Beach: A River of Sand

Dr. Roy Dokka thought it important that this video be available on the C4G channel for people who he thought wanted a better understanding of how sediments are turned into beach sand and how this sand is effected by manmade structures and the forces of nature.

[waves crashing]

(male narrator) If you ask a man
who lives along this beach in California

what a beach is made of,

he'll probably say " light-colored sand."

[waves crashing]

However, this beach in Hawaii

is made of small grains
of black volcanic rock.

This beach at La Jolla, California
is made of pebbles and cobbles.

[waves crashing]

In Southern Florida,

the beaches are composed
mostly of small bits of seashells.

And some English beaches are
made up of small, flat rock fragments

called shingles.

Actually, beaches are composed of
whatever loose material is available.

People say this California beach
is made of light-colored sand,

but what is the sand composed of?

Tiny grains of quartz and feldspar,

the two most common minerals
found in solid rock.

Where could the billions
of grains of minerals

that make up this beach
have come from?

And how did they get here?

All along this coast there are streams
that flow down to the beaches.

And when the stream is dry,

we can see that its bed
is actually a trail of sand.

If we go up one of these trails,

we should be able to see
where the sand comes from.

[gentle running water]

Up in the mountains,

we come to a place where
the stream flows over solid rock.

[rushing water]

Here, because of rain, heat, cold,

and chemical change
over thousands of years,

the solid rock breaks down
into bits of quartz, feldspar,

and other minerals.

[rain splashing]

Soon, the rock debris
is washed into a stream

and is on its way to the ocean.

By the time the rock debris
has reached the coast,

it has been refined and sorted out.

The bigger, heavier chunks of
rock have been left upstream.

The smallest particles
have been washed out to sea.

What is left are hard, durable
grains of quartz and feldspar,

the typical raw materials
of a sand beach.

Now let's find out something
about how beaches are formed.

[scratching]

Anyone who has built a sand
castle below the high tide line

knows something about the
processes that shape beaches.

[waves crashing]

The waves have restored
the beach to its original condition.

When a wave washes up on a beach,

sand grains are lifted up by the water.

Each wave picks up millions
of sand grains and moves them.

What effects do these
movements have on the beach

over long periods of time?

Still photographs of this beach

have been taken from
the same camera position

over a period of years.

Let's compare some
of these photographs.

The sand comes and goes
according to the season.

At the end of a summer,
the beach is piled high.

At the end of a winter,
the sand is gone.

The following summer,
the sand returns,

but why?

In summer, the waves that
wash up on this beach are small

and carry less energy
than the winter waves,

which are bigger and more powerful.

Such seasonal changes
in wave size may be the cause

of the seasonal changes in the beach.

Let's check this idea.

This is a model beach
in a wave tank.

We'll be able to make waves
of different kinds in the tank

and see what effect
they have on the beach.

First, we'll make some small waves,

the kind that are most
common in summer.

To speed up the process,

we'll use the time-lapse camera

and condense two hours
into 30 seconds.

The small summer waves push
the sand toward the shore

in the form of migrating sand bars.

[rushing water]

Eventually, the waves push
enough sand onshore

to form a steep beach face.

Now watch what happens
when we make bigger waves,

the kind that strike the beach in winter.

The bigger winter waves gouge
out sand from the steep slope

and deposit it as sand bars offshore.

The result is a beach face
that looks like this.

Now watch what happens
when we make summer wave again.

[rushing water]

The sand that was taken away
from the slope by the big waves

is put back again
by the smaller waves.

In other words,
the sand moves back and forth

between the exposed beach face

and the underwater part
of the beach slope.

[rushing water]

If sand moves only on and
offshore with the seasons,

why doesn't it pile up at the mouths
of the rivers that deliver it?

Why does it form into beaches

that stretch for hundreds
of miles down the coast?

You may have noticed

that waves usually approach
the coast at an angle,

not straight on.

The reason for this is that most
waves are created by storm winds

blowing far out at sea.

If a storm occurred
anyplace except here,

say in one of these areas,

then the waves created by the storm

and traveling out
from the storm area,

would approach the beach
at an angle,

not straight on

regardless of which way
the beach is facing.

Today, the waves are coming
in from the northwest.

[waves crashing]

Notice what happens to the waves

when they enter
the shallow coastal waters.

They bend and tend to become
parallel to the shoreline.

But as you can see,
the bending is not always complete.

The waves pass through
the surf zone at an angle

and strike the beach face at an angle.

Let's find out what effect waves
like these have on a beach.

[waves crashing]

First, let's find out what happens
to the sand on the beach face--

the exposed part of the slope.

These red markers will show
how the water moves.

[waves crashing]

Let's watch the red markers again,

this time tracing their movement
along the beach face.

[waves crashing]

The sand grains on the beach face
must be following a similar path.

[waves crashing]

What's happening to the sand
on the part of the beach slope

that's under deeper water
in the surf zone?

The waves passing overhead
move the sand back and forth

toward the shore
and away from the shore.

But are these the only directions
in which the water is moving?

Watch.

