>>Dr. Doug Elmore: This will be the summary for barrier islands. So what are barrier islands?
They're elongate islands that parallel the coast. Generally very long, but not very wide.
They mostly occur on shallow trailing edge shelves like the Atlantic Coast of North America.
They're separated by inlets, and they're separated from the mainland by bays, lagoons, or marshes.
How do barrier islands form? Well, the most accepted theory is called the submerge beach
ridge theory. Sea level was much lower, about 85 meters lower in the Pleistocene than today.
During the Pleistocene there were large glaciers, and because there were large glaciers in the
continents sea level was lower. Well as the ice started to melt, sea level started to
rise. And there were perhaps beach ridges along the shore line, but as sea level rose
some of those beach ridges were drowned or became separated from the mainland by a lagoon.
A rapid rise in sea level would leave that sand out in the ocean as a ridge. And that
would become a barrier island. Then over time as sea level continued to rise, the barrier
island move landward with the rising sea level. We'll talk about the process that allows that
to occur in some of the following slides.
These are the major environments on a barrier island. First on the right you have the shore
face sediments. These are the beach sediments. You have ridges, and the beach berm. Think
of that as basically the beach. Then you get to the dunes, sand dunes. Sometimes in many
places these sand dunes are cut by what are called overwash fans. You can see an example
of one right there. The island may also contain forests or grassland, just depends on the
type of island. The back part of the island consists of marsh, and then you may even have
a tidal flat, and then a lagoon. So those are, these are the major settings or depth
of shore environment on a barrier island.
Here's a picture of a beach in Louisiana. It's composed of sand, there's symmetrical
ripples, and a group of students right here digging a trench on the beach. The dunes:
generally cross-bedded sand, only flooded occasionally during large storms. Sometimes
a sandy soil. You have some plants that occur in the dunes, and those are actually very
important plants because they stabilize the dunes. So you shouldn't be walking on those
plants because it will destabilize them and destroy the dunes. And we don't want to destroy
dunes. The salt marsh, the back part of the barrier island. A very mucky, organic rich
sediment, sometimes very shallow water. You get a lot of crabs and oysters and things
like that. Various types of grasses: cordgrass, sawgrass. You get mussels, some clams, birds.
Okay, what are the major processes that shape barrier islands? And this is the real guts
of what I want to talk about in terms of barrier islands, and there are four major processes.
Longshore drift, inlet migration, the summer-winter change in the beach, and overwash fans. All
of these are very important. So, before we talk about longshore drift, we need to talk
a little bit about waves. So what are waves? A lot of students think they are movement
of current through the water. They're not. They're the movement of energy through the
water. Not a current of water. They're generated by wind, the size of waves are controlled
by the strength of the wind, the fetch, the distance that the wind acts on the water.
And there's a theory we have called the symmetrical oscillatory wave theory that explains how
waves move and form. And there are various variables that are important in understanding
waves. These include the celerity or the speed of the wave, the wave length the wave height.
You also have the period, which is the time for two crests to pass one point. And you
have the depth of water. These are the important variables.
Well, the symmetrical oscillatory wave theory, in this theory the water particles oscillate
in circular orbits, as this little diagram shows you. The water moves but it follows
circular orbits as the waves move. For deep water waves, orbital diameter decreases downward.
For depths less than one half the wave length, there's little disturbance of the bottom sediment.
However, when the depth becomes very shallow you get into what are called shallow water
waves. And the waves, we say, start to feel the bottom. They start to disturb the sediment
at the bottom. You get a decrease in the wave length, decrease in the velocity of the wave,
you get an increase in the wave height, and when you increase the wave height, eventually
what's going to happen? The wave's going to break. The orbits change from circular to
elliptical, but the period stays the same. So that's what a shallow wave may look like.
You get more elliptical orbits.
Well, longshore drift comes about after the waves break. So let's assume that the waves
are coming into a beach at an angle, as this diagram shows you. You can see the arrows
there. So the waves come in, they break and they move up the beach. But then the water
wants to go back, it's called the backwash. Is it going to follow the same path that it
did for coming in? No, it's going to follow the steepest and shortest path and that's
going to be straight down the beach. So imagine one grain of sand or one particle of water.
It comes in at an angle, it moves back straight down, then it's picked up by the next wave,
moves down the beach. This produces what's called the longshore drift or the movement
of sand along the beach. It's a very important process because it's very common on almost
all beaches.
To understand tidal inlet migration, which is another process that operates on barrier
islands, we need to understand something about tides. What causes the tides? Well, it's the
gravitational attraction of the moon and the sun. When the gravitational traction of the
moon and sun are combined you get the spring tides, neap tides when they're opposed. The
flood tide is when the tide comes in; the ebb tide is when it goes out. Here's a picture
of what's called a herringbone cross stratification. Where you see, right there is a pattern of
flow in one direction, then flow in the opposite direction. Bimodal flow during the ebb and
flood, that's indicative of tides. That's from an ancient rock-- about 300 million years
old.
Well, because of longshore drift, sand moves along barrier islands. Inlets form because
tides have to come in and go out. Here's a picture of an inlet that formed after Isabel,
which was a large storm...on the North Carolina coast. Inlets also migrate and they migrate
because of longshore drift. As you can see in this diagram, the longshore drift is in
one direction and it adds sediment to one end of the inlet. Well, the tides still need
to come in and out. So the tides want to maintain the size of that moonlet, so what do they
do? They erode on the downstream side of the inlet. So the inlet actually migrates because
of the longshore drift.
The third process that operates on barrier islands is the summer-winter change that occurs
in the beach. During the winter, there's more wave energy and the beaches are smaller and
the sand is stored offshore in sandbars. But during the summer, there's less wave energy
and more regular wave energy, and sand is added to the beaches and the beaches are wider.
What happens is that sandbars migrate onto the beach and make the beach wider. So that's
the change between summer and winter.
The fourth process is a very important one. It's called overwash fans. And you can see
some overwash fans here in that top photograph. You see those fan shaped bodies that extend
from the beach to the back part of the barrier island. And in the diagram below you can see
a schematic diagram of what one of these overwash fans looks like. These, as I said, are a very
important process. They occur during storms and what happens is that sand is moved from
the beach to the back part of the barrier island. And you deposit the sand as a tongue
or a fan of sand. The reason these are important is that they move islands landward. As sea
level rises, that causes the islands to move landward. This is a cross section of a barrier
island. We call it a transgressive barrier island because it's migrating landward. You
can see here the beach, the dunes, and then the lagoon and marsh. And notice that the
beach part and the dunes part are migrating over the grassland and the marsh. You can
sort of think of it like a tank tread-- is that the sand is keep being pushed from the
beach to the back part of the barrier island and move the whole island landward.