GEOLOGY/GEOPHYSICS 101 Program 28

Waves, Beaches, and Coasts

Much of the world's population lives within a few miles of a coastline, so coastline processes are important, and changes in sea level can be expensive and damaging. Human activity of various kinds has had drastic effects on coastline processes as we try to modify our environment to make it easier to live in it, but most of the time these modifications are disruptive, and on the long term are harmful although we may be fooled into thinking otherwise at least for a short time, so in today's program we will consider the shoreline processes and features and some of the effects of our modification of these processes. Let me remind you of the text assignment for today.

It's Chapter 20, pages 453 to 469 and pay special attention to Box 20.1 on page 456 which is about rip currents and Box 20.2 about rising sea level, and of course, follow the study plan and the study guide and review the objectives and be sure you've learned each one.

There are quite a few objectives for this lesson, so I'm not going to read them for you but go check them out in the study guide.

Well, let's begin this discussion of coastline by looking at sea level. Sea level is not a fixed quantity; that is, it changes over time. There are both long term changes and short term changes. The long term changes are affected by several different factors, for one thing is the subsidence and uplift of the land. Also, sea level can be affected by the amount of water in the ocean, and we've already seen how a glacial period can remove water from the oceanic reservoir to cause lowering of sea level, but we also might note that during tectonic changes the size of the ocean basins may change as continents move around and the ocean floor is squeezed. All of these things can affect sea level in one way or another.

We don't know exactly what sea level has been because it's hard to sort out the effects of all of these different changes, but we do know that it's been increasing worldwide since the last retreat of the glaciers about 10,000 years ago, and, in fact, it's increased about two inches in the last century. Estimates are that sea level may be as much four feet higher by the year 2100.

These predictions, of course, are based on estimates of global warming; no one knows for sure, but four feet is a lot when you consider how many people live close to the sea and how much land area arise of four foot of sea level would entail.

Sea level also undergoes short term changes from tides and storms, for example. The tides are caused by the pull of the sun and the moon, and these general occur in regular and predictable cycles. Storm surges from hurricanes and other high winds may also increase the sea level; in fact, much of the damage from a hurricane near shore is caused by the storm surge as happened here in Hawaii when Hurricane Iniki hit the south coast of the Island of Kauai, so at any given time the sea surface itself is a complex pattern of many waves which are superimposed, in other words, all existing in the same place at the same time.

These waves are generated by wind during storms at sea. The storms themselves create waves of many different sizes, heights, and wave lengths, and these waves travel thousands of miles across the ocean losing only a small amount of energy; they do disperse as they travel to become swell, and they lose wave height as the energy spreads out, so generally we find higher waves closer to storms, and this is important because it causes seasonal differences in wave height at any one location, and in general we find the highest waves generally moves towards the Equator from storms that develop in the Arctic Seas both North and South Hemisphere.

Well, as we'll see in the video, wave motion is cyclical, and the water actually moves in circular orbits. The water itself is not transported until the wave breaks; it's only energy that's transported through the ocean. This is analogous to earthquake waves which travel through the rock without actually moving the rock; in other words, the rocks are put back into their equilibrium positions after being deformed. The motion of an ocean wave generally extends down to a depth of about half a wavelength, so that the bottom of the sea floor, in other words, has no influence on the wave motion if the bottom is deeper than about one half of the wave length. At shallower depths, the bottom begins to interfere with the wave motion and cause changes in the waves.

The bottom also affects the speed of the waves if the waves are in extremely shallow water; in fact, it controls the wave speed if the depth of the bottom is less than about one twentieth of the wave length, so as the waves approach shore, they increase in height, they slow down, and they get closer together, and eventually the wave becomes unstable as the orbits interfered with at the bottom, and eventually the wave breaks and causes water to surge toward the shoreline in the surf zone.

At this point, water is actually transported shoreward throughout the surf zone, and this water which is transported must find some way to escape; the water can't pile up indefinitely on shore, so it does so through narrow rip currents which transport the water back seaward past the surf zone. It's worthy of noting here that these rip currents are usually narrow and often very intense. Well, not extremely intense, but certainly too strong a current to swim against, so the general rule is if you're caught in a rip current swim parallel to shore and you'll soon be out of the rip current and be able to swim back.

