Beach: A River of Sand

Summary

This script explores the diverse composition of beaches, from black volcanic rock in Hawaii to seashells in Florida. It delves into the geological processes shaping beaches, including erosion and sediment transport by waves and longshore currents. The video demonstrates seasonal beach changes, the impact of coastal structures on sand movement, and the necessity of maintaining natural sand transport systems, emphasizing the delicate balance of coastal ecosystems.

Takeaways

• 🏖 Beaches are composed of various materials depending on what is available in the local environment, such as black volcanic rock in Hawaii, seashells in Florida, and shingles in England.
• 🌊 The sand on many beaches is primarily made up of tiny grains of quartz and feldspar, which are the most common minerals in solid rock.
• 🏞️ The process of sand formation involves weathering and erosion of rocks in the mountains, which break down into smaller particles and are transported to the sea by streams.
• 🌊 Beaches are shaped by the action of waves, which can lift and move millions of sand grains, leading to seasonal changes in beach size and shape.
• 📸 The script describes a method of observing beach changes over time using still photographs taken from the same position over several years.
• 🌊 Seasonal variations in wave size and energy can cause sand to accumulate on beaches in the summer and recede in the winter.
• 🌊 The angle at which waves approach the shore influences the movement of sand along the beach, creating a longshore current that transports sand down the coast.
• 🏗️ Human-made structures like breakwaters and harbors can disrupt the natural flow of sand along beaches, leading to the need for dredging to maintain navigation and beach stability.
• 🌊 The sand on beaches moves in a cyclical pattern, being deposited and eroded by waves, which can lead to the formation of sandbars and changes in beach profiles.
• 🏞️ Submarine canyons can act as natural drains for the sand transported along beaches, carrying it away from the shore and down to the ocean floor.
• 🌳 The natural balance of sand movement along coasts is part of a larger system that includes rivers and the ocean, and human interference can necessitate active management to maintain this balance.

Q & A

What is the common misconception about the composition of a beach according to the man living along the California beach?

The common misconception is that a beach is made of light-colored sand.

What type of material is the beach in Hawaii made of, as mentioned in the script?

The beach in Hawaii is made of small grains of black volcanic rock.

What are the two most common minerals found in the sand of the California beach?

The two most common minerals found in the sand are quartz and feldspar.

How do the processes of weathering contribute to the formation of sand grains on the beach?

Weathering processes such as rain, heat, cold, and chemical changes over thousands of years break down solid rock into bits of quartz, feldspar, and other minerals, which eventually become sand grains.

What happens to the sand grains when a wave washes up on the beach?

When a wave washes up on the beach, sand grains are lifted up by the water, and each wave picks up millions of sand grains and moves them.

Why do beaches appear to change their shape and size with the seasons?

Beaches change shape and size with the seasons due to the differences in wave size and energy. Smaller, less powerful waves in summer cause the beach to pile up with sand, while larger, more powerful winter waves remove sand from the beach.

What is the term for the movement of sand on the beach face and in the surf zone?

The movement of sand on the beach face and in the surf zone is called 'longshore transport.'

How do waves entering the shallow coastal waters affect the direction of their approach to the beach?

As waves enter the shallow coastal waters, they bend and tend to become parallel to the shoreline, but often they still strike the beach face at an angle due to the influence of storm winds from far out at sea.

What is a longshore current and how does it affect the movement of sand along a beach?

A longshore current is a flow of water that moves along the coast, caused by waves breaking at an angle. It affects the movement of sand by carrying it not only back and forth but also down the coast, contributing to the longshore transport.

Why does the beach appear to end near the head of a submarine canyon?

The beach appears to end near the head of a submarine canyon because the sand is drained off down the canyon to the ocean bottom, effectively ending the beach.

What is the consequence of building a dam across a river that contributes sand to the beaches?

Building a dam across a river traps the sand that would normally move downriver to the beaches. This requires periodic draining of the reservoir and removal of accumulated sand to prevent the loss of beaches.

How does human interference with the natural sand transport system affect coastal areas?

Human interference with the natural sand transport system can upset the natural balance, requiring human and mechanical intervention to perform the work that nature previously did, such as dredging sand to maintain harbors and beaches.

