- Describe how energy is carried by waves and the relation of wave energy to features associated with the shoreline
- Explain how breakers occur
- Describe wave refraction and its contribution to longshore currents and the movement of sand along the coast
- Explain how longshore currents cause the formation of spits and baymouth bars
- Distinguish 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.
Coastlines are the great interface between the 29% of earth’s surface that is land and the 71% of earth that is covered by the oceans. Therefore, it is the longest visible boundary on earth. To understand the processes that take place at this interface, we must first consider the energetic action at this boundary; namely, waves. More information on these topics can be found in a physical oceanography text. The importance of this interface is seen in the study of ancient shorelines, and particularly for natural resources, a process called sequence stratigraphy.
12.1 Waves and wave processes
Waves are created when wind blows over the surface of water. Energy is transferred from wind to the water by friction and carried in the upper part of the water by waves. Waves move across the water surface with individual particles of water moving in circles, the water moving forward with the crest and moving backward in the trough. This can be demonstrated by watching the movement of a cork or some floating object as a wave passes.
Important terms to understand in the operation of waves include: 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 depends on the amount of energy carried in the wave. Even before reaching shore, wave height increases with increasing wave energy. Wavelength is the horizontal distance between adjacent wave crests or corresponding features of the wave. Wave velocity is the speed by which a wave crest moves forward, which is also related to the energy carried by the wave. 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. The vertical reach of waves in the water is called wave base. Looking at incoming waves at a beach, one will appreciate that most ocean waves have a wavelength on the order of a few tens of feet. Thus, seawater is typically disturbed by wave motion to a depth of a few tens of feet. This is known as fair weather wave base. In strong storms such as hurricanes, both wave length and the maximum depth of water disturbance increase dramatically. The effective depth to which waves can erode sediment is thus called storm wave base, which is approximately 300 feet .
Waves are generated by wind blowing across the ocean surface. The amount of energy imparted to the water depends on the wind velocity and the distance across which the wind is blowing. This distance is called fetch. Waves striking a shore were typically generated hundreds of miles from the coast by storms and may 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. As wave trains spread from different areas of generation, they may move in different directions and carry different amounts of energy. Interaction of these different wave trains produces the choppy sea surface seen in the open ocean. Also of interest is that many wave lengths are produced in a given wave train from a fetch region. Longer waves travel at a faster velocity than shorter wavelengths thus there is a sorting of wavelengths that takes place during travel of the wave train with the longer waves arriving first at a distant shore. This is a process 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. Where crests converge with other crests (called constructive interference) they add together producing peaks, a process referred to as wave amplification. Constructive interference of troughs produces hollows. Where crests converge with troughs, they cancel each other out (called destructive interference). As waves approach shore and begin to make frictional contact with the sea floor (i.e., water depth is a half wavelength or less) they begin to slow down, but the energy carried by the wave remains the same so they build up higher. Remember that the water moves in a circular motion as the wave passes, with the water that feeds each circle being drawn from the trough in front of the advancing wave. As the wave encounters shallower water at the shore, there is eventually insufficient water in front of the wave to supply a complete circle, and the crest pours over creating a breaker.
A special type of wave is generated by any energetic event affecting the sea floor, such as earthquakes, submarine landslides, and volcanic eruptions. Such waves are called tsunamis and, in the case of earthquakes, are created when a portion of the seafloor is suddenly elevated by movement in the crustal rocks below that are involved in the earthquake. The water is suddenly lifted and a wave train spreads out in all directions from the mound carrying enormous energy and traveling very fast (hundreds of miles per hour). 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, each wave makes contact with the shallow seafloor, friction increases, and the wave slows down. Wave height builds up and the wave strikes the shore as a wall of water a hundred or more feet high. The massive wave may sweep inland well beyond the beach. This is called the tsunami runup, which destroys structures far inland. Tsunamis deliver a catastrophic blow to observers at the beach as the water in the trough in front of it is drawn back toward the tsunami wave, exposing the seafloor. 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
Many different erosional and depositional features exist in the high energy of the coast. The coast or coastline includes all parts of the land-sea boundary area that are directly affected by the sea. This includes land far above high tide and well below normal wave base. But the shore or shoreline itself is the direct interface between water and land that migrates with the tides and with deposition and erosion of sediment. Processes at the shoreline are called littoral processes.
