Chem 1103: Lecture 11: The Dynamic Shoreline: Chap. 11 and Coastal Habitats:
Chap. 12
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Why important
o Beach and coast erosion
o Structure effects
o Loss of property
Coastal water movement
o Waves provide the energy that changes the shape and texture of the beach
deposits
 As waves shoal (touch bottom) in shallow water, speed decreases,
wavelength decreases, wave height increases, waves become less
stable, and refraction occurs (changes in waves discussed previously)
 Refraction is the bending of waves towards shallower water so
that they break almost parallel to the shore
 Waves become unstable and break in very shallow water
o Waves generate longshore currents that flow parallel to the beach and rip
currents that flow perpendicularly to the beach
 Angle of wave approach is the acute angle (less than 90o) between the
wave crest and the beach
 The angle is rarely larger than 10o
 The greater the angle of approach, the stronger will be the
longshore current generated for the same wave size
 The direction of longshore current varies with the direction of wave
approach
 Longshore currents can also be generated by wave set-up
 Wave set-up is a process whereby waves pile water against the
shore
 Larger waves transport more water shoreward, producing a
higher pile of waver than do smaller waves
 The sea surface slopes from areas of larger waves towards the
areas of smaller waves, thereby creating a pressure gradient
down which water flows as a longshore current parallel to the
shore
 Wave set-up is especially important along irregular coasts with
pronounced wave refraction
 Where two opposing longshore currents collide, they form a swift
narrow seaward rip current, drains excess water from the surf zone
Beaches
o The beach is the part of land that touches the sea. It can be divided into the:
 Off shore – the area seaward from where waves first begin to break,
breaker zone
 Nearshore – the area from the offshore to where waves wash back and
forth across the beach
 Near shore can be divided into the:
o Breaker zone – where the waves begin to break
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o Surf zone – where the waves expend most of their
energy
o Swash zone – where waves wash back-and-forth across
the beach face
 Backshore – the land that adjoins the near shore
o Position of the divisions of the beach varies with the tides, advancing
landward with high tide and retreating seaward with low tide
o Beach sediments are moved by currents and waves, especially breakers
 Beaches undergo seasonal changes and changes due to weather
 A beach profile is a cross section of the beach along a line that is
perpendicular to the shoreline
 By comparing a series of beach profiles along the same line
made at different times, it is possible to tell if the beach is
expanding or eroding
 Beaches display seasonal cycles of expansion and contraction
related to wave size
 A swell profile is concave upward with a wide, broad berm (relatively
flat backshore) and steep intertidal beach face
 This profile typically develops during summer when the
weather is fair and the dominant waves are flat swells, which
transport sediment shoreward and enlarge the beach
 A storm profile displays erosion of the berm and a broad flat intertidal
beach face
 This profile typically develops during winter when the weather
is more unsettled and the dominant waves are high, steep, and
erosive
 Finer sediment is transported seaward, leaving coarser
sediment behind on the beach
 Some of the sediment transported seaward forms a longshore
bar, which later migrates landward as the swell profile begins
to redevelop
 A sand budget is the balance between sediment added to and sediment
eroded from the beach
 Sediment input to the beach comes from rivers, sea cliff
erosion, and on-shore sediment transport
 Beach sediment is removed by longshore current, off-shore
transport, and wind erosion
 A balance between sediment gains and loss results in a steadstate condition
 If sediment loss is greater than sediment gain, a negative sand
budget exists and the beach will begin to erode
 If sediment loss is less than sediment gain, a positive sand
budget exists and the beach will begin to expand
Coastal Dunes
o Sand dunes are formed by winds blowing sand landward from the dry part of
the beach
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Well developed dunes typically have a sinusoidal profile with the
primary dune at the landward edge of the beach and possible
secondary dunes located farther inland
 Dunes can extend up to 10 km into the interior
 The area between adjacent dunes is called a valley or swale
 Small, irregular foredunes commonly occur a the upper edge of
the beach
Vegetation on the dunes traps windblown sand on their downwind side
