Waves and Coastlines
Chapter 13
Oceans, Winds, Waves, and Coastlines
Geology Today
Barbara W.
Murck
Brian J.
Skinner
N. Lindsley-Griffin, 1999
Sea arches, California
Ocean Waves
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
Wave Properties
Ocean
Waves
A single particle at the surface moves in a
circular path as each wave passes.
Motion decreases to nothing, at a depth equal
to one-half the wavelength.
N. Lindsley-Griffin, 1999
Waves Near Shore
Waves “feel” bottom
As bottom shallows, waves are forced into smaller
space. Their shape distorts as the front slows down.
Waves Near Shore
Breakers form as the wave
oversteepens and topples
N. Lindsley-Griffin, 1999
Waves Change
Near Shore
Fig. 13.16, p. 387
N. Lindsley-Griffin, 1999
Erosion by Waves
The part of a wave that “feels” bottom first, slows down.
The part still in deeper water moves faster, and the entire wave
line curves, forcing wave energy towards headlands.
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
Erosion
by Waves
Headlands are eroded
by waves refracted
towards them.
Less resistant rock
(like shale) will erode
faster.
More resistant rock
(like sandstone) will
be left behind as sea
stacks.
N. Lindsley-Griffin, 1999
Erosion
by Waves
Sea stacks
Oregon coast
Sea cliffs, sea stacks
northern California
coast
N. Lindsley-Griffin, 1999
Waves move away
from the prevailing
wind direction,
approaching coastlines
at an angle.
N. Lindsley-Griffin, 1999
Transport of Sediments
As waves approach shore at an angle, they generate a current
parallel to shore called the longshore current.
Longshore Current
N. Lindsley-Griffin, 1999
Wind
Transport of
Sediments
Waves run up shore at
an angle
Water runs straight
back along steepest slope
Sediment particles are
transported along shore
in a zigzag path
Net motion = beach drift
N. Lindsley-Griffin, 1999
Transport of Sediments
Longshore current and beach drift combine to transport large
quantities of sediment parallel to the shore
Fig. 13.18, p. 389
N. Lindsley-Griffin, 1999
Sediment transport by longshore current
N. Lindsley-Griffin, 1999
Human Interference
Groins - Sand
is deposited by
longshore
current on the
up-current
side; eroded
by wave action
on the downcurrent side.
J. Wiley, J.R. Griffin, N. Lindsley-Griffin, 1999
Human Interference
Erosive effect of
wave action
decreased by
breakwater;
beach builds
out.
Asymmetry of
beach indicates
longshore drift
direction from
bottom to top.
J. Wiley, J.R. Griffin, 1999
N. Lindsley-Griffin, 1999
Transport of Sediments
Water carried onshore must find a path back to sea
Rip currents flow out to sea along narrow channels and
broad sheets
J. Wiley; N. Lindsley-Griffin, 1999
Rip currents transport sediments out to deeper
water, can create dangerous undertows.
J. Wiley; N. Lindsley-Griffin, 1999
Waves Near Shore
Wave-cut bench and
sea cliffs, Oregon
Breaking waves apply
force in a narrow plane
Erosion is greatest in a
knife-edge right at water
level
Effects:
wave-cut benches
undermined sea cliffs
sea arches and caves
sea lion perches
N. Lindsley-Griffin, 1999
Sea lions, La Paz, Mexico
Waves Near Shore
Wave-cut notches and sea caves
represent the early stage of
undermining that eventually
results in mass-wasting collapse
of cliffs and sea stacks
Sea Cave,
California
Wave-cut notch, New Brunswick
N. Lindsley-Griffin, 1999
Waves Near Shore
J. Wiley; N. Lindsley-Griffin, 1999
Coastal Landforms
Wavecut bench at water level, uplifted wavecut bench
- north of San Francisco, California
N. Lindsley-Griffin, 1999
Coastal Landforms
Fig. 13.19, p. 390
N. Lindsley-Griffin, 1999
Two wave-cut benches, with headlands and
beaches (Tongue Point, New Zealand)
Coastal
Landforms
Modern sea stack
N. Lindsley-Griffin, 1999
Tectonically uplifted wave-cut bench
with fossil sea stacks
(north of San Francisco, CA)
Fossil sea stack
Coastal Landforms
Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999
Coastal Landforms
Sea arches form where waves cut through headlands.
Eventually wave action and mass wasting will remove the
top of the arch and leave sea stacks behind.
N. Lindsley-Griffin, 1999
Coastal Landforms
Longshore drift deposits sand in a southpointing spit at the mouth of the Russian
River, northern California
Coastal Landforms
A tombolo is a spit or bar that connects an
island or sea stack to the mainland.
Big Sur coast, California
Coastal Landforms
J. Wiley, J.R. Griffin, 1999
Tombolo seen from space
Coastal Landforms
A baymouth bar blocks the
mouth of a bay.
(Oregon coast)
Human Interference
Houghton Mifflin 1998 N.
Lindsley-Griffin, 1999
Beach houses attacked by
hurricane, North Carolina