The Dynamic Shoreline

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The Dynamic Shoreline
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Breaking waves provide the energy that changes
the shape and texture of the beach deposits.

• As waves shoal (touch bottom) in shallow water
  celerity decreases, wavelength decreases, wave
  height increases, waves become less stable and
  refraction occurs.
• 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.
• The beach is the part of the land that touches
  the sea. It can be divided into the Off shore,
  Near shore (breaker zone, surf zone, swash zone),
  and the Back ShorePosition of the divisions of
  the beach varies with the tides, advancing
  landward with high tide and retreating seaward
  with low tide.

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Saltational Transport
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Waves generate longshore currents that flow
parallel to the beach and rip currents that flow
perpendicularly to the beach.
11-1
Coastal Water Movement

• Angle of wave approach is the acute angle (less
  than 90o) between the wave crest and the beach.
• The direction of longshore current varies with
  the direction of wave approach.
• Longshore currents can also be generated by wave
  set-up.
• Where two opposing longshore currents collide,
  they form a swift, narrow, seaward rip current.

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Beach sediments are moved by currents and waves,
especially breakers.
11-2
Beaches

• A beach profile is a cross section of the beach
  along a line that is perpendicular to the
  shoreline.
• A swell profile is concave upward with a wide,
  broad berm (relatively flat backshore) and steep
  intertidal beach face.
• A storm profile displays erosion of the berm and
  a broad flat intertidal beach face.
• A sand budget is the balance between sediment
  added to and sediment eroded from the beach.

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Sand dunes are formed by winds blowing sand
landward from the dry part of the beach.
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Coastal Dunes

• 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.
• 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.
• 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.

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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.
11-4
Barrier Islands

• 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 the beach and include the
  nearshore zone, dune field, back-island flats and
  salt marshes.
• Barrier islands are created in many ways
  including sand ridges isolated by rising sea
  level, Sand spits breached during a storm,
  vertical growth and emergence of longshore sand
  bars.

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11-4
Barrier Islands

• As sea level rises, barrier islands migrate
  landward as washover transports sediments from
  the seaward side of the island to the landward
  side.

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11-3

• Sand saltates (bounces) up the windward side of
  the dune, collects in the wind-shadow at the top
  and periodically slides down the leeward face of
  the dune when the accumulation of sand becomes
  over-steepenedresulting in dune migration.
• Wave erosion of sand dunes transports sand
  offshore and creates a steep scarp at the base of
  the dune.
• 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.

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Storm surge is the high water created by the
accumulation of wind-blown water against the
shore and the mound of water generated by the low
atmospheric pressure of the storm.
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Barrier Islands

• The elevated water level allows waves to reach
  much farther inland than usual, especially if the
  storm surge coincides with a high tide.
• Waves more easily breach the island and wash over
  lower areas.
• New tidal channels may form during a storm surge.

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Coastlines are desirable areas for human
habitation, but human activity conflicts with the
dynamic state of coastal systems.
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Impact of People on the Coastline

• Humans try to stabilize the coastline in two
  ways by interfering with longshore sand
  transport, and by redirecting wave energy to
  prevent erosion.
• Preventing of sand drift involves jetties and
  groins.
• Redirecting wave energy involves breakwaters and
  seawalls.
• Beach nourishment with 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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A sea cliff is an abrupt rise of the land from
sea level.
11-5
Cliffed Coasts

• A sea cliff is most vulnerable to erosion at its
  base because waves that slam against the cliff
  compress air inside cracks which expands
  violently, sediment is hurled against the cliff
  by the waves, and sea water 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.

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Sea Stacks
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11-5
Cliffed Coasts

• Rate at which the cliff recedes is dependent
  upon
• Composition and durability of 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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1910
1890
1920
1990
1970
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A delta is an emergent accumulation of sediment
deposited at the mouth of a river as it flows
into a standing body of water.
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Deltas

• Deltas were named after the Greek letter delta ?.
• The three major areas of a delta are delta plain
  ,delta front and prodelta.
• In cross section, a deltas deposits can be
  divided into three sets of beds topset beds,
  foreset beds, and bottomset beds.
• As sediment accumulates the delta expands seaward
  with foreset beds burying bottomset beds and
  topset beds covering foreset beds.
• Shape of the delta can be altered by tides, waves
  and river deposition.
• Reduction in the supply of sediment to a delta
  results in delta erosion and subsidence as the
  sediments of the delta compact.

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Floods of 1849 and 1850, which caused widespread
damage in the Mississippi River Valley, revealed
the national interest in controlling the mighty
river. Major floods again occurred in 1912, 1913,
and 1927. The flood of 1927 was the most
disastrous in the history of the Lower
Mississippi Valley. An area of about 26,000
square miles was inundated. Levees were breached,
and cities, towns, and farms were laid waste.
Crops were destroyed, and industries and
transportation paralyzed.
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• The Flood of '27
•        The nation's most destructive flood began
  with the heavy rains that pounded the central
  basin of the Mississippi in the summer of 1926.
  By September, swollen tributaries were pouring
  through Kansas and Iowa. On New Year's Day, 1927,
  the Cumberland River threatened Nashville,
  topping levees at 56.2 feet. By May, the
  Mississippi below Memphis was a watery oval as
  wide as 60 miles across. Twenty-seven thousand
  square miles were inundated to depths up to 30
  feet.

