Physical Geology – Barrier Islands – Vitrual Field Trip


>>Dr. Doug Elmore: Let’s take a virtual field
trip to several barrier islands. Here’s a picture of a beach in Louisiana. It’s composed
of sand, there’s symmetrical ripples, and there’s seaward-dipping planar laminae, which
I’ll talk about more in the next slide. It’s of course it’s covered daily, twice daily,
with saltwater. And it’s inhabited by burrowers, crabs, worms, and birds, and a group of students
right here digging a trench on the beach. Here’s that trench they dug, and what you
can see here is there’s some faint laminae, you can see some lighter and darker sediment.
And these are seaward-dipping laminae, dipping toward us, toward you, which is where the
ocean is. And you also see some shell layers, which are probably a storm deposit at the
base of the trench. You also find burrows on the beach, and here are a couple examples
of burrows. The salt marsh — the back part of the barrier island. Here’s a picture of
a little hole we dug in lagoonal mud. It was very dark, organic rich, and it smelled like
rotten eggs. You also find a lot of fecal pellets from crabs and other organisms on
the marsh area. And all these little round things there are fecal pellets. Here’s an
army of crabs in the marsh. And here’s some ripple marks with fecal pellets in the troughs
of the ripples. I just happen to like that photograph. Here’s some pictures of Louisiana.
There’s New Orleans, we’re going to be down here to the southwest of New Orleans. And
what we’ve got in the lower picture are these islands. The red shows what they were like
in 1887, and the green shows what they were like in 1988. Notice the difference. Well
one, they’ve moved landward, and two, there’s been significant land loss. This is a picture
from Trinity Island on the Mississippi Delta, there are two pictures here. You see so the
upper one has some students standing on the beach. And you’ll notice it’s not a very nice
beach, there’s a lot of mud there. And if you look closely at the picture on the right,
you’ll see there are a lot of plants. That is actually marsh material—material that
formed in the marsh and it’s now on the beach. Why? Because the barrier island has moved
over its own marsh, it’s migrating landward, leaving the marsh material behind so it’s
eroded up on the beach. So at one point in time in that upper photograph, the barrier
island was out where the waves are breaking right now. But the whole island has moved
landward. And this is a picture of an overwash fan on Trinity Island. The island is in the
background there, and that lobe of sand there came from the beach and was deposited on the
back part of the barrier island, an overwash fan. And then there’s a student for scale. If there’s one point you need to remember
about barrier islands, it’s that the sand is moving in a number of different ways. It’s
moving along the shoreline, and it’s moving perpendicular to the shoreline. And this picture
here illustrates that. This is the end of the seawall on Galveston Island. There’s a
large seawall that protects the city of Galveston in Texas. They need that seawall otherwise
the city would be in serious shape. Well, eventually that seawall stops, and you’re
looking at the place where the seawall has stopped. Notice what’s happened to the island
where there is no seawall. It has moved landward compared to the part of the island that’s
protected by the seawall. You can also notice out in the water that the waves are breaking
in several different places: right along the shoreline, and then two other areas just offshore.
Those are those offshore bars that migrate landward during the summer and add sand to
the beach. So what do we do about this problem? If sea level is rising, and barrier islands
are moving landward, what do we try to do about that? Well, one thing we do, as illustrated
here, is to build a seawall. We also build breakwaters, groins, and use a process called
beach replenishment. This is another view of the Galveston Island seawall. You’ll notice
that they’ve put a lot of what’s called riprap out in front of the seawall to protect the
seawall. These large rocks reduce the wave energy and help protect the seawall. But there’s
a problem with seawalls. Seawalls concentrate wave energy, which undermines the wall and
results that you need to build a bigger seawall. And eventually they may have to—they will
have to do this on Galveston Island. Also notice that when you produce a seawall, you
don’t have much of a beach left. Do you want to go swimming off that particular seawall
right there? Probably not. There are other places along Galveston Island where you can
swim with a seawall, but in those areas they add sand to the beach every year, usually
out in front of the hotels. This is another picture from Galveston Island. It’s down the
coast. There’s no seawall there. But to protect houses in this subdivision, they’ve used what
are called geotubes, which are these large, black bags. They’re filled with sediment,
and they’re kind of a cheap seawall that might help protect the houses for a short amount
of time. One of the interesting things about this picture, it was taken in 2003, is that
they’re building a very nice house right next to the beach. I don’t think that’s a very
good idea [laughs] because the next big storm’s going to take that house out. This is a breakwater
from down in Louisiana. It’s that pile of rocks out there. It tends to reduce the wave
energy and can help protect the beach in terms of help reduce energy, the wave energy, and
reduce erosion. That’s on Grand Isle in Louisiana. We also build groins on beaches. Now groins
are generally wood structures, sometimes concrete structures, built perpendicular to the shoreline.
And they’re meant to trap the longshore drift. But there’s a problem with building groins.
Let’s say you have a house, they build a groin, they trap sand in front of their house. But
what happens down current? If there were a house just below that one house here, that
person’s got less beach. So what do they have to do? They have to build a groin too. You
notice up, in the upper part of the photograph many people have built groins because they
don’t have much of a choice. If they don’t have a groin, they won’t have much beach.
Another way we address beach erosion is beach replenishment. And this is a case where we
dredge sand, generally offshore, and pump it to the barrier island, the upstream part
of the barrier island, so the longshore drift will carry it down the barrier island. The
problem is this is very expensive, and studies have shown that most of the sand is gone within
two, and certainly four years. So it’s a very temporary solution. It’s not a long-term solution. So here are some pictures of what can happen
to houses on a barrier island. These are from Galveston Island, and were taken after Hurricane
Ike hit the shoreline. You’ll notice there are two houses here. The one in the lower
photograph is pretty much destroyed. The ones in the upper picture, the surf zone is right
under the houses. And those will eventually fall into the ocean. Here are some pictures
of what happened when Ike came ashore in Galveston Island. On the right you see the path of the
storm, and on the left you see two photographs. The upper one shows before the storm and there
are two arrows indicating two houses. And then the lower one shows what happened after
the storm. Those two houses are still there, shown by the arrows, but most of the houses
in front of them are gone. This is also from Ike, it’s a little north of the last couple
of pictures I showed you. And what’s kind of strange about this picture is there used
to be houses all along that road. Most of them are gone, but there’s one house still
standing. It was a very well built structure and so it withstood the storm. However, there
was a lot of water damage in the house, but the structure still stood. But most of the
houses, as you can see, are gone. That area was devastated by the storm. And some more
slides here from the Galveston area. The ones on the left show before and after. Showing
how houses were destroyed along the shoreline. And the one on the right shows this structure,
this jetty (it was I think a restaurant) that was built offshore before the storm, and then
it’s gone after the storm, destroyed by this strong wave energy. Orrin Pilkey, who’s a
geologist at Duke University and is well known for his work on barrier islands in North Carolina
and around the world, one of his quotes here states this: “Nature tries to shift the outer
banks, [referring to the outer banks of North Carolina] but man keeps shoveling it back.”
This may be one of the best solutions. If you have a house that’s very close to the
beach, what should you do about it? It’s probably going to be destroyed if another big storm
moves in. But one thing you can do, is you can move that house to the back part of the
barrier island, and you may have it for a little longer time than you would if you just
left it near the beach.