Geology
Lecture Outline
Introduction
– Where Ocean Meets Land
Classifying Coastlines – Processes and Features
Erosion-Dominated Coastlines – Rugged,
Irregular, and Rocky
Deposition-Dominated
Coastlines –Straight, Regular and Sand-rich
Beaches – Where Surf Meets the Sand
Estuaries
and Deltas – Where Rivers Meet the Sea
Reefs and Atolls – Coral Gardens Beneath the Waves
Other
Coastal Features – From Barrier Islands to Tombolos
U.S.
Shorelines – West versus East Coasts
Humans Assault on Shorelines - Mother
Nature Knows Best
Tsunami – When Hell Breaks Loose on a
Shoreline
II. Introduction
A. Coastlines are Long, Narrow Geographic Features
1. Occur wherever ocean and land meet at or
near sea level
·
Margins of continents
·
Edges of
inner-continental seaways
·
Island rims
2. Typically are only up to a kilometer or two wide
·
The term coastline or coastal zone has a much broader meaning than
shoreline and includes many other habitats and ecosystems associated with
terrestrial and marine processes
·
The coastal zone includes beaches, wave-cut terraces,
offshore bars, bluffs, back beach dunes, deltas, estuaries, lagoons, swamps,
and marshes
·
A shoreline is more limited to the beach, or littoral
zone
3. World Ocean is bounded by roughly 440,000
kilometers (273,000 miles) of
shoreline.
·
Equals 1 1/2 times the
distance to the Moon
B.
Shorelines are the Most Dynamic Regions on Earth
1. Comprises a special geography where
atmosphere, land and ocean meet at
a triple interface
2. A geographic region affected by many
natural agents
·
Wind
·
Surf
·
Tides
·
Tsunami
·
Storms
·
Flooding
·
Sea level fluctuation
·
Glaciation
·
Biologic forces
·
Earthquakes
·
Volcanism
·
Tectonics-
Uplift/Subsidence
·
Human interference
3. The location and shape of coastlines
are always changing
·
Location controlled by two factors:
ü
Tectonics and
ü
Ocean volume
·
Shape is controlled by several factors:
ü
Uplift
ü
Subsidence
ü
Eustatic sea level
fluctuations
ü
Erosion
ü
Deposition
ü
Volcanism
C.
Much of World's Population Lives At/Near the Coast
1. Coastal
dwellers are exposed to similar dynamic agents that shapes the
coast
2. Coastal dwellers have responded to dynamic change
along coastlines by
attempting
to stabilize coastal features
·
Jetties, seawalls,
dikes, breakwater, groins, etc.
III. Classifying Shorelines
A.
Classification Schemes for Coastlines are Based on the Dynamic Factors that
Shape Them
1. Common geotectonic origin
·
Leading edge coasts - active coasts
ü
Plate boundary
coastlines
ü
Tectonically active
ü
Active magmatism
·
Trailing edge coasts - passive coasts
ü
Innerplate coastlines
ü
Little to no tectonic or
magmatic activity
2. Eustatic
(worldwide) sea level fluctuations
·
Variation in total ocean
water volume
·
Variation in ocean basin
volume
·
Variation in water
column density
3.
Local changes in local sea level
·
Tectonic uplift or
subsidence
·
Isostatic adjustment
·
Local ocean surface
fluctuations
o
Storm surges
o
Waves
o
Tsunami
o
El Nino and La Nina
4. Coastlines are classified into two
distinctive types, based on the source of
dominate influence(s)
·
Primary - Dominated by Terrestrial Influences
·
Secondary - Dominated by Marine Influences
C. Erosional Coastlines
1. Erosional coasts typically have the following
character:
·
Rugged Relief
·
Irregular
·
Rocky
2. Erosional coasts are
dominated by erosional agents of change
·
Waves and Tides
·
Ocean Currents
·
River erosion -
downcutting of river valleys
·
Runoff and Wind
·
Glacial erosion - fiords
3. Features found along
an erosional coastal zones
·
Rocky headlands
o
Sea bluffs, caves, sea
arches, and sea stacks
·
Small pocket beaches
o
Coarse grained material
like, cobbles and gravel
·
Exposed wave-cut benches
o
Lack of sediment along
shoreline
4. Erosional coastlines are found in many parts of the
world
·
Example: Big Sur and the
Oregon Coast
D.
