Transcript Chapter 11: The Coast: Beaches and Shoreline Processes

Chapter 11: The Coast: Beaches
and Shoreline Processes
Coastal region
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Shore between low tide and highest
elevation affected by storm waves
Coast from shore to farthest inland
ocean features
Fig. 11-1
Beach
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Actively changing
Shore: backshore, foreshore, nearshore,
offshore
Berm, beach face, longshore bar, longshore
trough
Fig. 11-1
Beach composition and shape
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Locally available material
Coarser sediment
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Beach steeper
Finer sediment
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Beach more gentle
Sand movement
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Swash and backwash
Sediments moved up and down beach face
 Smaller, low energy waves sand moved up
beach face
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Summertime beach
Larger, high energy waves sand moved off
shore
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Wintertime beach
Sand movement
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Longshore current
 Longshore drift
or transport
 Parallel to shore
 Increasing
strength
More wave
energy
 Higher wave
frequency
 Steeper beach
 Greater angle
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Longshore current main way
sediments moved along coasts
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Southward along both U.S. coasts
most of the time
Erosional-type shore
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Headlands eroded
Sea arches
 Sea stacks
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Wave-cut cliffs
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Sea caves
Tectonically active coasts
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Example Pacific coast U.S.
Depositional-type shores
Sediments re-distributed by ocean
processes
 Typical in tectonically passive areas
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Depositional features
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Fig. 11-7
Spit
Tombolo
Bay barrier or
baymouth bar
Barrier islands
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Fig. 11-10
Ocean beach
Dune
Barrier flat
High salt marsh
Low salt marsh
Lagoon
Rising sea level
pushes barrier
island landward
FIG. 11.11b
Deltas
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Rivers deposit
sediment at coast
If more fluvial
influence than
ocean influence
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Bird’s foot delta
If more ocean
influence than
river influence
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Arcuate delta
Shepard
classification of
coasts
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Primary
 Geologically
youthful
 Controlled by
non-marine
processes
 Examples
 “Drowned”
river or glacial
valleys
 Volcanic coasts
 Coasts shaped
by recent
tectonic
movements
Secondary coasts
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Geologically mature
Controlled by
marine processes
Tectonically passive
 Examples
 Straightened
coasts
 Coral reefs
 Marsh grass
Fig. 11-13f
Emergent coasts
Uplift or sea level drops
 Once marine is now land
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Marine terraces
 Stranded beach deposits
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Submergent coasts
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Subsidence or sea level rises
Once land (or shallower) is now
underwater (or deeper)
Drowned beach deposits
 Drowned river/glacial valleys
 Submerged dunes
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Tectonic and isostatic changes
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Tectonic uplift and subsidence
Larger scale: major parts of continents
 Smaller scale: local deformation
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Isostatic adjustments to loading
Sediments or ice
 Regional and local effects
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Affect sea level
Eustatic changes in sea level
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Fig. 11-15
Global
Changes in spreading
rates of ocean floor
Changes in ice
volumes
Changes in ocean
temperature
 Global warming
 Sea level rises
 Ice melts, ocean
warmer
Atlantic coast of U.S.
Mainly submergent
 Barrier islands
 Glaciers shaped some
shorelines
 Eroding coasts
 Subsidence in most areas
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Gulf coast of U.S.
Mainly submergent
 Barrier islands
 Mississippi delta
 High rates of erosion
 Tectonic subsidence
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Pacific coast of U.S.
Mainly emergent
 Less erosion
 Mainly rocky shoreline
 Dams on rivers beach starvation
 Tectonically active
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U.S. coastal erosion, deposition
Fig. 11-17
Hard stabilization
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Groin and groin field
Perpendicular to shore
 Traps sand between groins
 Interrupts longshore current
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Jetty similar
Built to protect harbor entrance
 Sand on upcurrent side
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Breakwater
Parallel to shore
 Offshore
 Deposition behind breakwater
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Seawall
 Built on beach
 Parallel to beach
Erosion enhanced
seaward of wall
Seawall destroyed
Beach narrowed
Fig. 11-25
Alternatives to hard
stabilization
Limit construction near shore
 Relocate businesses/homes
destroyed by wave erosion
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End of Chapter 11: The Coast:
Beaches and Shoreline Processes