[waves crashing]

The dye shows that the water
is moving down the coast as well.

Now we'll repeat the experiment,

this time putting another spot of dye
just outside the breaking waves.

The second spot of dye shows that

the water outside the breaking
wave hardly moves at all,

while the dye within the surf zone
moves rapidly downcoast.

When the waves enter the surf zone,

they break at an angle

and cause this downcoast flow of
water called a longshore current.

Now let's see what this current
does to the sand in the surf zone.

The sand being moved onshore
and offshore by the waves

is also being moved downcoast
to the left by the longshore current.

From the air the pattern is clear.

The waves approach
the shore at an angle.

Even though they bend somewhat,
they strike the beach face at an angle.

The sand on the beach face

is carried in a series
of arcs down the coast.

In the surf zone,

the sand grains are being moved
not only back and forth,

but also down the coast
by the longshore current.

Such movement of sand on
the beach face and in the surf zone

is called longshore transport.

So, we can think of the beach
as a river of sand.

The beach face is
one bank of the river,

the outer edge of the surf
zone is the other.

Much more sand is moved in the surf
zone than along the beach face.

These groins built
along a nearby beach

provide further proof
of longshore transport.

The sand has piled up on
the same side of each barrier,

thus showing the direction
in which the sand is moving.

Measurements of such
accumulations of sand

along both coasts of the United States

show that the sand moves southward
in most places most of the time.

These figures show the number
of cubic yards of sand

that move south each year
by these locations.

Let's take a closer look
at one of these places.

We know the sand is moving
downcoast along this beach

toward the harbor at Santa Barbara.

Why does the beach
appear to end here?

And why has a sandspit
over 300 yards long

formed off the end of the breakwater?

We can answer these questions

by observing a model of
the harbor in a wave tank.

[click] [splash]

The waves strike
the breakwater at an angle

and bend around its end
into the harbor.

Now we'll add some sand
and create a beach.

Longshore transport carries
the sand along the shore.

The breakwater, acting as a dam,
stops the sand,

but only temporarily.

When the sand reaches
the end of the breakwater,

the incoming waves carry
the sand into the harbor.

Once inside, the sand settles
out into the quiet water

behind the breakwater
and a spit is formed.

Now we know that
the sand flows underwater

along the outside of the breakwater
and feeds the spit.

Let's watch this process once again.

[moving water]

In time, the sand closes off the harbor.

The problem at Santa Barbara
is how to keep the harbor

from being sealed off
by accumulating sand.

The solution is to take
the sand out of the harbor

and put it back into the natural
longshore transport system.

This is done with a dredge.

The dredge digs up sand from
the end of the spit

at the rate of about 280,000
cubic yards per year

and it works the year round.

The dredge picks up a mixture
of sand and water here,

pumps it through a pipe,

and dumps it here.

The sand spilled out onto
the beach below the harbor,

flows down the beach
towards the surf.

Once the sand reaches the surf,

it is picked up
by the longshore current

and is once again on its way
down the coast.

Eighty miles down the coast
are this breakwater

and pier at Santa Monica.

The breakwater was built
to provide a place

where small boats could anchor

and be protected from incoming waves.

Notice the bulge in the beach
opposite the breakwater.

The bulge was not there
before the breakwater was built,

but it appeared soon after.

Why?

The answer is that the breakwater
prevented the waves

from reaching the beach
and the river of sand

was deprived of the energy
that keeps it moving.

The sand movement along
the beach slowed down.

The sand accumulated
and the bulge was formed.

In time, the bulge would grow
until it reached the breakwater

and the boat anchorage
would be filled with sand.

To prevent this,

the sand is dredged regularly

and dumped farther down the coast

where the river of sand
is flowing normally.

One hundred twenty miles
farther down the coast,

the river of sand is interrupted again
but in a different way.

Although 200,000 cubic yards
of sand per year

are moving southward along this beach,

the beach narrows down and ends here.

And there is no piling up of
sand against the rocky point.

Where does the sand go?

Just offshore is a branch
of a submarine canyon.

The canyon is about 20 miles long

and extends to a depth
of more than 3000 feet.

Now we know why the beach ends

near the head of the submarine canyon.

The river of sand is
drained off down the canyon

and onto the ocean bottom.

The canyon is located here.

Farther upcoast there are
two other submarine canyons,

each just offshore
where a beach ends.

A system of rivers feeds sand
to each of the beaches.

The sand is carried down the rivers

and is moved southward
along the beaches.

The beaches end where
the sand is drained off

down the underwater canyons.

What happens when a dam is
built across one of the rivers?

The sand that would normally move
downriver to the beaches is trapped.

The reservoir has to
be drained periodically

and the accumulated sand removed.

[engines]

What would happen if
all the rivers were blocked?

Eventually, the beaches
would disappear.

So, the rivers of sand that
move along our coasts

are actually parts of
much larger systems.

Whenever man interferes
with such a system,

he becomes involved in its operation

to the degree that man upsets
the natural balance of the system,

he and his machines must do
the work that nature did before.

[water splashing]