Deposional Coastlines
1. Deposional coasts have
a recognizable character:
·
Generally straight,
low-lying, and rich in loose sediment
o
Example: Texas Gulf
Coast
2. Depositional coasts
are dominated by depositional agents of change
·
River sediment input
·
Eroded bluff material
input
·
Longshore currents
·
Tidal action
·
Biological activity
3. River and bluff
sediments are the primary sources of coastal sediment
·
Coral reefs are also
important in many tropical regions
·
Particularly during
storm events
4. Typical features of
depositional coasts include:
·
Barrier islands
·
Sand spits and tombolos
·
Bay mouth bars
·
Beaches
6. Ocean wave and
longshore current action over time will ultimately
straighten the
shoreline
·
Wave refraction
intensifies energy at headlands
·
Wave refraction dissipates
energy in bays
·
See figure 12.13 (page
298)
·
Overall effect of
shoreline straightening
o
Wear away headlands
o
Fill in the bays
o
Build up and accumulate
beaches
IV. Beaches and Beach Processes
A.
Beaches Defined
1. The beach is a zone of unconsolidated sediment that
covers all or part of the
shoreline
·
Beaches extend from
beyond the breaker zone to the landward edge of the coastal zone
·
Consists of sand and/or
pebbles and/or cobbles
·
Another term for the
beach is the littoral zone
2. Beaches can
be divided into three regions:
·
Offshore – 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
o
Near shore can be
divided into the:
§
Breaker zone – where the waves begin to break
§
Surf zone – where the waves expend most of their energy
§
Swash zone – where waves wash back-and-forth across the beach
face
·
Backshore – the land that adjoins the near shore
o
Also termed the
backbeach
3. Position of
the divisions of the beach varies with the tides, advancing
landward
with high tide and retreating seaward with low tide
B.
Beach Profile and Anatomy
1.
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
·
There is a general
relationship between beach material
composition and beach slope angle
o
More gradual - the finer
the sediment
o
More steeper - the
coarser the sediment
o
See Table 12.1 (p. 299)
for comparing
·
Study text figure
showing a beach profile
2. 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.
3.
A storm profile displays erosion of the berm
into a beach scarp 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 sets of longshore bars and
troughs, which later migrates landward
as the swell profile begins to redevelop
C.
Ocean Breakers and Currents
1. Ocean waves that meet the shoreline will break
·
Breaking waves, or breakers, and the resultant white water is termed surf
·
The size of surf is
dependent on swell size, coastline shape, and bottom conditions
·
The shape of the
breaking waves is dependent mainly on bottom conditions
2. Breaking waves turn swell energy into translational
kinetic energy –
shoreward-moving turbulent water termed "white water"
3. Surf energy goes to work in several ways:
·
Generates longshore
currents
·
Generates rip currents
·
Moves beach material
perpendicular to shore
o
Combination of saltation
and suspension
·
Erodes bedrock base -
creates wave-cut platform
·
Erodes and undermines
sea cliffs
4. Waves that
approach the shore at an angle will break at an angle to the
beach and
result in a peeling wave
·
Angled breakers have a
translational (white water) component that is parallel to shore
·
The shoreline-parallel
component of white water over time
generates a nearshore current called a longshore current
·
Longshore
(parallel-to-shore) currents move in the direction
opposite that of the direction the swell
arrives from
o
Northerly swells
generate a southward-bound longshore
current along a west or east-facing beach
o
Southerly swells
generate a northward-bound longshore
current along a west or east-facing
beach
·
Beach material moved by
a longshore current is termed
longshore
transport or drift
·
Study Text Figure for
longshore transport
5. Waves approaching the shore at an angle will
disturb the bottom sediment
prior to
breaking
·
Sediment will be moved
in similar direction to the longshorecurrent
·
This is termed offshore transport
6. Longshore currents cause transport (drift) of beach
material in the direction
of the
current
·
The dominant longshore current and transport
direction is south, due to the predominant northerly swell directions of winter swell
7. White water that piles up along a section of
beach must find a means to exit
back out to sea to maintain a water mass
balance along the seashore –
gravity maintained
·
This is commonly
accomplished by the generation of narrow seaward-bound currents running perpendicular
to shore
o
These currents are
called rip currents
8.
