Transcript Chapter 11 The Dynamic Shoreline

Introduction to
Oceanography
Physical Oceanography
-Coastlines 1-
Classifying Coasts
Coasts can be classified by a method that takes into account tectonic
position and sea level.
Primary coasts are young coasts dominated by terrestrial influences.
 Land erosion coasts
 Coasts built out by land processes
 Volcanic coasts
 Coasts shaped by earth movements
Secondary coasts are older coasts that have been changed by
marine influence.
 Waves and currents
 Stream erosion
 Abrasion of wind-driven particles
 Freeze/thaw cycles
 Slumping
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Primary Coasts
Many of the islands of Martha’s Vineyard and Nantucket are
examples of a primary coast.
Secondary Coasts
Waves are one of the forces that shape secondary coasts.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Large Scale Features of Secondary Coasts
These features are associated with secondary coasts.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Beaches
A beach is a zone of loose particles that covers a shore.
Note that larger particles
are associated with
more sloped beach.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
11-1
Coastal Water Movement
Breaking waves provide the energy that changes the
shape and texture of the beach deposits.
• As waves shoal (touch bottom) in shallow water celerity
decreases, wavelength decreases, wave height increases,
waves become less stable and refraction occurs.
• Refraction is the bending of waves towards shallower water so that they break
almost parallel to the shore.
• Waves become unstable and break in very shallow water.
• The beach is the part of the land that touches the sea. It can be
divided into the: Off shore, Near shore (breaker zone, surf
zone, swash zone), and the Back Shore
• Position of the divisions of the beach varies with the tides,
advancing landward with high tide and retreating seaward with
low tide.
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Coastal Water Movement
Waves generate longshore currents that flow parallel
to the beach and rip currents that flow
perpendicularly to the beach.
• Angle of wave approach is the acute angle (less than 90o)
between the wave crest and the beach.
• The direction of longshore current varies with the direction of
wave approach.
• Longshore currents can also be generated by wave set-up.
• Where two opposing longshore currents collide, they form a
swift, narrow, seaward rip current.
11-2
Beaches
Beach sediments are moved by currents and waves,
especially breakers.
• A beach profile is a cross section of the beach along a line that is
perpendicular to the shoreline.
• A swell profile is concave upward with a wide, broad berm
(relatively flat backshore) and steep intertidal beach face.
• A storm profile displays erosion of the berm and a broad flat
intertidal beach face.
• A sand budget is the balance between sediment added to and
sediment eroded from the beach.
Coastal Cells
Sections of coast in which sand input
and sand output are balanced are
referred to as coastal cells.
Coastal Dunes
11-3
Sand dunes are formed by winds blowing sand
landward from the dry part of the beach.
• Well developed dunes typically have a sinusoidal profile with the primary dune at
the landward edge of the beach and possible secondary dunes located farther
inland.
• Vegetation on the dunes traps windblown sand on their downwind side and
promotes dune growth and stability.
• Blowouts are wind-scoured breaks in the dune or depressions in the dune ridge
and commonly occur if vegetation is destroyed.
• Dunes are best developed if sand is abundant, onshore winds are moderately
strong and persistent, the tidal range is large and the beach is wide and gently
sloping.
• Sand saltates (bounces) up the windward side of the dune, collects in the windshadow at the top and periodically slides down the leeward face of the dune when
the accumulation of sand becomes over-steepened—resulting in dune migration.
• Wave erosion of sand dunes transports sand offshore and creates a steep scarp at
the base of the dune.
• Dunes act as a natural barrier and prevent inland flooding.
• Human activity that damages vegetation leads to dune destruction by blowouts
and washover by storm waves.
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Barrier Islands
Barrier islands are islands composed of sediment
that parallel the coast and form where sand supply is
abundant and a broad sea floor slopes gently
seaward.
• The islands are separated from the mainland by shallow bodies
of water which are connected to the ocean through tidal inlets.
• A series of distinct environments develop across the island
parallel the beach and include the nearshore zone, dune field,
back-island flats and salt marshes.
• Barrier islands are created in many ways including: sand ridges
isolated by rising sea level, Sand spits breached during a storm,
vertical growth and emergence of longshore sand bars.
• As sea level rises, barrier islands migrate landward as washover
transports sediments from the seaward side of the island to the
landward side.
11-5
Cliffed Coasts
A sea cliff is an abrupt rise of the land from sea
level.
• A sea cliff is most vulnerable to erosion at its base because
waves that slam against the cliff compress air inside cracks
which expands violently, sediment is hurled against the cliff by
the waves, and sea water dissolve some rock types.
• When sufficient rock at the base of the cliff has been removed,
the upper part of the cliff collapses.
• Collapsed material protects the base of the sea cliff from
additional erosion until it is destroyed and removed.
• Rate at which the cliff recedes is dependent upon:
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Composition and durability of cliff material.
Joints, fractures, faults and other weaknesses in the cliff material.
Amount of precipitation.
Steepness of the cliff.
• The wave-cut platform is the gentle sloping area in front of the
sea cliff that was produced by sea-cliff retreat.
Deltas
11-6
A delta is an emergent accumulation of sediment
deposited at the mouth of a river as it flows into a
standing body of water.
• Deltas were named after the Greek letter delta .
• The three major areas of a delta are delta plain ,delta front and
prodelta.
• In cross section, a delta’s deposits can be divided into three sets
of beds: topset beds, foreset beds, and bottomset beds.
• As sediment accumulates the delta expands seaward with
foreset beds burying bottomset beds and topset beds covering
foreset beds.
• Shape of the delta can be altered by tides, waves and river
deposition.
• Reduction in the supply of sediment to a delta results in delta
erosion and subsidence as the sediments of the delta compact.
Types of coastal setting
The term coast has a much broader meaning than
shoreline and includes many other habitats and
ecosystems associated with terrestrial and marine
processes.
• The six major coastal settings are: estuary, lagoon, salt marsh,
mangrove swamp and coral reef.
• Shorelines are one of the most productive ecosystems and
because they are shallow, they strongly respond to the effects of
waves, tides and weather.
Estuaries
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Estuaries are semienclosed bodies of
water where fresh
water from the land
mixes with sea water.
• Estuaries originate as:
drowned river valleys,
fjords, bar-built
estuaries, and tectonic
estuaries.
• Salinity typically grades
from normal marine
salinity at the tidal inlet
to fresh water at the
mouth of the river.
Estuaries
12-1
Estuaries can be subdivided into three types based
upon the relative importance of river inflow and tidal
mixing.
• Salt-wedge estuaries are dominated by the outflow from
rivers.
• Partially-mixed estuaries are dominated by neither river
inflow nor tidal mixing.
• In well-mixed estuaries tidal turbulence destroys the
halocline and water stratification.
• Because river discharge and tidal flow vary, conditions
within an estuary can also change, being well-mixed when
river flow decreases relative to tidal mixing, to becoming a
salt-wedge estuary at times of maximum river discharge.
12-1
Estuaries
The widely fluctuating environmental conditions in
estuaries make life stressful for organisms.
• Estuaries are extremely fertile because nutrients are brought in
by rivers and recycled from the bottom because of the
turbulence.
• Stressful conditions and abundant nutrients result in low
species diversity, but great abundance of the species present.
• Despite abundance of nutrients, phytoplankton blooms are
irregular and the base of the food chain is detritus washed in
from adjacent salt marshes.
• The benthic fauna strongly reflects the nature of the substrate
and most fishes are juvenile forms living within the estuary
until they mature and migrate to the ocean.
Chesapeake Bay