Outlines

00:00

🌊 Beach Composition and Formation

This paragraph explores the diverse composition of beaches around the world, highlighting that they are made from locally available materials such as black volcanic rock in Hawaii, seashells in Florida, and shingles in England. The focus then shifts to the California beach, which is primarily composed of quartz and feldspar, derived from the erosion of solid rock in the mountains. The process of sand formation is traced from the mountains to the ocean, detailing how the rock debris is refined and sorted by natural elements like rain and heat. The paragraph also delves into the dynamics of beach formation through the action of waves, which lift and move sand grains, shaping the beach over time. Seasonal changes in wave size and energy are suggested as a factor in the shifting sand levels on the beach.

05:17

🌪 Seasonal Impact on Beach Sand

The second paragraph examines the seasonal variations in beach sand levels, particularly the contrast between the high sand accumulation during summer and the reduced levels in winter. It explains that smaller, less energetic summer waves push sand towards the shore, creating steep beach faces, while larger, more powerful winter waves remove sand, forming offshore sandbars. A wave tank experiment demonstrates how different wave sizes affect the beach's shape, showing the sand's movement back and forth between the beach face and the underwater slope. The paragraph also discusses the role of wave angles in beach formation, as waves usually approach the coast at an angle, causing a longshore current that moves sand down the coast.

10:20

🏖️ Longshore Transport and Coastal Dynamics

This paragraph delves into the concept of longshore transport, illustrating how sand is moved along the beach face and in the surf zone by the combination of wave action and longshore currents. The use of dye in experiments reveals the down-coast movement of water and sand. The beach is likened to a river of sand, with the beach face and the outer edge of the surf zone as its banks. The paragraph discusses the evidence of longshore transport, such as the accumulation of sand on one side of groins and the southward movement of sand along the coast of the United States. It also explores the impact of coastal structures like breakwaters and piers on sand movement, leading to the formation of sand spits and the need for dredging to maintain coastal equilibrium.

15:24

🚧 Human Intervention in Coastal Sand Systems

The final paragraph discusses the consequences of human intervention in the natural coastal sand systems. It describes how structures like breakwaters and dams can disrupt the flow of sand, leading to the formation of sand bulges and the need for regular dredging to prevent coastal infrastructure from being overwhelmed by accumulating sand. The paragraph also considers the broader implications of such interventions, noting that if all rivers were blocked, beaches would eventually disappear. It concludes by emphasizing the interconnectedness of coastal sand systems and the necessity for humans to understand and manage their impact on these natural processes.

Keywords

💡Beach Composition

Beach Composition refers to the materials that make up a beach. The video script highlights that beaches are not universally composed of the same materials, but rather consist of whatever loose material is available in the area, such as light-colored sand in California, black volcanic rock in Hawaii, pebbles and cobbles in La Jolla, and seashells in southern Florida. This concept is central to understanding the diversity of beaches and their formation processes.

💡Quartz and Feldspar

Quartz and feldspar are two of the most common minerals found in the Earth's crust and are often the primary components of beach sand. The script explains that the sand on a California beach is made up of tiny grains of these minerals, which have been weathered and transported from the mountains to the ocean over time. These minerals are hard and durable, making them typical raw materials for a sand beach.

💡Longshore Transport

Longshore transport is the process by which sand and other materials are moved along the coast by waves and currents. The video script illustrates this concept by showing how sand is carried in arcs down the coast on the beach face and also moved down-coast by the longshore current in the surf zone. This process is crucial in the formation and maintenance of beaches and is responsible for the distribution of sand along coastlines.

💡Wave Action

Wave action is the effect of waves on the shoreline, which includes the erosion, transportation, and deposition of sediment. The script describes how waves lift and move sand grains, shaping the beach over time. Seasonal changes in wave size, from the smaller and less powerful waves of summer to the larger and more powerful waves of winter, influence the amount of sand on the beach and its distribution.

💡Seasonal Changes

Seasonal changes refer to the variations in natural phenomena that occur over the course of a year. In the context of the video, seasonal changes are evident in the size of waves and the amount of sand on the beach. The script notes that beaches pile up with sand in the summer and lose sand in the winter, which is attributed to the differences in wave energy during these seasons.

💡Migrating Sandbars

Migrating sandbars are sand deposits that move along the shore due to wave action. The video script describes how small summer waves push sand towards the shore, forming these sandbars. They are an example of how the dynamic process of wave action can lead to the temporary formation of features on the beach that change with the seasons.

