By the end of this chapter, students should be able to:
- Describe how waves occur, move, and carry energy
- Explain wave behavior approaching the shoreline
- Describe shoreline features and zones
- Describe wave refraction and its contribution to longshore currents and longshore drift
- Explain how longshore currents cause the formation of spits and baymouth bars
- Distinguish between submergent and emergent coasts and describe coastal features associated with each
- Describe the relationship between the natural river of sand in the littoral zone and human attempts to alter it for human convenience
- Describe the pattern of the main ocean currents and explain the different factors involved in surface currents and deep ocean currents
- Explain how ocean tides occur and distinguish among diurnal, semidiurnal, and mixed tide patterns
The Earth’s surface is 29% land and 71% water. Coastlines are the interfaces between, and as such, the longest visible boundaries on Earth. To understand the processes that occur at these boundaries, it is important to first understand wave energy. The importance of the coasts is seen in the study of ancient shorelines, and particularly for natural resources, a process called sequence stratigraphy.
12.1 Waves and Wave Processes
Wind blowing over the surface of water transfers energy to the water through friction. The energy transferred from wind to water causes waves to form. Waves move as individual oscillating particles of water. Wave water moving forward is the crest. Wave water moving backward is the trough. To see wave movement in action, watch a cork or some floating object as a wave passes.
Important terms to understand in the operation of waves include: the wave crest is the highest point of the wave; the trough is the lowest point of the wave. Wave height—equal to twice wave amplitude—is the vertical distance from the trough to the crest and is determined by wave energy. Wavelength is the horizontal distance between consecutive wave crests. Wave velocity is the speed at which a wave crest moves forward and is related to the wave’s energy. Wave period is the time interval it takes for adjacent wave crests to pass a given point.
The circular motion of water particles diminishes with depth and is negligible at about one-half wavelength, an important dimension to remember in connection with waves. Wave base is the vertical depth at which water is disturbed by waves, above which waves will ripple shore sand. Wave base is measured at about one-half wavelength deep, where the water particles’ circular motion diminishes. If waves approaching a beach crest at about 6 m (20 ft) intervals, this wave motion disturbs water to about 3 m (10 ft) deep. This motion is known as fair-weather wave base. In strong storms such as hurricanes, both wavelength and wave base increase dramatically to a depth known as storm wave base, which is approximately 91 m (300 ft).
Waves are generated by wind blowing across the ocean surface. The amount of energy imparted to the water depends on wind velocity and the distance across which the wind is blowing. This distance is called fetch. Waves striking a shore are typically generated by storms hundreds of miles from the coast and have been traveling across the ocean for days.
Winds blowing in a relatively constant direction generate waves moving in that direction. Such a group of approximately parallel waves traveling together is called a wave train. Because wave trains generate from different areas, they can spread in different directions and carry different amounts of energy. When different wave trains interact, they produce the choppy sea surface seen in the open ocean. Also, a wave train coming from one fetch can produce various wavelengths. Longer wavelengths travel at a faster velocity than shorter wavelengths, so they arrive first at a distant shore. Thus, there is a wavelength-sorting process that takes place during the wave train’s travel. This sorting process is called wave dispersion.
12.1.1 Behavior of Waves Approaching Shore
On the open sea, waves generally appear choppy because wave trains from many directions are interacting with each other. Constructive interference occurs where crests align with other crests. The aligned wave height is the sum of the individual wave heights, a process referred to as wave amplification. Hollows occur where troughs align with other troughs. Destructive interference occurs where crests align with troughs and cancel each other out. As waves approach shore and begin to make frictional contact with the sea floor at about one-half wavelength or less, they begin to slow down. However, the energy carried by the wave remains the same, so the waves build up higher. Remember that water moves in a circular motion as a wave passes, and each circle is fed from the trough in front of the advancing wave. As the wave encounters shallower water at the shore, there is eventually insufficient water in the trough in front of the wave to supply a complete circle, so the crest pours over creating a breaker.
A special type of wave called a tsunami is generated by energetic events affecting the sea floor, such as earthquakes, submarine landslides, and volcanic eruptions (see Chapter 9 and Chapter 4). During earthquakes, tsunamis can occur when the moving crustal rocks below the sea abruptly elevate a portion of the seafloor. Water is suddenly lifted and a wave train spreads out in all directions traveling over 322 kph (200 mph) and carrying enormous energy. Tsunamis may pass unnoticed in the open ocean because the wavelength is very long, and the wave height is very low. But, as the wave train approaches shore and each wave makes contact with the shallow seafloor, friction increases, and the wave slows down. Next, wave height builds up and the wave strikes the shore as a wall of water over 30 m (100 ft) high. The massive wave, called a tsunami runup, may sweep inland well beyond the beach destroying structures far inland. Tsunamis deliver a catastrophic blow to people at the beach. As the trough water in front of the tsunami wave is drawn back, the seafloor is exposed. Curious and unsuspecting people on the beach may run out to see exposed offshore sea life only to be overwhelmed when the breaking crest hits.
