Coasts Chapter12

8/12/2019 Coasts Chapter12

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Chapter 12Coasts

OceanographyAn Invitation to Marine Science, 7th

Tom Garrison


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Chapter 12 Study Plan

Coasts Are Shaped by Marine and TerrestrialProcesses

Erosional Processes Dominate Some Coasts

Beaches Dominate Depositional Coasts Larger-Scale Features Accumulate on Depositional

Coasts

Biological Activity Forms and Modifies Coasts

Fresh Water Meets the Ocean in Estuaries

The Characteristics of U.S. Coasts

Humans Interfere in Coastal Processes


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Chapter 12 Main Concepts

The location of a coast depends primarily on globaltectonic activity and the oceans water volume.

The shape of a coast is a product of many processes:uplift and subsidence, the wearing-down of land byerosion, and the redistribution of material by sedimenttransport and deposition.

Coasts are classified as erosional coasts (on whicherosion dominates) or depositional coasts (on whichdeposition dominates).

Beaches change shape and volume as a function of waveenergy and the balance of sediment input and removal.

Human interference with coastal processes has generallyaccelerated the erosion of coasts near inhabited areas.


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Coasts Are Shaped by Marine

and Terrestrial ProcessesSea levels past and future. (a)Sea level rose rapidly at theend of the last ice age asglaciers and ice caps meltedand water returned to theocean. The rate of rise has

slowed over the past 4,000years and is now believed to bebetween 1.0 and 2.4millimeters per year. (b)Projections of sea level throughthe year 2100. Seven researchgroups (represented here bycolored lines) have estimatedfuture sea level based onhistorical observations andclimate models. Even the mostconservative of thesepredictions estimates a 20-

centimeter (8-inch) rise.


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(top-left) The southeastern coastof the U.S. looked much different18,000 years ago, during the lastice age. Because of lower sealevel, the position of the gentlysloping coast has been as muchas 200 km (125 miles) seawardfrom the present shoreline,leaving much of the continentalshelf exposed.

(bottom-left) In the distant future,

if the ocean were to expand andthe polar ice caps were to meltbecause of global warming, sealevel could rise perhaps 60meters (200 feet), driving thecoast inland as much as 250kilometers (160 miles)

Coasts Are Shaped by Marine

and Terrestrial Processes


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Erosional Coasts Often Have

Complex Features

Features of an erosional

coast at low tide.

Wave erosion of a seacliff produces a shelf-

like wave-cut platform

visible at low tide.


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Shorelines Can Be Straightened

by Selective Erosion

Wave energy converges on headlands and diverges in the adjoiningbays. The accumulation of sediment derived from the headland in thetranquil bays eventually smoothes the contours of the shore.

Marine erosion is usually most rapid on high -energy coasts, areas

frequently battered by large waves. Low-energy coastsare only infrequently attacked by large waves.


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Beaches Often Have a Distinct

ProfileA beach is a zone of loose particles that covers a

shore.

What are the features of a beach?

Berm

Berm crest

Backshore

Foreshore Beach scarp

Longshore trough

Longshore bars


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Waves Transport Sediment on

Beaches

A longshore current movessediment along the shorelinebetween the surf zone and theupper limit of wave action.


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Sand Input and Outflow Are

Balanced in Coastal Cells

Coastal sediment transport cells.(a) The general features of coastal cells. Sand is introduced by rivers, transported southwardby the longshore drift, and trapped within the nearshore heads of submarine canyons.

(B) Example of a sand budget. If sediment gains and losses are approximately equal, thenearshore system is in equilibrium. If losses exceed gains, as shown here, the beaches withinthe cell will shrink and possibly disappear.

Sections of coast in which sand input and sand output are balanced are referred to as coastalcells.


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Large-Scale Features Accumulate

on Depositional Coasts

(above) A composite diagram of the large-scale features of an imaginarydepositional coast. Not all these features would be found in such close

proximity on a real coast.

A sand spi tforms where the longshore current slows as it clears a headland andapproaches a quiet bay.

A bay mouth barforms when a sand spit closes off a bay by attaching to aheadland adjacent to the bay.

