Chapter 12:

Coasts

(after a brief review of Tides)

1

Questions from previous classes:

What happens when a wave

meets a current?

wave = people walking

current = bus

If wave goes with the current, the wave will go faster and it will stretch out

(wavelength is greater with higher speed).

If going in opposite direction, wave can break (like a person tripping).

Example: Columbia River + ocean

It gets complicated when waves come from angle, but basically waves

elongate or shorten. Energy and momentum are conserved.

Question about springs/geysers?

Waves are a transfer of energy.2

Neap tides:

High tides are reduced when Sun and Moon at right angles.

This also means that the low tides are the highest low tides.

Is this image showing

spring tides or neap tides?

3

Moon attracts ocean

Center of mass

Moon

Earth’s motion creates opposing dome on the other side, away from the Moon.

The Earth and Moon act as

one system that has a single

center of mass.

When the Earth rotates

around that center of mass, it

creates a dome on the

opposite side of the Moon.

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Moon

Combined result:

5

Diurnal tides –

1 high tide in 24 hrs.

Semidiurnal tides –

2 high tides in 24 hrs.

Mixed tides –

Combination

Why?

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Diurnal tides –

1 high tide in 24 hrs.

Semidiurnal tides –

2 high tides in 24 hrs.

Mixed tides –

Combination

Why?

“A detailed analysis of all the variables that affect

the tides at any particular coast reveals that nearly

400 factors are involved … a completely

mathematical model of the tides is beyond the

limits of marine science…

Successful models must take into account at least

37 independent factors related to tides (the two

most important are the Moon and the Sun) …”

- Essentials of Oceanography textbook7

Diurnal tides –

1 high tide in 24 hrs.

Semidiurnal tides –

2 high tides in 24 hrs.

Mixed tides –

Combination

Why?

• positions of continents

• depth of the ocean

(remember, a tide is a

type of wave)

• coastline shape

Example:

Diurnal tides are

common in shallow

inland seas.

8

Notice how at each of these places the surrounding color—the tidal force for that

region—is blue, indicating little or no apparent tide. These convergent areas are

called amphidromic points. Tide waves move around these points, counterclockwise

in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Amphidromic points: where no tides occur

9

Amphidromic points: where no tides occur

Why?

The two domes of water on either side of the globe cannot really exist this way, due to

the rotation of the Earth.

Instead, the domes are broken up into several tidal cells that rotate around an

amphidromic point.

What do you think is

happening near the

North Pole?

Red =

High tides are the

highest,

Low tides are the lowest.

10

Coasts

• Classified by processes shaping them …

such as?

• Affected by:

1) Waves (primary)

2) Volcanoes

3) Sea levels

4) River transport

5) Tectonic activity11

Coasts can also be classified by the amount of energy

they experience.

(Energy from what?)

WAVES

12

Coasts can also be classified by the amount of energy

they experience.

(Energy from what?)

WAVES

Classify the coasts or coastal features

you see on the next slides:

high energy?

or

low energy?13

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15

16

17

18

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Coasts - balance between

creation

&

destruction

deposition

erosion

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Learning Goals: Coasts

• A coast’s location depends on:

• Movement of the Earth’s plates and how this impacts the crust

• The ocean’s water volume (in other words, sea level)

• A coast’s shape depends on:

• Wave energy

• Whether the plates are moving up or down (remember: isostasy)

• How erosion is impacting the land

• How sediments are moved along the shoreline

• Biological activity (examples: coral reefs and mangroves)

• Two major types of coasts:

1) Erosional

2) Depositional

• Humans try to “fix” coasts, but tend to cause more erosion, rather than less

erosion.

21

High energy coasts –

larger waves caused by

continuous storms.

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Features of

High Energy

Coasts

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Where is most of the erosion happening

in this picture?

24

Ocean wave refraction erodes protruding regions first.

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Low energy coast - sediments accumulate on shore.

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Low Energy Beaches

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Low Energy Coasts

Berms – sediment accumulation by wave action

Beach scarp – wall from furthest berm caused by highest tide

Bars – sediment that accumulates offshore

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Rip currents – When waves enter faster then they can leave.

A narrow channel of outflowing water forms. 30

Summer vs Winter Conditions

31

La Jolla, California – Summer Conditions

32

La Jolla, California – Winter Conditions

33

Tectonic Activity

34

This beach was created by tectonic activity,

and it’s in Alaska.

What do you think created it?

35

This former seafloor at Prince William

Sound, Alaska, was raised 3.5 meters (12

feet) above sea level by tectonic uplift

during the great earthquake of 27 March

1964. The exposed surface, which slopes

gently from the base of the sea cliffs to the

water, is about 400 meters (14 mile) wide.

The light-colored coating on the rocks

consists mainly of the dried remains of

small marine organisms.

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Longshore drift - movement of sediment along a shore by wave action.

