Our bias is that a beach never stays the same. The grains of sand as well as pebbles and cobbles on a beach are moved and sorted by a combination of waves, ocean currents, tidal currents, and winds. As the grains move, the shape of the beach changes, and so the next combination of waves, currents, tides, and winds work on a beach that is slightly to significantly different in comparison to the previous one.
Go to a beach during a storm and you will find a very different place compared to the same beach on a calm, sunny day (this is good advice for those imprudent enough to consider buying a house near a beach). Other, less dramatic changes take place on the beach when different types of waves and currents are at work. For example, the combination of a strong wind and strong current going in the same direction produces a very different effect from wind and current moving in opposite directions. On New Year’s Day 1989, at Topsail Island, North Carolina, 40 mph (64 km/h) winds were blowing from the north exactly parallel to the shoreline, causing the water to move to the south like a mountain stream, complete with standing waves oriented perpendicular to surf-zone waves. The surf zone was a 150 yd (about 140 m)-wide swath of water discolored by its high sand content. Eight-foot (2.4 m) waves were breaking just offshore in the zone of rapid water movement, stirring up the bottom and adding to the current’s sand load. During about eight hours of wind activity, the volume of sand moved in the surf zone probably equaled two or three years of normal sand transport on this particular beach. Had the wind been blowing offshore, the sand transport would have been negligible.
Plunging breakers like this curling crest form as waves move into shallow water. After the initial break, the wave re-forms and runs onto the beach. Note the curtain of spray rising off of the breaker, indicating a strong wind blowing seaward.
Waves are the single most important force determining the nature of a beach. Their ability to move sediment around and change the shape of beaches can be measured in terms of the amount of energy they carry. This energy transfer, in turn, depends very much on the height of the waves. During storms, when wave heights are at a maximum, much more sediment (and much bigger grains) can be moved than during normal wave conditions.
Waves are formed by wind blowing across the water surface. Wind transfers its energy to the ocean surface by friction as it blows, forming waves. The longer and harder the wind blows, the bigger the waves. The greater the distance over which the wind can blow to form waves (the fetch), the bigger the waves.
The waveform then moves that energy (not the water itself) through the sea surface toward the shoreline. Finally, the energy is expended on the beach when the waves reach the shore. The energy is spent or dissipated in three ways: the waves break, other types of waves and currents are formed, and sediments are moved.
Diagram showing the changes in wave length and wave height as a wave approaches the shore. Water particle motion in deep water is circular (orbitals), with little forward motion of the water mass, but as the wave begins to drag bottom in shallow water, the orbitals become elliptical and the wave breaks in the surf zone, carrying the water mass forward to move beach sediment. The depth at which the wave first feels bottom is the wave base, usually about half of the wavelength. Drawing by Charles Pilkey.
It is an interesting fact that in deep water, beyond the surf zone, the water mass does not move forward with the wave; rather, the wave is simply energy passing through the water mass. As a wave moves through a particular spot, the individual water particles move in a circular orbital pattern, up and down but with only slight forward motion. The diameter of the orbitals of the water’s motion decreases with depth until the wave is no longer disturbing the deeper water. In this way, the energy, not the water, is carried toward the shore. As the wave runs into shallow coastal water, the base of the wave intersects with the seafloor, begins to expend energy on the bottom sediments, and undergoes a change in wave orbital shape. This depth is known as the wave base.
Waves are described according to the wave height (the difference in elevation between the crest and the trough), the wavelength (the distance between successive crests), and the wave period (the time between two crests passing the same point measured in seconds). Wave buoys have been placed all around the world to measure wave character, and you can view records for many of these buoys on the National Data Buoy Center Web site at http://www.ndbc.noaa.gov. For example, such wave buoys off western Ireland record open ocean waves larger than 40 ft (more than 12 m) two or three times each year. In the wide expanses of the ocean, waves can get bigger than 50 ft (more than 15 m), but most beaches normally experience waves of 3 ft (1 m) or less.
The amount of energy in a surface wave on the open ocean is proportional to the wave height squared, so a small increase in wave height can mean a big increase in wave energy. A wave more than 13 ft (4 m) high, for example, has not four, but sixteen times more energy than a wave that is just over 3 ft (1 m) high.
These buildings are elevated on stilts at the landward edge of a very wide, flat, low-tide beach on the coast of Brazil. Note the yellow building and sign for reference points in comparison to this location in the high-tide photo.
The same location at high tide is a good example of how a small rise in sea level can move the shoreline position a considerable distance landward, a good analogy for understanding the impact of the longer-term sea-level rise. The low wave energy at this location does not put these houses at high risk; however, during infrequent storms considerable damage can take place. Photo set courtesy of Allen Archer.
