Geoscience Reference
In-Depth Information
As noted previously, the various combinations of wave/
beach interactions are too numerous to describe in this text.
The simplistic pattern just described nicely explains the pat-
tern of littoral drift along the coast of Southern California
where the strong, persistent longshore current freely moves
sediment from northwest to southeast. Along the eastern
shore of Lake Michigan, however, seasonal prevailing winds
shift from northwesterly in winter to southwesterly in sum-
mer. This shift in wind direction causes sediment to move
up and down the coast to some degree. Still another pattern
is observed along the concave southeastern coast of Texas
where waves approach from the southeast (Figure 19.12). In
the central part of this portion of the Texas coast, the waves
approach in a perpendicular fashion. On the flanks of this sec-
tion of coast, however, the curvature of the shore causes the
waves to approach obliquely. This interaction results in op-
posing longshore currents that converge in the central part of
the coast.
Backwash moves
sand down beach
perpendicularly
Swash moves
sand up beach
at an angle
Beach
Beach drift
Longshore current
Wave crests move
toward shore at
an oblique angle
Figure 19.11 Littoral drift.
A longshore current develops in
places where waves strike the coast obliquely. Sediment sub-
sequently moves down the shore as littoral drift. Littoral drift in-
cludes beach drift, which is the transport of sediment down shore
due to the zig-zag pattern of swash and backwash, and longshore
drift, which is the movement of sediment by the longshore current.
is deflected downwind when it interacts with the shore.
This deflection causes the formation of a
longshore cur-
rent
that flows parallel to the shore, one that is capable of
moving sediment down the coast in a process called
long-
shore drift
.
Obliquely approaching waves also cause an interesting pat-
tern of sediment transport on the beach proper. In these situa-
tions the surf also flows onto the beach (known as
swash
) at an
angle. After the swash reaches its apex on the beach, it flows
back to the ocean at a right angle to shore as
backwash
. Sedi-
ment is picked up in this fashion and transported down the coast
in a zig-zag pattern known as
beach drift
. Taken together, the
combined processes of longshore drift and beach drift are called
littoral drift
.
KEY CONCEPTS TO REMEMBER
ABOUT WATER MOVEMENT ALONG
COASTLINES
1.
Coastlines are the places on Earth where the hydro-
sphere, lithosphere, and atmosphere intersect over a
large spatial extent.
2.
The amount of water in the oceans fluctuates with
time due to adjustments in the hydrologic cycle. These
changes are called eustatic changes. Many coastlines
show evidence of long-term eustatic change in the form
of rias and fjords.
Tides
The formation of tides involves the complex geometric inter-
action of the Earth, Moon, and Sun. These interactions cause
gravitational forces that pull on the Earth's oceans in such a
way as to cause predictable oscillations in local sea level. To
see how these forces appear in animated form, go to the
Geo
Media Library
and select
Tides
. This animation will allow you to
better visualize the various forces acting on the Earth's oceans
and how they move water. Once you complete it, be sure to
answer the questions at the end to test your understanding of
this concept.
Longshore current
The current that develops parallel to a
coast when waves approach the coast obliquely and forward
momentum is deflected.
Beach drift
Sediment that is transported in the surf zone by
swash and backwash, which form due to the oblique approach
of waves.
Transport of sediment by the longshore
Littoral drift
Sediment that is transported through the com-
bined processes of longshore drift and beach drift.
Longshore drift
current.