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Nobody knows exactly how many died in the great
disaster. Historians once estimated the death
toll at 250 victims, but deaths due to disease
and exposure after the immediate flood are hard
to tally. John Barry's Rising Tide (1997)
estimates that up to 1,000 people, mostly black,
died in the Yazoo basin alone. At Mounds Landing
near Greenville, for example, a flood surge blew
out a dike where thousands of terrified workers
were building a bunker of sandbags. Swirling
west, the flood ravaged 2.7 acres of farmland
before rejoining the mainstem of the Mississippi
en route to Natchez and New Orleans.
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The Flood Control Act of 1928 authorized work
that would give the various basins protection
against Mississippi River floods only, although
the tributary streams within the basins caused
frequent flood damage that could not be prevented
by the main stem Mississippi River protective
works. Later amendments to this act have
authorized work that provides alleviation of the
tributary flood problems.
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One of the most important modifications to the
project was made in 1954 when Congress authorized
the feature for the control of flow at Old River
to prevent the capture of the Mississippi by the
Atchafalaya River.
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• The problem is that the Mississippi is on the
  verge of switching to a new channel along what is
  now the Atchafalaya River. The Atchafalaya River
  has already captured the Red River which flows
  from the west and used to be a tributary of the
  Mississippi. Already 30 percent of the flow of
  the Mississippi goes into a channel called the
  Old River and thence into the Atchafalaya River.
  The configuration is roughly in the form of an H
  in which the the Atchafalaya-Red Rivers form the
  left leg and the Mississippi the other with the
  Old River being the cross branch.
• The Old River Control Project of the Corp of
  Engineers is working to prevent the capture of
  100 percent of the Mississippi by the
  Atchafalaya. But the Corps of Engineers doesn't
  want to cut off all flow through the Old River
  because agricultural and marine development along
  the Atchafalaya River would be hurt. The Corps is
  committed to maintaining the 30 percent diversion
  that now exists.

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The U. S. Army Corps of Engineers estimates that
the present rate of coastal land loss is 25
square miles a year. The U. S. Fish and Wildlife
Service places that figure even higher at about
34 square miles a year. The latter number is
based on measuring the loss in coastal land are
between 1978 and 1990. Many areas of the Gulf of
Mexico coastline are retreating. If allowed to
continue, this retreat can cause irreversible
damage to the coastal environment, economy, and
quality of life. All Gulf states have serious
coastal erosion problems. Parts of Louisiana
retreat 65 feet or more per year, while erosion
rates of 15 feet year can be found in many other
areas of the Gulf.
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Wetland Loss in Louisiana

• (1) Sediment Deficit(2) Canals(3) Reclamation
  (4) Wave Erosion (5) Subsidence (6) Sea Level
  Rise (7) Saltwater Intrusion
• Nutria?????

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Are Nutria Really the Problem?
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Measures for Curtailing Wetland Loss in Louisiana

• Barrier Island RestorationWetland
  RestorationMarsh ManagementCanals and Land
  UseDiversion

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• On March 26 the Davis Pond Freshwater Diversion
  Project opened. Davis Pond is the largest
  freshwater diversion project in the world.
• Capable of diverting up to 10,650 cubic feet of
  water per second, officials from the Army Corps
  of Engineers hope that the 120 million project
  will help revive ailing wetlands in the
  Barataria-Terrebonne Estuary region. The plan is
  to divert freshwater from the Mississippi River
  into the estuary, pushing back salt water
  encroaching from the Gulf of Mexico.
• The plan met with some opposition, mostly from
  oyster farmers who were working in the wetlands.
  With the water restored to a freshwater state,
  the oysters could not survive. A relocation
  program was established for the farmers.

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• The key feature of the project, built into the
  Mississippi River Levee, is a reinforced concrete
  diversion structure with four 14-by-14-foot
  gates. It flows into a ponding area covering
  10,084 acres and is expected to help preserve
  33,000 acres of marsh and 777,000 acres of
  marshes and bays in the Barataria-Terrebonne
  Estuary region. A smaller, similar project in St.
  Bernard parish has already helped restore nearly
  70 percent of the freshwater vegetation in the
  area.

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The Caernarvon Project diverts fresh water and
its accompanying nutrients and sediments from the
Mississippi River to coastal bays and marshes in
Breton Sound for fish and wildlife enhancement.
Benefits include restoration of former ecological
conditions by controlling salinity and
supplementing nutrients and sediments. The bays
are important to oyster production and as
breeding areas for shrimp and food fishes, while
the marsh areas produce food for fur-bearing
animals, alligators, and migratory waterfowl. A
total of 16,000 acres of marshland will be
preserved and 77,000 acres of marshes and bays
will be benefited by the project.
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• The Breaux Act or the Coastal Wetlands Planning,
  Protection and Restoration Act (CWPPRA) in now in
  motion. The CWPPRA Task Force, the State
  Wetlands Authority and the Department of Natural
  Resources Coastal Zone Management Authority will
  establish it as a unifying strategic plan of
  action. It will become the CWPPRA restoration
  plan and Louisianas overall strategic coastal
  plan. Proposed projects will be measured against
  the strategies in the Coast 2050 Plan before
  being approved.
• Using the Coast 2050 Plan as a guide, we could
  restore and maintain more than 90 percent of the
  coastal land existing today.

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In one way or another, everyone in the nation
will feel the enormous loss of land along
Louisianas coast, and current restoration
efforts will only prevent 22 percent of the land
loss projected to occur within the next 50 years.
However, we know that a comprehensive restoration
program, using the Coast 2050 Plan as a guide,
could restore and maintain more than 90 percent
of the coastal land existing today. The price
tag is 14 billion to construct more than 500
projects that would be needed, but the price of
infrastructure alone that would be lost is more
than 150 billion.
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Coastal Development
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LUMCON