Geology Reference
In-Depth Information
taken at two different times, in 1954 and in 1973 after
19 years of erosion (Figure 11.4), show the changes for
the same area of the coast.
Between 1954 and 1973 a portion of the shore
eroded away. Shore erosion occurs through the combined
effects of waves and currents. Waves, especially dur-
ing storms, attack the bluffs along the shore, causing
them to fail. (See Exercise 8.) Waves and wave-formed
currents transport the sand along the shore. These
longshore currents flow in a predominant direction.
Any obstruction traps the sediment carried by the cur-
rents on its up-current side (the side from which the
current is coming). On the down-current side of the
obstruction the currents pick up a new load of sand.
Because sand, in the form of a wide beach, is an effec-
tive buffer against storm waves, groins cause deposi-
tion of sand that slows bluff erosion on the up-current
side. On the down-current side additional erosion
occurs.
4. Refer to Figure 11.2. What was the maximum change in
feet from high to low water level at Toledo during the
December 1987 storm?
5.
If the water-level differences between Buffalo and Toledo
were due mainly to atmospheric pressure differences, over
which end of the lake would the low-pressure area have
been at midnight on December 15,1987?
6.
Refer to Figure 11.3 and determine average annual rates of
coastal retreat between 1876 and 1937. Measure distances
perpendicular to the coast and use the scale given. Record
measured distances, too.
a.
Half-inch from the east margin of the diagram? Rate =
ft/yr
b.
Half-inch from west margin of the diagram? Rate =
ft/yr
c. Coast with the least change? Rate =
ft/yr
QUESTIONS 11, PART A
7. Compare the 1954 and 1973 photos (Figure 11.4). Describe
the physical and cultural changes that have occurred along
the coast. (The photos are almost the same scale; Figure 11.8
shows lake level change.)
Refer to Figure 11.1 to answer Questions 1 and 2. For Ques-
tions 7 and 8 you will need a sheet of tracing paper the size
of the photos (about 4" by 8"). (Questions 7-18 are modified
from Kennedy and Mayer, 1979.)
1.
Which coast of the United States (north, south, east, or
west) has the least problem with severe erosion?
8.
The straight objects jutting into the water are
groins,
struc-
tures designed to protect the bluffs by trapping sand and
gravel. What changes have occurred from 1954 to 1973 in the
number of groins?
2.
According to Figure 11.1, which Great Lake has the highest
percentage of U.S. shore in the severe erosion category?
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9.
List the physical processes involved in the changes that
3.
Many factors affect rates of coastal erosion (see Part B).
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have occurred on this coast. Our next task is to compare the
1954 and 1973 shorelines and bluff bases by sketching on
tracing paper these and other coastal features. (Another
approach is to make a transparency copy of the 1954 photo,
