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Chapter 16
The Coast and
Coral Reefs
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Coast
Zone of interaction between the sea and the land
Where waves, sea currents and winds act on the land
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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
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Chapter 16: The Coast and Coral Reefs
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
. Ber!
aised part of the beach on which vegetation often grows
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Chapter 16: The Coast and Coral Reefs
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Chapter 16: The Coast and Coral Reefs
Agents of coastal change
1. "inds
!ain agent of coastal change
"ransport sand and deposit it along the coast
#enerate waves as they blow across the water surface
2. C#rrents Bodies of water moving through the sea in a certain direction
either hori$ontally or vertically
%urrents formed when waves approach the coast at an angle
and brea& obli'uely against the coast are called longshore
currents
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3. $wash
(dvance of sea water up a beach after the brea&ing of a
wave
4. Backwash
eturn flow of sea water down the beach following the swash
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Parts of a wave
%rest ) "he highest part of a wave
"rough ) "he lowest part of a wave
Wave height ) "he vertical distance between the crest
and the trough
Wave length ) "he distance between two consecutivewave crests
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Wave energy
*epends on the si$e of the wave
"he si$e of the wave increases as the speed of the
wind increases
"he greater the e+panse of water over which the wind
blows termed fetch./, the larger the wave
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Wave action
Water particles move in a circular path within a wave
(s a wave approaches the shallow waters near the
shore, the wave path becomes more oval-shaped and
the wave length decreases
*ue to friction between the wave and the sea bed, the
wave slows down
"he waves behind move at a faster speed and push
against the preceding wave
(s a result, the wave height increases while the wave
length decreases
"he waves eventually brea&
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Chapter 16: The Coast and Coral Reefs
When a wave brea&s, the crest is thrown forward and
crashes against the shore
"he water rushes up the shore as swash, carrying withit sediments which may be deposited on the shore
#ravity pulls the swash bac& to the sea as bac&wash
!aterials are carried bac& to the sea with the bac&wash
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%onstructive waves
Waves that result in deposition of materials
When the swash is stronger than the bac&wash
0ccur on gently-sloping beaches
When they brea&, they are called spilling brea&ers
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*estructive waves
Waves that encourage erosion
!aterials on the beach are carried into the sea by the strongerbac&wash
0ccur on beaches with steep slopes
Waves brea& violently as plunging brea&ers
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Chapter 16: The Coast and Coral Reefs
Constructive Waves Destructive Waves
Long wa%e length $hort wa%e length
Low wa%e height High wa%e height
$&ill o%er when 'reaking (l#nge o%er when 'reaking
Co!!on on gentl)*slo&ing shores Co!!on on stee& coastal slo&es
+e&osit on the coast ,rode the coast
Less than ten wa%es 'reaking&er !in#te
-ore than ten wa%es 'reaking&er !in#te
Characteristics of constructive and destructive waves
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Chapter 16: The Coast and Coral Reefs
Processes of wave erosion
1. H)dra#lic action
epeated crashing of waves against the coast
"he roc& structure wea&ens and the roc&s brea& down
2. Ca%itation
Brea&ing waves enter the crac&s and 1oints in the roc&s "he water traps and compresses the air within the 1oints
"he compressed air e+erts pressure on the crac&s and 1oints
When the waves return to the sea, the pressure is released
and the air e+pands
epeated contraction and e+pansion of the air enlarges the
crac&s and 1oints
"he roc&s eventually brea& into smaller fragments
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Chapter 16: The Coast and Coral Reefs
3. $ol#tion corrosion/
oc&s may contain water-soluble minerals such as calcium
carbonate When these minerals dissolve upon contact with sea water,
pores are left in the roc&s
0ver time, the roc&s wea&en and disintegrate
4. 0'rasion corrasion/
oc& fragments carried by the water are thrown against the
coast, brea&ing up the coastal roc&s
5. 0ttrition oc& particles carried by the water collide with each other,
becoming smaller, smoother and rounder particles
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Chapter 16: The Coast and Coral Reefs
actors affecting !arine erosion
1. Hardness of the rocks
Less resistant roc&s are eroded faster
2. $tr#ct#re of the rocks
oc&s with more lines of wea&ness such as 1oints are eroded more
rapidly
3. "a%e energ)
2tormy weather causes more erosion as the waves are bigger due
to the strong winds
Larger waves usually have stronger bac&wash and more erosive
energy
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Chapter 16: The Coast and Coral Reefs
4. H#!an action
3n some places, seawalls
and brea&waters havebeen built to slow down
coastal erosion
5. i!e
0lder roc&s are more eroded since they have been e+posed
to wave action longer than more recent roc&s4
"he duration of a storm affect the amount of erosion
Structures to slow down
sea erosion
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Chapter 16: The Coast and Coral Reefs
"andfor!s caused #y !arine erosion
1. otches cliffs and wa%e*c#t &latfor!s
Waves act on a line of wea&ness on the roc& surface through
the processes of hydraulic action and abrasion
"his line of wea&ness enlarges to become a notch
Further erosion enlarges the notch into a cave
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Chapter 16: The Coast and Coral Reefs
"he roof of the cave eventually collapses and a steep cliff is
formed
(t the cliff base is a flat terrace called a wave-cut platform "he wave-cut platform will e+tend farther inland as the cliff
retreats
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Chapter 16: The Coast and Coral Reefs
2. Blowholes and sea inlets Waves pounding on a sea cave trap air in the crac&s and
1oints of the roc&s
"he compressed air e+erts pressure on the crac&s and 1oints
When the waves retreat, the air e+pands
0ver time, the roc&s are bro&en down and an opening called
