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Coastlines
Beaches and Coasts
• The coastal zone and beaches along the
continental margin are among the most
dynamic geologic setting on earth
• Human settlements are concentrated
along the coast and most of the world’s
population lives along the coast
• Human activities along the coast
oftentimes accelerate coastal erosion
processes
Coastal Zone
• World’s coastlines extend for 440,000km
• Coastal zone comprises < 0.05% of
landmass area
• > 50% world’s population lives within <
100km of coastline
• Most critical part of earth surface in terms
of global economy
Open Coasts
• Dominantly Erosional (West Coast)
– Narrow
– High relief
– Rocky
• Dominantly Depositional (East Coast)
– River deltas, barrier island systems
– Strand plains, reef coasts, glaciated coasts
– Complexity of depositional environments
EROSIONAL COAST
DEPOSITIONAL COAST
Nearshore Circulation
• Wave Propagation,
Refraction, Defraction,
Reflection
• Longshore Currents
• Rip Currents
WAVES
• Most water waves begin as wind (the generating
force) blows over the water and friction causes
wrinkles to form on the surface.
• These are called ripples or capillary waves and their
restoring force is surface tension.
• Small areas of capillary waves can appear and
disappear rapidly giving the impression that they are
jumping from point to point over the surface.
• These rapidly moving patches have been called cat’spaws.
Waves
• As more energy is transferred to the water, the waves
will grow in size.
• This will increase the roughness of the surface and
make it even easier for the wind to transfer energy to
the water so the waves will grow in size rapidly.
• As the waves grow, gravity will become the restoring
force and the waves will be called gravity waves.
• These are the waves we typically see at the coast or
any other water surface
Ideal Wave
Anatomy of a Wave:
Definitions
• The amount of time necessary for one wavelength to
pass by a stationary point is called the period of the
wave.
• Period is usually measured in seconds/cycle (cycle is
another term that can be used for wavelength in this
case). L=1.56T2m
• The number of wavelengths that pass a stationary
point in a unit amount of time is the wave frequency.
• Frequency is usually measured in cycles/second, the
reciprocal of the wave period.
Moving wave form sets water particles in circular motion
Size of Waves
• Actual transfer of energy from wind to
water complex, but size of wave depends
on:
– How fast the wind blows (velocity)
– The length of time the wind blows (duration)
– Distance over the water that the wind blows
(fetch)
Wave Speed
• The speed of a wave, its wavelength, and its period are
all related to one another
• Wave speed is usually represented with the symbol C
which stands for celerity which is derived from a Latin
word meaning swift.
• The speed of a wave (C ) is equal to its wavelength (L )
divided by its period (T ).
C =(L/T )
Wave Refraction
• As waves approach the shore they will
bend or refract because of their interaction
with the seafloor
• As the wave enters shallow water it will
slow down as a consequence of friction or
drag with the seafloor; however the wave
portion in deep water still moves relatively
faster
• The wave front then turns or refracts
Figure 9.12
Wave Diffraction
• As a wave passes an impermeable object
such as a jetty, breakwater or other type of
structure, the wave energy is spread along
the crest behind the obstacle
Wave Reflection
Reflection is dependent on the breaking state of the wave as well as
the slope and hardness of the surface. It can vary from 0% for breaking
waves on low-sloped, porous beaches to nearly 100% for non-breaking
waves in deep water on a hard, vertical surface such as a sea wall.
wave reflection.
Longshore Drift- Littoral Transport
• Sediment movement along the coastline is
done by longshore currents and is termed
longshore drift
• Longshore current confined to surf zone
Rip Currents
• Rip currents are very strong currents that
flow out to sea in the surf zone
• These narrow currents flow at speeds of a
few tens of centimeters per second and
can transport sediment
• Size, orientation, and spacing of rip
currents are related to wave conditions
and nearshore bathymetry
Rip currents represent the primary circulation system in the nearshore zone
The seaward extent of rip currents can vary from just beyond the line of
breaking waves to hundreds of yards offshore, extending up to a maximum of
2.5 times the surf zone width.
Video Animation.