Geology Reference
In-Depth Information
water races ahead until it too encounters wave base. The net
effect of this oblique approach is that waves bend so that they
more nearly parallel the shoreline, a phenomenon known as
wave refraction
(Figure 16.6).
Even though waves are refracted, they still usu-
ally strike the shoreline at some angle, causing the water
between the breaker zone and the beach to fl ow parallel to
the shoreline. These
longshore currents
, as they are called,
are long and narrow and fl ow in the same general direction
as the approaching waves. These currents are particularly
important in transporting and depositing sediment in the
nearshore zone.
the amount of water builds up until the excess moves out to
sea through the breaker zone.
Relief on the seafl oor plays an important role in deter-
mining the location of rip currents. They commonly de-
velop where wave heights are lower than in adjacent areas,
and differences in wave height are controlled by variations in
water depth. For instance, if waves move over a depression,
the height of the waves over the depression tends to be less
than in adjacent areas, forming the ideal environment for rip
currents.
Beaches, the most familiar coastal landforms, are ab-
sent, poorly developed, or restricted to protected areas on
seacoasts where erosion rather than deposition predominates.
Erosion creates steep or vertical slopes known as
sea cliffs.
During storms, these cliffs are pounded by waves (hydraulic
action), worn by the impact of sand and gravel (abrasion)
(
Rip Currents
Waves carry water into the nearshore zone,
so there must be a mechanism for mass transfer of water
back out to sea. One way in which water moves seaward
from the nearshore zone is in
rip currents
, narrow sur-
face currents that fl ow out to sea through the breaker zone
(
Figure 16.7). Surfers commonly take advantage of rip
currents for an easy ride out beyond the breaker zone, but
these currents pose a danger to inexperienced swimmers.
Some rip currents fl ow at several kilometers per hour, so if a
swimmer is caught in one, it is useless to try to swim directly
back to shore. Instead, because rip currents are narrow and
usually nearly perpendicular to the shore, one can swim
parallel to the shoreline for a short distance and then turn
shoreward with little diffi culty.
Rip currents are circulating cells fed by longshore cur-
rents that increase in velocity from midway between each rip
current (Figure 16.7a). When waves approach a shoreline,
◗
Figure 16.8), and more or less continuously eroded by dis-
solution involving the chemical breakdown of rocks by the
solvent action of seawater. Tremendous energy from waves
is concentrated on the bases of sea cliffs and is most effective
on those composed of sediments or highly fractured rocks.
In any case, the net effect of these processes is erosion of the
sea cliff and retreat of the cliff face landward.
◗
Wave intensity and the resistance of shoreline materials
to erosion determine the rate at which a sea cliff retreats
landward. A sea cliff of gla-
cial drift on Cape Cod, Mas-
sachusetts, erodes as much as
30 m per century, and some parts
of the White Cliffs of Dover in
England retreat landward at
more than 100 m per century. By
comparison, sea cliffs of dense
igneous or metamorphic rocks
erode and retreat much more
slowly.
Sea cliffs erode mostly as a
result of hydraulic action and
abrasion at their bases. As a
sea cliff is undercut by erosion,
the upper part is left unsup-
ported and susceptible to mass
wasting processes. Thus, sea
cliffs retreat little by little, and
as they do, they leave a beveled
surface called a
wave-cut plat-
form
that slopes gently seaward
(
Figure 16.9). Broad wave-
cut platforms exist in many
areas, but invariably the water
over them is shallow because
◗
◗
Figure 16.6
Wave Refraction Wave refraction (wave crests are indicated by dashed lines). These
waves are refracted as they enter shallow water and more nearly parallel the shoreline. The waves
generate a longshore current that fl ows in the direction of wave approach, from upper left to lower
right (arrow) in this example.
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