12 – Chapter 12 Oceanography: Coasts (concept – Flashcards

The place where ocean meets land is usually called the shore. Coast refers to the larger zone affected by the processes that occur at this boundary.

Sea level depends on the amount of water in the world ocean, the volume of the ocean's "container," and the temperature of the water (water expands as it warms). Tectonic forces of uplift and subsidence (along with isostatic equilibrium) determine the position of the shore.

Erosional coasts are new coasts in which the dominant processes are those that remove coastal material. Depositional coasts are usually older coasts that are steady or growing because of their rate of sediment accumulation.

Erosional coasts are shaped and attacked from the land by stream erosion, abrasion by wind-driven grit, glacial activity, rainfall, dissolution by acids from soil, and slumping. From the sea, large storm surf routinely generates tremendous pressures. Tiny pieces of sand, bits of gravel, or stones hurled by the waves are effective at eroding the shore.

Common features include sea cliffs, sea caves, and wave-cut platforms just offshore. Much of the debris removed from cliffs during the formation of these structures is deposited in the quieter water farther offshore, but some can rest at the bottom of the cliffs as exposed beaches.

Because of wave refraction, wave energy is focused onto headlands and away from bays by wave refraction .Over time, coastal erosion tends to produce a straight shoreline.

As we saw in Chapter 4, most islands that rise from the deep ocean are of volcanic origin. If the volcanism has been recent, the coasts of a volcanic island will consist of lobed lava flows extending seaward, common features in the Hawaiian Islands. Craters at the coast may fill with seawater after volcanic activity has slowed.

Over time, erosional shorelines can evolve into depositional ones.

The most familiar feature of a depositional coast is the beach.

Tidal range, pattern, and height - coupled with wave action - are the most important factors determining beach profile.

The movement of sediment along the coast, driven by wave action, is referred to as longshore drift. Longshore drift occurs in two ways: the wave-driven movement of sand along the exposed beach, and the current-driven movement of sand in the surf zone just offshore.

The natural sector of a coastline in which sand input and sand outflow are balanced may be thought of as a coastal cell. Sand enters a cell from rivers or streams, and exits as it falls into a submarine canyon.

A bay mouth bar forms when a sand spit closes off a bay by attaching to a headland adjacent to the bay.

Barrier islands are narrow, exposed sandbars that are parallel to but separated from land. Unlike barrier islands, sea islands contain a firm central core that was part of the mainland when sea level was lower.

A broad continental shelf must be present to provide a platform on which sediment can accumulate to form a delta. Tidal range must be low, and waves and currents generally mild. Deltas are most common on the low-energy shores of enclosed seas (where the tidal range is not extreme) and along the tectonically stable trailing edges of some continents.

Coral animals, some forms of cyanobacteria, and mangroves are effective at modifying coastlines. The greatest of all biologically modified coasts is the Great Queensland Barrier Reef in Australia.

[Unsure] Coral reefs are classified by: Fringing reefs, barrier reefs, and atolls Charles Darwin proposed this classification cheme.

An estuary is a body of water partially surrounded by land, where fresh water from a river mixes with ocean water.

By origin, estuary types are drowned river mouths, fjords, bar-built, or tectonic.

By water circulation patterns, estuaries are classified as salt-wedge, well-mixed, partially mixed, and reverse estuaries.

Estuaries often support a tremendous number of living organisms. The easy availability of nutrients and sunlight, protection from wave shock, and the presence of many habitats permit the growth of many species and individuals. Estuaries are frequently nurseries for marine animals; several species of perch, anchovy. Unfortunately for their inhabitants, the high demand for development is incompatible with a healthy estuarine ecosystem.