💡Longshore Current

A longshore current is a current that moves parallel to the shoreline, typically generated by the angle at which waves approach the coast. The script demonstrates how these currents affect the movement of sand along the beach, contributing to longshore transport and the shaping of the coastline.

💡Breakwaters

Breakwaters are structures built to protect the coast or harbors from the force of waves. The script uses breakwaters as examples of how human-made structures can interrupt the natural flow of sand along the coast, leading to the formation of sandbars or the need for dredging to maintain navigation channels.

💡Dredging

Dredging is the process of removing sediments, such as sand, from bodies of water to deepen channels or maintain harbors. The video script describes how dredging is used to manage the accumulation of sand in harbors caused by longshore transport, ensuring that sand does not block navigation channels or create unwanted landforms.

💡Submarine Canyons

Submarine canyons are deep valleys on the seafloor that can extend for miles and reach great depths. The script explains that these canyons can act as drains for the 'rivers of sand' moving along beaches, carrying sand from the shore to the deep ocean floor. This concept illustrates the connection between coastal processes and deep-sea geology.

💡Human Intervention

Human intervention refers to the actions taken by people to alter natural processes. The video script discusses how human activities, such as building breakwaters or dams, can disrupt the natural flow of sand along coastlines. This can lead to the need for ongoing maintenance, such as dredging, to counteract the effects of these interventions on coastal ecosystems and human infrastructure.

Highlights

Beaches are composed of various materials depending on what is available in the local environment.

The composition of a typical sand beach includes tiny grains of quartz and feldspar, common minerals in solid rock.

Beaches receive their sand from the erosion of rocks in the mountains, carried by streams to the ocean.

The process of sand formation involves natural elements like rain, heat, cold, and chemical changes over thousands of years.

Seasonal changes in wave size and energy influence the amount of sand present on a beach throughout the year.

Waves play a crucial role in shaping beaches by lifting and moving sand grains along the shore.

Beaches undergo a seasonal cycle of sand accumulation and erosion, visible through comparative photography over time.

Laboratory models demonstrate how different wave sizes affect the formation and erosion of beaches.

Longshore transport is the process by which sand moves along the coast due to the angle at which waves approach the shore.

The direction of longshore transport can be determined by the accumulation of sand on one side of coastal structures.

Submarine canyons act as natural drains for the longshore transport of sand, carrying it away from the beaches.

Human interference with the natural sand transport system, such as building breakwaters, can disrupt the balance of coastal ecosystems.

Dredging is a method used to manage sand accumulation in areas affected by human structures, like harbors.

The natural balance of sand transport systems can be restored by reintroducing dredged sand back into the coastal environment.

Blocking rivers that feed sand to beaches can lead to the eventual disappearance of those beaches if not managed properly.

The transcript highlights the interconnectedness of sand transport systems and the importance of maintaining their natural flow for coastal health.

Human activities must consider and sometimes replicate the work of nature to prevent negative impacts on coastal environments.

Transcripts

(roaring surf)

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."

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.

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 a 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.

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

Here, because of rain, heat, cold and  chemical change over thousands of years,  

the solid rock breaks down in to bits  of quartz, feldspar and other minerals.

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.

(rapid digging)

Anyone who has built a sandcastle below the high tide line

knows something about the  processes that shape beaches.

(lapping waves)

The waves have restored the  beach to its original condition.

When a wave washes up on the 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 sandbars.

Eventually, the waves push enough sand  on shore 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 sandbars offshore.

The result is a beach face that looks like this.

Now watch what happens when  we make summer waves again.

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.

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.

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.

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.

Let's watch the red markers again,

this time tracing their movement along the beach face.

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

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.

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 waves hardly moves at all,  

while the dye within the surf  zone moves rapidly down-coast.

When the waves enter the surf zone, 

they break at an angle and cause this down-coast  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 down-coast 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 down-coast along  this beach toward the harbor at Santa Barbara.

Why does the beach appear to end here?

And why has a sand spit 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.

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 in 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.

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 toward 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.

80 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.

120 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 3,000 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 on to the ocean bottom.

The canyon is located here.

Farther up-coast there are two other submarine canyons,

each just offshore where a beach ends.

A system of rivers feed 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.

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.

(whirring engine fades into gushing water)