12.2.1 Shoreline Zones
Shoreline zones can be viewed by looking at the beach profile, which is divided into four primary zones – offshore, nearshore, foreshore, and backshore. The offshore is below any shoreline-derived process, but is still geologically active due to cascading sands of turbidites and deeper currents (with deposits called contourites). The nearshore is affected by the waves, i.e., that part of the shore where water depth is a half wavelength or less. The width of this zone thus depends on the maximum wavelength of the approaching wave train and with the slope of the seafloor. The nearshore area, when looking at rocks deposited in this zone, is typically called the shoreface, and is broken into two segments: upper shoreface, which is affected by everyday wave action, typically consisting of finely-laminated and cross-bedded sand, and lower shoreface, the area only moved by storm waves, which has hummocky cross-stratified sand. The surf zone is where the waves break. The area (mostly overlapping the surf zone) that is periodically wet and dry, because of wave action and tides, is called the foreshore, which is made of planer-laminated, well-sorted sand. The beach face is where the swash of the breaking wave runs up and the backwash flows back down. Above the beach face are low ridges 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 be piled onto the beach. Behind the summer berm is commonly a low ridge of sand representing the winter berm. Beachgoers walk across this winter berm from the parking lot to the summer berm where they park their items. Higher winter storm energy moves the summer berm sand off the beach and piles it in the near from which it will be replaced next year as it is moved back onto the summer berm. There may be a zone of dunes behind the berms representing sand blown behind the beach by onshore winds. This area behind the berms that is always above the ocean in normal conditions is known as the backshore. Shorelines are actually simplified for this discussion; they are a dynamic and geologically-complicated place.
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 waves of the train slowing down first. This causes the waves to bend toward the beach. Such bending of the waves as they enter shallower water is called wave refraction, which produces the appearance from the beach that waves are approaching the beach face generally straight on, parallel to the beach. However, refracted waves on average approach the shoreline at somewhat of an angle creating a slight difference between the swash as it moves up the beach face and the backwash as it flows back down. This results in a net movement of the water along the beach creating a current called the longshore current. Sand stirred up by waves in the surf zone is thus moved along the shore by longshore drift. Longshore drift along both the west and east coasts of North America moves sand on average from north to south.
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 means to do this is a jetty, often built of concrete or large stones, forming a long barrier to deflect the sand away from the harbor mouth or other ocean waterway in which transport is desired. If the jetty does not succeed in deflecting the sand far enough out, it may continue to flow along the shore, building a spit around the end of the jetty. A more expensive but effective method is then to dredge the sand from the growing spit, put it on barges, and deliver it back to the drift downstream of the harbor opening. A even more expensive (but effective) means 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 inconveniences are sometimes not equal to the task or require ongoing modifications. The community of Santa Barbara, California, tried several methods to keep their harbor open before settling on pumps and piping .
Another coastal phenomenon related to longshore currents is the presence of rip currents. These involve the nearshore configuration of the seafloor and/or the arrival of wave trains straight onto the shore. In areas where wave motion pushes water directly toward the beach face, or the shape of the nearshore seafloor refracts and focuses the water movement toward a point on the beach, the water piling up there must find an outlet back to the sea. The outlet is provided by relatively narrow rip currents that carry the water directly away from the beach. Swimmers caught in such currents find themselves being carried out to sea. They may attempt to return to shore by swimming directly against the current. This is generally a fruitless effort because they tire against the strong current. A better solution is to ride it out to where it dissipates, then swim around it and return to the beach or swim laterally, parallel to the beach, until out of the current, then return to the beach. Awareness of the presence of rip currents with a plan is the key or avoid them all together.
12.2.3. Emergent and submergent coasts
Coastlines that have a relative fall in sea level, either caused by tectonics or sea level change, are called emergent. Where the shoreline is rocky, perhaps with a sea cliff, waves refracting around headlands attack the rocks behind the point of the headland.
They may cut out the rock at the base forming a sea arch which may collapse to isolate the point as a stack. Rocks behind the stack may be eroded away and sand eroded from the point collects behind it forming 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. Wave energy expended at the base of a sea cliff may cut a wave notch.
Sea cliffs tend to be persistent features as the waves cut away at their base and higher rocks calve off by mass wasting. If the coast is emergent, these erosional features may be elevated relative to the wave zone. Wave-cut platforms become marine terraces, perhaps with remnant sea cliffs inland from them.
