>>Today we will discuss barrier islands. What
are barrier islands? They are thin, long islands
that are separated from the mainland by a
bay, lagoon, or marsh, and they are separated
from each other by tidal inlets. Major parts
of a barrier island include the beach, the
dunes, and the marsh. And there's several
processes that shape barrier islands. There
are four of these processes. One of the most
important ones is called longshore drift.
As you see in this figure, as waves come in
at an angle, they break and move up the beach.
But when the water moves back down, it's going
to follow the steepest path, so the water
and the sediment that's being moved by the
water follows a zigzag pattern. And so sand
and water move down the beach. That's called
the longshore drift. It's a very important
process. One consequence of longshore drift
is if you have a tidal inlet and the longshore
drift is, say, one direction, what's going
to happen? It's going to add sand to one side
of the tidal inlet, but the tidal inlet wants
to maintain its opening because the tides
have to go in and out. So you get erosion
on the other side of the tidal inlet. As a
result, tidal inlets can migrate. The third
process that operates on a barrier island
is the summer/winter change, which is caused
by changes in wave energy. In the summer there's
lower wave energy, and sand is added to the
beach. Sand bars move on shore and the beach
becomes wider. In the winter there's higher
wave energy, and the sand is eroded and stored
offshore. The fourth process is overwash fans,
and this occurs during large storms or hurricanes.
What happens is sand is moved from the beach
to the back part of the barrier island. It
usually shows up as a tongue of sand that
goes from the beach to back into the marsh
area, the back part of the barrier island.
It's a very important process because it moves
barrier islands landward. So what is the key
message about barrier islands? If there's
one thing you need to remember about them,
it's that the sand is moving, and that barrier
islands are dynamic. The sand is always moving
on a barrier island. What are the implications
of this movement of sand? Well, think about
going to a barrier island. If you have a house
on that barrier island, you probably don't
want to have the surf zone in the middle of
your house. So what you do? One of the things
we do is we build sea walls. And see this
picture of the sea wall at Galveston. Look
out in front of the sea wall. Do you want
to swim there? It wouldn't be very safe. Sea
walls protect cities and property, but they
don't produce very good beaches. And another
problem with sea walls is they concentrate
the wave energy, and little seawalls begat
bigger seawalls. In other words, they concentrate
it, they undermine the seawall so you have
to build a bigger seawall. Another thing people
will commonly do on beaches is to build what
are called groins. Now groins are a wood structure,
it may be a three or four feet high that extends
perpendicular out into the surf zone from
the beach, usually in front of your house.
Why do you do that? Well, it's a dam, if you
will, to trap the longshore drift. And it
works. It will trap the longshore drift, but
the problem is you will end up having a wider
beach in front of your house, but what about
your neighbor which is just down current in
terms of longshore drift? They have less beach.
They have to build a groin too. So once you
start building groins, they extend all the
way down the beach. Another thing we commonly
do along beaches is called beach replenishment.
It's where we add sand to the beach, every
year sometimes. This is very expensive, and
the sand is commonly gone in less than four
years, so it's not clear it is a long-term
solution. I love going to barrier islands,
but what do we do about barrier islands? We
will debate this later.