Geologists often categorize beaches according to how much wave energy they normally receive (the Armstrong Price classification, discussed in chapter 2). Thus, we refer to beaches as low, moderate, or high energy. High-energy beaches are found on exposed oceanic coasts, such as the northwest coast of Ireland, the south coasts of Iceland and Australia, and the west coasts of Morocco, Taiwan, Chile, and the United States, while low-energy beaches are typical of sheltered seas or bays, such as those in southeast Florida (protected by the Bahama Banks), on the west coast of Turkey, on Spencer Gulf in Australia, and on the west coast of Madagascar. As noted in chapter 2, the width of an adjacent continental shelf also influences the wave energy reaching any particular open-ocean coast. The friction on waves that cross a wide shelf reduces the energy of the waves that finally break in the surf zone.
Storms can cause very dramatic changes on beaches. In the middle to high latitudes of both the Northern and Southern Hemispheres, low-pressure systems circle the globe, producing frequent storms. Beaches at these latitudes can expect to be affected by several storms every year. In the tropics, however, tropical storms, hurricanes, and typhoons occur seasonally but might make a direct hit on a particular beach only once every few decades. Right at the equator, few storms occur, which is why houses (on stilts) are built right on the beach, seaward of the high-tide line, as on the Gurupi Islands south of the mouth of the Amazon River in Brazil.
Any visitor to the beach will occasionally see regularly spaced waves with long, continuous crests and similar wave heights approach the beach. These waves are called swell, and they are usually seen at the beach on a calm day. Swell, which is caused by distant storms, is most common on beaches that face a large expanse of ocean. Swell waves usually have large spacing between successive crests; the time between two crests passing the same point (wave period) is usually greater than 7 seconds and can be as much as 15 seconds.
On other days, the sea surface can look like the inside of a washing machine, with waves of many different sizes moving in several different directions in a chaotic pattern. These types of waves are referred to as sea. The difference between a sea and swells is really a matter of time and distance. When you find sea waves at the beach, it will usually be windy; a local wind has produced the waves on the spot. Sea doesn’t have the beautiful continuous crests of swell; instead, the waves occur in discontinuous lines. These waves are usually also smaller (they have less wave height) than swell and have shorter wavelengths and wave periods (3 to 6 seconds).
At the point where the waves originally form by transfer of energy from wind to ocean, the waves are sea waves. As they move away from the point where they were formed, they begin to get sorted out according to size. Waves of similar size travel at similar speeds; each group is known as a wave train. The largest of the waves travel fastest, and they become the swell. Surfers know that along California beaches, swell may come from thousands of miles away, both from the Aleutians and from the Southern Ocean in the vicinity of New Zealand. Local weather forecasts from “surfers’ stations” provide warnings of impending swells, sometimes days ahead of time, allowing surfers to make plans to catch these waves!
For large parts of the world, the waves are driven by persistent winds that come from a consistent direction, such as the trade winds. Examples include the waves on the outer coasts of the Windward Islands (e.g., St. Lucia, Martinique) and Barbados. In other areas, the wind direction is less predictable, and some areas have distinct seasonal variations in wind and wave direction. For example, the beaches on the East Coast of the United States are affected mostly by northerly winds in the winter and southerly winds in the summer.
The Hawaiian Islands are typical of island chains in the middle of an ocean basin. Depending in part on the time of the year, they receive swells from any direction, including the trade wind waves from the northeast, the Kona storms from the southwest, the North Pacific swell from the Aleutians, and the South Pacific swell from the Southern Ocean.
Waves slow down as they move into shallow water because the waves start to interact with, or feel, the seabed. In this process of shoaling, the waves become higher and steeper and eventually become unstable and break. Wave steepness is defined as the ratio of wavelength to wave height. When the steepness gets to about 7, the wave becomes unstable and breaks. Large waves start to break quite far offshore, while smaller waves travel into shallower water before they break.
There are three types of breakers in the surf zone: spilling, plunging, and surging. The slope of the seabed and the size and steepness of the original waves coming in from deep water determine the breaker type. On gentle slopes, large waves (which are often low-steepness swell) usually break gently far from the shoreline as spilling breakers, the most common type of breaking wave. In this gentle type of breaking, the wave loses energy gradually as it continues to move onshore. Sometimes a single wave can form several lines of spilling breakers. This is the type of wave that often forms rip currents, described later in this chapter.
On steeper beaches with relatively steep waves, the crest of the breaker moves forward faster than its base, trapping air beneath it before it connects with the water again. This plunging breaker (sometimes called a dumping breaker) occasionally produces the “tube” of trapped air of which surfers are so fond. Sometimes these waves occur where there is a sudden rise in the seafloor, created, for example, by a large sandbar or the edge of a coral reef. Plunging breakers expend most of their energy all at once, unlike spilling breakers, which expend their energy while crossing wide, dissipative beaches.
On the steepest beaches, surging breakers form when waves rush up the beach in a smooth, sliding movement but no “curl” forms at the front of the waves. These waves can be dangerous to unwary swimmers, especially children, because of the strong currents associated with backwash. Surging breakers are most common on reflective beaches, where the incoming wave crashes into the steep, often gravel, slope.
