20-shores and beaches - impacttectonics.orgcoasts are characterized by long sandy beaches, deltas,...
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TCNJ PHY120 2013 GCHERMAN
SHORES AND BEACHES
Sources:
www.google.com
en.wikipedia.org
Thompson Higher Education 2007; Monroe, Wicander, and Hazlett, Physical Geology
Trunk Bay, St Thomas, US Virgin Islands
TCNJ PHY120 2013 GCHERMAN
• Shorelines and Coasts
• Role of Coral Reef in Shoreline Protection
• Wind-Generated Waves
• Near-shore currents and Tides
• Shoreline Erosion
• Shoreline Deposition
• Barrier Islands
• Types of Coasts
• Storm Waves and Coastal Flooding
• Shoreline Protection Efforts
SHORES AND BEACHES
TCNJ PHY120 2013 GCHERMAN
SHORELINES are continually being modified by waves, longshore
currents, and tidal currentsTwo views of the US Pacific Coast
TCNJ PHY120 2013 GCHERMAN
SHORELINES and COASTS
• Shoreline is the area of land between land and high tide reaches.
• Shoreline and coast are commonly used interchangeably, but coast is
considered to be a more inclusive term that includes the shoreline as
well as area seaward and landward of the shoreline.
•For example, coasts can include near-shore islands, cliffs, barrier dunes,
marshes, and other landforms found near shorelines.
TCNJ PHY120 2013 GCHERMAN
WIND-GENERATED WAVES are most common but waves can also
be produced by submarine faulting, volcanic eruptions, and rockfalls.
• When wind blows
over water the
frictional drag of one
fluid over another
transfers some of the
energy to the water,
causing the water
surface to oscillate.
• Wind-generated seas of various sizes form beneath atmospheric storms.
TCNJ PHY120 2013 GCHERMAN
HURRICANE SANDY 10-27/28-2012
HURRICANE SANDY 10-27/28-2012
TCNJ PHY120 2013 GCHERMAN
TCNJ PHY120 2013 GCHERMAN
WIND-GENERATED WAVES
• Storm-generated swells move away
from the area where they formed.
• The size of wind generated waves is
limited by the fetch of the water body;
that is the open distance across which
the wind blows.
TCNJ PHY120 2013 GCHERMAN
WAVES are oscillations of the water surface which transmit energy, in the
direction of wave movement.
• Wave base is that depth to which surface waves affect the water
and sea floor, and is equal to one-half of the wavelength.
TCNJ PHY120 2013 GCHERMAN
WAVES
• Deep-water swells caused by
wind or tectonic processes move
across open seas toward shore.
•The orbital motion of water
within them is stationary until it is
disrupted when reaching shallow
water at depths of ½ wavelength.
•Here waves do move water in the
direction of wave advance,
wavelengths begin to decrease,
and wave height increases causing
oversteepend waves that plunge
toward shorelines as breakers.
TCNJ PHY120 2013 GCHERMAN
BREAKERS form when the orbital
motion of water molecules is disrupted
by their entry to shallow water.
plunging breaker
spllling breaker
• The merging of waves of
different lengths in the
breaker zone accounts for
periodic variations in the
size of breaking waves.
• Commonly several times higher than
deep-water waves, and when they
plunge forward their kinetic energy is
expended on the shoreline.
TCNJ PHY120 2013 GCHERMAN
WAVE BASE AND BREAKERS
Waves with about 2 meters
and a wave base ~ 1m
Waves about
2 m high and
a wide break
zone
TCNJ PHY120 2013 GCHERMAN
THE NEARSHORE ZONE is the area extending seaward from the
upper limits of the shoreline to just beyond where the waves break.
• It includes a
breaker zone and
a surf zone.
Surf
zone
•Two important currents work here; longshore and rip currents.
TCNJ PHY120 2013 GCHERMAN
WAVES REFRACT (or bend) when reaching shorelines at oblique angles
as they are dragged by friction to near-parallel alignment with the shoreline
• This motion
generates long
shore currents
flowing in the
direction that
the waves
approach.
The surface trace of wave crest lines are dashed red
TCNJ PHY120 2013 GCHERMAN
LONGSHORE CURRENTS are produced by waves which encounter the
shoreline at an oblique angle.
• Long and
narrow an run
parallel to shore.
•They can
produce
significant
erosion,
transportation
and deposition.
TCNJ PHY120 2013 GCHERMAN
RIP CURRENTS are
narrow surface current
that flow out to sea
through the breaker zone.
• Form circulation cells
that feed long shore
currents.
•Often focused in areas
having the smallest
waves, like over shallow
depressions.