Okay, it's also worthy of note here that after the wave breaks some energy remains so that smaller waves then can progress towards shore to interact with the shoreline once again. It's these interactions of the waves with the shoreline that we're concerned with in our study of coastlines. One such effect is what's called wave "refraction." The word "refraction" simply means "bending," so waves are bent when they enter shallow water at an angle to the shoreline. This is because the one part of the wave is slowed down more than another sort of like a column of soldiers or the columns of a marching band walking through soft material or walking through mud. They slow down so the rest of the column gets ahead of them.

The effect of this is that the waves strike the shoreline at a lesser angle than the angle of approach; in other words, the waves strike more nearly parallel to shore, not completely parallel, but more nearly parallel. The effects of wave refraction also tend to concentrate wave energy on headlands or protrusions that stick out of the coastline and tend to defocus or diverge the energy in embayments. This is important for our purposes because the refraction of waves affects erosion, transportation, and deposition along the coastline.

Well, and again, we find erosion at headlands because that means material is removed from the headlands and transported into embayments where the wave energy is less. The effect of this is that it tends to straighten out a coastline over time smoothing out irregularities, so as a coastline matures, it tends to lose all these various irregularities and tends to become much straighter and smoother, so let's take a look specifically at coastal erosion. In coastal erosion currents and tides play a role.

Currents are generally weak, but they're still capable of moving material. The tides also produce currents, but the tides may also extend the area over which waves can act by raising and lowering sea levels, and the tidal currents themselves may be quite strong especially in narrow inlets. San Francisco Bay, for example, has notoriously strong tidal currents, and it was these tidal currents, among other things, that made Alcatraz island a nearly escape-proof prison, so most coastal erosion, especially of bedrock, is due to waves either directly or indirectly so let's look at the direct effect of waves.

As you know, when waves break on the shore they dissipate tremendous energy and can exert tremendous forces, so the waves like running water in streams can act either by hydraulic action; that's simply the force of the water, but also by abrasion. Now, the hydraulic action can be extreme. The pressures exerted by a wave breaking on the shore may be in the range of tons per square foot. An abrasion, of course, moves sand, pebbles, gravel back and forth, which further rounds the beach sediment, but also breaks and fractures the rock and tends to scour and polish the coastline.

The force is enough many times to fracture the bedrock and move large boulders, so the waves have much erosive power, and they also participate to some degree in mechanical weathering at the coastline which breaks the rocks down to smaller fragments, and by the way, the rate of erosion at a coastline is surprisingly high; for example, in Southeast England the Cliffs of Dover has retreated about two kilometers since 1066 during the Norman Invasion, and here in Hawaii it's a little slower than that, but it averages about five centimeters per year.

Now, again, that doesn't sound like much, but five centimeters per year is quite excessive, quite fast for geologic processes.

Okay, let's take a look at some erosive features, and I'm really trying to sum this up so be looking for these features in the video; I can't go into great detail on all of these things. Okay, one erosive feature are called "terraces." This is basically a leveling of the shoreline to about sea level or a little bit below. Waves can cut and transport a terrace, transport material over a terrace over several miles and eventually may completely level an island to form a shoal.

This is what's happened in the leeward Hawaiian Islands, and these flat topped islands after they subside will become guyots. It's also worthy of noting that the sediment often covers the terrace and becomes part of the beach system.

Okay, waves operating on the shoreline can also create features called caves and nips. Both of these are simply undercutting of an overhanging ledge at or near sea level. The caves simply represent more easily eroded rock, which you might think of a cave as a more intense version of a nip. A seacliff is formed by mass wasting after wave erosion undercuts the base of the cliff, and the material above becomes too unstable and falls into the water. Other features that we observe especially on new coastlines are "arches" and "stacks."

An "arch" is basically a rocky ridge which remains above a cave which is cut through the ridge, which when the ridge protrudes into the sea.

A "stack" is an islandlike erosional remnant that remains after an arch collapses, and here in Oahu Chinaman's Hat or Kualoa is the best example that we have. We find many of these things, for example, along the Southern Washington or Northern Oregon coast on the west coast. Wave erosion can also form features called "benches."

A "bench" is just a level platform that's above sea level. Here in Hawaii benches are very common, especially in soft tuff that forms the northeastern coast of Oahu around Hanauma Bay Koko Head. Basically here we find that rock that's continually wetted and dried by wave action has less resistance to erosion, so the bench tends to form aided by organic processes like organisms that live on the bench and also by potholing.

Remember from streams, potholing is the action of pebbles that are swirled around into depressions by hydraulic action of water.

