Chapter_16_The_Coast_and_Coral_Reefs[1].ppt

Chapter 16
The Coast and
Coral Reefs

Coast
• Zone of interaction between the sea and the land
• Where waves, sea currents and winds act on the land
Chapter 16: The Coast and Coral Reefs

Zonation of the coast
1. High tide shoreline
• Level reached during high tide
2. Low tide shoreline
• Level reached during the lowest tide
3. Coastline
• Highest level reached by storm waves
• Beyond the high tide shoreline
4. Offshore
• Zone submerged below the low tide shoreline

5. Foreshore
• Zone between the low tide and high tide shorelines
6. Backshore
• Zone between the high tide shoreline and the coastline
7. Beach
• Feature formed by deposition of sand, gravel and pebbles on
a wave-cut platform
8. Berm
• Raised part of the beach on which vegetation often grows

Agents of coastal change
1. Winds
• Main agent of coastal change
• Transport sand and deposit it along the coast
• Generate waves as they blow across the water surface
2. Currents
• Bodies of water moving through the sea in a certain direction
either horizontally or vertically
• Currents formed when waves approach the coast at an angle
and break obliquely against the coast are called longshore
currents

3. Swash
• Advance of sea water up a beach after the breaking of a
wave
4. Backwash
• Return flow of sea water down the beach following the swash

Parts of a wave
• Crest : The highest part of a wave
• Trough : The lowest part of a wave
• Wave height : The vertical distance between the crest
and the trough
• Wave length : The distance between two consecutive
wave crests

Wave energy
• Depends on the size of the wave
• The size of the wave increases as the speed of the
wind increases
• The greater the expanse of water over which the wind
blows (termed ‘fetch’), the larger the wave

Wave action
• Water particles move in a circular path within a wave
• As a wave approaches the shallow waters near the
shore, the wave path becomes more oval-shaped and
the wave length decreases
• Due to friction between the wave and the sea bed, the
wave slows down
• The waves behind move at a faster speed and push
against the preceding wave
• As a result, the wave height increases while the wave
length decreases
• The waves eventually break

• When a wave breaks, the crest is thrown forward and
crashes against the shore
• The water rushes up the shore as swash, carrying with
it sediments which may be deposited on the shore
• Gravity pulls the swash back to the sea as backwash
• Materials are carried back to the sea with the backwash

Constructive waves
• Waves that result in deposition of materials
• When the swash is stronger than the backwash
• Occur on gently-sloping beaches
• When they break, they are called spilling breakers

Destructive waves
• Waves that encourage erosion
• Materials on the beach are carried into the sea by the stronger
backwash
• Occur on beaches with steep slopes
• Waves break violently as plunging breakers

Constructive Waves Destructive Waves
Long wave length Short wave length
Low wave height High wave height
Spill over when breaking Plunge over when breaking
Common on gently-sloping shores Common on steep coastal slopes
Deposit on the coast Erode the coast
Less than ten waves breaking
per minute
More than ten waves breaking
per minute
Characteristics of constructive and destructive waves

Processes of wave erosion
1. Hydraulic action
• Repeated crashing of waves against the coast
• The rock structure weakens and the rocks break down
2. Cavitation
• Breaking waves enter the cracks and joints in the rocks
• The water traps and compresses the air within the joints
• The compressed air exerts pressure on the cracks and joints
• When the waves return to the sea, the pressure is released
and the air expands
• Repeated contraction and expansion of the air enlarges the
cracks and joints
• The rocks eventually break into smaller fragments

3. Solution
• Rocks may contain water-soluble minerals such as calcium
carbonate
• When these minerals dissolve upon contact with sea water,
pores are left in the rocks
• Over time, the rocks weaken and disintegrate
4. Abrasion
• Rock fragments carried by the water are thrown against the
coast, breaking up the coastal rocks
5. Attrition
• Rock particles carried by the water collide with each other,
becoming smaller, smoother and rounder particles

Factors affecting marine erosion
1. Hardness of the rocks
• Less resistant rocks are eroded faster
2. Structure of the rocks
• Rocks with more lines of weakness such as joints are eroded
more rapidly
3. Wave energy
• Stormy weather causes more erosion as the waves are
bigger due to the strong winds
• Larger waves usually have stronger backwash and more
erosive energy

4. Human action
• In some places, seawalls
and breakwaters have
been built to slow down
coastal erosion
5. Time
• Older rocks are more eroded since they have been exposed
to wave action longer than more recent rocks.
• The duration of a storm affect the amount of erosion
Structures to slow down
sea erosion