After breaking, any remaining energy in the waves is used up in the swash, where the water rushes up the beach and either seeps into it or rushes back as backwash. It is this phenomenon that is responsible for determining the slope of beaches between the high- and low-tide lines (discussed further in chapter 5). As wave swash rolls up a fine-sand beach, very little of the water soaks into the beach. Thus, backwash can carry some of the sand in a seaward direction and flatten the beach. If the beach consists of potato-size cobbles, the swash moving up the beach in large part disappears in the crevices between the rocks. There is little or no backwash to carry sediment seaward to flatten the beach.
Usually the slope of the beach is just sufficient to cause all the incoming energy to be used up as the waves move onshore. During storms, of course, extra energy is in the waves and the beach might not be able to absorb all the incoming energy at the shoreline. When this happens, the extra energy is used up in different ways. Sometimes the waves erode dunes or cliffs at the rear of the beach, whereas at other times, waves pass right over the beach, carrying beach sediment as washover into the dunes or completely across a barrier into a lagoon or bay.
Giant waves up to 100 ft (30 m) high have been reported in the open ocean and are referred to as rogue waves. They form when waves coming from more than one direction combine to form a giant wave, which commonly disappears after a while as the component waves separate and continue on their original paths. On February 13, 2010, however, a rogue wave struck Mavericks Beach in Half Moon Bay, California, during a surfing contest. Not only were surfers upended, but onlookers were knocked down, and their cameras, cell phones, and backpacks were washed away. No deaths were reported, but a number of people suffered broken bones. Similarly, on July 3, 1992, an 18 ft (5.5 m)-high wall of water hit Daytona Beach, Florida, causing great damage to parked cars and slightly injuring seventy-five people. It is still not clear whether this was a rogue wave (weather conditions were calm at the time) or a rare North American Atlantic tsunami. If it was a tsunami, it probably formed as a result of a giant landslide on the continental slope 50 mi (80 km) offshore.
One of the most important phenomena associated with waves is their capability to change their direction as they move into shallow water. This directional change happens because as a wave begins to feel the seabed, it slows down. If the wave is moving at an angle to the shoreline, then only part of the wave is feeling the seabed, while other parts are still in deep water. The part of the wave in shallow water moves more slowly than the part in deep water, and so the wave crest begins to bend. This process, in which the crest of the wave bends, or refracts, changes the angle between the waves and the shore.
Wave refraction is a complex phenomenon that depends primarily on the wavelength, the size and type of the wave, the direction from which the wave is coming, and the nature of the seafloor topography. Long waves (such as swell) feel the bottom in relatively deep water and so begin to bend far from the shore. It is quite common for swell waves to be completely refracted by the time they reach the shore, so that, although they may have arrived at the nearshore area at a steep angle to the beach, they break in a line of surf that is almost parallel to the beach. Shorter sea waves feel the seabed only in shallower water and may not be completely refracted at the shoreline, so it is more common for these waves to arrive at an angle of between 1 and 5 degrees to the beach.
Refraction around islands and across an irregular seabed can produce unexpected patterns as the resulting wave crests converge (concentrating their energy), diverge (dispersing their energy), and cross (creating turbulent conditions as the waves interact). In a classic 1947 study by Walter Munk from the Scripps Institution of Oceanography, the impact of continental shelf topography on waves on the beach was investigated. On the Southern California coast, Munk noted that submarine canyons cause waves to diverge on the beach, producing zones of low wave energy at the heads of canyons. A number of California fishing piers are located at the heads of submarine canyons, unknowingly taking advantage of the low wave energy produced by wave refraction in the canyon.
Waves can also experience a process known as diffraction, which involves a change in the direction of the waves as they pass through a narrow gap between two rocks, between jetties, or around a barrier in their path. This is the process that brings waves to the lee side of an object such as an offshore breakwater or other obstacle. It is the means by which the energy of incoming waves spreads laterally, perpendicular to the direction of the incoming waves.
Wave reflection is common off of rock cliffs and seawalls. Here at Portrush, Northern Ireland, the sea waves are approaching from right to left, and the spilling breakers are on the left of the wave crest. These waves are reflecting off of the seawall in the background, producing small waves that are spilling in the seaward direction (the wavelets visible in the middle of the photo, and the small wave in the far center background). These reflected waves are moving from left to right and collide with the incoming waves (note the colliding waves creating the large patch of white water in the center background). Sometimes a checkerboard pattern results, in which the wave-crest lines of the two wave sets cross. Photo courtesy of Norma Longo.
Wave reflection is a process seen on rocky coasts, seawalled shorelines, and steep natural beaches. As the name implies, a portion of the energy of the wave is reflected and moves backward toward the ocean, through the incoming surf. Such waves may be partly responsible for the troughs in front of seawalls.
Waves produce currents when they approach the shore. Probably the best known of these are longshore currents and rip currents. Longshore currents, sometimes called littoral drift, form when waves approach the shoreline at an angle and continue to push the water in the same direction as the wave breaks. Such currents are strongest in the wave-breaking zone and are easily observed by anyone standing on the beach. The greater the angle between the wave crest and the shoreline (up to 30 degrees) and the larger the wave height, the stronger the longshore current. Winds blowing alongshore can also cause or augment existing longshore currents, as previously described for a storm on Topsail Island, North Carolina. Tidal currents, produced purely by the rise and fall of the tides, can also create currents that flow along the shore. Longshore currents are the principal means by which sand is moved along beaches in a process known as longshore transport.