Okay, we can turn our attention also to coastal deposition then, and the main feature that's formed by deposition along the coastline is the beach. Everybody has been to the beach, but what do you see when you go to the beach. If you ask a hundred people exactly what a beach is, you'd probably get a hundred different answers. Basically the beach is all the sediment and materials which are accumulated along the coastline. We generally think of the beach as that sandy part of the coastline that's above sea level, but actually the beach is quite a bit more complicated than this. The beach itself is that sediment that extends all the way out to the surf zone, and the beach consists of whatever material is available and whatever material that can be moved into that area or out of that area by waves. It may be boulders, or gravel, or sand, or silt, or clay, and, in some cases, it may be manmade materials.

There was a beach in Northern California which was formed entirely from trash tin cans, and those tin cans behaved the same way that other natural sediments behave. The finer materials on a given beach are separated by wave action and carried out beyond the terrace eventually to be deposited in deep water, so what's left behind is the coarser grained material that the water can't move in suspension.

Here in Hawaii our beaches are composed mainly of basalt fragments, and coral fragments, and shell fragments, but also a small amount of black sand and occasionally a little bit of olivine, so the sand on the beach basically rests on bedrock. It rests usually on a bench above sea level and on a terrace below sea level.

There are several features of this part of the beach that we need to become familiar with. Most beaches contain a long low ridge of sand, which is on shore but parallel to the coast; this is called a "berm." The berm usually represents the highest seasonal wave influence on that particular beach, and in general the winter berm is high, and steep, and far back from the shore; whereas, the summer berm is low, and gentle, and closer to the shore.

Okay, the offshore equivalent of the berm is "sandbars." These are ridges of sand offshore which are parallel to the coast. These are submerged. Usually there are several parallel bars going outward from the shore of various sizes, and these also change seasonally becoming larger in winter.

Okay, the beach face itself is the slope of the beach. This is where the water runs up onto the berm, and on the beach face we find that both the grain size and the slope of the beach face depend upon the average wave height, and there are again seasonal variations here. We would find on a typical beach that in the winter time the beach face is steeper, and the sands grains are composed of coarser material than in the summer.

Okay, it's important to understand that a beach is a transient feature. The sand that you see on the beach today may not be the same sand that's there tomorrow. The sand moves along the beach both onshore and offshore. The onshore movement is largely due to wave rush up. The waves break, rush up onto the beach, and move the sand in a zigzag pattern down the beach, and it's surprising how fast this movement actually takes place. A typical sand grain may move as much as 30 to 40 meters in a given day parallel to the beach in the zigzag pattern. Offshore the sand grains are carried by turbulence and long shore currents inside the surf zone. The long shore currents are caused by the waves striking the shoreline not completely perpendicular. In addition to this long shore movement of sand, sand also moves both onshore and offshore. Wave energy moves sand perpendicular to the shore as well. Large waves tend to move sand from the beach seaward; whereas, small waves overall tend to move the beach material shoreward from the sea to the land, so we find that the winter beach is mostly underwater in sandbars, and the summer beach is mostly above water in berms.

Okay, some other features really quickly. Spits and barrier islands represent deposition by long shore currents, and there are also seasonal changes. We can generally characterize coastlines of several types. The study guide talks about erosional, depositional, emergent, submergent, and organic coastlines, and we can classify these, I think, in a fairly simple way.

"Emergent" coastlines are those coastlines where the land is rising. These are usually erosional coastlines, and we find them on active continental margins like the Western United States where there's a narrow continental shelf. Here we find lots of sea cliffs, arches and stacks, and terraces, and also relatively small pocket beaches.

On the other hand are "submergent" coastlines. This is where the land is subsiding, and these usually have depositional features. We find these on passive continental margins like the Eastern United States where there's a broad continental shelf. Here we find drowned valleys, and estuaries, and lots of barrier islands and spits, so I think this gives a fairly good background for looking at these features as we see them in the video so let's watch the video.

When we look at a sunset we see waves of light energy that have traveled an immense distance to reach our eyes.

When we look at an ocean, we see waves of water energy that may have journeyed thousands of kilometers to reach our shores. Most waves derive their energy from the wind. As the wind blows over the ocean, some of its energy is transferred to the surface forming waves that move through the water, and it is in large part the power of these waves that makes the coastal environment such a dynamic place. Coastal areas are among the most beautiful and desirable places anywhere on Earth.

The coast and coastal landforms like this beach are the result of a dynamic interaction between two competing geologic agents, the rocky land masses and the energy of the ocean. People tend to think of these as separate and independent from one another, but by ignoring the intimate connection between land and sea, they fail to realize that this delicately balanced system is subject to continual change. Building walls, and boardwalks, and homes on a shifting coastline is a gamble with nature that sometimes pays off with disastrous consequences.

