Dynamic Shoreline
• Why do we care?
– Loss of land
– Damage to structures
– Recreation
Coastal Water Movement
• Waves provide the energy
– Through breaking
• As waves shoal
– Speed decreases
– Height increases
– Wavelength decreases
– Period remains the same
– Become unstable
– Waves refract
• Bending of wave fronts toward shallow
water as they shoal
– Waves generate currents
• Longshore
• Rip
Generation of Longshore
Currents
• Longshore currents caused by waves
or wave set-up
• By waves
– Angle of wave approach is the acute
angle (less than 90o)
– This angle usually less than 10o
– The greater the angle of approach, the
stronger the longshore current
Longshore Currents
Longshore Currents by
Wave Set-up
• Wave set-up – process whereby
waves pile water against the shore
• Larger waves produce more wave
set-up than smaller ones
• Sea surface slope from where larger
waves hit shore to where smaller
waves hit shore causes pressure
gradient
• This pressure gradient causes a
current from high pressure to low
pressure (Coriolis not a factor)
Generation of Rip Currents
• Where two opposing longshore
currents collide (convergence), they
form a a swift, narrow seaward rip
current
• Rip current drains excess water from
the surf zone
Rip Currents
Beaches
• The part of the land that touches the
sea
• Parts of the beach
– Offshore – seaward of where waves
begin to break
– Nearshore – from the offshore to where
the waves wash back
• Breaker zone – waves begin to break
• Surf zone – waves expend most of their
energy
• Swash zone – waves wash back and forth
• Backshore – land that adjoins the nearshore
Beaches
• Positions of the divisions of the
beach vary with the tides
– Advances landward with high tide
– Retreats seaward with low tide
• Beach sediments are moved by
currents and waves, especially
breakers
• Beaches undergo seasonal changes
and changes due to weather
– Sand moves offshore in winter and
during big storms
– Sand moves onshore in summer and
during calm weather
Beach Profile
• A cross-section of the beach along a
line that is perpendicular to the shore
– By comparing beach profiles along the
same line taken at different times, it is
possible to see changes in the beach
• Beaches display seasonal cycles of
expansion and contraction related to
wave size
Swell Profile
• Made during calm weather
• Typical summer
• Concave upward with a wide, broad
berm, relatively flat backshore, and
steep intertidal beach face
Storm Profile
• Made when dominant waves are
high and steep
• Made during storms
• Typical winter profile
• Finer sediment is transported
seaward, leaving coarser sediment
on beach
• Longshore bar forms, which
migrates landward during calmer
seas
Sand Budget
• Balance between sediment added to
and eroded from the beach
• Input from rivers, sea cliff erosion,
and on-shore sediment transport
• Removal by longshore currents, offshore transport, and wind erosion
• Steady state – balance between gains
and losses
• When Negative– losses exceed gains
• When Positive– gains exceed losses
Coastal Dunes
• Sand dunes are formed by winds
blowing sand landward from the dry
part of the beach
• Well developed dunes typically have
a sinusoidal profile with the primary
dune at the landward edge of the
beach
• Secondary dunes can be located
farther inland
• Dunes can extend 10 km into interior
• Swale - area between dunes
• Vegetation on dunes traps
windblown sand and promotes dune
growth and stability
Dunes
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Foredune
Primary dunes
Secondary dunes
Back dune
Dune Blowouts
• Wind scoured breaks in the dune or
depressions in the dune ridge
• Commonly occur when vegetation is
destroyed
• With time blowouts can enlarge and
eliminate the dune
Dunes
• Dunes are best developed if sand is
abundant, onshore winds are
moderately strong and persistent, the
beach is wide, and the tidal range is
large
• Sand saltates (bounces, nothing to do
with salt) up the windward side of
the dune, collects at the top in the
wind shadow, and slides down the
leeward side
• Erosion of windward side and
deposition on leeward side results in
dune migration
Importance of Dunes
• Dunes act as a natural barrier and
prevent inland flooding
• Human activities can damage
vegetation and lead to dune
destruction by blowouts and
washover by storm waves
– Washover forms a washover fan on the
landward side of the dune
Barrier Islands
• Islands composed of sediment
• Parallel the coast
• Occur where sand supply is
abundant and the sea floor slope is
gentle (e.g. East coast of U.S.)
