by keith meldahl - kau · 2009. 8. 16. · beaches, sand spits, bars, and barrier ... division of...
TRANSCRIPT
© 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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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!
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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!
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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).
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
The four types
of estuaries.
See the previous
slide for an
explanation of
the differences
between these
types.
Estuaries
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
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.