Clearly, then, the coast is a part of our world that needs to be observed and understood. Consider the waves, for example. Their rhythmic motion and sound has made watching them a popular pastime, and yet few people have a real understanding of exactly what a wave is and how it works, and understanding ocean waves is vital to predicting their impact on not only the beach environment, but on coastal development.

When a wave approaches the beach, it's not the water itself that's advancing, but a serge of energy which is moving through the water. It's like the ripple that runs across a field of grain when the wind blows. The individual stocks don't run across the field; they simply bend as the wind strikes them or take the wave at a football game which creates the allusion that the spectators are rippling around the stadium when all they're actually doing is standing up or sitting down.

The same principle applies to water waves. Consider what happens to a floating object as a wave of energy passes through the water. That object tends to stay more or less in the same place tracing a circular motion as it bobs up and down. The individual particles composing the wave behave in a similar way. As the crest of the wave arrives, it lifts the particle up and forward, and then when the trough of the wave follows, the particle falls down and backward. Like the stalk of grain or the football fan, the particle returns to its original position after the disturbance is passed.

At the water's surface, the circular orbit of the water particle has a diameter that is roughly equal to the height of the wave. As one looks below the surface, however, the orbit gets smaller and smaller until there is virtually no motion of water at all. The downward limit of wave motion in the water is called the wave "base," and it's directly related to how far apart the waves are at the surface. The depth of the wave base is equal to about half the wave length, which is the distance between the crests and two waves. As the wave approaches the shore, and the water becomes shallower, the sea floor intersects the wave base confining the wave energy. The wave now starts to slow down as the sea floor begins to interfere with the orbital motion. This forces the wave up and shortens its length because waves behind it still in deeper water are advancing faster and begin to overtake it. As this happens to a succession of waves, they bunch up like cars in a traffic jam. As the bottom of each wave is slowed by the frictional drag of the seabed, the top continues to surge forward making the wave steeper and steeper. Eventually this steep front can no longer support the wave, and it breaks into surf.

Perhaps the ultimate ocean wave is the seismic sea wave, otherwise known as the "tsunami." Tsunamis can strike coasts without warning. With wave heights sometimes exceeding 30 meters, these waves have a potential for death and destruction that makes them the subject of legend throughout the world. Unlike ordinary wind- generated waves, tsunamis are caused by a much more powerful force, earthquakes. Undersea and coastal earthquakes can cause the ocean floor to shift suddenly. This movement of the ocean floor displaces a vast volume of the overlying water creating these unusual waves. Tsunamis are tremendously fast moving, some traveling in excess of 800 kilometers per hour. The wave length of a tsunami may be 150 kilometers, and so the movement of the orbiting water particles within the wave will stir up deep sea sediments even in the mid ocean.

Remarkably, however, such a tsunami may measure only a meter or so high in the open ocean, but as tsunamis approach the coast, they bunch up and rise monster like from the sea. In a few minutes a tsunamis can completely devastate a coastal community. One coastal community that experienced the crushing power of a tsunami firsthand was Hilo, Hawaii.

On April 1, 1946 following an earthquake off the coast of Alaska, one of the most destructive tsunamis of modern time sped across the Pacific and obliterated the entire shore zone at Hilo. The death toll that day was 159.

Fortunately, tsunamis are not everyday events, but even ordinary waves have some impact on the shoreline.

One very important process at work here is "refraction," the bending of wave fronts as they approach the shore. When a wave approaches the shore at an angle, the near shore stretch of wave front reaches the shallow water first and is, therefore, slowed down first. This local decrease in velocity causes the wave front to bend or refract because the deeper water portion of the wave continues to moves at its original speed. As a consequence of this refraction, the waves near shore tend to approach the coast nearly headon while those in deeper water continue along their original course.

Wave refraction has its greatest effect on irregular shorelines with deep bays and projecting headlands. Waves are refracted toward headlands smashing into them from both sides. at the same time they're spread out in bays; in other words, wave energy is concentrated on headlands and dispersed along the shoreline of bays.

The net effect of refraction on irregular coastlines is to straighten them out. As the waves crash against the headlands, they erode sediment, then deposit it as sand in the bays. So the waves perform a double action, simultaneously wearing away the headlands and filling up the bays. The erosion of coastal headlands is by no means the only source of sand. Most beach sand comes from sediment that is brought down to the ocean by rivers and streams.