The Pacific coast is an actively rising margin on which volcanoes, earthquakes, and other indications of recent tectonic activity are easily observed. Most of the sediments on the Pacific coast originated from erosion of relatively young granitic or volcanic rocks of nearby mountains. The Atlantic coast is a passive margin, tectonically calm and subsiding because of its trailing position on the North American Plate. Subsidence along the coast has been considerable—3,000 meters (10,000 feet) over the last 150 million years. A deep layer of sediment has built up offshore, material that helped produce today's barrier islands. The Gulf coast experiences a smaller tidal range and—hurricanes excepted—a smaller average wave size than either the Pacific or Atlantic coasts. Reduced longshore drift and an absence of interrupting submarine canyons allow the great volume of accumulated sediments from the Mississippi and other rivers to form large deltas, barrier islands, and a long raised "super berm" that prevents the ocean from inundating much of this sinking coast.

Steps taken to preserve or "improve" a stretch of coast may have the opposite effect, and coastal residents do not always learn by example. Intervention in coastal processes is almost always costly and temporary.

Because sediment flow into coastal cells has lessened due to dams and sediment diversion, United States beaches are generally shrinking.

Erosional coasts are often rough and irregular. The ocean has not had time to modify the terrestrial features provided by changes in sea level, the scouring of glaciers, deposition of sediment at the mouths of rivers, volcanic eruptions, or the movement of the Earth along faults. We could see sunken river valleys; deep, narrow embayments known as fjords; deltas; and coasts pocked by volcanic craters and lava flows. Depositional coasts are those coasts that have been significantly changed by wave action and other marine processes after sea level stabilized. Land erosion and marine erosion both work to change an erosional coast to a depositional coast. Depositional coasts are shaped and attacked from the land by stream erosion, the abrasion of wind-driven grit, the alternate freezing and thawing of water in rock cracks, the probing of plant roots, glacial activity, rainfall, dissolution by acids from soil, and slumping. Erosive forces can produce a wave-cut shore showing some or most of the features illustrated in Figure 11.12. The most familiar feature of a depositional coast is the beach (Figure 11.11). A beach is a zone of unconsolidated (loose) particles that covers part or all of a shore. As with all coastal situations, change is the dominant condition in both erosional and depositional coasts.

The net amount of sediment (usually sand) that moves along the coast, driven by wave action, is referred to as longshore drift. Longshore drift occurs in two ways: the wave-driven movement of sand along the exposed beach, and the current-driven movement of sand in the surf zone just offshore. If sediments have accumulated to form a beach, water from breaking waves will usually rush up the beach at a slight angle (waves rarely approach the shore exactly at a right angle), but return to the ocean by running straight down hill under the influence of gravity. The millions of sand grains disturbed by the wave will follow the water's path, moving up the beach at an angle but retreating down the beach straight down the slope. Net transport of the grains is "longshore," parallel to the coast, away from the direction of the approaching waves. Sediment is also transported in the surf zone in a longshore current. The waves breaking at a slight angle distribute a portion of their energy away from their direction of approach. This energy propels a narrow current in which sediment already suspended by wave action can be transported downcoast. The speed of the longshore current sometimes reaches almost 4 kilometers per hour (about 2.5 miles per hour). Net sand flow along the U. S. East and West coasts is usually to the south because the waves that drive the transport system usually approach from the north, where storms most commonly occur.

The features found on depositional coasts are usually composed of sediments rather than solid rock. Beaches are the dominant form. Most beach sediment reaches the coast in rivers. Not all the incoming sediment joins the longshore drift; some of the fine particles moved by rivers will stay in suspension long enough to be transported to the outer continental shelf and beyond. If the rate of deposition of larger particles exceeds the ability of the longshore transport system to remove and distribute the material along the coast, the sediments may build up at the river mouth to form a fan called a delta. The landward limit of a beach may be vegetation, a sea cliff, relatively permanent sand dunes, or construction such as a seawall. The seaward limit occurs where sediment movement on- and offshore ceases—a depth of about 10 meters (33 feet) at low tide. Such places include the calm spots between headlands, shores sheltered by offshore islands, and regions with usually quiet surf. Sometimes the sediment is transported a very short distance—grit may simply fall from the cliff above and accumulate at the shoreline—but more often the sediment on a beach has been moved for long distances to its present location. Wherever they are found, deltas and beaches are in a constant state of change. They may be thought of as rivers of sand—zones of continuous sediment transport. High-energy beaches tend to look different than low-energy beaches; fine-grain beaches different from coarse-grain ones. On fine-grain beaches, the ability of small sharp-edged particles to interlock discourages water from percolating down into the beach itself, so water from waves runs quickly back down the beach carrying surface particles toward the ocean. This process results in a very gradual slope. Broad flat beaches also have a large area on which to dissipate wave energy and can provide a calm environment for the settling of fine sediment particles. In contrast, coarse particles (gravel, pebbles) do not fit together well and readily allow water to drain between them. Onrushing water disappears into a beach made of coarse particles, so little water is left to rush down the slope, thereby minimizing the transport of sediments back to the ocean. Thus, larger particles tend to build up at the back of the beach, increasing its steepness.

