The Dynamic Shoreline

1
The Dynamic Shoreline
2
Breaking waves provide the energy that changes
the shape and texture of the beach deposits.
11-1
Coastal Water Movement

• 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 Shore

3
11-1
Coastal Water Movement

• Position of the divisions of the beach varies
  with the tides, advancing landward with high tide
  and retreating seaward with low tide.

4
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.

5
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.

6
Sand dunes are formed by winds blowing sand
landward from the dry part of the beach.
11-3
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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11-3
Coastal Dunes

• 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.

8
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.

9
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.

10
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.
11-4
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.

11
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.

12
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.

13
A delta is an emergent accumulation of sediment
deposited at the mouth of a river as it flows
into a standing body of water.
11-6
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.

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11-6
Deltas

• 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.

15
Coastlines are desirable areas for human
habitation, but human activity conflicts with the
dynamic state of coastal systems.
11-7
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.