Once the sand reaches the ocean, the waves distribute it along the coastline. This occurs as a result of wave movement up on to the sloping part of the beach, then back down again. Each cycle of wave movement carries particles up and down the beach slope. Because waves usually break at a slight angle to the shore, the grains of sand in this cycle are gradually worked along the shoreline in a zigzagged path. Sand gets moved along the beach face by the waves approaching the coastline at an angle, and when the waves break they have the momentum from their falling forward at that angle, so the waves rush up the beach face in the swash zone at an angle, but then gravity's going to pull that water straight back down the beach face, so what you and I see is kind of an arc shape of water swashing up and then going straight back down, and the result is that as this occurs thousands of times a day, the sand moves and has a zigzag motion up and down the beach face.

The yellow dye shows this movement. This flow of water along the shoreline is known as the "longshore current." Sand spits and baymouth bars are common products of long shore currents. What happens is that the sand is being carried along the coastline, the beach sand, and when the coastline reaches, say, a right angle turn, an abrupt bend, the beach will tend to be carried still by that long shore current straight along the coastline, so that the beach will start building out in creating an extension of the beach that will not necessarily follow the bed of the coastline.

In this case, a sand spit has formed off the end of this breakwater. This wave tank shows how the sand spit built up. The waves strike the breakwater at an angle and bend around it into the harbor When sand is added, the waves carry it into the harbor where it builds up into a spit behind the breakwater. To prevent the harbor from being sealed off and the beach beyond from being deprived of sand, engineers installed a dredge to pump the sand back into the long shore current by picking it up in the harbor and dumping it further down the coast.

Not only do beaches change continuously as sand is moved through them by the long shore current, but seasonal changes occur as well. The beaches change from season to season. By summertime, the waves are fairly low and gentle, and that has a tendency to drag sand toward the beach and build up the beach and make it broader, wider, and as it piles up, they say it has a fairly gentle slope. In the wintertime, though, the larger waves, more energetic waves, pick up that sand, tend to move it offshore, and store it in large sand waves almost like underwater sand dunes, and so the beach becomes very narrow.

What sand is there is very, very steep in slope, and most of the beach is really located offshore finding a more stable position under that bigger storm wave. The beach is just one part of a much larger system that regulates the formation, supply, and deposition of sediment along the shore. This system includes the mountains where weathering processes turn rock into sediment, the rivers which transport that sediment to the coast, and coastal processes like the long shore current that redistribute the sediment along the shore. As we have seen with breakwaters, however, people can easily disrupt the natural balance of this system and alter its ability to operate normally.

Dams are another example of our attempts to control natural processes. These structures serve a variety of valuable functions. The generation of hydroelectric power, the establishment of lakes for recreational purposes, and, in this case, flood control and the storage of water for drinking and irrigation. Despite their value, dams are not without significant drawbacks. Sediment that is normally carried downriver to the beaches is trapped in the reservoir instead.

Beaches that don't receive a steady supply of river sediment will soon disappear. It is tempting to cast people as the villains in this apparent conflict with nature, but the issue is not that simple. What would happen if we didn't dam rivers? Would we be willing to risk the exposure to catastrophic floods and to give up the electrical power and the fresh water that dams provide? If not, is the damage they cause to coastal property and to the beach environment too high a price to pay? These are difficult choices, and there are no perfect solutions.

Problems often arise as the result of special circumstances. During severe storms, for example, crashing waves can batter coastlines. Such storms occur only once every few decades, but in the quiet periods in between, people tend to ignore the historical record of erosion and build along the edges of the shore. To protect the ocean view homes and hotels that are perched atop sea cliffs and along beaches, sea walls have been erected that reflect the energy of the waves away from the coast and slow down erosion; however, what may have sounded fairly straight forward in theory has become quite controversial in practice.

Those in favor of seawalls argue that the cliffs must be protected to safeguard the real estate above them; those who are opposed maintain that in the long run sea walls do more harm than good because they represent a threat to the beach itself.

Coastal erosion is a natural process, and as we begin to put houses on the edges of coastlines, I mean we're concerned about losing some of those homes, and so you want to slow the erosion. Well, you're trying to slow something that's quite natural. When you do that, you upset the balance of things. Seawalls to limit erosion are also cutting sand supply, and so putting in a seawall will for a short period of time lessen the amount of erosion, but what the result is that sediment is no longer there to be taken to the beaches.

