Geology Reference
In-Depth Information
just fi ve years to replenish the beach and build a seawall at
Ocean City, Maryland. Furthermore, barriers retard erosion
only in the area directly behind them; Galveston Island west
of its seawall has been eroded back about 45 m.
Another option, adopted by both Atlantic City, New
Jersey, and Miami Beach, Florida, is to pump sand onto the
beaches to replace that lost to erosion. This, too, is expensive
as the sand must be replenished periodically because erosion
is a continuing process. In many areas, groins are constructed
to preserve beaches, but unless additional sand is artifi cially
supplied to the beaches, longshore currents invariably erode
sand from the downcurrent sides of the groins.
Geo-Recap
Chapter Summary
Shorelines are continuously modifi ed by the energy of
waves and longshore currents and, to a limited degree, by
tidal currents.
The gravitational attraction of the Moon and Sun causes
the ocean surface to rise and fall as tides twice daily in
most shoreline areas.
Waves are oscillations on water surfaces that transmit
energy in the direction of wave movement. Surface waves
affect the water and seafl oor only to wave base, which is
equal to half the wavelength.
Little or no net forward motion of water occurs in waves
in the open sea. When waves enter shallow water, they
are transformed into waves in which water moves in the
direction of wave advance.
Wind-generated waves, especially storm waves, are
responsible for most geologic work on shorelines, but
waves can also be generated by faulting, volcanic explo-
sions, and rockfalls.
Breakers form where waves enter shallow water and
disrupt the orbital motion of water particles. The waves
become oversteepened and plunge forward or spill onto
the shoreline, thus expending their kinetic energy.
Waves approaching a shoreline at an angle generate a
longshore current. These currents are capable of consid-
erable erosion, transport, and deposition.
Narrow surface currents called rip currents carry water
from the nearshore zone seaward through the breaker
zone.
Many shorelines are characterized by erosion. Such
shorelines have sea cliffs and wave-cut platforms. Other
features commonly present include sea caves, sea arches,
and sea stacks. Depositional coasts, on the other hand, are
characterized by long sandy beaches, deltas, and barrier
islands.
Beaches, the most common shoreline depositional fea-
tures, are continuously modifi ed by nearshore processes,
and their profi les generally exhibit seasonal changes.
Spits, baymouth bars, and tombolos all form and grow as
a result of longshore current transport and deposition.
Barrier islands are nearshore sediment deposits of uncer-
tain origin. They parallel the mainland but are separated
from it by a lagoon.
The volume of sediment, or nearshore sediment budget,
in a nearshore system remains rather constant unless the
system is somehow disrupted, as when dams are built
across streams that supply sand to the system.
Submergent and emergent coasts are defi ned on the basis
of their relationship to changes in sea level.
Coastal fl ooding during storms by waves and storm surge
is an ongoing problem in many areas.
Important Terms
backshore (p. 416)
barrier island (p. 420)
baymouth bar (p. 418)
beach (p. 416)
beach face (p. 416)
berm (p. 416)
breaker (p. 412)
crest (wave) (p. 410)
emergent coast (p. 426)
fetch (p. 411)
foreshore (p. 416)
headland (p. 415)
longshore current (p. 414)
marine terrace (p. 415)
 
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