Landforms caused by marine erosion
1. Notches, cliffs and wave-cut platforms
• Waves act on a line of weakness on the rock surface through
the processes of hydraulic action and abrasion
• This line of weakness enlarges to become a notch
• Further erosion enlarges the notch into a cave

• The roof of the cave eventually collapses and a steep cliff is
formed
• At the cliff base is a flat terrace called a wave-cut platform
• The wave-cut platform will extend farther inland as the cliff
retreats

2. Blowholes and sea inlets
• Waves pounding on a sea cave trap air in the cracks and
joints of the rocks
• The compressed air exerts pressure on the cracks and joints
• When the waves retreat, the air expands
• Over time, the rocks are broken down and an opening called
a blowhole is formed at the roof of the cave
• The blowhole may
enlarge until the cave
collapses, resulting in a
deep, long and narrow
inlet called a geo

3. Headlands and bays
• Develop along coasts with alternate bands of resistant and
less resistant rocks
• The resistant rocks are eroded more slowly and protrude into
the sea to form headlands
• The less resistant rocks form bays between the headlands
• Can also develop when destructive waves erode along lines
of weakness in rocks to form bays

Headland
and bay

• At the bays, waves
curve out, resulting in
wave energy being
dispersed and thus
encouraging deposition
• Deposition in the
bay results in a
straighter shoreline
• Wave refraction occurs in areas where
there are headlands and
bays
• Waves concentrate their energy on the
headlands by curving in on
them

4. Caves, arches, stacks and stumps
• Waves attack lines of weakness in rocks along the base of
the headland cliff to form notches
• Over time, the notches enlarge to become caves
• Continued erosion of caves on two sides of the same
headland cuts through the headland, creating an arch
A B C

An arch

• The arch widens and the roof eventually collapses
• This leaves an isolated pillar known as a stack
• The stack is gradually eroded down into a stump
Arch collapses
Stack
Notch
Stump

A coast showing typical coastal features like stacks, stumps
and cliff

Wave transportation and deposition
• Occurs mainly through longshore drift along the coast
• When waves approach the shore at an angle, the swash rushes up
the shore diagonally, carrying sediment up the shore
• The backwash brings
sediment back into the
sea
• As a result, sediment
is moved in a zig-zag
manner along the
shore
Longshore drift
Swash
zone
Breake
r zone
Swash
Backwash
Movement of material by
longshore current
Direction of wind
Beach

Landforms caused by wave deposition
1. Beaches
• Accumulation of rock debris and sediment on or along a
wave-cut platform
• Constructive waves deposit materials on the coast
• Coarser materials
are deposited
farther inland
while finer
materials are
found nearer
the sea
A cobblestone beach at
Georgetown, St. Vincent

2. Spits
• Long, narrow low-lying strips of sand and shingle
projecting from the shore towards the sea
Formation of a spit

3. Bars
• Narrow ridges of deposited material lying away from and
parallel to the coast (off-shore bar)
• A spit may grow across an estuary, a lagoon or a bay to
become a bar (bay-bar)
4. Tombolos
• Formed when a spit or
a bar extends to join
an offshore island
The Cocal Spit in Mayaro,
Trinidad
Recurved spit
Maria River

Causes of marine erosion
1. Wave action and longshore drift
• Erosion occurs when the amount of materials deposited is
less than the amount carried away
• Large storm waves cause coastal erosion which can lead to
long-term loss of sediments or temporary redistribution of
sediments
• Destructive waves erode the beach by carrying materials out
to sea, whereas constructive waves increase the size of the
beach by depositing materials on it
• Longshore drift moves sediments from one part of the coast
to another part farther down the coastline

2. Rock structure and strength
• Rocks with joints and fractures are eroded faster
• Less resistant rocks such as limestone are more vulnerable to erosion
3. Natural hazards
• Hurricanes, volcanic eruptions and earthquakes can cause dramatic
changes in coastlines
• They can destroy coastal features
Changes to Coconut Beach (Dominica) during the 1995 hurricane season

4. Atmospheric processes
• Onshore winds pick up sediments and move them up the
beach to form sand dunes
• Rain helps to carry sediments down to the beach
5. Human actions and interference
• Dam construction and river channelisation reduce the
amount of sand that reaches the shore
• Humans sometimes remove beach sand as raw material for
the construction industry
Building groynes is one way
humans can interfere in
coastal formation