and promotes dune growth and stability
Blowouts are wind-scoured breaks In the dune or depressions in the
dune ridge and commonly occur if vegetation is destroyed
 With time the blowout can enlarge and eliminate the dune
Dunes are best developed if sand is abundant, onshore winds are
moderately strong and persistent, the tidal range is large and the beach
is wide and gently sloping
Wave erosion of sand dunes transports sand offshore and creates a
steep scarp at the base of the dune
 The scarp reflects the wave energy and lessens additional
erosion of the dune by the breakers
Dunes act as a natural barrier and prevent inland flooding
Human activity that damages vegetation leads to dune destruction by
blowouts and washover by storm waves
 Washover forms a washover fan on the landward side of the
dune
Barrier Islands
o Barrier islands are islands composed of sediment that parallel the coast and
form where sand supply is abundant and a broad sea floor slopes gently
seaward
 The islands are separated from the mainland by shallow bodies of
water which are connected to the ocean through tidal inlets
 A series of distinct environments develop across the island parallel to
the beach and include the nearshore zone, dune field, back-island flats,
and salt marshes
 The back island flats are washover fans deposited during
storms as water flooded across the lower parts of the island
 Salt mashes are protected areas on the back side of the island
where mud collects
 Barrier islands are created in many ways including:
 Sand ridges on the coastal plain which paralleled the coast and
were later isolated as lowlands were submerged by rising sea
level
 Sand spits that were breached during a storm and remained
separated from the mainland by a tidal inlet
 Vertical growth and emergence of alongshore sand bars
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As sea level rises, barrier islands migrate landward as washover
transports sediments from the seaward side of the island to the
landward side
o Storm surge is the high water created by the accumulation of wind-blow water
against the shore and the mound of water generated by the low atmospheric
pressure of the storm.
 The elevated water level allows waves to reach much farther inland
than usual, especially if the storm surge coincides with high tide
 Waves more easily breach the island and wash over lower areas
 New tidal channels may form during a storm surge
 Most tidal inlets are eroded from the landward side of the
barrier island seaward
 The bay becomes swollen with rainwater, runoff, and inflow
from the sea
 The onshore wind piles the water against the mainland and
after the storm has passed, a seiche can develop which then
raises the water level against the landward side of the barrier
island
 Storm winds may also reverse direction on the rear of the storm
and blow offshore, piling additional water against the barrier
island
 If water breaches the island, its seaward flow may create a new
inlet
 Most inlets are quickly filed with sediment because of
longshore drift
BREAK
 Cliffed Coasts
o A sea cliff is an abrupt rise of the land from sea level
 A sea cliff is most vulnerable to erosion at its base because:
 As waves slam against the cliff, air is compressed inside cracks
and then expands violently as the wave recedes
o This can eventually shatter the rock
 Sediment is hurled against the cliff by the waves
 Sea water can dissolve some rock types
 When sufficient rock at the base of the cliff has been removed, the
upper part of the cliff collapses
 Collapsed material protects the base of the sea cliff from additional
erosion until it is destroyed and removed
 Rate at which the cliff recedes is dependent upon:
 Composition and durability of the cliff material
 Joints, fractures, faults and other weaknesses in the cliff
material
 Amount of precipitation
 Steepness of the cliff
 The wave-cut platform is the gentle sloping area in front of the sea
cliff that was produced by sea-cliff retreat
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Deltas
o A delta is an emergent accumulation of sediment deposited at the mouth of a
river as it flows into a standing body of water
 The three major areas of a delta are:
 Delta plain – flat, low-lying area at or below sea level that is
drained by a system of distributaries
 Delta front – shoreline and broad submerged area of the delta
that slopes gently seaward
 Prodelta – far off shore area of the inner shelf that receives fine
sediment from the river
 In cross sections, a delta’s deposits can be divided into theree sets of
beds:
 Topset beds – flat-lyign beds of sand and mud of the delta plain
deposited by the distributaries in their channels and in the interchannel areas
 Foreset beds – thick silts and sands of the delta front that slope
gently seaward and form the bulk of the delta
