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Chapter 10 — Transcript

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0:01Hello, I'm Dr. Jeff Ashley and welcome
0:03to chapter 10. Let's get
0:05started. Chapter 10 deals with the
0:08coast. And as I said before, around 50%
0:10of the world's population lives by a
0:13coast. Knowing the coast definitions and
0:16some of the processes will help us look
0:18at the human impacts along these
0:20coastlines. So, what is the coast? Well,
0:24the coast can be divided into certain
0:27sections or compartments. Really the
0:30coast is this and it starts at the
0:33coastline and heads towards the land.
0:38What is the beach? Well, the beach is
0:40further subdivided into nearshore and
0:43shore. Let's just look at nearshore.
0:45This is the area where we talked about
0:47the three different forms of waves.
0:49These are where we see breakers. The
0:51waveforms breaking and transferring
0:54their energy in the form of forming a
0:58breaker. The shore is really defined as
1:00that coastline area towards the low tide
1:04shoreline. Further subdivided is the
1:07shore into the foreshore and the
1:09backshore. Take a look at the foreshore
1:11that is from the low tide shoreline to
1:13the high tide shoreline. And the
1:16backshore includes that burm area which
1:19is from the high tide shoreline to the
1:22coastline.
1:23So although we deal with the coast in
1:25this chapter, we'll really be talking
1:27about the from offshore point to the
1:31coast. So we'll be looking at beaches
1:33and the coast or the coastline. I do
1:37want to remind you something from our
1:38wave chapter. The three forms of waves
1:42that we have when they go from a long
1:45deep wave to a shallow wave. And this
1:49happens because they begin to fill the
1:51bottom as we said and they exert energy
1:54in the form of these three
1:55characteristic waveforms either
1:57spilling, plunging or surging. And as we
2:00said, spilling waves form on a gentle
2:03slope or flatter beaches. You can often
2:06see these if you're standing on the
2:07beach. You can see these gentle foaming
2:10white forms here coming into the uh
2:13coast.
2:16Plunging happens on more steeply sloped
2:18shores, as we said before, where there's
2:20a sudden slowing of the wave and that
2:22wave gets higher and it dissipates the
2:25energy in a curling function like this.
2:28Surfers love these types of waves,
2:30plunging waves. And the last type of
2:33waves is when you have the steepest
2:34shoreline and that wave form quickly
2:36exerts its energy and topples over in a
2:39surging fashion like this. I do also
2:42have to mention at this point because I
2:44didn't before a little bit about rip
2:45currents. As these waves come in and
2:48that wave energy comes into the beach
2:51area, all that energy has to be
2:55dissipated somewhere and all that water
2:57has to be dissipated. So imagine you're
2:59forming breakers here and breakers here.
3:02What we have is these areas of returning
3:04energy and returning water flow to the
3:07offshore area. These are known as rip
3:10currents. You might have heard them
3:12referred to as undertoe or rip tides,
3:15but these ocean phenomena are actually
3:17rip currents. Rip currents are narrow
3:20currents in the surf zone that move
3:22quickly away from shore. A typical rip
3:24current ranges from 50 to 100 ft wide
3:27and can extend 100 yard or more
3:29offshore. It can reach speeds of over 5
3:32mph. That's faster than an Olympic
3:35swimmer. That makes them dangerous and
3:37potentially deadly. And scientists want
3:40to learn more about them so we can
3:41better forecast when and where they will
3:43form and keep beachgoers safe. Here's
3:46what we know. Waves don't have to be
3:48huge for a rip current to form. 2 or 3
3:51feet are all it takes. And the weather
3:53doesn't have to be bad for a rip current
3:55to emerge. They often occur in the nice
3:57days after a storm. They're usually
4:00strongest near low tide, but can form at
4:02any time. Rip currents often form where
4:05sand bars are near the shore. They occur
4:07at breaks or channels in the bar.
4:09They're often difficult to see, but you
4:11can spot them in areas where waves
4:13aren't breaking or where there's foam,
4:15seaweed, or discolored water being
4:17pulled offshore. It's easier to see a
4:19rip current from higher up, such as from
4:21the beach access over dunes or a
4:23lifeguard's tower. Rip currents are a
4:25hazard for beachgoers, but by knowing
4:27the dangers and what to look for, you
4:29can avoid being caught in the grip of
4:32the rip. So, here we have an image of
4:34these rip currents in Chile. And we can
4:37see that they are easily sometimes
4:40spottable from the shoreline. And as
4:42that video said, higher up on the dunes,
4:44you can look at this area where we have
4:47the wave action clearly dissipating that
4:49energy in the form of waves spilling or
4:51plunging over. And then we have these
4:53areas that are seemingly quiescent. But
4:56if you put this into action, you can see
4:57that the water is actually flowing
4:59offshore in these areas called rips or
5:02these rip currents. A great way if you
5:05get stuck in one of these to get release
5:07from it is not instinctively to swim in
5:11this direction as you'll be swimming
5:13counter that current and that will tire
5:15you out. But swim perpendicular to the
5:18shoreline. So, if I'm caught in a rip
5:20and I'm heading this way, don't swim
5:22back to shore. Swim perpendicular to
5:26that current or that is to say parallel
5:29to the shore. You'll soon come out of
5:31that rip current and end up in the surf
5:34zone and that surf zone will then
5:36hopefully send you back to the beach.