Depositional coasts can also develop narrow, exposed sandbars that are parallel

to but separated from land - known as barrier islands. A long, shallow body of seawater isolated from the ocean is known as a lagoon.


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Coasts Are Formed And Modified

By Biological Activity

The development of an atoll.

(a) A fringing reef forms around an island in the tropics.

(b) The island sinks as the oceanic plate on which it rides moves away from a spreading center. In

this case, the island does not sink at a rate faster than coral organisms can build upward forming a

barrier reef.

(c) The island eventually disappears beneath the surface, but the coral remains at the surface as an atoll.


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Estuaries Are Classified by Their Origins

An estuary is a body of water inwhich fresh river water mixes withocean water. Estuaries can beclassified by their origin:

Drowned river mouths

Fjords

Bar-built Tectonic

(a) Drowned river mouths: themouths of the James, York, andSusquehanna rivers; Chesapeake

Bay; Sydney Harbour, Australia.(b) Fjords: New Zealands MilfordSouth; the Strait of Juan de Fuca inWashington state.

(c) Bar-built: Albernarle and Pamlicosounds in North Carolina

(d) Tectonic: San Francisco Bay;Tomales Bay


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Estuary Characteristics Are Influenced

by Water Density and Flow

Types of estuaries invertical cross sections.The salinity values showthe amount of mixing

between fresh water andseawater in the varioustypes.

(a) Salt wedge estuary.(b) Well-mixed estuary.

(c) Partially mixedestuary.

(d) Fjord estuary.


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The Characteristics of U.S. Coasts

The Pacific Coast- An actively rising margin

where indications of recent tectonic activity

can be observed.

The Atlantic Coast- A passive margin onthe trailing position of the North American

plate.

The Gulf Coast- Smaller wave size and asmaller tidal range characterize the Gulf

Coast.

H H I t f d i t l


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Humans Have Interfered in oastalProcesses

What are some ways thathumans try to influencecoastal processes?

Groins

Seawalls Importing sand

(left) A few of the manytypes of measures taken toslow and prevent beach

erosion.

In many cases thesemethods help serve as areminder that shorelines andbeaches are constantly

changing, and are not underhuman control.

(a) Groin


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(a) GroinGroins are structures that extend fromthe beach into the water. They helpcounter erosion by trapping sand fromthe current. Groins accumulate sand ontheir updrift side, but erosion is worseon the downdrift side, which is deprivedof sand.

Current

(b) SeawallSeawalls protect property temporarily,but they also increase beach erosionby deflecting wave energy onto thesand in front of and beside them. Highwaves can wash over seawalls anddestroy them and property.

(c) Importing sandImporting sand to a beach is

considered the best response toerosion. The new sand often isdredged from offshore, can cost tensof millions of dollars, and can disturbaquatic biodiversity. Because it isoften finer than beach sand, dredgedsand erodes more quickly.

Stepped Art


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Humans Have Interfered in

Coastal Processes

A map showing shore erosion by region. One example of shoreerosion is the lighthouse on Cape Hatteras, which was movedduring 1998 and 1999 to protect it from destruction. It was

threatened by rising sea levels and a changing shoreline.


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Chapter 12 in Perspective

In this chapter you learnedthat the location of a coast depends primarilyon global tectonic activity and the oceans water volume, while the shape ofa coast is a product of many processes: uplift and subsidence, the wearing-down of land by erosion, and the redistribution of material by sedimenttransport and deposition. Coasts are classified as erosional coasts (onwhich erosion dominates) or depositional coasts (on which depositiondominates). Natural rock bridges, tall stacks, and sea caves are found on

erosional coasts. Depositional coasts often support beaches, accumulationsof loose particles. Generally, the finer the particles on the beach, the flatterits slope. Beaches change shape and volume as a function of wave energyand the balance of sediment input and removal. Coral reefs and estuariesare among the most complex and biologically productive coasts. Humaninterference with coastal processes has generally accelerated the erosion ofcoasts near inhabited areas.

In the next chapter you will learnthat the study of oceanography includesa marvelous variety of living things. The next chapters discussion of thegeneral nature and characteristics of marine life will launch us into thebiological part of our journey.

Coasts Chapter12

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