• Separated from mainland by shallow
bodies of water, which are connected
to the ocean through tidal inlets
Parts of a Barrier Island
• Nearshore zone
• Dune field
• Back-island flats – washover fans
deposited during storms
• Salt marshes – protected areas where
mud accumulates
• Barrier islands migrate landward
Creation of Barrier Islands
• Submergence of sand ridges on the
coastal plain
• Sand spits that were breached during
a storm
• Vertical growth and emergence of
longshore bars
Cliffed Coasts
• Sea cliff- an abrupt rise of the land
from sea level
• Erodes at its base
– Waves slam against the base, compress
air in cracks, air expands violently as
the water retreats
– Sediment is hurled against the cliff by
waves
– Sea water can dissolve some rock types
• When sufficient material at the base
is removed, the upper part of the
cliff collapses
Erosion of Cliffed Coasts
• Rate at which cliff recedes is
dependent on
– Composition and durability of the cliff
material
– Joints, fractures, faults and other
weaknesses in the cliff material
– Amount of precipitation
– Steepness of the cliff
• Wave-cut platform – gentle sloping
area in front of the sea cliff that was
produced by sea cliff retreat
Erosion of Cliffed Cloasts
Deltas
• Emergent accumulation of sediment
deposited at the mouth of a river as it
flows into a standing body
• Nile and Mississippi are famous
ones
Areas of a Delta
• Three major areas
– 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 offshore area of the inner
continental shelf that receives fine
sediment from the river
Types of Beds
• Topset beds
– Flat lying beds of sand and mud on the
delta plain
– Deposited by distributaries in their
channels and in the interchannels
– Delta plain
• Foreset beds
– Thick silts and sands of the delta front
that slope gently seaward
– The bulk of the delta
– Delta front
• Bottomset beds
– Flat lying silts and clays of the prodelta
that settle out of suspension offshore
– prodelta
Delta Growth and Shape
• As sediment accumulates
– the delta expands seaward
– Foreset beds bury bottomset beds
– Topset beds bury foreset beds
• Shape of delta can be altered by:
– Tides
– Waves
– Rivers
• Reduction in supply of sediment to a
delta results in delta erosion and
subsidence as the sediments compact
– Louisiana 20 cm/yr, 1 cm/yr sea level
rise
River, Wave or Tide domination
• River dominated
– In areas protected from large waves and small
tidal range
– Delta has ideal triangular shape
• Wave dominated
– Only a slight protrusion at river mouth
– Waves and longshore currents erode most of the
delta sediment
• Tide dominated
– Altered by ebb and flow of the tides
– Have long linear submarine ridges and islands
that radiate from the river’s mouth
Coastal Habitats
• Coastal has a broader meaning than
shoreline
• Major coastal settings are:
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Estuary
Lagoon
Salt Marsh
Mangrove Swamp
Coral Reef
• Productive ecosystems
• Shallow and respond to waves, tides,
and weather
Estuaries
• Semi-enclosed bodies of water where
freshwater from the land mixes with
sea water
• Examples:
– Lower Hudson River
– Narragansett Bay
Types of Estuaries
Estuaries and Mixing
Estuaries
• Due to the mixing, the
environmental conditions fluctuate
widely – makes life stressful
• Extremely fertile due to nutrient
inflow from land
• Stressful conditions and abundant
nutrients result in low species
diversity, but great abundance of the
species present
• Phytoplankton blooms are irregular
due to low light from turbidity
• Benthic fauna dependent on
substrate
Salt Wedge
Lagoons
• Isolated or semi-enclosed, shallow,
coastal bodies of water that receive
little if any fresh water inflow
• Occur at any latitude
• Some polluted by man; were
productive, now wasteland
Lagoons
Salt Marshes
• Intertidal flats covered by grassy
vegetation
• Found in protected areas with a
moderate tidal range
• Flood daily at high tide and drain
through a series of channels with the
ebb tide
• One of the most productive
environments
Parts of Marshes
• Low Salt Marshes
– From the low tide mark to neap high
tide
• High Salt Marshes
– From neap high tide to highest spring
tide
Salt Marshes
• Low Salt Marshes
– More productive area
– Nitrate is commonly the limiting
nutrient
– Plants die in autumn, partially
decompose and supply abundant
detritus
– Detritus either becomes food or
accumulates and eventually forms peat
• High Salt Marshes
– More terrestrial than marine in nature
– More diverse flora and fauna
Life in Salt Marshes
• Distribution and density of
organisms in salt marshes strongly
reflects availability of food, need for
protection and frequency of flooding
• Salt marshes serve as a nursery and
shelter for juvenile organisms
• Many salt marshes damaged by man
– filled in
Mangrove Swamp
• Mangroves are large woody trees with
a dense complex root system that grows
downward from the branches
•Distribution of the trees is largely
controlled by air temperature, exposure
to wave an current attack, tidal range,
substrate, and sea water chemistry
•Detritus from the mangrove forms the
base of the food chain
Coral Reefs
• An organically constructed, waveresistant, rock-like structure created
by carbonate-secreting organisms
• Composed of loose to well-cemented
organic debris of carbonate shells
and skeletons
• Living part is just a thin veneer on
the surface
Location of Coral Reefs
Coral
• Corals belong to Cnidaris
– The animal is the coral polyp
– The body of the polyp resembles a sac
with the open end surrounded by
tentacles
– The coralite is the exoskeleton formed
by the polyp
Coral
• Coral share a mutualistic
relationship with the algae call
zooxanthallae
• Zooxanthallae live within the skin of
the polyp
• Zooxantallae can comprise up to
75% of the polyp’s body weight
• The coral provides protection for the
algae and supplies them with
nutrients and CO2
• The algae supply the coral with
oxygen and food
• Recycling of nutrients between the
polyp and the algae allows the corals
to thrive in the nutrient-poor tropical
seas
Coral Survival
• Corals cannot survive in fresh,
brackish or highly turbid water
• Corals do best in nutrient poor water
because they are easily outcompeted
by benthic filter feeders in nutrientrich water where phytoplankton are
abundant
Atoll Formation
Lab time
Impact of People on the
Coastline
• Humans try to stabilize the coastline
– Interfere with longshore sand transport
– Redirect wave energy to prevent
erosion
• Build structures
• Beach nourishment – adding sand to
beach to restore beach
• Increase in sea level due to global
warming will cause more land to be
flooded and threaten more coastal
buildings
Structures
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Jetties
Groins
Breakwaters
Seawalls