5:38So, let's talk about beaches and what
5:40are beaches composed of? Well, beaches
5:42are composed of l locally sourced
5:45material, sediment. Uh this can be
5:47lethogous sediment or it can be
5:49biogenous sediment. So it's locally
5:52available material. It may be coarse or
5:54it may be fine. It depends upon wave
5:56action and the sources of these
5:58materials. If you're in an area that has
6:00a lot of cliffs, erosion from those
6:02cliffs could send large boulders to form
6:04your beach. If you have inputs of
6:07riverine water or river water, you could
6:09have sand or even mud from those
6:11delivering sediment and accumulating
6:14your beach. So the composition of your
6:15beach will be reflective of that
6:17riverine sediment or in some instances
6:20you could have significant biological
6:22material and this is often what you see
6:24in tropical beaches. So we'll take a
6:26look at some of those beaches in a
6:29moment. So here we have the uh beach in
6:32the Hawaiian islands and the Hawaiian
6:33islands are volcanically
6:36um active. So a lot of the volcanic
6:40material will end up being eroded and it
6:42will be comprised of the beach. So a lot
6:44of these are black beaches because
6:46they're lava or volcanic material. You
6:49can see in this beach that we have
6:51significant shells that is because of
6:54the wave action delivering calccarious
6:57or solicious cells uh uh shells to the
7:00beach. So it's biogenous or biogenic
7:03material here. Here we don't have the
7:05open ocean but we have a lake here in
7:08Ontario. And we can see that largely
7:10what is composing of the beach is large
7:14boulders and rocks. Here remember the
7:15Wentworth scale. These are large
7:17particle sizes. Here we have an
7:19indication of uh the adjacent land uh
7:24biological impacts here. And these are
7:26coral reefs off the shore um in the
7:28Maldes. And those coral reefs can break
7:30off through wave action and end up being
7:33composed of or compositing the beach. So
7:36let's talk about sand movement along the
7:38beach. And in two parts, sand is moved
7:42thanks to this swash and backwash which
7:44we'll describe and also thanks to this
7:46thing called longshore current. So swash
7:49swash and back law backwash are
7:52perpendicular to the shoreline. So it's
7:55toward and away the shoreline. Long
7:57shore current is parallel moving
8:00parallel to the shoreline. Let's zoom in
8:02more to look at these transport
8:04properties. So let's start off with the
8:06swash and the backwash. After a wave
8:08breaks, there's an uprush of water.
8:10That's called a swash on a beach. The
8:12sediment is moved towards the land.
8:15However, that sediment and water then
8:17returns to the open ocean. The sediment
8:20is moved away from the shore. So in a
8:22swash, you're delivering water and
8:24sediment to the beach. In a backwash,
8:26you are taking it away. In lightwave
8:29activity, the swash component dominates
8:32and this delivers sediment towards the
8:34shore, building up your seashore.
8:36Usually beaches are wider here. Swash
8:40versus backwash dominates when you have
8:42fair weather. In the northern
8:44hemisphere, fair weather is usually
8:45summertime beach. You don't get as large
8:47waves as you do in the winter. This is
8:49where your swash dominates and you're
8:51building up, so to speak, the beach. So
8:54here we have an image from California.
8:57Uh this would be summertime in the
8:58northern hemisphere here. And we
9:01have low energy waves delivering some of
9:05that offshore sand and sediment to the
9:07beach, but it's building it up. Those
9:09waves are not high enough energy to be
9:11eroding them or backwashing that
9:13sediment. So the swash is dominating,
9:16building up quite a nice large beach
9:18here. However, what happens is in
9:21wintertime we get the backwash that's
9:23dominant, heavy wave activity. We've got
9:26sediment moving away from that beach or
9:28the shore. We get a narrower beach and
9:30the composition of that beach changes
9:32often radically as we'll see. Sand forms
9:35offshore in these things called sand
9:37bars. As I said, this is a wintertime
9:39beach because winter in the northern
9:41hemisphere is usually stormier weather
9:43and it creates more waves of higher
9:46energy. So, same beach here and we see
9:49that backwash dominating and completely
9:52removing the finer grain sediment,
9:54leaving behind the larger grained uh
9:57boulders and rocks here. So, the beach
9:59looks entirely different here. What has
10:01happened to all that finer grain sand?
