By Keith Meldahl - kau · 2009. 8. 16. · beaches, sand spits, bars, and barrier ... division of Thomson Learning, Inc. Summary of Important Concepts, continued Longshore drift is

© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Lesson: Coastal OceanographyBy Keith Meldahl

corresponding to Chapter 12: Coasts

This map shows that many areas of the U.S.A. have significant shoreline

erosion problems.


Summary of Important Concepts

• Coasts are classified as primary, secondary, or biological.

PRIMARY COASTS: Young coasts that have not been

modified much by waves and currents. These coasts are rough

and irregular, because the ocean has not had enough time to

smooth and modify them much. Primary coasts occur mostly in

areas of tectonic activity, or areas of large river deltas.

SECONDARY COASTS: Older coasts that have been modified

extensively by waves and currents. These coasts have been

influenced by oceanic processes for long time spans, and thus

have many features created by waves and currents. Secondary

coasts occur in areas of little tectonic activity.

Major features of secondary coasts: sea cliffs and wave-cut

platforms; shoreline straightening over time; and the formation of

beaches, sand spits, bars, and barrier islands.


Summary of Important Concepts, continued

Longshore drift is one of the most important processes affecting

secondary coasts. Longshore drift is the movement of sand

along the shoreline in the direction of the waves.

***********

BIOLOGICAL COASTS: Coasts whose appearance results mostly

from the activities of animals and plants. Coral reefs and

mangrove coasts are examples.

Estuaries are important biological habitats along many

coasts. Estuaries are bodies of water is partially surrounded by

land where fresh river water and ocean water mix. Different types

of estuaries are distinguished by their origin, and by the mixing

pattern of the fresh and salt water.

• Human interference in coastal processes rarely helps stabilize

coasts for long, and in fact may lead to increased erosion.


Classifying Coasts

A coast can be classified as either PRIMARY, SECONDARY, or

BIOLOGICAL, according to the main processes that affect its

appearance.

Primary coasts are young coasts that have not been altered much by

oceanic processes (waves and currents).

Secondary coasts are older coasts that have been changed by

waves and currents.

Biological coasts are coasts primarily formed by the growth of

animals and/or plants.


Primary Coasts

Primary coasts have not been altered much by waves and currents.

Their appearance reflects either terrestrial (land-based) or tectonic

processes. The dominant processes affecting primary coasts are:

• Tectonic uplift or lowering of the coast.

• Drowning of the coast by a recent rise in sea level.

• Outward-building of the coast near a river delta (an area where a

river enters the ocean and deposits large amounts of sediment).

Primary coasts are generally rough and irregular, because they

have not been straightened out by waves and currents.


Secondary Coasts

Secondary coasts

have been

significantly

altered by the

actions of waves

and currents. For

example, erosion

by waves makes

sea cliffs and

wave-cut

platforms, as

shown in this

figure.


Secondary Coasts

An important feature

of most secondary

coasts is shore

straightening. A

geologically “new”

coast tends to be

irregular. But as this

figure shows, wave

refraction focuses

wave energy on

headlands, and

spreads out wave

energy across bays.

Thus over time the

headlands erode and

the bays fill with

sediment, making a

straighter shoreline!


Beaches

A beach is a zone of

loose particles along a

shoreline. The sizes of

particles vary widely

between different

beaches. Among other

things, the particle size

determines the slope of

the beach. As the table

shows, the larger the

particles, the steeper the

slope of the beach.


Beaches

In this figure, note the locations of the following beach features:

• backshore, foreshore, nearshore, and offshore zones

• berms, longshore troughs, and longshore bars

Longshore DriftLongshore

drift is the

movement of sand

along the beach in

the direction of the

waves. Breaking

waves create a

current of water

parallel to shore,

called a longshore

current. This

current moves sand

along the shore in

the direction the

waves are moving.

Large amounts of

sand move along

coasts by longshore

drift.


The sand on beaches generally comes from rivers. The sand moves

gradually along the beach in the direction of longshore drift. The sand then

eventually leaves the beach where it drains down submarine canyons. A

single section of coast with river input, longshore drift, and eventual output

down a submarine canyon is called a coastal cell.