 Bottomset beds – flat-lying silts and clays of the prodelta that
settle out of suspension far offshore
 As sediment accumulates the delta expands seaward with forest beds
burying bottomset beds and topset beds covering foreset beds
 Shape of the delta can be altered by tides, waves, and river deposition
 River dominated deltas form in areas protected from large
waves and with a small tidal ranges
o Delta displays the ideal triangular form
 Wave dominated deltas are so altered by wave erosion and
longshore drift that most of the delta sediment is distributed
along the coast and only a slight protrusion exists at the mouth
of the river
 Tide dominated deltas are altered by the ebb and flow of the
tides and into long linear submarine ridges and islands that
radiate from the river’s mount
 Reduction in the supply of sediment to a delta results in delta erosion
and subsidence as the sediments of the delta compact – Louisiana 1
cm/yr
Coastal Habitats
 The term coast has a much broader meaning than shoreline and includes many
other habitats and ecosystems associated with terrestrial and marine processes
 The six major coastal settings are:
 Estuary – semi-enclosed body of water where fresh and marine waters mix
 Lagoon – semi-enclosed body of water receiving no appreciable inflow of
fresh water
 Salt marsh – plant-covered intertidal zone
 Mangrove swamp – dense tree growth of the tropical and subtropical
shoreline
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 Coral reef – calcareous ridge constructed by corals and algae
 Shorelines are one of the most productive ecosystems and because they are
shallow, they strongly respond to the effects of waves, tides, and weather
Estuaries
 Estuaries are semi-enclosed bodies of water where fresh water from the land
mixes with sea water
 Estuaries originate as:
 Drowned river valleys – with the rise in sea level, the lower portions of river
valleys have flooded
 Fjords – as glaciers have retreated and sea level risen, the lower portions of
glacial valleys have flooded
 Fjords are typically long, narrow, and deep with steep cliff-like sides
 The bottom of fjords frequently are partially blocked by glacial moraines
(ridges of sediment deposited at the front of the glacier) which inhibit
current flow and can produce hypoxic to anoxic conditions at the bottom
 Bar-built estuaries – spits and sand bars may partially block the entrance of an
embayment, thereby restricting tidal flow
 Tectonic estuaries –uplift associated with plate tectonics can partially block
the entrance to an embayment
 Salinity typically grades from normal marine salinity at the tidal inlet of fresh
water at the mouth of the river
 In some estuaries, the water is well stratified with a strong halocline
separating the dense saline water below from the fresh water above
 Tidal flow provides the energy for mixing the fresh and saltwater masses
 If tidal flow is strong, stratification is weak
 Estuaries can be subdivided into three types based upon the relative importance of
river inflow and tidal mixing
 Salt-wedge estuaries are dominated by the outflow from rivers
 The outflow from rivers is much greater than the inflow from the tides
 The water column is highly stratified with a well-defined, strong halocline
that inhibits mixing
 Salt water forms a wedge that extends landward below the fresh water
wedge that extends seaward
 Strong turbulent currents in the fewsh water flow across the halocline and
generate internal waves
 As the internal waves steepen and break, they mix salt water into the fresh
wand it is swept seaward
 The continual loss of salt water into the fesh water generates a slow
current that flows in along the bottom and up along the underside of the
fresh water wedge
 The bottom current is too weak to carry much sediment into the estuary
from outside the tidal inlet
 Sediment distribution in the estuary consists of river sand at the landward
edge of the saltwater wedge and mainly river clays and silts elsewhere
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Partially-mixed estuaries are dominated by neither river inflow nor tidal
mixing
 Tidal currents promote greater mixing and both stratification and the
halocline are greatly weakened
 As more saltwater mixes into the fresh, a stronger bottom current is
generated
 The bottom of the seaward end of the estuary is covered with
sediments from the shelf, whereas the landward end is dominated by
river sediments
 In well-mixed estuaries, tidal turbulence destroys the halocline and water
stratification
 In wide estuaries, Coriolis deflects river outflow to one side and tidal
inflow to the other
 A salinity gradient extends across the estuary, but not vertically within the
water column
 Seawater flows in and fresh water flows out on opposite sides of the tidal
inlet at all depths.