10:04Well, it's been backwashed offshore
10:06creating this sandbar. This is an annual
10:09cycle going from a summertime beach
10:11which is large to a wintertime beach
10:13which is narrower and changes the
10:16composition often. Uh but it's it's an
10:19annual cycle. So it goes back and forth
10:21predominantly produced by either low
10:24wave action or high wave action. So
10:27another schematic here in summertime
10:29when we have calm weather we get that
10:31sand being stored on the beach versus in
10:33wintertime that sand is not totally
10:36gone. it is actually being stored or
10:39reposited uh offshore. Again, it's an
10:42annual cycle that flip-flops between
10:44storage offshore to storage onshore.
10:46Next time you go to the beach in the
10:48winter and the same beach in the
10:49summertime, notice the difference. So,
10:52as we said before, waves do not come in
10:54parallel to the shore. Not very often.
10:57More often than not, they do not. So,
10:59they come in at an angle. And we know
11:01what happens to the wave as it
11:03approaches or feels the bottom. here
11:05that both water and sediment particles
11:07are coming in at an angle. So that wash
11:09is coming in at an angle, but the
11:11backwash is directly back to the ocean.
11:15So we get this zigzag motion of both
11:17water and particles. Again, you can see
11:19this undulation often when you're on the
11:22beach that sets up this net long shore
11:25current. It's depicted here going this
11:27way because our zigzag is going this
11:30way, but it could go in opposite
11:31direction. That sets up a net movement
11:34of particles and water slightly offshore
11:37in what's called a longshore current or
11:39a longshore drift. Let's take a look at
11:41that zigzagging motion in this short
11:44little animation. So, we come in with a
11:46wave not perpendicular uh sorry, not
11:48parallel to the shore, but at an angle
11:51that moves both water and that little
11:52white particle there of sand coming in
11:55with a swash that is at an angle to the
11:59shore. But that backwash went directly
12:01back. So that was a linear
12:04bypass in the direction of the
12:08offshore. The net movement here, as
12:10you'll see, this is zigzagging motion.
12:12But again, the net movement is this
12:14longshore drift or longshore current. So
12:17although we saw that zigzag motion in
12:19the surf zone, the wave refraction
12:22causes water and sa sand to move
12:25parallel to the shore. So this is called
12:27longshore current or longshore transport
12:31and it moves water and sediment along
12:34parallel to that shore. Longshore
12:36transport is responsible for delivering
12:39millions of tons of sediment either
12:42removing it from beaches or piling them
12:45up in certain areas of a beach. So the
12:47direction of this transport changes due
12:49to wave approach. But in general along
12:52the Atlantic and the Pacific coast, it
12:55is a southward movement. And that's
12:58because the wave action again is not
12:59coming in parallel to the shore. It's
13:01coming in at an angle. And the net
13:03movement will be in a certain direction,
13:06usually southward. But if you go to the
13:08beach and you see it moving to the
13:10north, don't think that's strange. It
13:12can happen. The best way to look at
13:15longshore transport or longshore drift
13:18is to stand on a beach and throw an
13:20orange or a tennis ball, something that
13:22floats into the offshore area and you
13:25can see that movement of the floatable
13:27device moving. You can actually even if
13:30you know the distance, record how long
13:32it takes to move a certain distance and
13:34record the speed of that long shore
13:36current. Again, it's water moving but
13:38also what you don't see is sediment
13:40moving along with it. So if your wave
13:44direction is as such and you get a long
13:46shore current moving this way, it's
13:48scouring the beach and eventually
13:51uh narrowing that beach here but moving
13:54sediment in that direction. So let's
13:56talk about two different shorelines. One
13:58is called an erosional shoreline and
14:00one's called a depositional shoreline.
14:02For erosional shorelines, think of the
14:05west coast of the United States. We have
14:07a cliffy environment that's tectonically
14:10active. There are a whole bunch of
14:11features as we'll see that have weird
14:13names sometimes such as headlands, wave
14:16cuts, cliffs with sea caves, sea arches
14:19or sea stacks and you have a marine
14:21terrace. The wave erosion increases in
14:25these areas. So you get high wave action
14:28scouring the coastline. Again, you get
14:31these sometimes unique features here.
