>>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.
