Geology Reference
In-Depth Information
EXERCISE
1 1
Coastal Hazards
INTRODUCTION
Coastal zones are among the most highly populated
regions on Earth. In the United States, it has been
estimated that about 80 percent of the population
lives within 50 miles of a coast. Worldwide, the num-
ber is probably 50-60 percent. Coasts, therefore, have
great potential for either good or bad interactions
between natural processes and human activities. The
number of people living near coasts will rise in the
future, both as world population rises and as more
people move to cities, many of which are near coasts.
Processes in coastal zones are distinct from
other geologic processes such as earthquakes, vol-
canoes, floods, or landslides, in that coastal
processes are always active. Between earthquakes
there is no shaking. Between coastal storms, how-
ever, both water, in the form of waves and currents,
and sediments, in the form of sand (if we are looking
at a typical beach), are continuously in motion.
Indeed, coastal zones, like rivers, can be thought of
as integrated systems of both water movement and
sediment movement. Coasts, of course, can also be
impacted by other geologic processes such as land-
slides, river and coastal floods, earthquakes, and vol-
canic eruptions.
Thirty of the 50 states in the United States are
considered coastal states; that is, they have either
marine or Great Lakes shorelines. Nearly all of these
states have some coastal areas that are experiencing
moderate to severe erosion problems (Williams et al.,
1990), as shown in Figure 11.1.
Coasts are highly energetic zones, where dynamic
processes related to water and wind interact with
land-related processes. Natural and human activities
impact both water and land processes. Table 11.1
summarizes the causes of coastal land loss. Note that
sediment transport, and, in some cases, volcanic
eruptions, can also lead to coastal land gain.
Long-term observation of the interactions of peo-
ple and coasts has led to the development of several
principles, which are listed in Table 11.2.
The nature of coastal issues also depends on
whether a coast is predominantly broad sand
beaches or narrow beaches below steep cliffs. A
broad sand beach will be more vulnerable to high
waves, storm surges (water pushed toward shore by
the force of winds), and tsunamis, as it will let the
waves travel further inland. A steep cliff will break up
waves before they can travel inland. In some areas,
one type of coast will dominate for many miles; in
other areas coasts are diverse assemblages of rocky
cliffs, regolith bluffs, and small pocket beaches of
sand or gravel.
Worldwide sea-level rise, attributed partly to
global warming, has magnified the problems of the
ocean coasts in recent years. Natural long-term fluc-
tuations in lake levels have caused problems for
coastal installations and transportation systems on
large lakes.
In this exercise, we look at shoreline erosion (Part
A) and lake level trends (Part B) in the Great Lakes,
tsunami hazards along the west coast (Part C), and
storm and/or hurricane-related geologic hazards
along the east and south coasts (Part D). There is a
wide range of vulnerability of oceanic coasts to sea-
level rise. Throughout these exercises, look for
causes listed in Table 11.1 and applications of the
principles in Table 11.2. Note that the cases in this
exercise are general examples. Should you ever
decide to live along a coast, site-specific investiga-
tion of water, beach, and land processes will help you
assess the specific risks you will face.
PART A. COASTAL EROSION, LAKE ERIE
In many areas along the shores of the Great Lakes,
shore and bluff erosion present serious hazards to
structures. In this part of the exercise we examine
recession rates on the south shore of Lake Erie. (Refer
to Figure 11.1 for the location of Lake Erie.)
174