14:33Think of California or Oregon or
14:36Washington coastline where you do have
14:38at these beaches depositional areas of
14:40sand, but mostly because of the wave
14:42action here, you get a lot of erosional
14:46activity eroding that land and those
14:48cliffs creating sometimes sea arches or
14:52this was once land removing or eroding
14:55that land and creating a sea stack like
14:57this. So unique features. The opposite
15:00to that is depositional shorelines. And
15:02as the name implies, we're depositing
15:04stuff. So these are primarily dep
15:07deposition areas and they're depositing
15:10them by that longshore drift. I said it
15:13removes some of the beach uh sand, but
15:15it also piles it up in different areas.
15:18So here we have the east coast of the
15:19United States. And here the long shore
15:21drift is going from south to north. We
15:23just said it's often southerntherly.
15:25Here it's northerly. Doesn't matter. But
15:27here are some of the features. When you
15:28have depositional shorelines, you got
15:31get a lot of deposition of beach
15:33material. So you create these barrier
15:35islands, these essentially large sand
15:38bars that are separated from the
15:40mainland by a
15:42lagoon. You have an area if that bay
15:45barrier
15:46completely crosses the entire inbaitment
15:49here of the land, you get a bay barrier.
15:52So it's a barrier to that bay. It's no
15:54longer a bay, but it's providing a
15:56barrier. If there's a partial barrier,
15:58it's called the spit. Here you have an
16:00offshore island, but it's connected by
16:02sand. That's called this weird term,
16:05tambalo. And we'll also talk about
16:07deltas. Deltas are areas where river
16:09water meets ocean water in this very
16:12small area, but they uh create these
16:14almost dendritic like nerve- like uh
16:18beach deposits, and those are called
16:21deltas. As I said, the east coast of the
16:23United States is a great location to see
16:25some of these features. Here we see a
16:28bay barrier, but you can also see that
16:30we have a channel here that is a
16:33human-made channel. This would have been
16:35a complete barrier, but somebody has
16:37channelized that probably to get boats
16:39in here for navigation and recreational
16:42purposes. Not a total coverage, but a
16:45part coverage of the inbaitment, and
16:47that's called a spit. Great examples of
16:50barrier islands. You can go to New
16:52Jersey. You can go to Virginia,
16:55Maryland, and this one is from the
16:57Maryland coast. This is Asetique Island.
16:59This is a buildup of sand creating this
17:02barrier island. And here we have the
17:04lagoon. And here we have the land. Here
17:07we have a picture of Ombello, which is
17:09an island, an offshore island, but it's
17:11connected via depositional material,
17:14beach material that has formed here from
17:16these deposition or depositing areas.
17:19So, let's spend a little time discussing
17:21barrier islands because they're so
17:23important. They are long, narrow
17:25offshore deposits that are parallel to
17:27the shore. As we'll see in the video,
17:29they they were de developed geologic
17:32time scales ago. They're very common on
17:34the east coast of the United States and
17:36also the Gulf Coast and they're highly
17:39instructive to protecting mainland from
17:42highwave action such as when you have a
17:45tropical storm or a hurricane. So, some
17:48of the features of the barrier island
17:49are here. Um, I just want to set the
17:52stage for a video that I'm going to play
17:54out that says that barrier islands are
17:57constantly moving. They're constantly
17:58moving towards the land. So here's the
18:00ocean, the island, that lagoon. What we
18:03don't have here is the mainland. So the
18:05video is going to show you why these
18:07things are moving towards the land. You
18:10see this pete bed? Pete is high highly
18:13rich enriched in organic matter. It was
18:15once living material, usually
18:17vegetative, and really indicative
18:20indicative of salt marsh communities.
18:23But why is this strewn all along here?
18:25Well, we're going to play out the video,
18:27and the last part of that gets to it.
18:28So, let's do it. As I said, when you're
18:31watching this video, look for the
18:32indications or evidence that barrier
18:35islands are constantly moving to the
18:37land. And part of that has something to
18:39do with the movement of the top part
18:42here and the non-movement of that pete
18:45layer. So, this is great. We need an
18:47animation here. Let's watch it. So, what
18:50are barrier islands? They're elongate
18:53islands that parallel the coast.
18:55generally very long but not very wide.
18:58They mostly occur on shallow trailing
19:01edge shelves like the Atlantic coast of
19:03North America. They're separated by
19:06inlets and they're separated from the
19:09mainland by bays, lagoons, or
19:12marshes. How do barrier islands form?