Tectonic subsidence or sea level rise produces a submergent coast. Features associated with submergent coasts include estuaries, bays and river mouths flooded by the higher water. Fjords are former glacial valleys now flooded by post Ice Age sea level rise (see chapter 14). Elongated bodies of sand called barrier islands form parallel to the shoreline from the old beach sands, often isolated from the mainland by lagoons behind them . The formation of barrier islands is controversial; some workers believe as above that barrier islands were formed by rising sea level as the ice sheets melted after the last ice age. Accumulation of spits and far offshore bar formations are also mentioned as possible formation hypotheses for barrier islands.
Tidal flats or mudflats form where tides alternately flood and expose low areas along the coast. Combinations of symmetrical ripple marks, asymmetrical ripple marks from tidal currents, and mud cracks from drying form on these flats. An example of ancient tidal flat deposits is exposed in the Precambrian strata found in the central part of the Wasatch Mountains of Utah. These ancient deposits provide an example of applying Hutton’s Uniformity Principle. The presence of features common on modern tidal flats prompts the interpretation that these ancient deposits were formed in a similar environment. There were shorelines, tides, and shoreline processes acting at that time, yet the age of the ancient rocks indicates that there were no land plants to hold products of mechanical weathering in place so rates of erosion would have been different. The Uniformity Principle must be applied with some knowledge of the context of the application.
Typically tidal flats are broken into three different sections, which may be abundant or absent in each individual tidal flat. Barren zones are areas with strong, flowing water and coarser sediment, with ripples and cross bedding common. Marshes are vegetated with common sand and mud. Salt pans are the finest-grained parts of the tidal flats, with silty sediment, mud cracks, and is less often submerged .
Lagoons are locations where spits, barrier islands, or other features have partially cut off a body of water from the ocean. Estuaries are a (typically vegetated) type of lagoon where fresh water is flowing into the area as well, making the water brackish (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 terrestrial and marine geologic environment, where littoral, lacustrine, and fluvial processes can overlap.
12.2.4 Human impact on coastal beaches
Coasts are prime real estate land that attracts development of beach houses, condominiums, and hotels. This kind of interest and investment leads to ongoing efforts to manage the natural processes in coastal areas. Humans who find longshore drift is removing sand from their beaches often use groins (also spelled groyne) in an attempt to retain it.
Similar but smaller than jetties, groins are bits of wood or concrete built across the beach perpendicular to the shoreline at the downstream end of one’s property. Unlike jetties, they are used to preserve sand on a beach, rather than to divert it from an area. Sand erodes on the downstream side of the groin and collects against the upstream side. Every groin thus creates need for another one downstream. The series of groins along a beach develops a scalloped appearance for the shoreline.
Sand for longshore drift and beaches comes from rivers flowing to the oceans from inland areas. Beaches may become starved of sand if sediment carried by streams and rivers is trapped behind dams. To mitigate this, beach replenishment may be employed where 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 foreign microorganisms and can even bring in foreign objects that impact humans on replenished beaches. Visitors to one replenished east coast beach found munitions and metal shards in the sand which had been brought from abandoned test ranges from which the sand had been dredged .
Another approach to reduce erosion or provide protected areas for boat anchoring is construction of a breakwater, an offshore structure against which the waves break, leaving calmer waters behind it. Unfortunately, this means that waves can no longer reach the beach to keep the longshore drift of sand moving. The drift is interrupted, the sand is deposited in the quieter water, and the shoreline builds out forming a tombolo behind the breakwater, eventually covering the structure with sand . The image shows this result at the breakwater constructed by the city of Venice, California in an attempt to create a quiet water harbor. The tombolo behind the breakwater is now acting as a large groin in the beach drift.
12.2.5 Submarine Canyons
Submarine canyons are narrow and deep canyons located in the marine environment on continental shelves. They typically form at the mouths of large landward river systems, both by cutting down into the continental shelf during times of low sea level, and also by continual material slumping or flowing down from the mouth of the river or a delta. Underwater currents rich in sediment pass through the canyons, erode them, and drain onto the ocean floor. Steep delta faces and underwater flows of sediments are released down the continental slope as underwater landslides, called turbidity flows. Erosive action of this type of flow continues to cut the canyon and eventually fan-shaped deposits develop on the ocean floor beyond the continental slope . See chapter 5.5.2 for more information.