a blowhole is formed at the roof of the cave
"he blowhole may
enlarge until the cavecollapses, resulting in a
deep, long and narrow
inlet called a geo
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Chapter 16: The Coast and Coral Reefs
3. Headlands and 'a)s
*evelop along coasts with alternate bands of resistant and
less resistant roc&s "he resistant roc&s are eroded more slowly and protrude into
the sea to form headlands
"he less resistant roc&s form bays between the headlands
%an also develop when destructive waves erode along lines
of wea&ness in roc&s to form bays
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Chapter 16: The Coast and Coral Reefs
Headland
and bay
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Chapter 16: The Coast and Coral Reefs
(t the bays, waves curve
out, resulting in wave
energy being dispersed
and thus encouraging
deposition
*eposition 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
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Chapter 16: The Coast and Coral Reefs
4. Ca%es arches stacks and st#!&s
Waves attac& lines of wea&ness in roc&s along the base of
the headland cliff to form notches 0ver time, the notches enlarge to become caves
%ontinued erosion of caves on two sides of the same
headland cuts through the headland, creating an arch
A $ C
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Chapter 16: The Coast and Coral Reefs
An arch
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Chapter 16: The Coast and Coral Reefs
"he arch widens and the roof eventually collapses
"his leaves an isolated pillar &nown as a stac&
"he stac& is gradually eroded down into a stump
Arch collapses
%tac&
'otch
%tu!p
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Chapter 16: The Coast and Coral Reefs
A coast in Bathsheba, Barbados, showing typical coastal
features like stacks, stumps and cliff
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Wave transportation and deposition 0ccurs 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
Chapter 16: The Coast and Coral Reefs
"he bac&wash brings
sediment bac& into the
sea (s a result, sediment
is moved in a $ig-$ag
manner along the
shore
"ongshore drift
%wash
(one
$rea&er
(one
%wash
$ac&wash
)ove!ent of !aterial #y longshore
current
Direction of wind
$each
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Chapter 16: The Coast and Coral Reefs
"andfor!s caused #y wave deposition
1. Beaches
(ccumulation of roc& debris and sediment on or along a wave-cut platform
%onstructive waves deposit materials on the coast %oarser materials
are deposited
farther inland
while finer
materials are
found nearer
the sea
A cobblestone beach at
Georgetown, St. incent
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Chapter 16: The Coast and Coral Reefs
2. $&its
Long, narrow low-lying strips of sand and shingle
pro1ecting from the shore towards the sea
or!ation of a spit
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Chapter 16: The Coast and Coral Reefs
3. Bars
5arrow ridges of deposited material lying away from and
parallel to the coast off-shore bar/ ( spit may grow across an estuary, a lagoon or a bay to
become a bar bay-bar/
4. o!'olos
Formed when a spit or
a bar e+tends to 1oin
an offshore island
!he Cocal Spit in "ayaro,
!rinidad
Recurved spit
'ariva River
Ch t 16 Th C t d C l R f
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Chapter 16: The Coast and Coral Reefs
Causes of !arine erosion
1. "a%e action and longshore drift
6rosion occurs when the amount of materials deposited is
less than the amount carried away
Large storm waves cause coastal erosion which can lead to
long-term loss of sediments or temporary redistribution of
sediments
*estructive waves erode the beach by carrying materials out
to sea, whereas constructive waves increase the si$e 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
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2. ock str#ct#re and strength oc&s with 1oints and fractures are eroded faster
Less resistant roc&s such as limestone are more vulnerable to erosion
3. at#ral haards Hurricanes, volcanic eruptions and earth'ua&es can cause dramatic
changes in coastlines
"hey can destroy coastal features
Changes to Coconut Beach #$ominica% during the &''( hurricane season
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Chapter 16: The Coast and Coral Reefs
4. 0t!os&heric &rocesses
0nshore winds pic& up sediments and move them up the beach to form sand dunes
ain helps to carry sediments down to the beach
5. H#!an actions and interference *am 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
Ch t 16 Th C t d C l R f
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Chapter 16: The Coast and Coral Reefs
Coastal !anage!ent
1. ro)nes
2tructures built out from the shore and into the sea
%onstructed at a right angle to the sea
6ffective 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, theycontribute to erosion of the beach behind them by cutting
off the supply of sediments to the beach
2. e&lenishing the 'each 2and is sometimes added artificially to badly eroded
beaches
"he sand is ta&en from e+ternal sources
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Chapter 16: The Coast and Coral Reefs
3. $eawalls
Walls constructed on the inland part of the coast to deflect
oncoming waves Built parallel to the coast
7sually made of hard roc&s or concrete
%an be sloping or vertical
!ay cause erosion in the long run
"he energy of the bac&wash is reflected from the wall and
erodes the beach materials beneath and in front of the
wall
2couring occurs at the base of the seawall, wea&ening it
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4. Breakwaters
2tructures built either offshore or pro1ecting out into the sea
from the shore to dissipate the energy of oncoming waves !ade of roc&s or concrete
%an be fi+ed or floating
"he erosive energy of oncoming waves is concentrated on
the brea&water
!aterials are deposited
behind the brea&water
"he nearby unprotected
section of the coast stops
receiving fresh supplies
of depositional materials
and becomes more
vulnerable to erosion
A breakwater protecting the coast
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Case study: $eacherosion in $ar#ados
"he beaches are an importantsource of income
"hey are protected by coral reefssurrounding most parts ofBarbados
But the reefs are fast
disappearing Beach erosion is aggravated byhurricanes
"he government has ta&en stepsto protect the beaches
Beach enhancement and
stabilisation wor&s were carriedout between 8998 and 899: onoc&ley %hrist %hurch/ andWeston 2t ;ames/
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* + 1*&!