12.2 Shoreline Features
Coastlines are dynamic, high energy, and geologically complicated places where many different erosional (see Chapter 5) and depositional (see Chapter 5) features exist. They include all parts of the land-sea boundary directly affected by the sea, including land far above high tide and well below normal wave base. But, the shoreline itself is the direct interface between water and land that migrates with the tides. These interfaces at the shoreline are called littoral processes. The combination of waves, currents, climate, coastal morphology, and gravity, all act on this land-sea boundary to create shoreline features.
12.2.1 Shoreline Zones
Shorelines are divided into five primary zones—offshore, nearshore, surf, foreshore, and backshore. The offshore zone is below water, but it is still geologically active due to cascading sands of turbidites and deeper currents with deposits called contourites. The nearshore zone is the area of the shore affected by the waves where water depth is one-half wavelength or less. The width of this zone depends on the maximum wavelength of the approaching wave train and the slope of the seafloor. The nearshore zone includes the shoreface, which is where rocks are deposited. The shoreface is broken into two segments: upper and lower shoreface. Upper shoreface is affected by everyday wave action and consists of finely-laminated and cross-bedded sand. The lower shoreface is the only area moved by storm waves and consists of hummocky cross-stratified sand. The surf zone is where the waves break.
The foreshore zone overlaps the surf zone and is periodically wet and dry due to waves and tides. The foreshore zone is made of planer-laminated, well-sorted sand. The beach face is the part of the foreshore zone where the breaking waves swash up and the backwash flows back down. Low ridges above the beach face in the foreshore zone are called berms. During the summer in North America, when most people visit the beach, the zone of footprints and beach umbrellas is the summer berm. Wave energy is typically lower in the summer, which allows sand to pile onto the beach. Behind the summer berm is a low ridge of sand called the winter berm. In winter, high storm energy moves the summer berm sand off the beach and piles it in the nearshore zone. The next year, the sand is replaced again as it is moved back onto the summer berm. The backshore zone is the area always above the ocean in normal conditions. In the backshore zone, onshore winds blow sand behind the beach and behind the berms, creating dunes.
12.2.2 Refraction, Longshore Currents, and Longshore Drift
As waves enter shallower water, they slow down. Waves usually approach the shoreline at an angle, with one end of the wave train slowing down first. This angle causes the waves to bend toward the beach. This bending action is called wave refraction. From the beach face, this bending looks like waves are approaching straight on, parallel to the beach. However, as refracted waves approach the shoreline at an angle, they create a slight difference between the swash as it moves up the beach face and the backwash as it flows back down. The swash and backwash along the beach create a current called the longshore current. Waves stir up sand in the surf zone and move it along the shore. This movement is called longshore drift. Longshore drift along the west and east coasts of North America moves sand north to south on average.
Longshore currents can carry longshore drift down a coast until it reaches a bay or inlet where it will deposit sand in the quieter water. Here, a spit can form (see Chapter 11). As the spit grows, it may extend across the mouth of the bay forming a barrier called a baymouth bar. Where the bay or inlet serves as boat anchorage, spits and baymouth bars are a severe inconvenience. Often, inconvenienced communities create methods to keep their bays and harbors open.
Longshore drift can be carried down the coast, until it reaches a bay or inlet where it begins to deposit in the quieter water. Here, a spit begins to form. As the spit grows, it may extend across the mouth of the bay forming a baymouth bar. Where the bay or inlet serves as anchorage for boats, such spit growth and baymouth bars are a severe inconvenience. Communities thus affected attempt measures to keep their harbor open.
One way to keep a harbor open is to build a jetty. A jetty is a long concrete or stone barrier formed to deflect the sand away from a harbor mouth or other ocean waterway. If the jetty does not deflect the sand far enough out, sand may continue to flow along the shore, forming a spit around the end of the jetty. A more expensive but effective method to keep a bay mouth open is to dredge the sand from the growing spit, put it on barges, and deliver it back to the drift downstream of the harbor mouth. An even more expensive but effective option is to install large pumps and pipes to draw in the sand upstream of the harbor, pump it through pipes, and discharge it back into the drift downstream of the harbor mouth. Because natural processes are at work continuously, human efforts to mitigate inconvenient spits and baymouth bars require ongoing modifications. For example, the community of Santa Barbara, California, tried several methods to keep their harbor open before settling on pumps and piping.