19:16Well, the most accepted theory is called
19:18the submerge beach ridge theory. Sea
19:22level was much lower about 85 meters
19:26lower in the pleaene than today. During
19:29the pleaene there were large glaciers
19:32and because there were large glaciers on
19:33the continent sea level was lower. Well,
19:36as the ice started to melt, sea level
19:39started to rise and there was perhaps
19:42beach ridges along the shoreline. But as
19:46sea level rose, some of those beach
19:48ridges were drowned or became separated
19:51from the mainland by a lagoon. A rapid
19:54rise in sea level would leave that sand
19:57out in the ocean as a ridge and that
20:00would become a barrier island. Then over
20:03time, as sea level continued to rise,
20:07the barrier island moved landward with
20:10the rising sea level. We'll talk about
20:13the process that allows that to occur in
20:16some of the following slides. These are
20:19the major environments on a barrier
20:22island. First on the right, you have the
20:27shoreface sediments. These are the beach
20:29sediments. You have ridges and the beach
20:33berm. Think of that as basically the
20:36beach. Then you get to the dunes, sand
20:40dunes.
20:41Sometimes in many places these sand
20:43dunes are cut by what are called
20:45overwash fans. You can see an example of
20:47one right there. He's going to talk to
20:50that in a moment about that being a
20:52critical process in the formation of
20:56higher sand dunes, but also in the
20:58movement of this barrier island. This is
21:01exactly the depiction that I had except
21:04it's reversed by the way. So over here
21:07you have the land. Over here you have
21:10the open ocean. So it's just been
21:12rotated by 180. Let's play a little bit
21:14more out and then we'll eventually get
21:17to this landward migration
21:21process. The island may also contain
21:24forests or grassland just depends on the
21:27type of island. The back part of the
21:29island consists of
21:31marsh and then you may even have a tidal
21:33flat and then a lagoon. So those are
21:36these are the major settings or
21:38depositional environments on a barrier
21:41island. Here's a picture of a beach in
21:45Louisiana. It's composed of sand.
21:48There's symmetrical ripples and a group
21:50of students right here digging a trench
21:52on the
21:53beach. The dunes generally cross-bedded
21:58sand only flooded occasionally during
22:01large storms. sometimes a sandy soil.
22:05You have some plants that occur in the
22:07dunes and they're actually very
22:08important plants um because they
22:11stabilize the dunes. So, you shouldn't
22:13be walking on those plants because it
22:15will destabilize them and destroy the
22:18dunes and we don't want to destroy
22:20dunes. The salt marsh, the back part of
22:23the barrier island, very mucky organic
22:27rich sediment, sometimes very shallow
22:30water. Get a lot of crabs and oysters
22:33and things like that. Various types of
22:35grasses, cord grass, sawrass. You get
22:38muscles and clams,
22:41birds. All that organic matter in the
22:43salt marsh will eventually die and it
22:46will form part of a sediment record that
22:49will be buried. Highly enriched organic
22:52matter from a vegetative source. So when
22:54we talk about this pete layer, it's
22:56going to crop up in our depiction that
22:58we just saw and it's going to crop up
22:59momentarily. It's that stuff and it's
23:02formed on that lagoon side. Okay? But it
23:06provides us evidence for landward
23:08migration of the barrier islands. The
23:11rest of this I am going to leave to you.
23:14The rest of this middle you can watch.
23:16It goes into wave action. The different
23:19types of waves as deep waves approach
23:22the shore. We discussed that in the wave
23:24chapter. Long shore drift which we just
23:26introduced important in sediment
23:28transport a bit on tides and tidal
23:31influence. But then I'm going to come to
23:33this last part which is the migration of
23:36the barrier islands. Let's play it out.
23:39Well, because of longshore drift, sand
23:42moves along barrier islands. Inlets form
23:46because tides have to come in and go
23:49out. Here's a picture of an inlet that
23:52formed after Isabel, which was a large
23:55storm on the North Carolina
23:58coast. Inlets also migrate, and they
24:01migrate because of longshore drift. As
24:05you can see in this diagram, the long
24:07shore drift is in one direction, and it
24:09adds sediment to one end of the inlet.
24:12Well, the tides still need to come in
24:14and out. So, the tides want to maintain
24:16the size of that inlet. So, what do they
24:18do? They erode on the downstream side of
24:23the inlet. So the inlet actually
24:27migrates. You probably heard stories of
24:30after storms, these inlets either
24:32disappearing or appearing, islands, new
24:35islands forming uh in these barrier
24:37islands, which happens all the time. So
24:40take-home message here, they're in
24:42complete and natural movement all the
24:44time.