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Case study: $each erosion in $ar#ados
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Coral limestone being undercut at )istins, Barbados
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,ow to descri#e coastal
scenery "rend or direction, e4g4 5-2
2hape, e4g4 straight, gentlyundulating or deeply indented
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Chapter 16: The Coast and Coral Reefs
,ow to descri#e coastal
scenery "ransportation as evidenced by longshoredrift accumulating sediment on one side of
groyne
*eposition features such as spits, bars and
deltas 0ffshore features such as cays and coral reefs
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Corals
!ade up of the limestone s&eletons of tiny marine
organisms called coral polyps
Conditions for the growth of coral polyps
2ea temperature between >?% and @?%
2hallow sea water less than A?m deep
%lear salt water
olyps thrive on the seaward side of coral reefs where
waves and currents bring an abundant supply of
o+ygen and food
6+tensive coral formations develop between latitudes
@?5 and @?2, on the eastern side of land masses
where there are warm currents
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Chapter 16: The Coast and Coral Reefs
-rigins of Corals
2everal theories proposed
2ubsidence of islands proposed by %harles *arwin
%oral growth e+tends to surface as island
subsides
Wider and deeper lagoon results %ontinued subsidence results in island being
completely submerged
(toll a horseshoe-shaped reef/ remains
3sland subsidence caused by volcanoes becominge+tinct
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Chapter 16: The Coast and Coral Reefs
Types of coral reefs
1. Fringing reef
( narrow coral platform separated from the coast by a
shallow lagoon
2. Barrier reef
( coral platform separated from the coast by a deep widelagoon
3. 0toll
( circular coral reef which
encloses a lagoon
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Coral reef structure
!ost reefs are fairly narrow
"he tops lie near to low tide level
"hey are steep on the seaward side
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0n the landward
side, sand is
deposited by the
brea&ing waves lants readily
inhabit these
sand deposits
Cross*section of a coral
reef
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Coral reefs in the Cari##ean
%oral reefs have declined significantly
%oral cover decreased from more than :?C in 89AA to
8?C in >??8, a total loss of D?C
eefs at is& pro1ect >??E/ found that EC of
%aribbean reefs were being threatened by high levels
of human activities
"he reefs in eastern and southern %aribbean, the
#reater (ntilles, Florida Geys, ucatan and the
!esoamerican Barrier eef are under threat
"hough natural factors also influence reef development,humans are the main culprits responsible for destroying
the coral reefs of the wider %aribbean
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Chapter 16: The Coast and Coral Reefs
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5atural factors
Hurricanes destroy coral reefs and the organisms that
live there 5atural predators such as the crown-of-thorns starfish
reduce the population of coral polyps
7pwelling of warm water may raise sea water
temperatures and inhibit reef development
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A Crown*of*!horns starfish in
the midst of corals
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Chapter 16: The Coast and Coral Reefs
Human factors
1. (oll#tion
Land-based sources of pollution and sediments threaten@:C of the reefs
Waste materials from factories and holiday resorts pollute the
sea water
esticides washed off farms contaminate coral colonies
ollution from ships threatens 8:C of the reefs
(reas under threat are ;amaica, Hispaniola, uerto ico, the
high islands of the Lesser (ntilles, Beli$e, %osta ica and
anama
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p
2. O%er*fishing
(s the world.s population increases, so does the demand for
food, including seafood 0ver-fishing in reefs may result in algae blooms which inhibit
the growth of corals
3. Coastal de%elo&!ents and acti%ities
%oastal developments disrupt currents and cause sedimentdamage to the fragile corals
"hese include the reclamation of reef areas to build airports
and the development of marinas, groynes and causeways
ecreational activities such as boating, windsurfing,
waters&iing and diving in reef areas also damage corals by
stirring up sediments, thus bloc&ing out sunlight