Longshore Drift & Coastal Cells


Longshore Drift, Sand Spits, & Bay Mouth Bars

As sand moves along the coast due to longshore drift, it may form

sand spits where the sand slows down at the mouth of a bay. If the

sand closes off the mouth of a bay, a bay mouth bar is formed.

The longshore current slows down after it goes around a

headland. The sand gets deposited at the mouth of the bay,

forming a sand spit.


Primary Coasts and Secondary Coasts are formed by physical processes,

as discussed above. But some coasts are formed by biological activity.

Examples include coral reefs and coasts dominated by mangroves.

Coral Reefs are the most dramatic example of biological coasts. Coral

animals make three main types of reefs:

Fringing Reefs cling to the margin of land.

Barrier Reefs are separated from land by a lagoon.

Atolls are ring-shaped islands of coral reef enclosing lagoons.

Biological Coasts


Biological Coasts

Charles Darwin was the first to propose that the three types of

reefs (fringing, barrier, and atoll) reflected a single process. The

first type of reef to form around a new island is a fringing reef. As

an island gradually sinks, a fringing reef grows upward to

become a barrier reef. As the island sinks below sea level, only a

ring of coral -- an atoll -- is left at the surface!


Coral Reefs and Plate Tectonics

The progression from fringing reef to barrier reef to atoll occurs because islands sink slowly over time. Why does this happen? Recall that many volcanic islands form over hot spots in the mantle. Once an island forms, it gets carried away from the hot spot by the moving oceanic plate on which it is riding. As the plate cools down, it gets denser, and so sinks lower into the mantle. This is an example of isostatic equilibrium.


Estuaries

Estuaries are important features of many coasts. An estuary is a body of water, partially surrounded by land, in which fresh river water mixes with ocean water. Estuaries are very rich biological habitats, but also highly vulnerable to pollution and urban development.

Estuaries can be classified by their origin into four types.

Drowned river mouths (where a river valley has been flooded by rising sea level since the last Ice Age). Fjords (where a glacial valley has been flooded by rising sea level since the last Ice Age). Bar-built (where a barrier island separates a lagoon from the open ocean). Tectonic (where faults have dropped down a section of the earth’s crust in a coastal area).


The four types

of estuaries.

See the previous

slide for an

explanation of

the differences

between these

types.

Estuaries


Estuaries are areas of

mixing between fresh

water and salt water.

Estuaries are thus

categorized into five

types according to the

pattern of mixing

between fresh and salt

water:

• salt wedge

• well-mixed

• partially mixed

• fjord

• reverse

Estuaries


The Pacific Coast - An active continental margin where recent tectonic

activity has caused many areas to be uplifted. The Pacific Coast is

mostly a primary coast. It is characterized by steep, rugged shorelines

and a narrow continental shelf.

The Atlantic Coast - Because the Atlantic Coast is a passive continental

margin on the trailing edge of the North American plate, it has been

either stable or slowly subsiding for a long period of time. The Atlantic

Coast is mostly a secondary coast, having been highly altered by waves

and currents. It is characterized by gently sloping shorelines and a wide

continental shelf.

The Gulf Coast - In tectonic setting similar to the Atlantic Coast.

However smaller wave size and a smaller tidal range mean that oceanic

processes have less effect than the other coasts. The dominant coastal

feature here is the Mississippi River Delta.

Characteristics of U.S. Coasts


Human Interference in Coastal Processes

Efforts to stabilize shoreline areas often do not work for long, and

can even make erosion problems worse.

• Groins are short extensions of rock or other material placed at right

angles to the beach. Their intent is to trap sand moving along the beach

by longshore drift. But they usually make erosion worse on the down-drift

side of the groin.

•Seawalls are built to protect coastal cliffs and bluffs from eroding, but

their flat faces reflect wave energy back onto the beach, eroding the

beach. The beach forms a natural barrier to waves, so as the beach gets

smaller, wave erosion worsens.

•Importing sand (adding sand to the beach from elsewhere) is commonly

done to restore eroded beaches. This expensive procedure usually

involves dredging sand from offshore and putting it on the beach. But this

offshore sand is often finer (made up of smaller particles), so it erodes

away within a short period of time.

Documents

By Keith Meldahl - kau · 2009. 8. 16. · beaches, sand spits, bars, and barrier ... division of Thomson Learning, Inc. Summary of Important Concepts, continued Longshore drift is