24:45The bummer is when you build upon these
24:47either a bridge connecting the inlet
24:50from one side to the other side or beach
24:52homes and now you have this migration or
24:57towards land uh movement. That's where
25:00it becomes a problem because of the long
25:02shore
25:03drift. A third process that operates on
25:06barrier island is the summer winter
25:08change that occurs in the
25:10beach. During the winter, there's more
25:13wave energy and the beachers are smaller
25:16and the sand is stored offshore in sand
25:19bars. But during the summer, there's
25:22less wave energy and more regular wave
25:24energy and sand is added to the beaches
25:27and the beaches are wider. What happens
25:30is that sand bars migrate onto the beach
25:34and make the beach wider. So that's the
25:37change between summer and winter. The
25:39fourth process is a very important one.
25:42It's called overwash
25:44fans. And you can see some overwash fans
25:47here in that top photograph. You see
25:50those fan shaped bodies that extend from
25:53the beach to the back part of the
25:54barrier island. And in the diagram
25:57below, you can see a schematic diagram
26:00of what one of these overwash fans looks
26:03like. These, as I said, are a very
26:06important process.
26:08They occur during
26:10storms and what happens is that sand is
26:13moved from the beach to the back part of
26:16the barrier island and you deposit the
26:18sand as a tongue or a fan of sand. The
26:22reason these are important is that they
26:24move islands landward. As sea level
26:28rises, that causes the islands to move
26:32landward. This is a cross-section of a
26:35barrier island. We call it a
26:37transgressive barrier island because
26:39it's migrating landward. You can see
26:42here the beach, the dunes, and then the
26:46lagoon and
26:48marsh. And notice that the beach part
26:51and the dunes part are migrating over
26:54the grassland and the marsh. You can
26:57sort of think of it like a tank tread is
27:00that the sand is keep being pushed from
27:02the beach to the back part of the
27:04barrier island and move the whole island
27:07landward. All right? So remember that
27:09pete area in the marshy area. So this is
27:12all pete sediment. But as you smear that
27:15layer of sand over, you're going to
27:17cover some of that pete. But eventually
27:20cuz this migration this part of the
27:22beach at one point of time is going to
27:24be the beach is going to be here. It's
27:27going to be here. The pete deposit is
27:29not changing. So eventually if you're on
27:32a beach and the evidence of movement is
27:35this that you can dig down and find that
27:37pete area that was once the organically
27:42rich lagoon area now being evidenced to
27:45support this migration of the barrier
27:48island. So again we can see it here that
27:50the older pete deposits are now found on
27:52the ocean beach. You can dig down and
27:55sometimes you can find that. This is
27:57also cool too. If you're on a beach, you
27:59might find Pete deposits if you dig down
28:02far enough. But you could find this. And
28:04these are tree stumps. These are tree
28:06stumps that were growing on the high
28:08dune or near the marsh. And because of
28:11that landward movement, now the sand
28:14being moved landward, but now it exposes
28:17that once high dune area. Another way to
28:20build up a coastline is through delivery
28:22of sediments by way of uh rivers. So
28:26riverine sediments can be delivered to
28:28the ocean. They accumulate there in what
28:30we call a delta. So there's a great
28:33example of a very large delta called the
28:35Mississippi delta in the Gulf of Mexico.
28:38Here we see a different type of delta in
28:40North Africa, the Nile River Delta, but
28:42it delivers a lot of that lethogenous
28:44sediment to the coastal area. So three
28:47different types of coast geographically
28:50in the United States. We have the
28:51Atlantic coast, the Gulf Coast, and also
28:53the Pacific coast. What are some common
28:56characteristics? The Atlantic coast is
28:59open to some wave attack, but we also
29:01see barrier islands are very common and
29:04dominate the landscape. Sea level is
29:07occurring here. Sea level rise is
29:08occurring here at around.3 m per century
29:12around 1 ft. Uh we saw that drown river
29:15valleys are common. So this goes back
29:17geological time scales when we had lower
29:19sea levels, rising sea levels because of
29:22glaciers melting.
29:24drowned those river
29:27valleys. The Atlantic coast, as I said,
29:29could be dotted with barrier islands.
29:31And we see that here's us in the
29:33Delaware Bay, the upper part of the
29:35Delaware River, Delaware Bay. And here
29:37we have a whole bunch of barrier
29:39islands. So, the closest one, take a
29:41drive from Philadelphia to the shore.
29:43You can get there in about an hour. And
29:44you can see these barrier islands as you
29:46cross over them on these
29:48bridges. the Gulf Coast of Mexico, low
29:52tidal range, generally low wave action
29:55unless there's a storm. There's a
29:57tectonic subsidance happening below. And
30:00we have sediments that are being
30:02delivered to this area dominating by the
30:05Mississippi River. The Pacific coast, we
30:08said, is tectonically active. It has a
30:11high exposure to high energy waves. So,
30:13this is an erosional shoreline, not a
30:16depositional uh shoreline. and we have
30:18fairly high rates of erosion. These are
30:21uh 0.005 meters, but this is not per
30:25decade, it's per year. I'd like to shift
30:27the attention to a certain
30:29characteristic feature of coastlines,
30:32and these are special areas called
30:33estuaries. And estuaries are the meeting
30:35place of river water and seawater in a
30:38semi-encclosed area. So, we have four
30:41types of estuaries that are based on
30:43origin. Let's go through them.