•Pose a risk to
inexperienced swimmers
TCNJ PHY120 2013 GCHERMAN
TIDES are twice daily
fluctuations in the surface
elevation of standing water
bodies caused by the
gravitational attraction of
the Sun and Moon
• Spring tides occur when the
Moon and Sun are aligned
every two weeks, the
gravitational pull is highest, and
tidal effects are amplified.
•Neap tides occur during the
Moon’s first and third quarters
when the Moon, Earth and Sun
are at right angles causing the
lowest high tides, and the
highest low tides
TCNJ PHY120 2013 GCHERMAN
TIDAL CURRENTS have little affect on shore erosion.
• The Sun is 27 million times more
massive than the Moon, but it is 390
times farther away from Earth so its
gravitational force is just less than half
of that from the Moon.
•The Moon generates lunar tidal bulges
whereas the Sun generates solar tidal
bulges.
•There is a 50-minute time lag each day from when high tide occurs on consecutive
days owing to the constant change of position of the Moon with respect to Earth’s
latitude.
TCNJ PHY120 2013 GCHERMAN
SHORELINE EROSION
• Erosion processes develop
sea cliffs, wave-cut platforms,
sea arches, and sea stacks can
develop along shorelines.
•Most geologic modification of shorelines
is accomplished by wind-generated
waves, especially storm waves.
•Erosion rather than deposition
characterizes many shorelines.
TCNJ PHY120 2013 GCHERMAN
SHORELINE EROSION processes
include hydraulic action, abrasion, and
corrosion.
•Most geologic modification of shorelines is
accomplished by wind-generated waves,
especially storm waves.
The rocks in the lower part of this image on a small
island in the Irish Sea have been smoothed by
abrasion in comparison to the higher reaches
beyond normal wave action
Hydraulic action and abrasion have undercut these sea cliffs near
Bodega Bay, California.
TCNJ PHY120 2013 GCHERMAN
SHORELINE EROSION
• Sea caves and arches form
because waves refract around
objects and erode headlands on
both sides.
TCNJ PHY120 2013 GCHERMAN
SHORELINE EROSION
•Wave-cut platforms are broad, smooth,
submarine surfaces sloping gently seaward.
• Sea stacks are erosion remnants of wave-cut
platforms and collapsed sea arches
TCNJ PHY120 2013 GCHERMAN
A BEACH is a deposit of unconsolidated sediment extending landward fro
low tide to a change in topography such as a line of sand dunes, or sea cliff, or
the point where vegetation begins.
• Beaches are the most
common shoreline
depositional feature.
A pocket beach at Julia Pfeiffer State Park, California
TCNJ PHY120 2013 GCHERMAN
BEACHES have several components including backshores, berms, beach
faces, and foreshores
• BACKSHORE is usually dry,
being covered by water only
during high tides.
• BERMS are nearly
horizontal platforms
composed of sediment
deposited by waves
•FORESHORE is the part covered by
water during high tide but exposed during
lo tide
• BEACH FACES are the part of the
foreshore that forms below berms that
are exposed to wave wash
TCNJ PHY120 2013 GCHERMAN
MORE BEACHES
Oahu on Hawaii is composed of fragmented marine shells
California beach composed
mostly of quartz sand
Maui beach in
Hawaii is
composed of
small fragments
of basalt and
obsidian
BEACHES are continually modified by the action of waves, longshore currents,
tides, and storms, and exhibit profiles typical of seasonal change which affect wave
direction and strength.
TCNJ PHY120 2013 GCHERMAN
Seasonal changes in a beach profile
SUMMER BEACHES are covered in sand, possess a wide berm, a steep
beach face, and a smooth offshore profile.
TCNJ PHY120 2013 GCHERMAN
WINTER BEACHES are
attached more vigorously by
storm-generated waves, and has
little or no berm, a gentle slope,
and offshore sandbars that
parallel the shoreline.
• The gentle swells from the onset of the summer season return the offshore sand to
the beach.
LONGSHORE DRIFT is the
deposition of sediment along the
shoreline in the direction of wave
propagation by longshore currents
• Groins or jetties help protect the
beach from longshore drift
TCNJ PHY120 2013 GCHERMAN
Groins along Cape May beach in New Jersey trap
sand as longshore drift
TCNJ PHY120 2013 GCHERMAN
SPITS, BAYS, and TOMBOLOS are formed by longshore currents
and are composed of sand and/or gravel.
TCNJ PHY120 2013 GCHERMANTCNJ PHY120 2013 GCHERMAN
SPITS are fingerlike projections of a beach into a body of water.
• Composed mostly of sand and more
rarely, gravel.
• Where some spits are modified by waves so that their free ends are curved,
they are sometimes called hooks, or recurved spits.