The longshore current is set up by waves approaching the shore at an angle, in this case from the lower left in the diagram. The left end of the wave crest line enters shallow water first, slowing down due to friction with the bottom. Where the wave is in deeper water (to the right), it continues without slowing down, so the wave crest line appears to bend as it is refracted landward. The wave is piling water up in the nearshore, and ultimately that water must flow laterally, creating the longshore current. Such currents have velocities high enough to move sediment set in motion by wave turbulence. Drawing by Charles Pilkey.
The amount of sand moved on beaches by longshore transport is highly variable from beach to beach. Such volumes are difficult to measure or even estimate accurately, and the volumes of sand moved vary greatly from year to year. For example, an estimated 10,000 cu. yd. (about 7,650 m3) are moved to the west on Bogue Banks, North Carolina, in a typical year. This number is so small and the accuracy of such estimates is so poor that it is considered to be essentially zero annual transport of sand. More commonly, beach sand transport volumes on the order of 100,000 cu. yd. (about 76,500 m3) per year are estimated. The net southward sand volume transport at Nags Head, North Carolina, is assumed to be 500,000 cu. yd. (more than 382,000 m3) per year. The word net here is important because there are many times when the wind, waves, and currents reverse and move sand in the other direction. For the barrier island beaches of southeastern Iceland, the estimate is a net of 4 million cu. yd. (well over 3 million m3) of sand moved to the east every year. To understand the meaning of these numbers, remember that a good-size dump truck carries 10 cu. yd. (7 to 8 m3).
Rip currents, always a hazard to swimmers, are narrow bands of water that flow directly or obliquely offshore. They are caused when waves pile water in the surf zone, and the water has to escape somewhere. Often rip currents occur through gaps in an offshore bar. Under these circumstances, the currents will remain in one place for hours and even days. The locations of long-term rip currents are occasionally marked by indentations, holes, or depressions in the beach, a common occurrence in some Australian beaches. Sometimes rip currents are connected to longshore currents, dividing the coast into a series of circulating cells with water moving alongshore until it joins a rip current and then moves offshore. Other rip currents are caused by obstructions in the surf zone, such as rock outcrops or groins. Surfers often use the offshore flowing currents next to groins and jetties to float with relative ease out to where they can catch the next big wave at the swell-to-surf transition.
An example of a large rip current (center) on the coast of Tasmania, Australia, shows the seaward head of the rip, with suspended sediment in the water, and the zone where the current cuts across the surf zone, interrupting the lateral wave crest lines. Suspended sediment is also visible on the seaward side of the surf zone, suggesting that longshore transport is taking place. Onshore winds are building dunes on the landward sides of the pocket beaches. Photo courtesy of Andy Short.
There are other types of off-and onshore currents that can also bring sand to and remove sand from beaches. When strong winds blow offshore from a beach, they gradually move water on a broad front (sometimes miles wide), away from the beach in a seaward direction. A depression is formed near the shoreline, crudely analogous to removing a cup of water from a water-filled bucket, and water moves ashore along the bottom to replace the water that has moved away from the beach. This landward-directed bottom current is referred to as upwelling, and if there is sand suspended by wave activity, this current may at times be strong enough to carry sand ashore, which is why offshore winds occasionally may cause a beach to widen.
The opposite may happen when strong winds blow in an onshore direction. The water piles up or mounds up against the shore, setting up a situation in which water must sink and flow offshore to maintain a uniform sea level. This mounding and onshore movement of water is called a storm surge, and the resulting seaward-moving bottom current, or downwelling, may take sand away from the beach. Hurricanes push water through and over barrier islands with the surge and onshore winds, but as the storm passes, the winds reverse to offshore and the amount of water moving seaward after the storm passes, referred to as ebb surge, can be very large and can have a very strong current. Sand can in this way be carried from the beach, many miles out to sea, even beyond the reach of the shallow nearshore waves that sweep bottom sand back onshore after storms. Such sand carried to these depths is lost to the beach system.
Each beach will have a different response to onshore and offshore winds, and observant longtime beach watchers (such as lifeguards and surfers) can often provide insight into how a particular beach works under various wind conditions.
Beautiful Lighthouse Beach on the coast of New South Wales, Australia, is fronted by a series of well-defined rip currents. Each break in the surf line of what appears to be calm water is a strong seaward-flowing rip current. The cuspate indentations in the beach that are lined up with the rip-current channels create a typical pattern. Note also that the two narrow rock headlands (top and bottom) are acting as natural groins, creating pocket beaches and blocking the longshore transport of beach sand. A mature dune field is at the back of the beach but has been reactivated (set in motion again due to loss of vegetative cover) in two areas by blowouts and associated parabolic dunes. Photo courtesy of Andy Short.
Waves interact with the beach in a two-way relationship. The shape of the submerged part of the beach influences the way waves break and create currents, but the waves and their currents also cause the beach to change shape. They do this by sediment erosion, transport, and deposition.