 Since river discharge and tidal inflow vary, the type of estuary can change
 The widely fluctuating environmental conditions in estuaries make life stressful
for organisms
 Estuaries are extremely fertile because nutrients are brought in by rivers and
recycled from the bottom because of the turbulence (waves and tides)
 Stressful conditions and abundant nutrients result in low species diversity, but
great abundance of the species present
 Despite abundance of nutrients, phytoplankton blooms are irregular due to
low light (muddy water) and the base of the food chain is detritus washed in
from adjacent salt marshes
 The benthic fauna strongly reflects the nature of the substrate and most fishes
are juvenile forms living within the estuary until they mature and migrate to
the ocean
Lagoons
o Lagoons are isolated to semi-enclosed, shallow, coastal bodies of water that
receive little if any fresh water inflow
o Some lagoons are now polluted by man; were productive, now wasteland
Salt marshes
o Salt marshes are intertidal flats covered by grassy vegetation
 Mashes are most commonly found in protected areas with a moderate
tidal range, such as the landward side of barrier islands
 Marshes flood daily at high tide and then drain through a series of
channels with the ebb tide
 They are one of the most productive environments, despite harsh
conditions
 Marshes can be divided into two parts:
 Low salt marshes – extend from the low tide mark to neap high
tide
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o Along the Atlantic and Gulf Coasts, these areas are
dominated by a knee-high cordgrass
o Low marshes are the more productive area with
productivity of 800-2600 gm C/m2/yr
o Nitrate is commonly the limiting nutrient
o Plants die in autumn, partially decompose and supply
abundant detritus which becomes food for the
detritivores or accumulates and eventually forms peat
 High salt marshes –extend form neap high tide to highest
spring tide
o This area is flooded only at the highest spring tide or
during a storm surge
o It is more terrestrial than marine in nature and has a
more diverse fauna and flora
 Distribution and density of organisms in salt marshes strongly reflects
availability of food, need for protection, and frequency of flooding
 Salt marshes serve as nursery and shelter for juvenile organisms
 Many salt marshes damaged by man – filled in
Mangrove Swamps
o Mangroves are large woody trees with a dense, complex root system that
grows downward from the branches
 Mangroves are the dominant plant of the tropical and subtropical
intertidal area
 Distribution of the trees is largely controlled by air temperature,
exposure to wave and current attack, tidal range, substrate, and sea
water chemistry
 Detritus from the mangrove forms the base of the food chain
Coral Reefs
o A coral reef is an organically constructed, wave-resistant, rock-like structure
created by carbonate-secreting organisms
 Most of the reef is composed of loose to well-cemented organic debris
of carbonate shells and skeletons
 The living part of the reef is just a thin veneer on the surface
 Corals belong to Cnidaris
 The animal is the coral polyp
o The body of the polyp resembles a sac with the open
end surrounded with tentacles
 The corralite is the exoskeleton formed by the polyp. Its
interior is divided by septa, vertical partitions.
 Corals share a mutualistic relationship (mutually beneficial) with the
algae called zooxanthallae which live within the skin of the polyp and
can comprise up to 75% of the polyp’s body weight
 The coral provides protection for the algae and supplies them
with nutrients and carbon dioxide from the polyps metabolic
wastes
 The algae supply the coral with oxygen and food
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Recycling of nutrients between the polyp and the algae allows
the corals to thrive in the nutrient-poor tropical seas
 Corals can not survive in fresh, brackish water or highly turbid water
 Corals do best in nutrient poor water because they are easily outcompeted by benthic filter feeders in nutrient-rich water where
phytoplankton are abundant
o As a result of corals growing continuously upward towards the sunlight as sea
level rises and/or land subsides and, coral reefs pass through three stages of
development
 Fringe reefs form limestone shorelines around islands or along
continents and are the earliest stage of reef development
 As the land is progressively submerged and the coral grows upward,
an expanding shallow lagoon begins to separate the fringe reef from
the shoreline and the reef is called a barrier reef
 In the final stage, the land vanishes below the sea and the reef forms
an island ring or ring or islands, called an atoll, around a shallow
lagoon
END
Lab
Drawing wave orthogonals
Missing nos on Fig 12-15
 Impact of People on the Coastline
o Coastlines are desirable areas for human habitation, but human activity
conflicts with the dynamic state of coastal systems
 Humans try to stabilize coastlines:
 By interfering with longshore sand transport
 By redirecting wave energy to prevent erosion
 Intervention of sand drift is done by building jetties and groins
 Jetties – block sediment deposition at mouths of harbors, inlets,
or estuaries
o Jetties lead to sand buildup on the updrift side and
erosion on the downdrift side
 groins – function as dams by trapping sand
o groins also lead to sand buildup on updrift side and
erosion on downdrift side
 Redirecting wave energy involves breakwaters and seawalls
 Breakwaters are built offshore and parallel to a beach or harbor
mouth
 Seawalls are built along the shore as protection from storm
erosion
o They increase water turbulence at the front of the
barrier
o They deepen the nearshore zone
 Large waves can reach the beach causing
greater erosion and weakening of the seawall
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o They cause wave reflection, which can create strong rip
currents
o They reduce sediment input, which disrupts the sand
budget
Beach nourishment with artificially supplied sand is expensive
and temporary
An increase in sea level from global warming will cause more
land to be flooded and threaten more coastal buildings
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