TCNJ PHY120 2013 GCHERMANTCNJ PHY120 2013 GCHERMAN
TWO NOTABLE SPITS
TCNJ PHY120 2013 GCHERMAN
BAYS are large bodies of water connected to an ocean or sea formed by an
inlet of land due to the surrounding land blocking some waves and often
reducing winds.
A BAYMOUTH BAR is a
spit that has grown until it
completely closes off a bay from
the open
• A common way to protect bays and
other inlets is to build jetties and
breakwaters.
TCNJ PHY120 2013 GCHERMAN
TOMBOLOS are fingerlike projections
of a beach into a body of water.
• They are a rare type of spit that extends out
to seas and connects a seastack or an island to
the mainland
Somewhere along the Pacific coast, United States
TCNJ PHY120 2013 GCHERMAN
BARRIER ISLANDS are nearshore deposits of sand that parallel the mainland
and are separated from it by lagoons.
• They migrate during large storms by erosion on the seaward side of the island and
deposition on the lagoon side.
Aerial (left) and satellite (right) views of Padre Island, western Gulf of Mexico
TCNJ PHY120 2013 GCHERMAN
• Two hypotheses
are that:
a) they formed as
spits that became
detached from the
land, or
THE ORIGIN
OF BARRIER
ISLANDS is not
fully understood.
b) they formed as beach
ridges on coasts that
subsequently subsided
TCNJ PHY120 2013 GCHERMAN
BARRIER ISLAND MIGRATION is happening in many areas of the US
East coast as a natural response to sea-level rise
• Jetties built in the 1930’s to protect Ocean City, Maryland disrupted the southerly longshore drift,
starving Assateague Island, which has migrated 500 m landward in the Jetties’ wake since then. In 2002,
stabilization efforts were made with beach-replenishment projects.
TCNJ PHY120 2013 GCHERMAN
NEARSHORE SEDIMENT BUDGET is the gains and losses of
sediment in the nearshore zone.
• Barring disruption of
a nearshore system,
for example by
damming the streams
which supply sand to
the shoreline, the
volume of sediment
will remain fairly
constant despite
seasonal changes in
the beach and
nearshore profile.
TCNJ PHY120 2013 GCHERMAN
DEPOSITIONAL
COASTS are characterized
by long sandy beaches, deltas,
and barrier islands.
• Good examples of both are the
U.S. East and Gulf Coasts
SUMBMERGENT COASTS where
a pre-existing shoreline is drowned from rising
seal levels or subsiding crust.
TCNJ PHY120 2013 GCHERMAN
EROSIONAL COASTS are steep and
irregular and typically lack
well-developed beaches.
They can also be
EMERGENT COASTS if the land has
risen with respect to sea level and the
erosion and depositional processes have not
yet reached an equilibrium with the changing
environment.
• Many of the western U.S. beaches fall into
these categories
TCNJ PHY120 2013 GCHERMAN
GPS vertical crustal motion (NASA-JPL) represented with light blue TIN surface showing continental regions of uplift (U) and subsidence
(S) with respect to plate-motion vectors (red arrows), and earthquake seismicity zones in the mid-continental and eastern US regions
(orange polygons). Overlapping and interfering ring structures stem from the Chicxulub (~65mya) and Chesapeake (~35mya) impacts.
TCNJ PHY120 2013 GCHERMAN
Surf’s Up
• An analysis of long
tide gauge records,
corrected for
postglacial rebound
and other phenomena,
indicates that the
current rate of sea
level rise is about 1.8 ±
0.3 mm/year. EOS, October 1, 1996
-3 to -4
0 to -1
0 to 1
-1 to -2
-1 to -2
-2 to -3
• We also saw in Lab 6
that the GPS CORS data
show that the coast is
locally subsiding at
rates of up to
–3 to –4 mm per year.
TCNJ PHY120 2013 GCHERMAN
SEASHORES AND LAKE SHORES
• Waves, nearshore currents, and tides control sediment transport and deposition
along shorelines.
• Waves and nearshore currents are effective along both seacoasts and lakeshores,
but tides are most effective along seashores.
• Waves are mostly derived from wind blowing across the water surface.
• Wave height is a function of the duration, speed, and fetch of the wind.
• The fetch on ponds and lakes is much smaller than on oceans, and therefore the
waves are smaller, as are the nearshore currents
TCNJ PHY120 2013 GCHERMAN
STORM WAVES AND COASTAL FLOODING
• Although shorelines are battered by high winds during storms, especially
hurricanes, coastal flooding causes most fatalities.
