20-shores and beaches - impacttectonics.orgcoasts are characterized by long sandy beaches, deltas,...

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


• 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


SHORELINES are continually being modified by waves, longshore

currents, and tidal currentsTwo views of the US Pacific Coast


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.


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.


HURRICANE SANDY 10-27/28-2012


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.


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.


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.


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.


WAVE BASE AND BREAKERS

Waves with about 2 meters

and a wave base ~ 1m

Waves about

2 m high and

a wide break

zone


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.


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


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.


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


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


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.


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.


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.


SHORELINE EROSION

• Sea caves and arches form

because waves refract around

objects and erode headlands on

both sides.


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


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


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


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.


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.


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


Groins along Cape May beach in New Jersey trap

sand as longshore drift


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


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.


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


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


• 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


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.


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.


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.


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


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.


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.


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


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.


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!


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


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


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.


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.


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.


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.


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


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.


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.

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