30:46First, we have a river system that's
30:48meeting the sea system in a
30:50semi-encclosed area. Here's a great
30:52example, the Chesapeake Bay, where
30:54there's several river inputs to it
30:56delivering fresh water. Open to the open
30:59ocean. Here's the open ocean. This
31:01semi-encclosed body of water is known as
31:03an estuary. It will have mixed salinity.
31:06Up here, it may be fresh water. Here,
31:09brackish water, a mix between fresh
31:11water and salt water. and open ocean
31:13back to salt water at 35 parts per
31:16thousand. This is called a coastal plane
31:18estuary. The sea level has ridden and
31:20flooded what once was a river valley.
31:24Um, and this is showing the Delaware
31:26estuary right here and the Chesig
31:28eststerary, which is the largest estuary
31:29in the United
31:31States. When barrier islands form, they
31:34semi enclose an area called a lagoon.
31:37And there may be some freshwater inputs
31:39by rivers here. So here's a LANCAT image
31:42of the Outer Banks. So the barrier
31:43islands here, this is all lagoon water,
31:47semi-encclosed because in parts it is
31:49open to the open ocean. So you have
31:50seawater mixing with the riverine
31:53freshwater. That is another definition
31:56of an estuary. But here we call it a
31:58barbuilt estuary.
32:01Another way that you can build an
32:03estuary based on origin is if you have a
32:05very deep valley here and a river source
32:09meeting the open source here. This is
32:11called a fior estuary. Very deep
32:14estuary. So seawater is on this side.
32:17Fresh water coming in and again it's a
32:19semi-encclosed area or an area where
32:22fresh water meets saltwater.
32:25Lastly, the type great example here of
32:28the San Francisco estuary, which is this
32:30area here where the salt water interacts
32:33with riverine water in a plate
32:35tectonically active area. So here we
32:38have tectonic San Andreas fault forming.
32:41So through subsidance, we're forming
32:43this geological semi-encclosed system
32:46here where again seawater is interacting
32:48with freshwater semi-encclosed body of
32:50water. Freshwater seawater interaction
32:53definition of estuary. This time it's
32:55called a tectonic estuary. Estuaries can
32:59also be based not on origin but be can
33:02be named on the mixing patterns of the
33:04fresh water and the salt water. So we
33:07have a well-mixed estuary here. So in
33:10each of these we have the river source
33:12coming here and the seawater source
33:14being delivered from the open ocean over
33:16here. And just envision this a
33:18semi-encclosed body of water called an
33:20estuary. Here, if you were to park a
33:23boat here and then travel to the mouth
33:27of the estuary and take salinity
33:29readings, you would see salinity
33:31increasing from around five parts per
33:33thousand or close to zero parts per
33:35thousand freshwater to 35 parts per
33:38thousand. Right? And if you were to take
33:40a salinometer, which measures salinity,
33:43and drop it from your boat down here, do
33:45a vertical profile instead of a
33:48horizontal, a vertical profile, the
33:50salinity would change not very
33:53much. This is called a well-mixed
33:55estuary. It happens in a shallow basin
33:58um where it allows for complete mixing
34:00of fresh and sea water from top to
34:03bottom. Here you kind of look at the
34:05same thing, but we have a deeper estuary
34:08and this is called a slightly stratified
34:10estuary. Again, if you're traveling from
34:13the fresh water to the open ocean,
34:14naturally you're going to increase
34:16salinity. But look at this. The dense
34:18seawater is undercutting the freshwater.
34:21So, it's a layered system or stratified
34:24system, but only slightly. So, it's
34:26undercutting that dense seawater,
34:28undercutting the fresh water. So if you
34:31were to park your boat right here and do
34:33a salinometer, you would go from uh
34:36let's say 10 parts per thousand to 15 to
34:3925. You're doing a vertical profile.
34:42Salinity changes quite rapidly. It
34:45didn't do that before in the previous
34:48example. Here we have that's slightly
34:51stratified, but it's really stratified.