•Storm surge occur during
hurricanes because low
atmospheric pressure allows
the ocean surface to bulge up
beneath the eye of the storm,
so when the eye makes
landfall, the bulge and wind-
driven waves pile up in storm
surge that floods low-lying
areas.
TCNJ PHY120 2013 GCHERMAN
HURRICANE KATRINA, August 2005, was one of the most tragic
natural disasters in US National history.
• Gulf coasts areas Louisiana and
Mississippi were inundated by a storm
surge in places more than 7 meters high!
TCNJ PHY120 2013 GCHERMAN
Role of Coral Reef in Shoreline Protection from Tsunami
• Studies on the the impact of coral reef on damages wrought by the December 2004 tsunami
in the Indian Ocean show that where coral reefs were previously damaged from warm water
or humans (e.g., the coral is mined for cement), damage along the coastline was greater than
where the coral reef was healthy.
• Reefs, like barrier islands and other natural coastal
landforms provide a natural protection to inhabitants
of coastal areas. EOS, vol 86, p. 301, 2006.
Great Barrier Reef, Eastern Australia
TCNJ PHY120 2013 GCHERMAN
SHORELINE PROTECTION EFFORTS include a variety of measures
including beach nourishment programs, seawall and berm construction
Miami Beach before after the US Army Corp of
Engineers beach nourishment project Construction of the Galveston, Texas seawall began in 1902.
Use of rip rap to
build a berm
TCNJ PHY120 2013 GCHERMAN
RISING SEA LEVEL AND THE FATE OF VENICE
The Grande Canal in Venice
• Venice is at sea level, but the seas are rising, and Venice is sinking from loading and compaction
of mud and sand substrate, and groundwater extraction.
• It has sunk
about 10 m
since its
founding.
• Since 2003 the Italian government has been ‘sinking’ billions of dollars into sea gates that will
protect the lagoons.
TCNJ PHY120 2013 GCHERMAN
COMMON MISCONCEPTION
A world-wide flood, such as that described in the biblical story of Noah,
could have been possible.
Fact: The geological implications of Noah's flood make the story implausible.
• Note that flooding of all the land areas of Earth requires a large rise in sea level.
• Presently, over 97% of the world's water is already in the oceans, and the only other source
would be glaciers - including the polar ice caps - which contain about 2%.
• If the glaciers should all melt, sea level would rise about 70 m.
• This would cause flooding of coastal regions around the world, but all land areas would not
come close to being covered.
TCNJ PHY120 2013 GCHERMAN
QUESTION
How do rising sea level and coastal development, including
developments on barrier islands, complicate efforts to control
shoreline erosion?
ANSWER: With rising sea level, waves will feel bottom and break further
towards shore causing greater shoreline erosion.
• With coastal development, the numbers of people and structures impacted
by coastal erosion will likely be greater in the future.
TCNJ PHY120 2013 GCHERMAN
QUESTION
Why does an observer at a shoreline location experience two high and
two low tides daily?
ANSWER: Tides are caused by the gravitational attraction exerted on water by
the Moon and Sun.
• There are two high tides daily because the effect is greatest when the area
impacted is in direct line with the Sun and/or Moon, either facing them or on the
opposite side of Earth from them, which happens twice daily.
• The same is true for low tides except that they occur when the area impacted is
at right angles to the Sun and/or Moon.
TCNJ PHY120 2013 GCHERMAN
Why are most beaches made up of quartz sand?
ANSWER: Quartz is the most resistant of the common rock-forming minerals.
Because it is so abundant and so resistant, many beaches are made up of sand-sized
grains of quartz.
Are there other materials that make up beaches?
If so, what are they?
ANSWER: Local and regional erosion of headlands of mixed rock types will result in
sands with a variety of rock and mineral compositions, but with time and distance of
transport, sands will progressively be dominated by quartz.
QUESTION
TCNJ PHY120 2013 GCHERMAN
What are sleeper waves, and why are they dangerous?
QUESTION
ANSWER: Sleeper waves are
much larger waves in a wave
series.
• They are dangerous
because while most waves
will provide a uniform
motion, in height and
frequency or period, sleeper
waves will come as a
surprise, perhaps catching
bathers off guard.
TCNJ PHY120 2013 GCHERMAN
Methane Hydrates are mounds of methane ice rising from the ocean
floor have previously been recognized.
• It has been suggested
that such hydrates may
store enough carbon-
based fuel to double the
world's supply.
• Some bacteria even
seem to be associated
with the mounds.
•But recently, researchers from the Harbor Branch Oceanographic Institution have discovered
an apparently new species of centipede-like worm living on or in these mounds.
The mounds studied are in waters 1,800 feet deep.
EOS, August 5, 1997.