More energy is needed to pick up a sand grain from the beach surface than is required to transport it once the grain is in the water column. Most grains are picked up at the point where the wave breaks and stirs up the bottom, a process easily observed by snorkelers. These grains are thrust into the longshore and cross-shore currents, which carry them for distances proportional to the current’s strength. In the turbulent water of the storm surf zone, the grains may remain suspended for a long time, but under mild wave conditions, sand grains may move a few feet, then settle to the bottom, only to be kicked up again by the next breaking wave. The amount of sediment transport and the direction in which it moves are, of course, highly variable, depending largely on the size and orientation of the waves.
Bottom currents in the nearshore zone can be generated by strong directional wind or wave patterns. Upwelling occurs when the wind pushes surface waters offshore, causing bottom waters to flow landward to replace the seaward-moving surface water. Similarly, when the wind or waves push surface water onshore, there must be a seaward return flow of bottom water to maintain the sea level. The result is downwelling. These processes are important to the ecosystem as well because upwelling brings nutrients back into the surface waters, and downwelling takes oxygen-rich surface waters into the deep. Drawing by Charles Pilkey.
Some types of waves other than wind waves can affect beaches. Tsunamis (Japanese for “great harbor wave”) are formed by sudden disturbances such as underwater earthquakes, volcanic eruptions, giant underwater landslides, or extraterrestrial bodies (asteroids) landing in the sea. The sudden release of a huge amount of energy and displacement of a large volume of water at the seafloor forms a tsunami wave, which travels rapidly (hundreds of miles per hour) across the ocean surface.
Tsunamis have very small wave heights in deep water, barely discernable by ships at sea, but wavelengths may be hundreds of miles. Just like normal wind-driven ocean waves, tsunamis slow down in shallow water and rise to great heights before breaking onshore. Japanese fishermen, returning from uneventful times at sea, have found debris and bodies in the ocean as they neared the shore, illustrating the difference between deepwater and shallow-water tsunami behavior. In the days before radio communication, the fishermen had no inkling that a tsunami wave had passed by them and struck their villages onshore.
The best-known example to the current generation is, of course, the Indian Ocean tsunami of December 2004, which had a wavelength of more than 124 mi (200 km) but was only slightly over 1.6 ft (50 cm) high in deep water. That wave traveled at 466 mph (750 km/h) across the ocean, and when it reached the shorelines around the Indian Ocean, it rose to heights of more than 50 ft (15 m). The volume of water in a tsunami wave is much larger than that in a wind wave of the same height, hence the immense damage. Videos of the event also indicate that much of the destruction resulted from surging currents that funneled up topographic lows and between obstructions once the wave broke against the mainland. There were strong currents set up by the backflow as well, both over the land and in the nearshore. The tsunami killed more than 230,000 people in eleven countries that bordered the Indian Ocean. Throughout the area, pre-storm human impacts, such as loss of coral reefs and destruction of mangrove forests to make way for shrimp farming, played a significant role in increasing the damaging effects of the giant wave on shorelines. While this book was being prepared, the Chilean earthquake of February 2010 struck and generated a deadly tsunami on the Chilean coast. Although the rest of the Pacific Rim beaches were little affected, the fact that people in Australia went down to the beach to see the tsunami arrive attests to the general lack of understanding about coastal processes. The Chilean earthquake demonstrated another natural process of beach modification, namely, the catastrophic uplift or downwarping of coasts.
Post-tsunami videos and photos, however, demonstrate an important fact: The beach is still present in most localities. Static structures are destroyed, but the dynamic beach flexes.
Tides are formed by the gravitational pull of the moon and the sun on the rotating globe. For various reasons, the difference in water level between high tide and low tide at the coast is very important in shaping a beach. First, the higher the tidal range, the wider the beach is likely to be, simply because a greater distance of intertidal area is exposed. Second, if the tide is out, currents and waves do not reach the dry parts of the beach. At any point on a beach, the chances of waves moving sand depend on how often waves occur there, and these periods are reduced if the tidal range is great. Put simply, a large tidal range spreads out the energy of the waves over a wide surface area during a tidal cycle. Although wave action might be minimized on beaches in areas with large tidal ranges, beaches are still affected by strong, sediment-transporting tidal currents.
Some coasts have complex tides that rise and fall more, or less, frequently than the common, semidiurnal (twice-a-day) tides. On any coastline, tides also vary from day to day in a regular, predictable way as the Earth revolves around the sun and the moon revolves around the Earth. This variation, which fishermen and mariners are well aware of, is the change that occurs every seven days between the spring tide (largest tidal range) and the neap tide (smallest tidal range). Over the course of a year, the height of spring tides also varies, with the largest tides of the year occurring on the spring and autumn equinoxes (March and September): the equinoctial tides. There are many longer-term cycles in tides that are driven by planetary and lunar orbits and controlled mainly by the closeness of the sun and moon to the Earth. These include the 18.6-year lunar nodal cycle.
