Transcript Coastal Zones and Processes Chapter 7

Chapter 7
Coastal Zones and Processes
Coastal zones are important to the United States:
30 of the 50 states abut a major body of water
(Atlantic or Pacific Ocean, Gulf of Mexico, one of
the Great Lakes), and those of 30 states are home
to about 85% of the nation’s population. About half
of that 85% live in the coastal regions and that
number is growing.
Figure 7.1
Nature of the Coastline
• Factors that impact coastlines:
– Tectonic setting
– Materials present at the shore
– Energy of water striking the coast
• Active margin – associated with active
plate tectonics
– Often cliffs found near active margins
• Passive margin – far from active plate
tectonics
– Often a broad shelf, beaches and with sandy
offshore beaches
Figures 7.2 and 7.5 a and b the Coast of California
Figures 7.3 a and b, a beach
A beach is a gently sloping surface washed over
by the waves and covered by sediment
The beach face is the portion regularly washed by
the waves as tides rise and fall
The berm is the flatter part of the beach landward
of the beach face
Nature of the Coastline
• Beach – a gently sloping surface washed over by the
waves and covered by sediment
– Sediments are carried by longshore currents and placed on
the beach by littoral drift
• Waves – generated by wind are small undulations on the
ocean surface
– Waves move along an ocean and water molecules move only
in orbits
– Waves disappear below wave base
– Orbits are larger nearest the surface and become smaller with
depth
– Once a wave ‘feels’ the bottom the orbits are interfered with
and breaking occurs
– Erosion of solid rock along a coast takes place because of
milling
• Ocean surfaces appear to move in and out relative to the
shore because of tides
– Tides occur because of the gravitational pull on the oceans by
the moon and sun
Figures 7.4 a and b Waves and breakers
Figures 7.6 a and b Spring tides: Sun, moon, and
earth are all aligned; Neap tides: When moon and
sun are at right angles, tidal extremes are reduced
Figure 7.7 longshore currents and their effect on sand
movement. Waves push water and sand ashore at an angle
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
Fig. 7.10 consequences of storm surges
Emergent and Submergent Coastlines
• Long-term sea-level changes often are associated
with plate tectonics
– Continental margins may uplift and produce an emergent
coastline
– Large rivers drain water and sediments from the
continental interior and form large deltas in adjacent
oceans
– Loading of the crust produces submergent coastlines
• Signs of changing relative sea level
– A wave-cut platform will form at coastlines
• A number of wave cut platforms is an indication of uplifting land
or dropping sea level
– A drowned valley can record changes in sea level also
Fig. 7.11 Wave-cut platforms
Figure 7.12 a drowned valley
Figure 7.14
Coastal Erosion and Stabilization
• A rise in sea level will cause coastal erosion and
will require stabilization
– Sand may be needed, from dredging, to replace sand
lost to long shore currents
– Breakwaters, jetties, or groins act to stabilize beaches
• These artificial barriers have erosion and deposition
problems associated with them also
• Projects to stabilize coastlines and beaches are
required because of changes in sea level
Figures 7.15 a and b
Figures 7.16 a, b, and c
Figures 7.17 a and c
Figure 7.17b
Figure 7.18
Figures 7.19 a and b
Cliff Erosion
• 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
Figures 7.20 a and b
Figure 7.21
Figures 7.22 a and b
Barrier Islands
• Long, low, narrow islands parallel to the
coastline
– Unstable environments
• Easily washed over because of low relief
• Sand migrates constantly with the wind
• Constantly retreating landward with time
Figure 7.24
Figure 7.25
Estuaries
• Body of water along a coastline, open to
the sea, in which tides rise and fall
– Often rivers bring freshwater into a mix with
sea water producing a brackish water
environment
• Water circulation is limited
– Estuaries do not tolerate major changes in
salinity or chemistry of the water
– Heavily impacted by pollution
Figure 7.26
Construction and Reconstruction in
High Energy Environments
• Coastal areas are easily damaged by major
storms that frequent our oceans and seas
• People have traditionally flocked to the coast to
live and work
• Property values are high along coastlines and at
risk
• Governments spend large amounts of money to
maintain coastal areas
• Recognize the coastal hazards: building
locations
Figure 7.27
Figure 7.28a
Figures 7.28 b and c