The shape of a delta represents a balance between the accumulation of sediments and their removal by the ocean. For a delta to maintain its size or grow, the river that feeds the delta must carry enough sediment to keep marine processes in check. The combined effects of waves, tides, and river flow determine the shape of a delta. River-dominated deltas are fed by a strong flow of fresh water and continental sediments, and form in protected marginal seas. In tide-dominated deltas, freshwater discharge is overpowered by tidal currents that mold sediments into long islands parallel to the river flow and perpendicular to the trend of the coast. Wave-dominated deltas are generally smaller than either tide- or river-dominated deltas and have a smooth shoreline punctuated by beaches and sand dunes. Instead of a bird's-foot pattern of distributaries, a wave-dominated delta will have one primary exit channel. Deltas do not form at the mouth of every sediment-laden river. A broad continental shelf must be present to provide a platform on which sediment can accumulate, and, as befits an erosional coast, marine processes must not dominate—that is, tidal range should be low, and waves and currents generally mild. Deltas are most common on the low-energy shores of enclosed seas where the tidal range is not extreme, and along the tectonically stable trailing edges of some continents. The largest deltas are those of the Gulf of Mexico (the Mississippi), the Mediterranean Sea (the Nile), the Ganges-Brahamaputra river system in the Bay of Bengal, and the huge deltas formed by the rivers of China that empty into the South China Sea. Deltas tend not to form at West Coast river mouths because the continental shelf is narrow, river flow is generally low (except for the Columbia River), and beaches are usually high in wave energy.

Three factors determine the characteristics of estuaries: the shape of the estuary, the volume of river flow at the head of the estuary, and the range of tides at the estuary's mouth. The mingling of waters of different densities, the rise and fall of the tide, and the variations in river flow along with the actions of wind, ice, and Coriolis Effect guarantee that patterns of water circulation in an estuary will be complex. Estuaries are classified by their circulation patterns. The simplest circulation patterns are found in salt wedge estuaries, which form where a rapidly flowing large river enters the ocean in an area where tidal range is low or moderate. The exiting fresh water holds back a wedge of intruding seawater. Note that density differences cause fresh water to flow over salt water. The seawater wedge moves seaward at times of low tide or strong river flow, and returns landward as the tide rises or when river flow diminishes. Some seawater from the wedge joins the seaward-flowing fresh water at the steeply sloped upper boundary of the wedge, and new seawater from the ocean replaces it. Nutrients and sediments from the ocean can enter the estuary in this way. Examples of salt wedge estuaries are the mouths of the Hudson and Mississippi rivers. A different pattern occurs where the river flows more slowly and tidal range is moderate to high. As their name implies, well-mixed estuaries contain differing mixtures of fresh and salt water through most of their length. Tidal turbulence stirs the waters together as river runoff pushes the mixtures to sea. The mouth of the Columbia River is an example. Deeper estuaries exposed to similar tidal conditions but greater river flow become partially-mixed estuaries. Partially-mixed estuaries share some of the properties of salt wedge and well-mixed estuaries. England's Thames River, San Francisco Bay, and Chesapeake Bay are examples. Reverse estuaries can form along arid coasts when rivers cease to flow. The evaporation of seawater in the uppermost reaches of these estuaries will cause water to flow from the ocean into the estuary, producing a gradient of increasing salinity from the ocean to the estuary's upper reaches. Reverse estuaries—sometimes called lagoons—are common on the Pacific coast of Mexico's Baja peninsula and along the U.S. Gulf coast. Estuaries often support a large number of living organisms. The easy availability of nutrients and sunlight, protection from wave shock, and presence of many habitats permit the growth of many species and individuals. Estuaries are frequently nurseries for marine animals; several species of perch, anchovy, and Pacific herring take advantage of the abundant food in estuaries during their first weeks of life. Unfortunately for their inhabitants, estuaries are in high demand for development into recreational resources and harbors. Estuaries have become the most polluted of all marine environments.