Spring and neap tides result from different alignments of the Earth-moon-sun system. Spring tide, which is the time of maximum tidal range between low and high tide, occurs when the Earth, moon, and sun are in a line so that the gravitational pull of the moon and sun are cumulative. When the sun-Earth-moon alignment is at right angles, the tidal range is at its minimum, the neap tide. Drawing by Charles Pilkey.
For coasts with a significant tidal range (more than 6.5 ft [2 m]), a storm striking a shore at high tide will cause a greater penetration of waves, water, and sand onto the land behind the beach than would the same storm striking the same beach at low tide. This difference certainly applies to hurricanes, which usually make landfall in a short time interval. However, nor’easters (extratropical storms with extensive storm fronts) usually last through one or more complete tidal cycles, so their impact is sure to be great for any coast that has a high tidal range. On the other hand, if the tidal range is small, like the 1 ft (30 cm) tidal amplitude of Tahiti, the timing of a storm makes little difference in its impact on beaches. A large (more than 26 ft [8 m]) swell on the KwaZulu-Natal coast of South Africa in March 2008 took place during the peak of the 18.6-year tidal cycle, on an equinoctial high tide. Large swells in the past had done relatively little damage, but the water level of this storm was raised on top of the very high tide, so the waves reached parts of the coast behind the beach that were normally well out of reach. The result was that this storm’s impact was devastating, with widespread coastal erosion along what had been perceived as a stable coastline.
Although somewhat hidden in the shadows, this visible erosional impact and property destruction at Ballito on the KwaZulu-Natal coast of South Africa was caused by exceptionally high tidal levels during a 1997 storm. The waves atop the high tide not only undermined this house but also completely demolished another house that was in front of it.
The third role of tides on beaches is the production of tidal currents, as water flows into and out of harbors, embayments, estuaries, and tidal inlets. Usually wave-formed longshore currents move much more sand than tidal currents do. However, strong tidal currents are common through inlets between barrier islands and in restricted embayments with large tidal ranges (e.g., the Bay of Fundy, Canada). These currents move large volumes of sand and form tidal deltas and various types of sandbars and fields of sand waves. Such sand transport and sandbar formation may add to or subtract from the sediment supply to adjacent beaches.
In general, the height of tides depends on the width and slope of the continental shelf and the configuration of the coast (see chapter 2). The flatter and wider the shelf, the higher the tides will be. Beaches can be found under a whole range of possible tides, from low-tide conditions (less than 1 ft [30 cm]), for example, those of oceanic volcanic islands and enclosed seas, such as parts of the Mediterranean, Baltic, and Caribbean seas, right up to the highest tides on Earth, in the Bay of Fundy, where the water level can rise more than 42 ft (13 m) between low and high tide. Other extreme high tides (with approximate highest tides) can be found in the Bristol Channel in southwestern England (more than 39 ft [12 m]), the Gulf of St. Malo in northwestern France (37 ft [11.5 m]), the Strait of Magellan in southernmost South America (36 ft [11 m]), the Okhotsk Sea in Siberia (more than 25.5 ft [9 m]), and northeastern Australia (25.5 ft [9 m]). Tidal range also is a controlling factor in the formation of barrier islands. Coastal geologist Miles Hayes classified barrier islands, in part, on tidal ranges. Microtidal ranges (o to 6.5 ft [o to 2 m]) favor long, narrow (hot dog–shaped) barrier islands, while mesotidal ranges (6.5 to 13 ft [2 to 4 m]) favor shorter, drumstick-shaped islands. Barrier islands generally do not occur in areas of large tidal range (macrotidal, more than 13 ft [4 m]) because the strong tidal currents rework the sand into sandbars parallel to the direction of current movement, such as in the Bay of Fundy.
The rise and fall of the tides is obvious in the visible surface-water levels, but beaches are saturated with water at high tide; they drain of that water as the tide falls. This changing of the groundwater level in the beach forces water and air into and out of the beach. As the beach drains during an outgoing tide, the water within the sediment is replaced by air. The incoming tide pushes the air out as the next tide rises. Evidence of such water drainage and escaping air is provided by the variety of small bedforms and sedimentary structures that result (see chapter 7). This effect also allows seawater and freshwater that enters the landward part of the beach from rain and runoff to interact and to produce yet another microhabitat for organisms.
Apart from waves and tides, the water level at the shoreline can rise and fall in response to surges. A surge is the difference between the predicted elevation of the ocean, based on the stage of the tide, and the observed height of the water. Surges are associated mainly with storms that have low air pressure and strong winds. The pressure is lower beneath the central part of a storm (e.g., the eye of a hurricane), and the water surface rises as a result (water levels rise 1.13 ft for each inch of mercury fall or 1 cm for each millibar decrease in pressure). The surge elevation is often even more strongly affected by strong storm winds piling water up at the shoreline. Hurricanes with winds of 75 to 95 mph (120 to 150 km/h) will typically produce a storm surge of about 5 ft (1.5 m ), while winds of 130 to 155 mph (200 to 250 km/h) can create a storm surge as high as 18 ft (5.5 m). During Category 5 hurricanes, surges on the U.S. East Coast have reached more than 20 ft (6 m). Hurricane Katrina’s surge reached more than 32 ft (10 m) in Mississippi. Even during a Category 1 hurricane, typical surges are 6 to 7 ft (1 to 3 m). The exact height of the storm surge is a function of the onshore wind velocity, the duration of the wind, and the slope of the inner continental shelf. The largest storm surge recorded appears to be 42 ft (12.8 m ), which occurred on the coast of Australia in 1899.