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Longshore currents can create:

Sand spit – sand deposition downstream of a headland

Bay mouth bar – sand spits close off a bay

Barrier island – sand deposition apart from mainland

Tombolo – sand creating bridge to off-shore feature 39

Coastal sediment transport cells

40

A bay mouth bar. The inlet is now closed, but increased river flow (from inland rainfall)

or large waves combined with very high tides could break the bar.

For an indication of scale, note the freeway bridges at the top of the photograph.

41

Barrier islands off the North Carolina Coast. 42

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Humans try to protect coastlines,

but generally end up causing more erosion.

44

Groins (jetties) –

manmade structures

designed to limit

longshore drift.

45

When completed in 1870, the Cape

Hatteras lighthouse was located a safe

1,500 feet from the ocean. Even then,

however, storm-driven tides completely

washed over Hatteras Island, eroding sand

from the ocean side of the island and

depositing it on the sound side. By 1970,

this process, which has caused the gradual

westward migration of the Outer Banks for

at least the past 10,000 years, left the

lighthouse just 120 feet from the ocean’s

edge and almost certain destruction.

The Story of the Cape Hatteras Lighthouse:

Outer Banks of North Carolina

46

The Cape Hatteras lighthouse was

moved in 1999.

http://www.nps.gov/caha/learn/hi

storyculture/movingthelighthouse

.htm

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1930s:

Coast Guard installed the first sheetpile "groins" (walls built perpendicular to the

shore) to try to protect the tower.

1936:

They let the lighthouse get washed over by the ocean – and just moved its light

to a skeleton steel tower

1960s and 1970s: various attempts to "stabilize" the coast included beach

nourishment and three new groins installed north of the lighthouse.

1980: A severe storm in 1980 accentuated the island's westward movement

washing away the foundation of the first (1803) lighthouse, which had been 600

feet south of the existing lighthouse. In 1803, that lighthouse had been one mile

from the shoreline.

In 1980, the National Park Service began a three-year process for long-term

protection:

- Relocation was considered but quickly discounted as impractical.

- The option finally selected was a concrete and steel seawall revetment.

- The technology to move large structures improved.

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The National Academy of Science and

North Carolina State University

Investigated.

NCSU independently reviewed the

National Academy of Sciences’ report

and then issued its own report, Saving

the Cape Hatteras Lighthouse from the

Sea, in January 1997.

It not only endorsed the National

Academy of Sciences’ findings, but also

recommended that “the National Park

Service proceed as soon as possible

with its present plans to obtain the

financial resources necessary to

preserve the lighthouse by moving it.”

Funding was appropriated by Congress

to move the lighthouse.

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Mangroves: nature’s way to reduce erosion.

50Stopped here on 3/23

Deltas

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Delta – sediment deposits at river mouth.

Mississippi Delta

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Deltas can only form

if sediment deposit is

greater than removal.

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Atolls

Kayangel Atoll in Palau, in the tropical Pacific.54

Atoll formation

Fringing reef – corals that cling to land.

Barrier reef – as land subsides or erodes away, lagoon separates reef and land.

Atoll - When land completely subsides, ring-shaped island is left.

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Moorea

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Great Barrier Reef,

Australia

Largest barrier reef

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Estuaries

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Estuaries –

water surrounded by

land, exposed to both

fresh and salt water.

Very productive regions.

Types of

Estuaries

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a. Drowned river mouth –

sea level rise floods inland

rivers.

Chesapeake Bay

(largest estuary in the U.S.)

Types of

Estuaries

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Another drowned river valley: Sydney Harbour, AustraliaTypes of

Estuaries

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b. Fjords –

valleys cut by

glaciers.

Alaska

Puget Sound,

WA

(second

largest

estuary in the

U.S.)

Types of

Estuaries

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In fact, the movement

of glaciers have

carved many water

bodies and islands:

example 1

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Martha’s Vineyard –

terminal moraine.

In fact, the movement

of glaciers have

carved many water

bodies and islands:

example 2

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http://dep.state.fl.us/coastal/habitats/estuaries.htm

Types of

Estuaries

c. Bar-built

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Where is this and

what is it called?

Miami Beach –

Barrier Island

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A characteristically

straight fault coast at

Tomales Bay,

California. Point Reyes

is visible to the left

(west); the city of San

Francisco is out of

view to the south. The

San Andreas Fault

trace disappears below

sea level in the bay

(arrow); the straight

sides of the bay closely

parallel the submerged

fault.

Types of

Estuaries

d. Tectonic

68

Question for next class:

What are the primary factors that will influence the coastline in

Miami, over the coming years and decades?

The southeastern coast of the US, ~18,000 years ago

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Most important

vocabulary terms & processes

1. Coastal processes are impacted by waves (primarily), volcanoes, sea

levels, river transport, and tectonic activity.

2. Difference between erosion and deposition

3. Shapes of high energy coasts, compared to low energy coasts.

4. Longshore drift, longshore current and longshore transport

5. Ways that humans have modified coastlines, compared to natural

reinforcement

groin, seawall, beach nourishment

vs.

protection from mangrove forests, coral reefs, etc

6. Deltas, estuaries, atolls, moraine

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