Rip currents are narrow river-like channels of ocean water that move
seaward through the nearshore surf zone
and into the offshore region of
the beach where they dissipate
·
Rip currents typically
have an extended mushroom-like shape (from a
"bird's eye" view), and are often accentuated by rough, foamy
water
·
Rip currents form by a
combination of two phenomena at specific locations along a beach:
o
Convergence zones of
inbound white water energy
o
Inshore-bound waters
especially accumulate along
low-standing channels along the beach
bottom
·
Swimmers and surfers
caught in outgoing rip currents escape them by moving paddling perpendicular to
shore
D.
Seasonal Beach Changes
1. Beaches undergo seasonal changes
due to differences in marine weather
·
Times of the year with
persistent large stormy surf results in excessive beach erosion
o
Sand is moved offshore
into longshore bars
o
Beach lowers and
steepens
·
Times of the year with
persistently calm conditions results in
excess beach deposition
o
Sand is moved shoreward
back onto the exposed beach
o
Beach raises and
flattens
E.
Coastal Beach Cells
1.
Beach-lined coastlines are broken into separate unique segments called
coastal cells
2. Each coastal cell acts like a river of
sand
·
Driven primarily by
longshore currents
·
"Upstream and
"downstream" ends of a cell
3.
Each coastal cell has a sand budget
·
The sand budget is the
balance between sediment added
to and sediment removed from the cell
4. Major input sources of sediment for beach cells:
·
Rivers
·
Coastal cliffs
5. The boundary between coastal cells is marked
by a coastal geographic
barrier that diverts or terminates
longshore transport
·
Typically the barrier is
a submarine canyon or extensive headland
6.
Longshore and offshore currents transport beach material both parallel to
the shoreline
·
Parallel sand movement
is termed longshore transport
·
Sediment input to the
beach comes from rivers, sea cliff erosion, and on-shore sediment transport
·
Beach sediment is
removed by longshore current, off-short transport, and wind erosion
·
A balance between
sediment gains and loss results in a stead-state 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
7. The
typical final output destination for sediment in a coastal sand cell is an
underwater (submarine) canyon
·
Sand is funneled down
these underwater ravines via turbidity currents
·
Sand is permanently
removed from the coastal cell
V. Estuaries and Deltas
A. Estuaries
1. Defined:
·
Estuaries are
semi-enclosed bodies of water wherefresh water from the land mixes with sea
water
2. 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 byglacial 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.
3. 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
4. Tidal flow
provides the energy for mixing the fresh and saltwater masses.
§
If tidal flow is strong,
stratification is weak.
5. Estuaries
can be subdivided into three types based upon the relative
importance of river inflow and tidal mixing.
.
§
Salt-wedge estuaries
§
Partially-mixed
estuaries
§
Well-mixed estuaries
6. Salt-wedge estuaries are dominated by the outflow from
rivers
§
The outflow from rivers
is much greater than the inflowfrom 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 fresh 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 fresh 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
7. 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
8. Well-mixed estuaries
are dominated by tidal turbulence which 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
9. Since river discharge and tidal inflow vary,
the type of estuary can change
10. The widely
fluctuating environmental conditions in estuaries make life
stressful
for organisms
11. 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
12. 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.
B.
River Deltas
1. Defined:
·
A delta is an emergent accumulation
of sediment deposited at the mouth of a river as it flows into a standing body of water
2. 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
3. In cross sections, a delta’s deposits can be
divided into three 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 inter-channel 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
4.
As sediment accumulates the delta expands seaward with forest beds
burying bottomset beds and topset beds
covering foreset beds
5. 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
o
Example: the Mississippi River
·
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
o
Delta displays only a
slight protrusion at the mouth of
the river
o
Example: the Sao Francisco River
·
Tide dominated
deltas are altered by the ebb and flow
of the tides and into the ocean
o
Delta displays long
linear submarine ridges
and islands that radiate from the
river’s mouth
o
Example: the Ganges River
6.
Reduction in the supply of sediment to a delta results in delta erosion and
subsidence as the sediments of the delta
compact
·
Erosion and Subsidence
Mississippi Delta = 1 cm/yr
VII. Reefs, Islands, Atolls, and Guyouts
A.
Coral Reefs Defined
1. 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 the
Animal Order Cnidaris
Ø
The animal is the coral
polyp
Ø
The body of the polyp
resembles a sac with the open
end surrounded with tentacles
Ø
The corralite is the
exoskeleton formed by the polyp.
Its interior is divided by septa,
vertical partitions.