The beaches receive part of their sand supply from cliff sides, and as you slow down the erosion of cliff sides, then the beaches are losing a source, an important source of their sediments. Another problem is that the flat surface of a seawall reflects much of the wave energy directly back toward the beach. Unfortunately, this can erode the sand at the foot of the wall eventually undermining it.

At the Scripp's Institution of Oceanography, scientists deal with this controversial issue on a continuing basis. Scott Jenkins of Scripp's Center for Coastal Studies is one of those involved in the design of seawalls, breakwaters, and other coastal structures. The goal is to design structures that do the job with a minimal negative impact on the environment. Jenkins and his colleagues use a wave tank and scale models to test their designs, in this case a breakwater. Sensors placed around the tank measure the heights of the waves both inside and outside the breakwater giving Jenkins an indication of its effectiveness at reducing wave energy. Data from the experiment is fed into a computer allowing the scientists to refine and retest the design before an actual prototype is built.

When designing the seawall, Jenkins and his colleagues turn to nature for inspiration. The irregularly shaped surfaces of sea cliffs and coral reefs reflect a minimal amount of wave energy, so the Scripp's scientists decided to incorporate nature's energy absorbing design into their seawall. So far this wall has been a success.

The property's been protected from further erosion without destroying the beach at the base of the wall, but while Jenkins is committed to building the most effective seawalls he can, he recognizes that they are only a short term solution, and he is sensitive to the arguments of those who oppose attempts to redirect or in any way modify natural processes along the coast.

There's a wide variety of environmental groups, and there's a wide range of government officials and university professors who oppose construction and structural intervention on the shoreline, and the reason is philosophical, that we want to preserve the shoreline in its natural state.

Now, those who are going to lose property if erosion continues also have a concern, and those are the people who, of course, are going to favor these structures, and my personal belief is we should adopt the policy of maintaining the coast line in its natural state, and a large part of that policy would involve bypassing of sediments around dams and preventing further encroachment of coastal structures in the near shore area, and then I would say having made those fixes, let the system adjust to its own equilibrium.

There's far too much energy out there for man to compete against. Jenkins contends that doing a better job of transporting sediment around dams would be an important long term solution to the problem of beach erosion. Basically it's an earth moving problem, and we already have a very well developed technology in earth moving.

Now, in Southern California and in many other areas as well there are seasonal fluctuations in the lake level, and typically the lake levels are low in the summer, but whatever season they're low, earth moving equipment can come in and excavate these sands from the dry foreshore area. The foreshore deltas in these reservoirs contain most of the beach sized sand, and these will be high and dry when the lake levels are low, so they can be collected with standard earth moving equipment and either trucked directly to the beach or reintroduced to the streambeds downstream of the dam.

If there is technology and engineering available for transporting sand around dams, why isn't this being done? One reason may be that many scientists originally rejected the idea that dams actually contribute to erosion, but that is no longer the case. The problem currently seems to be that the value of sand as a coastal resource may still not be fully recognized.

A lot of the sand is already excavated by sands and gravel companies for construction material. It should be treated as a public resource and a fair market value paid for it. For instance, people on the beach would be willing to pay many dollars per cubic yard for nourishment sands, sands that sand and gravel companies haul away at just a fraction of a dollar a cubic yard, so this needs, I think to be regulated just like water and treating sand as a public resource.

Regardless of how the battle over seawalls and sediment supply eventually turns out, coastal dwellers will always have to deal with incursions from the oceans. In addition to problems caused by crashing waves, there are a number of other factors that affect the level of the water.

The most familiar is the action of the tides. Tides are primarily the work of the moon, and to a lesser degree, the sun. As the moon orbits the Earth, it exerts a powerful gravitational pull. This causes the ocean on the side of the Earth facing the moon to bulge out slightly. Another tidal bulge occurs on the other side of the planet as water lags behind due to weaker gravitational attraction from the moon.

These bulges create a high tide. High tides can create tremendous havoc, especially if they're combined with violent storms. This is what happened in 1970 in Bangladesh when a cyclone combined with a spring tide flooded the delta of the Ganges River drowning a quarter of a million people, but such tidal disasters are rare.

Most of the time the twice daily ebb and flow of tides only brings about small brief changes in the water level, but there is also a long term change going on all the while. Since the peak of the last ice age tens of thousands of years ago, melting glacial have spilled intense qualities of water into the oceans causing a rise in sea level of over 100 meters. Such a global change in the volume of water in the ocean is known as a "eustatic" change.