Rip currents are another coastal phenomenon related to longshore currents. Rip currents occur in the nearshore seafloor when wave trains come straight onto the shoreline. In areas where wave trains push water directly toward the beach face or where the shape of the nearshore seafloor refracts waves toward a specific point on the beach, the water piles up on shore. But this water must find an outlet back to the sea. The outlet is relatively narrow, and rip currents carry the water directly away from the beach. Swimmers caught in rip currents are carried out to sea. Swimming back to shore directly against the strong current is fruitless. A better solution is to ride out the current to where it dissipates, swim around it, and return to the beach. Another solution is to swim parallel to the beach until out of the current, then return to the beach. The best solution is to be aware of rip currents, have a plan, or avoid them all together.
Like rip currents, undertow is a current that moves away from the shore. However, unlike rip currents, undertow occurs under all approaching waves, and is strongest in the surf zone, where waves are high and water is shallow. Undertow is return flow for water transported onshore by waves.
12.2.3 Emergent and Submergent Coasts
Emergent coasts occur where sea levels fall. Submergent coasts occur where sea levels rise. Tectonic shifts and sea level changes cause the long-term rise and fall of sea level. Some features associated with emergent coasts include high cliffs, headlands, exposed bedrock, steep slopes, rocky shores, arches, stacks, tombolos, wave platforms, and wave notches.
In emergent coasts, wave energy, wind, and gravity erode the coastline. The erosional features are elevated relative to the wave zone. Sea cliffs are persistent features as waves cut away at their base and higher rocks calve off by mass wasting. Refracted waves that attack bedrock at the base of headlands may erode or carve out a sea arch, which can extend below sea level. When a sea arch collapses, it leaves a rock column called a stack.
When rock behind the stack erodes, sand can erode from the headland. Eroded sand that collects from the headland to the stack forms a tombolo: a sand strip that connects the stack to the shoreline. Where sand supply is low, wave energy may erode a wave-cut platform across the surf zone, exposed as bare rock with tidal pools at low tide. This bench-like terrace extends to the cliff’s base. When wave energy cuts into the base of a sea cliff, it creates a wave notch.
Submergent coasts occur where sea levels rise due to tectonic subsidence—when the Earth’s crust sinks—or when sea levels rise due to glacier melt. Features associated with submergent coasts include river mouths, fjords, barrier islands, lagoons, estuaries, bays, tidal flats, and tidal currents. In submergent coastlines, river mouths are flooded by the rising water. Fjords are glacial valleys flooded by post-ice age sea level rise (see Chapter 14). Barrier islands are elongated bodies of sand that formed from the old beach sands that used to parallel the shoreline. Often, lagoons isolate barrier islands from the mainland. Barrier island formation is controversial: some scientists believe that they formed when ice sheets melted after the last ice age, raising sea levels. Another hypothesis is that barrier islands formed from spits and bars accumulating far offshore.
Tidal flats—or mudflats, form where tides alternately flood and expose low areas along the coast. Tidal currents create combinations of symmetrical and asymmetrical ripple marks on mudflats, and drying mud creates mud cracks. In the central Wasatch Mountains of Utah, ancient tidal flat deposits are exposed in the Precambrian strata of the Big Cottonwood Formation. These ancient deposits provide an example of applying Hutton’s principle of uniformitarianism (see Chapter 1). Features common on modern tidal flats indicate that these ancient deposits were formed in a similar environment: there were shorelines, tides, and shoreline processes acting at that time, yet the ancient age indicates that there were no land plants to hold products of mechanical weathering in place (see Chapter 5), so erosion rates would have been different.
Geologically, tidal flats are broken into three different sections: barren zones, marshes, and salt pans. These zones may be abundant or absent in each individual tidal flat. Barren zones are areas with strong flowing water, coarser sediment, with ripples and cross bedding common. Marshes are vegetated with common sand and mud. Salt pans or flats, less often submerged than the other zones, are the finest-grained parts of tidal flats, with silty sediment and mud cracks (see Chapter 5).
Lagoons are locations where spits, barrier islands, or other features partially cut off a body of water from the ocean. Estuaries are a vegetated type of lagoon where fresh water flows into the area making the water brackish—a salinity between salt and fresh water. However, terms like lagoon, estuary, and even bay are often loosely used in place of one another. Lagoons and estuaries are certainly transitional between land and water environments where littoral, shallow shorelines; lacustrine, lakes or lagoons; and fluvial, rivers or currents can overlap. For more information on lagoons and estuaries, see Chapter 5.
