Beach: A River Of Sand (1965)

[Applause] [Applause] [Applause] 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 Hoya California is made of Pebbles and cobbles [Applause] in Southern Florida the beaches are composed mostly of small bits of seash shells 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 that California beach is made of light colored sand but what is the sand composed of tiny grains of quartz and felbar 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 into bits of quartz Feld Spar 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 is 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 Feld Spar the typical raw materials of a sand beach now let's find out something about how beaches are formed anyone who has built a sand castle below the high tide line knows something about the processes that shape beaches [Applause] 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 AC 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 upon 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 season 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 timelapse camera and condense 2 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 stand 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 sand bars offshore the result is a beach face that looks like 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 into 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 po 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 any place 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 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 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 D 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 D shows that the water outside the braking waves hardly moves at all while the Dy 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 why has a sand spit over 300 yd 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 Brak water at an angle and bend around its end into the harbor now we'll add some sand and create a beach long short transport carries the sand along the shore the break water acting as a dam stops the sand but only temporarily when the sand reaches the end of the break water 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 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 Mi 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 M long and extends to a depth of more than 3,000 ft 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 up Coast 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 down River 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 [Applause] [Applause]