Surges are largest in bodies of water that are shallow and confined by surrounding landmasses. Like tidal amplitude and wave height, a potential surge is higher when the shelf is flatter and wider. Thus, the Gulf of Mexico is prone to much higher surges than is the Pacific Coast of the Americas. Negative surges can also happen when high pressure or offshore winds occur. This means that tidal water levels do not reach the full elevation expected from the gravitational pull of the moon and sun.
El Niño is one-half of the phenomenon known as ENSO, or the El Niño–Southern Oscillation. This meteorological condition occurs when atmospheric pressure increases in the western Pacific (the area around Australia) and declines in the eastern Pacific (the area around Tahiti). The resulting change in the trade winds brings warm water to northwestern South America, changing a zone of upwelling into one of downwelling. In Peru, for example, this water mass is associated with enhanced storminess and a higher-than-normal sea level in an otherwise arid region. Once the coastal upwelling ceases and nutrients are no longer brought from depth into surface waters, the local fisheries collapse.
The storms and high sea levels of an El Niño event can wreak havoc on the shoreline. Along the heavily developed West Coast of the United States, stormy El Niño years are the occasion of greater and more damaging coastal erosion. In the early 1990s, a beach in front of a small village on the west coast of Colombia retreated 3 ft (1 m) per day during spring tides due to higher sea levels, raised by the El Niño effect. Although El Niño events do not occur with any regularity, they happen at least once a decade, and therefore are normal, if extreme, coastal processes.
Anyone who has been on a beach during a gale is inevitably amazed, at least the first time, at how much sand is collected by his or her hair. At the same time, sand grains will have blasted their legs. The ability of wind to transport sand on the beach depends on the grain size of the sand and how wet it is. Even wet sand can be moved. Studies have shown that when wind blows across a wet beach surface, a layer of sand one or two grains thick is dried out, allowing sand to be readily moved.
The wind can be responsible for the movement of large volumes of sand on the beach. The most obvious manifestation of the work of the wind is sand dune formation at the rear of the beach (see chapter 8), but wind blows along the length of the beach and into the water as well. The proof is in the sand accumulated in the lee of bottles, logs, and shells and a variety of surface features showing wind effects and direction. In short, the wind is a major player in shaping beaches.
Upper left Storm surges raise sea level during hurricanes well above the elevations of barrier islands, resulting in extensive flooding and overwash. Barrier islands in particular depend on such processes for their origin and evolution, as washover of sediment brings sand to the interiors and back sides of the islands. Houses, however, do not fare well when they are impacted by storm surges, as shown in this photo of Dauphin Island, Alabama. Note that even after a hurricane, the beach is still present, though it may have moved landward. Reports of “beach erosion” do not mean the beach has disappeared, though some of the houses have.
Lower left The Pacific coast of Colombia is retreating for a variety of reasons; however, during the sea level rise that occurs during El Niño events, the rate of retreat accelerates. This old plantation house was originally built a considerable distance inland, but by 1990 the shoreline was at its front door. On each high spring tide during El Niño, the scarp at the back of the beach retreated landward 3 ft (1 m) so that within a short time after this photo, the building was lost. The retreating beach remained.
Upper right Storm surges on any coast allow waves to reach parts of the beach that are normally beyond the influence of waves. Here the waves atop the surge water level are cutting into the toe of the sand dunes, forming a wave-cut scarp. Although infrequent, this is one way in which the beach adjusts during a storm; the dunes serve the important purpose of supplying extra sand to the beach and absorbing the wave energy.
Lower right The wind is sweeping this beach at Ostend, Belgium, parallel to the shoreline. It is blowing toward the bottom of the photo, as is demonstrated by the sand accumulating downdrift of shells in the foreground. Variable wind directions move sand landward, parallel to the shore, and seaward. If this were a natural beach, a changing landscape of dunes would probably be situated landward. Instead, a wall of high-rise buildings creates a static scene.
The fact that the level of the oceans is rising is indisputable, whether humans are the cause of global warming or not. Tide gauge measurements going back one hundred years and satellite measurements going back to the early 1990s show that sea level is rising at a global rate of a little more than 1 ft (30.5 cm) per century. Most observers think that the rate is accelerating and that by the year 2100, the sea level will have risen somewhere between 3 and 5 ft (1 and 1.5 m).