Although today's sea level is much more stable than it was at the time the ice age ended, a small eustatic change is still going on. The glaciers of Greenland and Antarctica are continuing to melt faster than they grow. This causes a small but steady rise in sea level worldwide; however, between now and the year 2100 there may be a significant increase in sea level due to the so- called Greenhouse Effect.

Carbon dioxide and water vapor in the atmosphere act like the glass of a greenhouse. They let in the sunlight but trap some of the reradiated infrared heat energy. Without this Greenhouse Effect, the Earth would become too cold to support human life, but since the Industrial Revolution began to mechanize our world in the late Eighteenth Century, we've been adding tremendous quantities of carbon dioxide to the atmosphere by burning fossil fuels.

The first of these was coal, the fossil remains of vegetation. Burning coal produced the steam which powered steamships, factories, and locomotives. It also released vast amounts of carbon dioxide, which until then had been stored underground for millions of years. Since the early days of the Industrial Revolution, the world's reliance on fossil fuels has increased dramatically. Today these fuels include not only coal, but gasoline and oil. If we continue to burn these at our present rate, the amount of carbon dioxide in the atmosphere will increase significantly. This could magnify the Greenhouse Effect to such an extent that air temperatures could rise by several degrees and accelerate polar ice melting which would result in a rise in sea level of a few meters.

This may not seem like much, but it would be enough to flood many of the world's coastal communities.

Although the coastline appears to be a stable and permanent fixture of the landscape, it's in fact a place of inevitable change. When people choose to live here, they become subject to that change and run the risk of losing everything either suddenly or steadily over time. Permanent protection for coastal development simply doesn't exist, and many protection schemes actually degrade the quality of the beach that attracted people here in the first place. As the result, it's becoming increasingly important to develop a wise coastal management policy that incorporates the most current scientific knowledge with the needs of the environment and of our community.

It's clear that there's a significant role for geologists and indeed for all of us to play in learning to protect the coastline for ourselves and for future generations. Funding for this program was provided by the Annenberg C.P.B. Project.

Basically we can look at any dynamic system in terms of something being added, something being removed, and something being stored in some sort of reservoir, so we find that if the input equals the output, the amount that's stored remains constant, and of course, a change in either the output or the input causes either a decrease or an increase in the amount of storage.

There are many examples we can look at, models, I should say, to try to get a sense of this. The simplest one would be simply a tub of water that has a drain and a faucet, and as long as the rate at which the water is input from the faucet equals the rate at which it goes out the drain, then the level of water in the tub stays constant. This we can also extend this model to think of the surface water system.

Water in the ocean behaves much in the same way. As long as input by precipitation and runoff equals output by evaporation, the level of the water in the ocean stays constant, and when there are disruptions to this system, for example, when glacial ice removes some of the runoff, then the water level in the sea falls.

We can also think of the Earth's exterior heat engine as a dynamic system. We're constantly receiving heat from the sun, and that heat is redistributed and stored here on Earth but is also reradiated back out into the atmosphere out of the atmosphere, and as long as the amount radiated away equals the amount input from the sun, then the Earth's temperature remains relatively constant, so we find that the characteristics of a dynamic system are that material and/or energy flows somehow through the system, and material you can think of is being moved somehow by various forms of energy, and the reason we bring this up at this point is because all natural systems are dynamic systems which are attempting to establish equilibrium; in fact, this idea of equilibrium or balance is one of the driving forces of nature and all natural systems.

The problem is that systems never quite make it to equilibrium because of changes either in input or output.

Streams, for example, try to establish an equilibrium of sedimentation, transportation, deposition, and erosion. Okay, so we can apply this now to the beach. The part of the beach that we see on the shoreline represents the stored material which is passing through the system, and the sand on the beach today may be quite different although the beach itself may be in the same place as it was last year sort of like the crowd at a trade show. The people who are in the building at any given time are not the same people because there are people coming and people going all day long, but they tend to remain in the same place, so basically we can look at the beach as sediment which collects in places where the energy of the water is not sufficient to keep the grains in suspension.

The input or the source of material for the beach is from streams or from offshore areas. Here in Hawaii it's coral reefs. The reef material is broken shells and coral, and, of course, we only find this in tropical regions where coral grows. The competency of the streams which are entering near the beach determines the size of the material introduced into the system, and, of course, the beach itself is determined by the strength of the long shore currents and the strengths of the waves.

