Sec 2 IDS, 2010
Coastal Management
Coastal processes
• Erosion, transportation and
deposition
• These processes are influenced by
waves and currents.
Coastal processes
• Waves are the primary agent of erosion.
• Erodes the coast by hydraulic action,
corrosion, attrition and solution.
• High energy waves will cause more
erosion.
Q: What factors determine the energy of waves?
Coastal processes
Wave energy is determined by
- wind speed
- fetch
- natural events (such as tsunamis)
- human events (such as sea traffic)
WAVES as agents of erosion
Waves form as wind blows over the ocean surface. Waves
increase in size with the following factors:
- greater wind speed – greater wind energy
- greater amounts of time that the wind blows
- greater fetch: the distance of water over which the wind
blows
WAVES as agents of erosion
How large waves cause coastal erosion?
• Swash is the movement of waves up the
shore towards the land. As the waves rush up
the shore, they transport materials up the
shore.
• As the swash moves up the coast, it loses
energy due to gravity and pulled back into the
sea. This backward movement is known as
backwash. They transport materials down to
the sea.
WAVES as agents of erosion
• If backwash is stronger than the swash –
wave energy is high – destructive waves –
removes materials from the beach rather than
deposit them
• Low energy or constructive waves lead to
deposition of materials. This occurs when
there is a strong swash and a weaker
backwash.
• Destructive waves – plunging breakers
• Constructive waves – spilling breakers
WAVES as agents of erosion
Long-term effect of wave translation:
Coastal erosion and beach modification
Identify the crest, swash and backwash of the wave.
Swash & Backwash
http://www.youtube.com/watch?v=SQDmbPX8KIg&feature=related
CURRENTS as agents of erosion
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Tides influence currents.
Tides are daily changes in the sea level due to
the movement of the Moon and the Sun.
Therefore the coast experiences a high/low tide
every 12 hours
Currents are flows of water that move either
horizontally or vertically in a certain direction
Changes in tides can create currents capable of
removing materials from the coast
Materials can be removed when water retreats
during low tides
CURRENTS as agents of erosion
Coastal processes - erosion
• Corrasion – when material carried by the waves are
thrown against the coast and breaking the rocks on
the coasts
• Attrition – when materials carried by the waves are
thrown against one another, breaking down into
smaller, smoother and rounded pieces
• Solution – soluble minerals in coastal rocks are
dissolved and removed by seawater
• Hydraulic Action – water surges into cracks, joints and
compress the air inside which exerts pressure on the
lines of weakness. When water retreats, there is a
release of pressure as trapped air expands. This
repeated action weakens and breaks down rocks
Transportation: Longshore currents
• Longshore currents:
currents flowing parallel
to the coast
• Horizontal movement of
a large volume of
seawater
 Due to oblique waves
(waves approaching the
coast at an angle)
 Due to differences in
water temperature
 Due to differences in
water salinity
 Global currents or
more local currents
Transportation: Longshore Drifts
• Swash pushing sediment onshore in an oblique
angle
• Backwash: Back flow of water and sediments
perpendicular to shore by gravity
• Net effect: Zigzag longshore beach drift
• Zigzag movement is one way in which sediments
are moved along the coast
• The movement of sediments is sometimes helped
by longshore currents which results in longshore
drift (movement of sediments parallel to the coast)
Animation: http://www.bbc.co.uk/schools/riversandcoasts/coasts/change_coast/pg_14_flash.shtml
Transportation: Longshore Drifts
The waves are hitting the beach at an angle. Therefore the
direction of the longshore current (the movement of water) and
the littoral drift (the movement of sand) are along the beach
toward the viewer. Littoral drift is measured by the volume of sand
moved per year (cubic meters).
Transportation: Longshore Drifts
Wave crest
Ethan left his soccer ball along the beach. Where will it
likely end up at the beach? Why?
Shore in 60 years
Shore in 30 years
Coastal
Erosion
Shore Today
~25% of homes
within 500 ft of the
world’s coastlines
(including shores of
the Great Lakes) will
fall victim to erosion
effects within the
next 60 years
Factors Influencing Coastal Erosion
• Erosion factors
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Tropical cyclones
Tsunamis
Tidal actions
Long-term rise of sea level
Human activities
Storms and Coastal Dynamics
• Storms are caused by low pressure
systems
– Low pressure cause a bulge of water to form
– Onshore winds, associated with storms,
approach the shore with unusually high tides
called a surge
– The exceptional force of the wind driven waves
and surge combine to erode beach faces,
dunes above the beach, and sometimes cliffs
behind the beach
– Hurricanes are power examples of these
storms
Coastal Erosion: Before Hurricane
Coastal Erosion: After Hurricane
U.S. Geological Survey
Cliff
• Waves and currents act to erode beaches and
eventually cliffs
• Cliffs are undercut
– Cliff face then slumps or slides off into the sea
– Cliff faces show landward retreat of the shoreline
• Wave refraction works to erode points of land
jutting out into the sea
– Wave base interaction with the ocean bottom slow the
progress of the wave
– Wave base encounters the bottom near a jutting point
before the coastline
Cliffs and shore platforms
Cliffs and shore platforms
http://www.bbc.co.uk/schools/riversandcoasts/coasts/change_coast/pg_07_flash.shtml
Cliff erosion: June 2002
Cliff erosion: October 2002
Stacks and Stumps
http://www.bbc.co.uk/schools/riversandcoasts/coasts/change_coast/pg_11_flash.shtml
Headlands and Bays
http://www.bbc.co.uk/schools/riversandcoasts/coasts/change_coast/pg_05_flash.shtml