During the twentieth century, the rise was caused mainly by thermal expansion of ocean water due to heating, plus a smaller contribution of water from both melting mountain glaciers and the Greenland ice sheet. As global warming proceeds, that relationship is changing: In the twenty-first century, meltwater from the West Antarctic Ice Sheet is expected to be the largest contributor, followed in importance by Greenland meltwater, the thermal expansion effect, and mountain glacier melting. If all of the West Antarctic Ice Sheet were to melt, sea level would rise 16 ft (nearly 5 m); the Greenland glaciers contain 20 ft (more than 6 m) of potential sea-level rise. All told, the mountain glaciers of the world have about 1.5 ft (more than 45 cm) of potential sea-level rise in them.
The actual amount of sea-level rise along the world’s shorelines varies from place to place. On many of the world’s river deltas, including the Ganges, Niger, Nile, Mekong, and Mississippi, the land is sinking, so the relative sea-level rise is large, locally as much as 4 ft (more than 1 m) per century in parts of the Mississippi Delta. At high latitudes in parts of Canada, Siberia, and Scandinavia, sea level is dropping because the land is recovering or rebounding from the release of the weight of glaciers that have recently melted away. Earthquakes often cause the land to sink or rise overnight, causing an instantaneous change in sea level, sometimes extending over hundreds of miles of shoreline.
Sea-level rise will do more than cause flooding. Storm waves will penetrate further inland; infrastructure will suffer serious losses (e.g., storm-water drainage systems, waste treatment systems); and salinization of groundwater and soils will occur. All of these effects already are occurring in some parts of the world. Ultimately, the sea-level rise will require massive retreat from the world’s beaches. Where there is no development, the beaches will not be impacted except to change their location in space. Where buildings, roads, and seawalls line the shore, the survival of beaches may depend on whether humans try to hold the shoreline in place. If future generations decide to fight nature, the beaches are doomed.
Beaches thrive in a hostile environment where human structures fail and cliffs collapse. To do this, beaches find a balance between their size, shape, and arrangement of sediments, on the one hand, and the actions of waves, sea level, currents, and wind, on the other. Their ability to change shape as these factors change is the secret of their success.
Geologists take various approaches in their efforts to understand beaches. One way is to make a sediment budget calculation. Like a financial budget, a sediment budget involves deposits, transfers, and withdrawals. A beach in which the sediment budget is balanced is said to be in equilibrium. If there is a negative sediment budget, the beach will erode; if the sediment budget is positive, the beach will advance and widen.
Many beaches exist in a dynamic equilibrium involving the following factors:
• Energy—waves, wave-formed currents, and wind
• Sea level—the water elevation at which the energy is delivered to the coast
• Materials—sediments, including amount, type, and grain size
• Shape and location—the three-dimensional shape of the beach and the surrounding solid geology
When one of the four parameters changes on a particular beach, the others change accordingly. For example, if a dam is constructed across a nearby river, the amount of sand coming down to a beach (materials) will be reduced. Loss of sand leads to erosion (shape and location).
One of the most famous examples of a dam cutting off the sediment supply to a coast is Egypt’s Aswan Dam. The Aswan High Dam, completed in 1970, almost completely cut off the sand supply to the beaches of the Nile Delta. Coastal erosion has ensued ever since, and the loss of nutrients brought to the delta floodplain and adjacent coastal waters has negatively impacted both agriculture and the commercial fishery. Similarly, in Southern California, between Point Conception and San Diego, dams on local rivers have reduced the sand supply (materials) by 50 percent for a number of beaches.
We know that the impact of storms (energy) may depend on the preexisting beach state (shape). Consequently, storms that strike in early fall will often make larger changes in beach shape than the same storm striking in early spring.
The gravel beach at the mouth of the Elwha River along the Strait of Juan de Fuca between British Columbia, Canada, and Washington State provides another example of how the equilibrium works. The present beach on the river delta is gravel, but before a dam was built on the Elwha River a hundred years ago, the beach was sandy, with only a small gravel content. After the dam was built, the supply of sand to the beach (materials) was almost entirely halted, but the beach depended upon a constant recharge of sand to maintain its grain size. Waves (energy) gradually removed the sand from the beach, leaving a lesser volume of gravel to accumulate to maintain a gravel beach that was narrower and steep (shape and location).
One problem that arose from the change in grain size was the demise of a clam habitat, eliminating an important source of food and income for a local Native American tribe. The Elwha Dam is scheduled to be removed in 2011 to restore both the salmon run and a sand beach for clam habitat.
And so it goes. The beach is one of the most dynamic environments on the Earth’s surface and is often associated with even more dynamic tidal inlets. The beach changes rapidly according to the whims of the waves, but it changes according to a set of rules that we now understand in a general way. How exciting it is to stand on a beach, to understand what is happening, and to guess where the future lies. For example, we could have guessed that the California and Nile Delta dams would have led to beach erosion, and any coastal scientist today would predict that the Elwha Dam might cause the loss of the beach clams at the river mouth. Now we are faced with a global sea-level rise, and there is no question that the beaches will be moving back at an ever-increasing rate in coming decades.
While we know the general rules of beach behavior, we also know that each beach behaves differently because of the infinite number of possible combinations of nature’s processes that affect them. This complexity makes beaches all the more exciting!