2.
Corals share a symbiotic 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
·
Recycling of nutrients
between the polyp and the algae
allows the corals to thrive in the
nutrient-poor tropical seas
3.
Corals cannot survive in fresh, brackish water or highly
turbid water
4. Corals do best in nutrient poor water
because they are
easily out-competed by benthic filter
feeders in nutrient-
rich water where phytoplankton are abundant
B.
Evolution of Coral Reef Systems
1. 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
reef development
·
Fringe reefs
·
Barrier reefs
·
Atolls
2. Fringe reefs
form limestone shorelines around islands
or along continents
·
Represents the earliest
stage of reef development
·
Form in areas with low
rainfall runoff
·
Leeward side of islands
·
Many Hawaiian reefs are
this type
3.
Over time, islands do two important things
·
Slowly subside with the
underlying ocean crust
·
Slowly wear down to sea
level by wave erosion
4. Upwards
reef growth keep ups with sinking island
·
Reefs grow upward at up
to 1 cm/year
5.
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 (second stage)
·
Barrier reefs occur at
lower latitudes than fringe reefs
·
Australia's Great
Barrier Reef is an example
6. 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 (final stage)
7. See Figure 12.27 in text (page 307)
C.
Atolls and Guyouts are Modified Oceanic Islands
1. Island eventually worn down to
below sea level, with
only the growing reef able to maintain at sea level
·
This stage of an island
is termed an atoll
2. Eventually reef growth lags behind rate of
atoll
subsidence, and entire atoll structure
becomes
permanently submerged
·
The submerged atoll is
termed a guyout
3.
See Figure 12.27 in text (page 307)
V. Other Geographic Features of Coastlines
A.
Coastal Bluffs and Cliffs
1. Defined:
·
A sea cliff is an abrupt
rise of the land from sea level
2. 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
3. Collapsed material protects the base of the
sea cliff from
additional erosion until it is destroyed and
removed
4.
The 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
5. The
wave-cut platform is the gentle sloping area in front
of the sea
cliff that was produced by sea-cliff retreat
B.
Coastal Dunes
1. Sand dunes
are formed by onshore winds blowing sand
landward from
the dry part of the beach
2. 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
2.
Vegetation on the dunes traps windblown sand on their
downwind side and promotes dune growth and
stability
3. Blowouts are wind-scoured breaks In the dune
or
depressions in the dune ridge and commonly
occur if
vegetation is destroyed
·
With time blowout can
enlarge and destroy the dune
4. Dunes are best developed in the following
conditions:
·
Sand is abundant
·
Onshore winds are strong
and persistent
·
The tidal range is large
·
The beach is wide and
gently sloping
5.
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
7. Dunes act as a natural barrier and prevent
inland
flooding
8. 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
C.
Barrier Islands
1. 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
2.
The islands are separated from the mainland by shallow
bodies of water which are connected to
the ocean
through tidal inlets
3. 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
4.
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
·
As sea level rises,
barrier islands migrate landward as
washover transports sediments from the
seaward side
of the island to the landward side
D. Sand spits
1. Sand spit defined:
·
linear feature made up of unconsolidated sediment that
grows
downcurrent of the longshore current
2. Sand spits are anchored
to coast off of natural or man-
made projections, such as sea stacks or jetties
3. Usually, the youngest
end of the spit is wide and/or
hook-shaped
E. Tombolos
1. Tombolo defined:
·
A sand spit that grows from the shore to an offshore
landmass
2. Tombolos are common
features of sandy shorelines that
have abundant
seastacks and islands in close proximity
to shore
F. Bay Mouth Bars
1. Bay mouth bar defined:
§
Form when spits grow across and block the entrance to a bay
2. If waves and tides are
strong enough, the bar will be
breached. Otherwise, a
lagoon will develop
G.
Lagoons
1.
Lagoons defined:
·
Lagoons are isolated to
semi-enclosed, shallow, coastal
bodies of
water that receive little if any fresh water inflow
2. Some lagoons are now polluted; were once
productive,
now are
wastelands
H.
Salt Marshes
1. Salt marshes defined:
·
Salt marshes are
intertidal flats covered by grassy vegetation
2. Mashes are most commonly found in protected
areas
with a moderate tidal range, such as the
landward side of
barrier islands
3.
Marshes flood daily at high tide and then drain through a
series of channels with the ebb tide
4.
