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OECS Regional Engineering Workshop
September 29 – October 3, 2014
Coastal Erosion and Sea Defense:
Introduction to Coastal Dynamics
David A Y Smith
OBJECTIVE
• To provide a basic understanding of the primary coastal processes: What
are the wave and current processes that dominate the nearshore zone
and how do they interact with the shoreline?
TOPICS
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Understanding Waves
Waves in motion
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Tides and Tidal Currents
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Diffraction
Refraction
Shoaling/Breaking
Reflection
How are tides generated
Tidal currents
Sediment Transport
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Alongshore transport
Cross-shore sediment transport
TYPES OF WAVES
WAVES IN MOTION – WIND WAVE PROPERTIES
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• In shallow water, the orbital motion
becomes a “flat ellipse” which
eventually becomes a to-and-fro
current
• Waves under the influence of winds are
called wind waves while those that
outlast this influence are called swells
Waves generated by winds
transmit energy through the water
column.
In deep water, an orbital current
flow is created below the crest of
waves which reverses direction
under the trough
WAVES IN MOTION – WAVE PROPERTIES
WAVES IN MOTION - REFRACTION
• REFRACTION
– Bending of wave crests as one part of the wave crest reaches shallow water
first, slowing down while the other part “catches up”.
• The distance between wave
crests (wave length) decreases
but the time between the wave
crests (wave period) does not
change.
• As waves approach shallow
water:
decreases
L
C = remains constant
decreases
T
WAVES IN MOTION - REFRACTION
• Wave orthogonal – imaginary
line drawn perpendicular to the
crest of the wave. This
essentially shows the direction
of progression of the waves.
• Waves react to the nearshore
contours and bend until they
become almost perpendicular to
the contours
• This causes focusing on some
sections of the shoreline such as
headlands and dispersion in
other areas such as bays
WAVES IN MOTION – SHOALING AND BREAKING
– Increase or decrease in wave height due to the change in depths as the wave
approaches shallow water
– As the waves bunch closer together in shallow water, they increase in height
– Waves increase in height until they become so steep they break
– As the waves break, the Kinetic energy (from the forward motion of the waves) is
converted to Potential Energy (increase in height of water) during breaking
Breaking
Wave crest
MSL
Seabed
WAVES IN MOTION - DIFFRACTION
• Diffraction occurs when waves pass
through a gap or against the edge of a
physical structure
WAVES IN MOTION - REFLECTION
• Vertical surfaces absorb, transmit and
reflect wave energy.
• Reflection occurs when the wave energy
that impacts on to a physical structure is
not totally absorbed by the structure or
transmitted through the structure.
• Reefs and rock breakwaters absorb and
transmit some of the energy to their lee
side (in the incident direction of the
waves) but they also reflect a portion of
this energy.
• Vertical walls reflect most of the energy
in the opposite direction of the incident
waves.
• Reflection can reduce or increase total
wave energy depending on where in the
wave cycle the impact occurs
THE RIVER OF SAND…WAVES TO SEDIMENT TRANSPORT
– Waves are generated in deep water by winds.
– As waves approach the shoreline they undergo
several transformation processes such as
refraction, shoaling and breaking.
– As waves break, the dissipated energy causes an
increase in water level
– Different water levels along a shoreline cause an
alongshore current.
– The breaking process also disturbs the sediments
on the seafloor, bringing them into suspension.
– The alongshore currents move the sediments
along the shoreline.
SEDIMENT TRANSPORT
• ALONGSHORE MOVEMENT
– When waves break, their Kinetic Energy is transferred to Potential
Energy in the form of an increased water level. The variations in water
level along the shoreline cause water flow (currents) from areas of
high to low water level. The turbulence from the breaking waves
brings sediments on the seabed into suspension. The wave-generated
currents transport these sediments along the shoreline.
• CROSS-SHORE MOVEMENT
– Waves breaking on a shoreline over a period of time shape the crossshore profile into a stable equilibrium profile. Higher waves result in
steeper beach slope formations. Therefore, the profile of beaches tend
to vary with the seasons.
– Larger than normal waves (eg. Hurricanes) can have a profound impact
on a beach profile, moving sand from the shoreline to the offshore
area and eroding beach dunes.
SEDIMENT TRANSPORT -ALONGSHORE TRANSPORT
• ALONGSHORE – ZONE OF TRANSPORT
Wave crests
Depth contours
Point of breaking
Surf Zone
shoreline
S.T.
SEDIMENT TRANSPORT -CROSS-SHORE
• CROSS-SHORE - SEASONAL BEACH PROFILE CHANGES
Dune
Storm or Winter Swell Waves
Summer waves
Area of erosion
Area of accretion
Cross-shore
sand movement
Steeper beach profile
after storm or during
seasons of high waves
SEDIMENT TRANSPORT -SOURCES AND SINKS
• SEDIMENT SOURCES
– Rivers
– Coral Reefs
– Seagrass/algae
– Erosion of cliffs
• SEDIMENT SINKS
– Canyons
– Shelves
– Beaches
– Structures
SEDIMENT TRANSPORT – IMPACTS OF STRUCTURES
• IMPACT OF NATURAL AND ARTIFICIAL STRUCTURES IN THE SEA ON SEDIMENT
MOVEMENT AND BEACH FORMATION.
– Reefs – Natural offshore formations that create calm areas in their lee
thereby reducing the potential for cross-shore movement of sand (and
alongshore movement to a lesser extent), often resulting in build up of
sand on the beach.
– Breakwaters – artificial shore-parallel reef structures that create build
up of sand depending on their level of submergence and distance from
the shoreline.
– Groynes – Shore-perpendicular structures that interrupt the
alongshore movement of sand. Groynes tend to create build up on the
upstream end and erosion on the downstream end of the dominant
sediment transport direction.
GROYNE IMPACT
BREAKWATER IMPACT
Breakwaters
TIDES
• Three principal forces are involved
in the production of tides: (1)
gravitational attraction between
the moon and the earth; (2)
gravitational attraction between
the sun and the earth; and (3) the
force of the earth's gravity, which
pulls every particle of the earth
toward the earth's center. The
moon is mainly responsible for the
tides (its effect is about 2.2 times
as great as the sun's).
TIDAL CYCLES
• Tidal currents and wave-driven
currents interact to produce
resulting nearshore current flows
• In aggressive wave environments
wave-driven currents usually
dominate in the nearshore
• Tidal currents and oceanic currents
always dominate in the offshore
OCEANIC CURRENTS
• Tidal currents and oceanic currents always dominate in the offshore