastal waters is when tides
31:06are entering semi-encclosed systems or
31:08traveling up rivers um you generate
31:11these things called tidal bores. They're
31:13tiger generated walls of water that seem
31:16to be moving up certain rivers. Certain
31:19conditions needed for this. They don't
31:20happen on every estuary or river. You
31:22need a large spring tidal range of about
31:256 meters. You need abrupt flood tide and
31:28short e tide phases, low-lying river
31:32with a seawward current. You've got a
31:34shallowing or landward
31:37seafloor and a narrowing of the basin in
31:40the upper reaches. This is all to say
31:41that you need these certain conditions.
31:43They don't happen in many places. One
31:46place is uh viewed here a tidal bore
31:48happening. This is the onrushing tide
31:51that is happening in France. So you can
31:53see that this is the the the head of the
31:55tide coming in. So it's a flooding tide
31:58coming up this river. And what people do
32:01is they surf these. Let's watch this
32:03video of a tidal bore happening on the
32:06Amazon
32:08River. What a
32:10perfect. Look at this thing.
32:13Absolutely
32:16perfect.
32:20Nobody. Oh my
32:23god. And then it got serious. What
32:26happened next stunned everyone.
32:33Perhaps it was the effect of the super
32:35moon.
32:37But then came the biggest, most
32:39perfectly formed poroca wave any human
32:43has ever seen. And it was all roles to
32:47ride.
32:55We're out here in the Amazon. You're
32:56away from cities and you know the
32:58pollution and stuff. You got just this
33:01the water's brown but it's not dirty.
33:02It's just so when tides are coming into
33:04these areas, we call them flooding. So
33:07water is rushing up a bay or river with
33:10the incoming tide or the flooding tide.
33:12We said if it's retreating, water drains
33:15from the bay or river as the tide goes
33:17out. It's called an ecurren. We have
33:19other terminology called high slack
33:21water. That's the peak of each high tide
33:23with no current motion. So high tides
33:26will come in and then there's a point in
33:29time when they then start to that's
33:31coming in or flooding before they
33:34maximize their high tide or at the point
33:37it's high tide then they're going to be
33:39no current motion for that one point in
33:42time and then they'll start ebbing out
33:44of the river or estuary. Low slack water
33:47is the same peak of each low tide where
33:49there's no current motion. So we can see
33:51that here on this chart. This is the
33:55tidal range. So we have a high tide and
33:59a high tide and a low tide and a low
34:01tide. Uh one phase here a dural phase.
34:06But look at the current velocity. So
34:09when you go to the low water or low tide
34:13that velocity goes to zero. Okay? And
34:16then you return to moving water. So that
34:19moving water in and out of these areas,
34:21this tidal motion actually on the high
34:24tide and on the low tide stop for one
34:26instant of time. That velocity is zero.
34:29Some weird stuff can happen if your
34:31tides are coming into or out of uh
34:34geomorphic areas that are unique. Uh,
34:38one thing that may happen is you have a
34:41maelstrom that's in the Arctic
34:45Norway and it is uh rapidly spinning
34:49seawater. It's called a vortex and these
34:52typically happen when you are in a
34:54restricted channel connecting two basins
34:56of different tidal
34:59cycles. So there's an image of the
35:01Norwegian maelstrom. So can we use ties
35:05to generate power? We said we certainly
35:07might try with waves. Can the same be
35:09done for tides? The answer is yes. And
35:11there's primary two ways here. Tidal
35:13water can be trapped behind coastal
35:15barriers in bays and estuaries and that
35:18turns electrical turbines or tidal
35:20currents in narrow channels can turn
35:22underwater turbines.
35:26The pros of tidal generated power are
35:28it's much like wave power, clean,
35:31renewable, lower operating costs than
35:33fossil fuel burning power plants despite
35:35the initial higher cost of building
35:37them. The United Kingdom proposed
35:39building the world's first largest tidal
35:41power
35:43plant. Possible environmental concerns
35:45is it changes the habitat of submarine
35:48or organisms. you're altering the
35:50ecology of these rivers or estuaries
35:52where you're sort of essentially damning
35:55them and can affect migrations. Marine
35:58animals can become trapped in the moving
36:01tidal power devices called turbines and
36:04possibly noise can disrupt
36:06habitats. So here's one that has been
36:09successfully producing tidal power since
36:111966 in St. Moro, France. So you see
36:14that this is an estuary. you have to
36:17enclose it in concrete. Uh you have
36:20tides that are coming in and out on a
36:23tidal range or a tidal period. When that
36:26tide is ebbing, it is then held into
36:31this area. So an ebbing tide will create
36:34a whole bunch of water in the upper
36:36reaches. it's blocked off and then as
36:39the tide retreats it's um it then that
36:43high tide water which is trapped behind
36:45this barrier then can flow into the
36:48lower low tide area and that's going to
36:51drive a turbine and again we said before
36:54anytime you drive a turbine you can
36:55generate
36:56electricity. So there we have it. That
36:59was the end of our chapter 9 dealing
37:01with tides. I hope you enjoyed it and I
37:04hope you found some marine exploitation
37:08or marine innovation towards the end of
37:10that that might spur your interest in
37:11creating your
37:14fact. Take care. See you later.

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