They are one of the most productive environments,
despite harsh conditions
5. Marshes can be divided into two parts:
·
Low salt marshes –
extend from the low tide mark to neap
high tide
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
I.
Mangrove Swamps
1. Mangroves defined:
·
Mangroves are large
woody trees with a dense, complex
root system that grows downward from the
branches
2.
Mangroves are the dominant plant of the tropical and
subtropical intertidal area
3. Distribution of the trees is largely
controlled by air
temperature, exposure to wave and current
attack, tidal
range, substrate, and sea water chemistry
4. Detritus from the mangrove forms the base of the
food
chain
J.
Effects of Storm Surges on Coastline Features
1. Storm surge defined:
·
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
2. The elevated water level allows waves to
reach much
farther inland than usual, especially if
the storm surge
coincides with high tide
3. During a storm surge, ocean waves more
easily breach
the island and wash over lower areas
4.
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
VI. Coastlines of
the U.S.A.
A.
Pacific Coastlines
1. Tectonically active coastline
·
Dominated by uplift
2. Typically rugged and irregular with abundant sea
cliffs
3. Sediment sources from nearby granitic and volcanic
mountains
via rivers far exceed local bluff input
4. Deltas tend not to form due to high energy
shorelines
B.
Atlantic Coastlines
1. Tectonically inactive coastline
·
Dominated by subsidence
2. Abundance of barrier islands and submerged river
valleys
3. Sediment sources are mainly from offshore deposits
4. Deltas are rare or absent
C.
Gulf Coast
1. Tectonically inactive coastline
·
Dominated by extreme
subsidence
2. Typically very low-lying and straight with abundant
broad
beaches and barrier islands
3. Absence of large waves (except infrequent hurricanes)
and
submarine canyons
4. Formation of large deltas
·
Excessive sediment input
from rivers
VII. Humans Assault on Coastlines
A. Beaches are Systems that Exist in a Natural Balance
Between
Erosion and Deposition
1. Natural input of material from rivers and
sea cliffs
2. Natural movement of material in longshore
transport
3. Natural seasonal changes in beach sand
budget
B. Humans Build Structures That Oppose Coastal
Processes and Typically Cause Cell
Imbalances
1. Breakwaters
·
Eliminate or reduce wave
influence
·
Disrupt longshore
current and transport
·
Sand piles up on beach
behind breakwater
2. Groins
·
Disrupts and intersects
longshore transport
·
Designed to trap and
hold sand on beach
·
Excess sand buildup on
"upstream" side of groin
·
Excess sand erosion on
"downstream" side of groin
3. Jetties
·
Disrupts and intersects
longshore transport
·
Excess sand buildup on
"upstream" side of jetty
·
Designed to prevent sand
from blocking harbor
channel
entrance
4. Seawalls
·
Blocks wave erosion of
sea cliff or bluff
·
Reflected wave energy
increases beach erosion in front
of seawall
·
Blocking of bluff
erosion reduces sediment input fro beach
replenishment
·
Designed to protect
bluff and/or structures behind seawall
IV. Tsunami!
A.
Defined
1. Tsunami are very long wavelength shallow water
progressive (gravity) waves caused by the rapid
displacement of ocean water
·
Tsunami is mistakenly
called a tidal wave
·
Seismic sea waves are
tsunami
·
Not all tsunamis are
seismic sea waves
B. The Nature of Tsunami
1. Tsunami are shallow water
waves because they always
travel in water depths shallower than ½ their wavelength
·
Tsunami wavelengths are
up to 200 kilometers
5. Tsunami waves
travel very fast
·
Calculated by the shallow
water wave equation: C = √gd
where C is speed, g is acceleration due to gravity,
and d is water depth
(typically 15,000 feet in the Pacific)
·
up to 500 miles per hour
·
Can cross the
4. Tsunami waves in open ocean only 1-2 meter in
height
·
Ocean vessels on the high
seas wouldn’t notice one
5. Tsunami resemble
a swiftly rising tide (a tidal bore) rather than a breaking
wave
when they make shore
·
Picture a super gigantic
“mush burger” wave
·
Unlike a normal sea wave,
the tsunami wave keeps driving onshore for minutes
C. How are Tsunami Generated
1. Tsunami
are generated by several water disturbing forces
which acts to displace surface
water
·
Earthquake/Faulting (sea
bottom displacement)
·
Shoreline or underwater
landslide event
·
Volcanic eruption
·
Bolide impact
2. Seismic sea waves
are generated when the ocean bottom is rapidly raised, or lowered, along an
underwater fault zone
during
a large earthquake, i.e. large fault rupture
·
Up-lifted sea bottom
causes an initial “bump” in the ocean surface
Ø
Crest of tsunami wave
forms first
·
Down-dropped sea bottom
causes an initial “dimple” in the ocean surface
Ø
Trough of tsunami wave
forms first
3. After a tsunami is generated, the wave typically
disperses
into
multiple crests
·
The first wave often is
not the largest
·
Because of the long wavelength,
the crests may be separated by 10s of minutes or even hours.