12.2.4 Human impact on coastal beaches
Humans impact coastal beaches when they build homes, condominiums, hotels, businesses, and harbors—and then again when they try to manage the natural processes of erosion. Waves, currents, longshore drift, and dams at river mouths deplete sand from expensive beachfront property and expose once calm harbors to high-wave energy. To protect their investment, keep sand on their beach, and maintain calm harbors, landowners find ways to mitigate the damage by building jetties, groins, dams, and breakwaters.
Jetties are large manmade piles of boulders or concrete built at river mouths and harbors. A jetty is designed to divert the current or tide, to keep a channel to the ocean open, and to protect a harbor or beach from wave action. Groins are similar but smaller than jetties. Groins are bits of wood or concrete built across the beach perpendicular to the shoreline and downstream of a property. Unlike jetties, groins are used to preserve sand on a beach rather than to divert it. Sand erodes on the downstream side of the groin and collects against the upstream side. Every groin thus creates a need for another one downstream. A series of groins along a beach develops a scalloped appearance along the shoreline.
Inland streams and rivers flow to the ocean carrying sand to beaches. When dams are built, they trap sand and sediment from reaching beaches. To replenish beaches, sand is hauled in from other areas by trucks or barges and dumped on the depleted beach. Unfortunately, this can disrupt the ecosystem that exists along the shoreline by exposing native creatures to foreign sandy material and microorganisms and by introducing foreign objects to humans. For example, visitors to one replenished east coast beach found munitions and metal shards in the sand, which had been dredged from abandoned test ranges.
An approach to protect harbors and moorings from high-energy wave action is to build a breakwater—an offshore structure against which the waves break, leaving calmer waters behind it. Unfortunately, breakwaters keep waves from reaching the beach, which stops sand moving with longshore drift. When longshore drift is interrupted, sand is deposited in quieter water, and the shoreline builds out forming a tombolo behind the breakwater. The tombolo eventually covers the breakwater in sand. When the city of Venice, California built a breakwater to create a quiet water harbor, longshore drift created a tombolo behind the breakwater, as seen in the image. The tombolo now acts as a large groin in the beach drift.
12.2.5 Submarine Canyons
Submarine canyons are narrow, deep underwater canyons located on continental shelves. Submarine canyons typically form at the mouths of large landward river systems. They form when rivers cut down into the continental shelf during low sea level and when material continually slumps or flows down from the mouth of a river or a delta. Underwater currents rich in sediment pass through the canyons, erode them, and drain onto the ocean floor. Underwater landslides, called turbidity flows, occur when steep delta faces and underwater sediment flows are released down the continental slope. Turbidity flows can continue to erode the canyon, and eventually, fan-shaped deposits develop on the ocean floor beyond the continental slope. See Chapter 5 for more information on turbidity flows.
12.3 Currents and Tides
Ocean water moves as waves, currents, and tides. Ocean currents are driven by persistent global winds blowing over the water’s surface and by water density. Ocean currents are part of Earth’s heat engine in which solar energy is absorbed by ocean water and distributed by ocean currents (remember the specific heat of water, Chapter 11.
12.3.1 Surface Currents
In the figure, the black arrows show global currents. Notice the large circular currents in the northern and southern hemispheres in the Atlantic, Pacific, and Indian Oceans. These currents are called gyres and are driven by atmospheric circulation—air movement. Gyres rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere because of the Coriolis Effect (see Chapter 13). Western boundary currents flow from the equator toward the poles carrying warm water. They are key contributors to local climate. Western boundary currents are narrow due to the Earth’s rotation and move poleward along the east coasts of adjacent continents. The Gulf Stream and the Kuroshio currents in the northern hemisphere and the Brazil, Mozambique, and Australian currents in the southern hemisphere are western boundary currents. Currents returning cold water toward the equator are broad and diffuse along the western coasts of adjacent land masses. These warm and cold currents affect nearby lands making them warmer or colder than other areas at equivalent latitudes. For example, the warm Gulf Stream makes Northern Europe much milder than similar latitudes in northeastern Canada and Greenland. Another example is the cool Humboldt Current, also called the Peru Current, flowing north along the west coast of South America. Cold currents limit evaporation in the ocean, which is one reason the Atacama Desert in Chile is cool and arid.
