 
We're going
to do a review here
of beach processes
and shorelines
in Southern California.
If you go to a typical
Southern California beach
in summer, you'll see usually
that it's wide and sandy.
But if you were to come back
to that same beach
in the wintertime,
you might see
that it's covered with rocks.
And the issue is not
"where did the rocks come from?"
The issue is
"where did the sand go?"
And the answer is, it went
out there,
just a few
hundred yards offshore.
And that gives us a narrow
rocky beach in the winter,
because the sand is out there
in those sandbars offshore.
But in the summertime
that sand comes back
and covers up the rocks
and typically produces a nice
wide, sandy beach.
So, there's this natural
seasonal change
that happens
between the winter
and the summer,
and the reason has to do
with the sizes of the waves.
In the summertime,
we typically have small waves
that usually push the sand
up toward the beach, and
give us a big, wide sandy beach.
But in the winter we
tend to get bigger storms
and large waves can come
in the wintertime,
and those big waves will break
hard on the beach,
and sweep the sand offshore
into these places
that we call longshore bars.
And usually in the winter is
when we typically have our
worst coastal erosion problems
and cliff failures,
with the big waves
and the narrow beaches
that aren't protecting the
cliffs from those waves.
Now, another thing
that sand does, rather
than just moving toward the
beach and away
from the beach according
to the size of the waves,
is that it also moves along
the beach,
parallel to the beach,
and the reason for that has
to do with situations
where the waves arrive
at the beach at an angle.
You can see in this picture
that the waves are hitting the
beach at an angle,
and the effect of that is
to cause the sand to be moved
to the right along the beach.
And this animation
illustrates this.
You can see here the waves
approaching the beach
at an angle,
and as they hit the beach
at an angle they sweep
up the beach at an angle,
and then wash back
down...up the beach at an angle
...and then wash back down,
and the results is
that the sand moves
in a series
of zig-zaggy arcs along the
beach in the direction
that the waves are going.
This is called "longshore drift"
or "longshore transport,"
and the movement
of sand occurs not just
on the beach itself, but also
in this area called the surf zone,
before the waves even
come to the beach,
because the movement
of the waves creates a current
that's called a
"longshore current."
Now, if you were to go look
at the movement of sand
by longshore drift along both
coasts of the United States,
what you would find is that the
net movement of sand is
to the south,
both along the West Coast
and the East Coast.
And the reason for that is
that most of our big waves
come from the North Pacific
for the West Coast,
or from the North Atlantic
if you're on the East Coast.
So, those waves come
from north
and they hit the beach
from north to south,
and push the sand
from north to south.
Now, we're going to zoom
in on Southern California here,
and look at the movement
of sand along our coast.
The sand moves south along
our coast by longshore drift
(or southeast because
of the orientation
of the coastline).
But what happens is the sand
doesn't just travel south
along the coast forever.
It ends up leaving the beach
where it drains off
down these areas
that we call
"submarine canyons."
And you can see them labeled
here: Redondo Canyon,
Newport Canyon,
and then notice La Jolla
Canyon down at the bottom
of the image there.
That's the canyon
that is taking all of the sand,
in what's called the Oceanside
Beach Compartment.
A "beach compartment" is an
area in
which sand arrives
at the beach,
moves along the beach,
and eventually leaves the
beach down the
submarine canyon.
What we're going
to do here is look more
closely at the Oceanside Compartment,
with a diagram
that will illustrate one
of these beach compartments.
So, we can imagine
that the sand would come
down to the beach from rivers.
(It also comes to some degree
from bluff erosion.)
And then the sand ends
up moving south along the
beach because of
longshore drift. And then
when the sand reaches the head
of one of these submarine
canyons, it ends up draining
down the canyon
out onto the ocean floor.
And once it does
that it's pretty much gone,
you can't get it back
very easily.
This next picture shows us
La Jolla Canyon.
And so the sand heads south
along our coast past Cardiff
and Del Mar
and eventually reaches the
head of La Jolla Canyon, not
very far from the beach there
down in La Jolla. And that's
where it heads
out onto the ocean floor.
You can actually go
out to La Jolla Canyon,
if you happen
to be a scuba diver, as I am,
and if you go
out to the canyon you can
actually swim
out to the canyon
from the beach,
and you can reach the head
of the canyon, and if you go
down into the canyon you can
actually witness the sand
draining away,
down onto the ocean floor.
It's kind of amazing to see
that sand and think
that that might have been sand
that somebody was walking on,
you know, 10 or 20 years ago!
But now it's leaving the beach
system, and there's really no
good way to get it back.
So, that's the story
of our Southern California
beaches, and how the sand moves
on the beaches.