D.
Tsunami Are Classified Into Two Categories
1. Based on
their proximity to origin
·
Local
·
Far traveled
2.
Local tsunamis primarily affect a small area
and are usually caused by landslides (often underwater) which are triggered by
earthquakes or volcanic eruptions.
·
These are sometimes very
severe and occur with
virtually no warning.
3. The largest local tsunami on record occurred in
·
From damage to trees, it
is estimated to have reached more than 1500 feet up the mountainside
·
A wave about 150 feet high swept down the bay
and out to sea
·
Four of six people aboard three boats
anchored in the bay survived.
4. Tsunami can hit coastlines that are thousands of
kilometers from the point of tsunami generation
5. They are free gravity waves like ocean swell
6. They do lose energy the further they travel
7. The Pacific basin is notorious for abundant
far-traveled tsunami
8. Tsunami can be predicted after an earthquake
D.
When Tsunami Meet the Shoreline
1. Fast-moving tsunami waves change radically when they
encounter
the shoreline very
·
Slows down
·
Wavelength shortens
dramatically
·
Tremendous increase in
wave height
·
First wave encountered may
be either the trough or the crest
Ø
Trough – Appears like a
Super low low tide
Ø
Crest - Looks like a humungous tidal bore
2. Low-lying areas along coastlines are at serious risk
when a
tsunami hits
·
A very rapid onslaught of
sea water rushes onshore
·
The driving surge pushes
inland as Sea level
3.
Examples of devastating far-traveled tsunami events
E.
Tsunami Prediction and Warning Systems
1) Coastal Tidal Gauge Network
2) The DART Open Ocean Network
F. Important Tsunami Safety Tips
1. If you are in a coastal community less than 50
feet above
sea
level, and you feel a severe earthquake (one that
makes it almost impossible to stand up, which is
causing substantial damage to buildings, or is
opening
cracks in
the ground), RUN for the
highest point you
can reach within minutes.
·
Once you see the wave,
you cannot outrun it. If all else fails, some people have survived by climbing
trees.
2. Even if you have felt no earthquake, or only a
mild one, a
sudden recession of water is always
a danger sign.
·
Run away from the water to high ground.
3. Remember -
more severe waves can follow for hours.
·
Do not return to low-lying areas
for 24 hours.
2.
Ships at anchor should
weigh anchor and head to sea.
3.
Ships at dock should also, if there is a
warning due to a distant earthquake.
·
However, if at dock during
a severe earthquake, it is questionable whether the best choice is to jump
ashore and run inland, or to try to ride it out aboard (loose mooring lines if
possible.)
4.
Tsunami warnings for
distant sites are still inexact.
·
They can warn that a
tsunami might occur, and approximately when, but the danger at a
particular location depends on topography, the particular characteristics of
the wave, and other factors
·
This results in many false alarms, leading
people to disregard alarms when they occur
XI. Coastline and Beaches Vocabulary -
Chapter 12
Active coast
Atoll
Backshore
Backbeach
Backwash
Barrier Island
Barrier reef
Bay Mouth Bar
Beach
Beach scarp
Berm
Berm crest
Breaker
Breakwater
Coast(line)
Coastal cell
Delta
Dissolution
Estuary
Eustatic sea level change
Fjord
Foreshore
Fringing reef
Groin
Guyout
High-energy coast
Lagoon
Longshore bar
Longshore current
Longshore transport (drift)
Low-energy coast
Primary coast
Reef
Rip current
Sand spit
Sea cave
Sea cliff
Seamount
Seawall
Secondary coast
Shore(line)
Submarine canyon
Surf
Swash
Tombolo
Turbidity current
Wave-cut platform