34:54This is called a salt wedge. wedge
34:56because that salt water much more dense
34:58than fresh water is wedging itself
35:00underneath that upper part which is the
35:02fresh water less dense. So again let's
35:05park our boat right here and take a
35:07depth profile starting at around 1015
35:10and then you rapidly go to uh a rapid
35:14change in salinity. Um that would be
35:17indicative of you have a salt wedge
35:19estuary. Again, these are strong river
35:22outflows when the river is flowing quite
35:25quickly here and uh a relatively deep um
35:30basin. The last one is again this is
35:34almost like the salt wedge on steroids.
35:37When you get a super wedge here, when
35:40that river flow is very strong, it's
35:42just smearing that fresh water on top.
35:45You're creating this two-layer system
35:47that we talked about in the open ocean
35:49that less dense water on top of more
35:51dense. This is really happening here. So
35:54these are four types of estuaries not
35:56based on their origin. They're based on
35:58their mixing. These are somewhat
36:00complicated. I just want you to get a
36:02vibe for the four types and begin to
36:04understand like the mixing of fresh
36:06water and salt water and the different
36:08permutations you can have. It gets
36:11complicated. It's based on the
36:12geomorphology of the basin here, water
36:15movement as well, the size of the
36:17estuary, the river flow, even rainfall
36:20as well. So, it gets complicated. We'll
36:23just leave it at that. As we said, a lot
36:25of people build on these coastal
36:27systems. How can we especially prevent
36:30barrier islands from being eroded or how
36:34can we prevent erosion from happening on
36:36the west coast as well? Here we have the
36:39ability to build hard stabilizations.
36:41These are structures built to decrease
36:43coastal erosion. We want to interfere
36:46with the sand movement, keep sand in
36:48place, build up a beach. There's various
36:50examples of ways to do hard
36:52stabilization. Groins and groin fields,
36:54jetties, breakwaters, and seaw
36:57walls. Here we have what's called a
36:59groin. If you have several of them, it's
37:01a groin field. You can see this on the
37:03coast of the Jersey Shore. You have a
37:05lot of these man-made structures.
37:07They're often either wood and rock or
37:09just rock and they're meant to trap that
37:13longshore uh transport of sediment in
37:17certain areas. They do it right before
37:19the groin, but you end up having
37:21erosional areas behind the groin. So,
37:24they're temporary structures or stopping
37:27mechanisms, but over large periods of
37:29time and certainly during storms, they
37:31don't work very well. You can see what
37:33happens here. Next time you go to the
37:35Jersey Shore, look, on one side you have
37:37a buildup of beach or sediment. On the
37:39other side you have an erosional uh
37:41zone. So depositional zone, erosion
37:44zone. So you're just trying to stop long
37:47shore transport, but it's not very
37:50effective. Another way to do it is
37:52through jetties. Jetties are like
37:53groins. They're humanmade structures,
37:55but they allow channelization of the
37:57barrier island. We saw this on a LANCAT
38:00image where we could see maybe boats
38:02coming in here. Yep. used for
38:04navigational purposes and again you get
38:06depending upon the longshore drift here.
38:08It's going this way. So you get this
38:11stops in part sediment being transport
38:13and it builds up here on this deposition
38:15zone but on the other side it's erosion.
38:18So here we have this
38:21channelization right and we see that
38:23buildup on one side. So longshore drift
38:25must be going this way from left to
38:29right. Here you have a erosional zone.
38:32You can have breakwaters which are
38:34basically like structures that try to
38:37break or just dissipate the energy of
38:39the waves. Again, you can form new
38:42deposition areas. They work in certain
38:44areas, but then in certain areas they
38:46are overcome by the energy of the waves
38:48or you get erosional
38:50areas. Here you have a series of these
38:53breakwaters as they're called, these
38:55man-made structures. And here it's been
38:57around. These are usually placed
38:59offshore, but these have been offshore
39:01so long that we see an erosional area in
39:04the middle and then a deposition area.
39:06They've actually uh formed these
39:08sandbars to that breakwater
39:12area. What about seaw walls? You can
39:14build seaw walls, but eventually they
39:16break. Here we have a seaw wall. Again,
39:18a natural uh sorry, a man-made structure
39:21used to dissipate that wave energy, but
39:24you get fractures in that such as this.
39:27So, minimally
39:29effective. I'm going to play this out
39:32momentarily, but I just want to say what
39:36are the alternatives to hard
39:37stabilization? You can relocate things
39:39like a lighthouse in North Carolina was
39:41relocated because of the loss of the
39:44beach and that barrier island migration.
39:46Or you can do a soft stabilization which
39:50is dune vegetation here. Planting dune
39:53grasses and shrubbery to stabilize that
39:56dune. These are effective to a degree
39:59but during large storms and certainly
40:01frequent storms they uh are not so
40:04effective either. That is chapter 10.
40:06We'll get back to that video in a
40:08moment.

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