Geology Reference
In-Depth Information
Exercise 11 • Coastal Hazards
179
base. Draw the first line parallel to and along the largest
groin. Mark this line C. Draw two more parallel lines (A and
B) one-inch apart to the west. Draw and label three lines (D,
E, F), again parallel to line C and one-inch apart, to the east.
13.
Compare and explain the changes in the coast west and
east of the groins.
14.
From your observations of the beach in the top photo and
the net change in the coast, what direction does the domi-
nant longshore current flow?
e.
Remove the 1954 tracing paper and place it over the
1973 photo. Align the streets where you can, so the trac-
ing and the photo can be accurately compared.
15.
What really protects the bluffs along the coast from the
energy of the waves when groins are installed?
f.
Using a colored pencil, trace the base of the bluff with a
solid line and the shoreline with short dashes. Label the
colored-pencil lines "1973."
In this part of the exercise we will use the changes in the
position of the base of the bluff, as measured along the six
reference lines, to determine the rate of recession of the coast
and to predict the future position of part of the coast.
11.
What is the maximum width of beach lost between 1954
and 1973?
16.
At each of the reference lines (A-F) on the tracing paper or
overlay, measure the distance between the solid lines (base of
the bluff) you drew for 1954 and the lines you drew for 1973.
Use the scale given in the photos to measure the distances.
Record the distances in the appropriate boxes below.
12.
Shade the area between the 1954 and 1973 solid lines rep-
resenting the base of the bluff. What does the shaded area
represent?
West lines
East lines
Distance between
1954 and 1973
bluffs, in ft
Profile
line
Distance between 1954
and 1973 bluffs, in ft
Rate of bluff
change, in ft/yr
Profile
line
Rate of bluff
change, in ft/yr
A
D
B
E
C
F
Average
Average
17.
Divide each measured distance between 1954 and 1973
bluffs by 19 years (the time between 1954 and 1973) to obtain
the average annual rate of change in the position of the bluff
for each profile line. Record these data in the indicated spots
in the table in Question 16.
20.
Plot on your tracing paper map, using long dashes, the
expected 1993 coastline between the two lines with the most
rapid recession. You have now used your understanding of
past geologic processes to predict future changes. Since 1993
is now past and a photograph is available (Figure 11.5), you
can determine how close your prediction is to the actual
change that occurred.
18.
Determine the average distance and rate for the western
and eastern part of the coast. What geologic processes can
explain the differences in the average rates?
=9
19.
Assuming the annual rates determined at the two refer-
ence lines with the most rapid recession, calculate how much
recession would be expected at these lines by 1993.
a.
At line
21.
Place your tracing paper map over the 1993 photograph
(which is approximately the same scale), and use another
color to draw a solid line to mark the location of the base of
the bluff in 1993.
, the recession in feet expected between
1973 and 1993 is