12.3 Currents and Tides
Water in the ocean, when moving, can move via waves, currents, and tides. Waves have been discussed above in chapter 12.1, and this section will focus on the other two. Currents in the ocean are driven by persistent global winds blowing over the surface of the water and water density. They are part of the Earth’s heat engine in which solar energy is absorbed by the ocean water (remember the specific heat of water). The absorbed energy is distributed by ocean currents.
12.3.1 Surface currents
In the above figure, notice the large sub-circular currents in the northern and southern hemispheres in the Atlantic, Pacific and Indian Oceans. These are driven by prevailing atmospheric circulation and are called gyres and rotate clockwise in the northern hemisphere and counterclockwise in the southern semisphere because of the Coriolis Effect (see Chapter 13). Currents flowing from the equator toward the poles tend to be narrow as a result of the Earth’s rotation and carry warm water poleward along the east coasts of adjacent continents. These are called western boundary currents, and they are key contributors to local climate. The Gulf Stream and the Kurosho currents in the northern hemisphere and the Brazil, Mozambique, and Australian currents in the southern hemisphere are such western boundary currents. Currents returning cold water toward the equator tend to be 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 flowing north along the west coast of South America. This cold current limits evaporation in the ocean and contributes to the arid condition of the Atacama Desert .
12.3.2 Deep Currents
Whether an ocean current moves horizontally or vertically depends on its density. The density of seawater is determined by factors such as temperature and salinity. Evaporation and influx of freshwater from rivers also affect salinity and therefore the density of seawater. As the western boundary currents cool, some of the cool, denser water sinks to become the deep water of the oceans. Movement of this deep water is called the thermohaline circulation (thermo refers to temperature, haline refers to salinity) and connects the deep waters of all the world’s oceans. This can be best illustrated by the Gulf Stream. After the warm water within the current promotes much evaporation and the heat dissipates, the resulting water is much colder and saltier. As the denser water reaches the North Atlantic and Greenland, it begins to descend, and becomes a deep water current. This worldwide (connected) shallow and deep ocean circulation is sometimes referred to as the global conveyor belt .
The gravitational effects of the sun and moon on the oceans create tides, the rising and lowering of sea level during the day . The earth rotates daily within the gravity fields of the moon and sun. Although the sun is much larger and exerts a more powerful gravity, its great distance from earth means that the gravitational influence of the moon on tides is dominant. The magnitude of the tide at a given location, 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. When the sun, moon, and earth line up with each other at full moon or new moon, the tidal range is at a maximum. This is called spring tide. Approximately two weeks later when the moon and sun are at right angles with the earth, tidal range is lowest. This is called neap tide.
The earth rotates within the tidal envelope so we experience the rising and ebbing of the tide on a daily basis.Tides are measured at coastal locations and these measurements and tidal predictions based on them are published for those who depend on this information (e.g. this NOAA website) . The rising and falling of the tides (tidal patterns) as experienced at a given shore location are of three types, diurnal, semidiurnal, and mixed.
Diurnal tides go through a complete cycle once in each tidal day. Keeping in mind that the moon orbits around the Earth, a tidal day is the amount of time the Earth rotates to the same location of the Moon above the Earth, which (considering the movement of the bodies) consists of slightly longer than 24 hours. Semidiurnal tides go through the complete cycle twice in each tidal day with the tidal range typically showing some inequality 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 with a diurnal overprint. The pattern at a given shore location and the times of arrival of tidal phases are complicated and determined by the bathymetry (depth) of the ocean basins and continental obstacles in the way of the tidal envelope within which the earth rotates. Local tidal experts use tidal charts (indicated in the example above) based on daily observations to make forecasts for expected tides for the next few days.
Typical tidal ranges are on the order of 3 feet. Extreme tidal ranges occur where the tidal wave enters a restrictive zone. An example is the English Channel between Great Britain and the European continent where tidal ranges of 25 to 32 feet have been observed. 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 40 feet and extremes of around 60 feet have been observed. At these locations of extreme tidal range, a person who ventures out onto the seafloor exposed during ebb tide may not be able to outrun the advancing water during flood tide. Additional information on tides can be found at this NOAA website.
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, longterm rising sea levels, or longterm falling sea levels. Human intervention in beach processes, like jetties and groins, have negative consequences that need to be mitigated.