Coastal management
1. Groynes
• Structures built out from the shore and into the sea
• Constructed at a right angle to the sea
• Effective in preventing longshore drift from moving sediments
from one point to another farther along the coastline
• While they protect one part of the coast from erosion, they
contribute to erosion of the beach behind them by cutting
off the supply of sediments to the beach
2. Replenishing the beach
 Sand is sometimes added artificially to badly
eroded beaches
 The sand is taken from external sources

3. Seawalls
• Walls constructed on the inland part of the coast to deflect
oncoming waves
• Built parallel to the coast
• Usually made of hard rocks or concrete
• Can be sloping or vertical
• May cause erosion in the long run
• The energy of the backwash is reflected from the wall and
erodes the beach materials beneath and in front of the
wall
• Scouring occurs at the base of the seawall, weakening it

4. Breakwaters
• Structures built either offshore or projecting out into the sea
from the shore to dissipate the energy of oncoming waves
• Made of rocks or concrete
• Can be fixed or floating
• The erosive energy of oncoming waves is concentrated on
the breakwater
• Materials are deposited
behind the breakwater
• The nearby unprotected
section of the coast stops
receiving fresh supplies
of depositional materials
and becomes more
vulnerable to erosion
A breakwater protecting the coast

Case study: Beach
erosion in Barbados
• The beaches are an important
source of income
• They are protected by coral reefs
surrounding most parts of
Barbados
• But the reefs are fast
disappearing
• Beach erosion is aggravated by
hurricanes
• The government has taken steps
to protect the beaches
• Beach enhancement and
stabilisation works were carried
out between 1991 and 1995 on
Rockley (Christ Church) and
Weston (St James)
0 5 10km

Corals
• Made up of the limestone skeletons of tiny marine
organisms called coral polyps
Conditions for the growth of coral polyps
• Sea temperature between 20C and 30C
• Shallow sea water less than 70m deep
• Clear salt water
• Polyps thrive on the seaward side of coral reefs where
waves and currents bring an abundant supply of
oxygen and food
• Extensive coral formations develop between latitudes
30N and 30S, on the eastern side of land masses
where there are warm currents

Types of coral reefs
1. Fringing reef
• A narrow coral platform separated from the coast by a
shallow lagoon
2. Barrier reef
• A coral platform separated from the coast by a deep wide
lagoon
3. Atoll
• A circular coral reef which
encloses a lagoon

Coral reef structure
• Most reefs are fairly narrow
• The tops lie near to low tide level
• They are steep on the seaward side
• On the landward
side, sand is
deposited by the
breaking waves
• Plants readily
inhabit these
sand deposits
Cross-section of a coral
reef

Coral reefs in the Caribbean
• Coral reefs have declined significantly
• Coral cover decreased from more than 50% in 1977 to
10% in 2001, a total loss of 80%
• Reefs at Risk project (2004) found that 64% of
Caribbean reefs were being threatened by high levels
of human activities
• The reefs in eastern and southern Caribbean, the
Greater Antilles, Florida Keys, Yucatan and the
Mesoamerican Barrier Reef are under threat
• Though natural factors also influence reef development,
humans are the main culprit responsible for destroying
the coral reefs of the wider Caribbean

Natural factors
• Hurricanes destroy coral reefs and the organisms that
live there
• Natural predators such as the crown-of-thorns starfish
reduce the population of coral polyps
• Upwelling of warm water may raise sea water
temperatures and inhibit reef development
A Crown-of-Thorns starfish in
the midst of corals

Human factors
1. Pollution
• Land-based sources of pollution and sediments threaten
35% of the reefs
• Waste materials from factories and holiday resorts pollute
the sea water
• Pesticides washed off farms contaminate coral colonies
• Pollution from ships threatens 15% of the reefs
• Areas under threat are Jamaica, Hispaniola, Puerto Rico, the
high islands of the Lesser Antilles, Belize, Costa Rica and
Panama

2. Over-fishing
• As the world’s population increases, so does the demand for
food, including seafood
• Over-fishing in reefs may result in algae blooms which inhibit
the growth of corals
3. Coastal developments and activities
• Coastal developments disrupt currents and cause sediment
damage to the fragile corals
• These include the reclamation of reef areas to build airports
and the development of marinas, groynes and causeways
• Recreational activities such as boating, windsurfing,
waterskiing and diving in reef areas also damage corals by
stirring up sediments, thus blocking out sunlight

Chapter_16_The_Coast_and_Coral_Reefs[1].ppt

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