The West Coast is an actively rising margin on which volcanoes, earthquakes, and other indications of recent tectonic activity are easily observed. West Coast beaches are typically interrupted by jagged rocky headlands, volcanic intrusions, or the effects of submarine canyons. Most of the sediments on the West Coast originated from erosion of relatively young granitic rocks of the coastal mountains. The particles of quartz and feldspar that comprise most of the sand were transported to the shore by flowing rivers. The volume of sedimentary material transported to west coast beaches from inland areas greatly exceeds the amount originating at the coast itself. Because West Coast beaches are usually high in wave energy, deltas tend not to form at West Coast river mouths. The predominant direction of longshore drift is to the south. The East Coast is a passive margin, tectonically calm and subsiding because of its trailing central position on the North American Plate. Subsidence along the coast has been considerable over the last 150 million years, and a deep layer of sediment built up offshore, material that produced the ancestors of today's barrier islands. Relatively recent subsidence has been more important in shaping the present coast, however. Coastal sinking and rising sea level have combined to submerge some parts of the East Coast at a rate of about 1/3 meter (1 foot) per century. This process has formed the huge flooded valleys of Chesapeake and Delaware Bays, the landward-migrating barrier islands, and the shrinking lowlands of Florida and Georgia. Rocks to the north (in Maine, for example) are among the hardest and most resistant to erosion of any on the continent, so beaches are uncommon in Maine. But from New Jersey southward, the rocks are more easily fragmented and weathered, and beaches are much more common. As on the West Coast, sediments are transported coastward by rivers from eroding inland mountains, but the transported material is trapped in sunken estuaries and therefore plays a less important role on beaches. Eastern beaches are typically formed of sediments from nearby erosional shores, or from the shoreward movement of offshore deposits laid down when the sea level was lower. The amount of sand in an area thus depends in part on the resistance or susceptibility of nearby shores to erosion. Sand moves generally south on these beaches just as it does on the West Coast, but the volume of moving sand in the East is less. The Gulf Coast experiences a smaller tidal variance and—hurricanes excepted—a smaller average wave size than either the West or East Coasts. Reduced longshore drift and an absence of interrupting submarine canyons allow the great volume of accumulated sediments from the Mississippi and other rivers to form large deltas, barrier islands, and a long raised "super berm" that prevents the ocean from inundating much of this sinking coast.

Beaches exist in a tenuous balance between accumulation and destruction, and human activity can tip the balance one way or the other. We often divert rivers, build harbors, and develop property with surprisingly little understanding of the impact our actions will have on the adjacent coast. Residents of erosional coasts can only accept the inexorable loss of their property to the attack of natural forces, but residents of depositional coasts are sometimes presented with alternatives. The choices are almost never simple. For example, should rivers be dammed to control devastating floods? If the dams are built, they will trap sediments on their way from mountains to coast. Beaches within the coastal cell fed by the dammed river will shrink because the sand on which they depend to replenish losses at the shore is blocked. Alarmed coastal residents will then take steps to hang onto whatever sand remains. They may try to trap "their" beaches by erecting groins to stop the longshore transport of sediments. This temporary expedient usually accelerates erosion downcoast. Diminished beaches then expose shore cliffs to accelerated erosion—wind wave energy that would have harmlessly churned sand grains now speeds the destruction of natural and artificial structures. Was protection from periodic flooding worth the loss of the beach? I suppose it depends on where your property is situated! Shores that look permanent through the short perspective of a human lifetime are in fact among the most ephemeral of all marine structures. The only way to prevent the loss of life and property is not to build close to shore.