Okay, the movement on a beach is not just like a river. I should say is not just a single flow in one direction, but there's a lateral as well as a parallel movement, so the actual path of a particular sand grain may be relatively slow and chaotic. The sand grain may move around on shore and off shore and along shore, but eventually it migrates downstream. We can think of a beach this way as a river of sand.

Okay, so the essence of what I'm getting at here is the amount of material on a beach, which represents that stored material, may fluctuate on both human and geologic time scales in large scale cycles, for example, which correspond to climatic cycles, and it also may happen in a noncyclical fashion as the landscape matures or is rejuvenated; in other words, as the landscape inward of a particular coastal area becomes more mature, more sediment is transported to the coastline, and so that material may represent a net accumulation of material on the beach. The output of material from the beach is to the deep sea floor. Some of it simply, the finer grain materials simply is swept over the wave cut terrace and disappears, but much of it accumulates at the heads of submarine canyons, and the submarine canyon may act as a conduit to transport this material eventually to the sea floor by turbidity currents.

Most submarine canyons transport material from beach cells rather than individual beaches. By a "beach cell" we mean that the sand may move from one beach to another, so that the output from one beach through long shore movement provides the input for another beach, and in this system the cell itself which may consist of two, four, seven, ten, some large number of beaches may actually represent a dynamic system as well, so that one beach may gain sand temporarily while another one up or down the stream loses, so the number of beaches in a cell varies from place to place, but each cell is in itself an equilibrium system, so the reason for bringing all this up now is that when humans and our culture interact with the beach system or, in fact, any dynamic equilibrium, we don't generally respect this concept of dynamic equilibrium and how changes can occur over time.

Keep in mind the idea of dynamic equilibrium means that changes will occur, small gains and losses here and there as time passes. Natural changes in our beach fronts are generally contrary to our usage plans. This is economically driven. We build buildings, highways, bridges, ownership of land are all dependent upon a beach or a particular coastline being in a certain place at a certain time. If you buy property, you want to have the property still be there next year, so if the sand starts disappearing, you want to move it, or you want to do something to change it.

Natural changes such as these are seeing to a threat to our usage plans, so we try to prevent these changes by modifying the beach. What we generally don't realize is that modifications some place on the beach will affect other portions of that same beach and also affect other beaches downstream in the cell. Changes may also affect beaches upstream as well. We do things like build drawings and seawalls to prevent erosion, which may temporarily work on a limited portion of one beach by interfering with the long shore sediment flow, but it generally causes erosion further down the beach by robbing the long shore currents of their sediment load, so the moving water establishes a balance between erosion, deposition, and transportation, and the water will carry as much sediment as it's capable of carrying, so if we take sediment away from it here on my growing down the beach, your sand may start to disappear because I've used that sand to deposit to build up my beach. We also interfere with this at the other end by channelizing streams in urban areas, and this alters the nature and amount of material that's input to the beach, and this reduced input may cause erosion to exceed deposition further down the beach, so that the beach may disappear entirely.

Dredged harbor channels also interfere with the sand movement, and we often have to continue dredging to keep the channels open. Beaches here in Hawaii, especially in Waikiki, have been so extensively modified that they no longer resemble natural systems. We've removed the sources of sediments by channelization. We've killed off the coral reefs. We've built groins and seawalls. So what's the solution?

Well, the solution in Waikiki has been to ship sand from other places to provide the supply. This requires expensive and frequent additions to maintain the economic viability of beaches of tourist attraction. The problem is that these other places where we get the sand from are coming in short supply, and in the past anyway, the effect on these other places due to the removal of sand hasn't been considered because of the limited population in these places. Waikiki is not the only place where these disruptions have required extensive and expensive measures to preserve the coastal area. The question that we need to ask here is have we learned from the mistakes we've made not only in Waikiki but in other kinds of things?

Other beach front areas in Hawaii are now experiencing rapid growth and development, and our environmental awareness has increased, and there's more of a sense of coexistence as opposed to our domination of natural areas, but only in a limited sense, so our human modification of beach systems serves as a good model for the concept of dynamic equilibrium and the disruption of these equilibrium systems. Beaches are rather small features geologically speaking.

The changes on beaches are usually immediately obvious over short time periods, a few years and take place on very short time scales, so here in our geology course, we're now down to the final two lessons which deal with our relationship with the planet. Specifically we now want to take these concepts and look at geologic hazards and resources, and specifically we want to approach this from our attitudes about our relationship with the Earth, so next time the Lesson 25, "Living with Earth: Geologic Hazards," so I'll see you next time.