12.3.2 Deep Currents
Whether an ocean current moves horizontally or vertically depends on its density. The density of seawater is determined by temperature and salinity. Evaporation and freshwater influx from rivers affect salinity and, therefore, the density of seawater. As the western boundary currents cool, some of the cool, denser water sinks to become the ocean’s deep waters. Deep-water movement is called thermohaline circulation—thermo refers to temperature, and haline refers to salinity. This circulation connects the world’s deep ocean waters. The Gulf Stream best illustrates thermohaline circulation. Warm water currents promote much evaporation; therefore, heat dissipates. The resulting water is much colder, saltier, and denser. As the denser water reaches the North Atlantic and Greenland, it begins to descend and becomes a deep-water current. This worldwide connection between shallow and deep-ocean circulation is sometimes referred to as the global conveyor belt.
Tides are the rising and lowering of sea level during the day and are caused by the gravitational effects of the Sun and Moon on the oceans. The Earth rotates daily within the Moon and Sun’s gravity fields. Although the Sun is much larger and the gravitational pull is more powerful, the Moon is closer to Earth; hence, the Moon’s gravitational influence on tides is dominant. The magnitude of the tide at a given location and the difference between high and low tide—the tidal range, depends primarily on the configuration of the Moon and Sun with respect to the Earth. Spring tide occurs when the Sun, Moon, and Earth line up with each other at the full or new Moon, and the tidal range is at a maximum. Neap tide occurs approximately two weeks later when the Moon and Sun are at right angles with the Earth, and the tidal range is lowest.
The Earth rotates within a tidal envelope, so tides rise and ebb daily. Tides are measured at coastal locations. These measurements and the tidal predictions based on are published on the NOAA website. Tides rising and falling create tidal patterns at any given shore location. The three types of tidal patterns are diurnal, semidiurnal, and mixed.
Diurnal tides go through one complete cycle each tidal day. A tidal day is the amount of time for the Moon to align with a point on the Earth as the Earth rotates, which is slightly longer than 24 hours. Semidiurnal tides go through two complete cycles in each tidal day—every 12 hours, with the tidal range typically varying in each cycle. Mixed tides are a combination of diurnal and semidiurnal patterns and show two tidal cycles per tidal day, but the relative amplitudes of each cycle and their highs and lows vary during the tidal month. For example, there is a high, high tide and a high, low tide. The next day, there is a low, high tide and a low, low tide. Forecasting the tidal pattern and the times tidal phases arrive at a given shore location
is complicated. Tidal phases are determined by bathymetry: the depth of ocean basins and the continental obstacles that are in the way of the tidal envelope within which the Earth rotates. Local tidal experts make 48-hour tidal forecasts using tidal charts based on daily observations, as can be seen in the chart of different tide types. A typical tidal range is approximately 1 m (3 ft). Extreme tidal ranges occur where the tidal wave enters a narrow restrictive zone that funnels the tidal energy. An example is the English Channel between Great Britain and the European continent where the tidal range is 7 to 9.75 m (23 to 32 ft). The Earth’s highest tidal ranges occur at the Bay of Fundy, the funnel-like bay between Nova Scotia and New Brunswick, Canada, where the average range is nearly 12 m (40 ft) and the extreme range is around 18 m (60 ft). At extreme tidal range locations, a person who ventures out onto the seafloor exposed during ebb tide may not be able to outrun the advancing water during flood tide. NOAA has additional information on tides.
Shoreline processes are complex, but important for understanding coastal processes. Waves, currents, and tides are the main agents that shape shorelines. Most coastal landforms can be attributed to moving sand via longshore drift, and long-term rising or falling sea levels.
The shoreline is the interface between water and land and is divided into five zones. Processes at the shoreline are called littoral processes. Waves approach the beach at an angle, which cause the waves to bend towards the beach. This bending action is called wave refraction and is responsible for creating the longshore current and longshore drift—the motion that moves sand along the coasts. When the longshore current deposits sand down the coast to quiet waters, the sand can accumulate, creating a spit or barrier called a baymouth bar, which often blocks bays and harbors. Inconvenienced humans create methods to keep their harbors open by creating jetties and groins, which negatively affect natural beach processes.
Emergent coasts are created by sea levels falling, while submergent coasts are caused by sea levels rising. Oceans absorb solar energy, which is distributed by currents throughout the world. Circular currents, called gyres, rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Thermohaline circulation connects the world’s deep ocean waters: when shallow warm water evaporates, the colder, saltier, and denser water sinks and becomes deep-water currents. The connection between shallow and deep-ocean circulation is called the global conveyor belt.
Tides are the rising and lowering of sea level during the day and are caused by the gravitational effects of the Sun and Moon on the oceans. There are three types of tidal patterns: diurnal, semidiurnal, and mixed. Typical tidal ranges are approximately 1 m (3 ft). Extreme tidal ranges are around 18 m (60 ft).