CHAPTER 13 Shorelines
324
Atlantic and Gulf Coasts
Much of the development along the
Atlantic and Gulf coasts has occurred on
barrier islands. Typically, barrier islands
consist of a wide beach that is backed by
dunes and separated from the mainland by
marshy lagoons. The broad expanses of
sand and exposure to the ocean have made
barrier islands exceedingly attractive sites
for development. Unfortunately, development has taken place more rapidly than our
understanding of barrier island dynamics
has grown.
Because barrier islands face the open
ocean, they receive the full force of major
storms that strike the coast. When a storm
occurs, the barriers absorb the energy of
the waves primarily through the movement
of sand. This process and the dilemma that
results have been described as follows:
Waves may move sand from the beach to offshore areas
or, conversely, into the dunes; they may erode the dunes,
depositing sand onto the beach or carrying it out to sea;
or they may carry sand from the beach and the dunes into
the marshes behind the barrier, a process known as overwash. The common factor is movement. Just as a flexible
reed may survive a wind that destroys an oak tree, so the
barriers survive hurricanes and nor’easters not through
unyielding strength but by giving before the storm.
This picture changes when a barrier is developed for
homes or a resort. Storm waves that previously rushed
harmlessly through gaps between the dunes now encounter
buildings and roadways. Moreover, because the dynamic
nature of the barrier is readily perceived only during
storms, homeowners tend to attribute damage to a particular storm, rather than to the basic mobility of coastal
barriers. With their homes or investments at stake, local
residents are more likely to seek to hold the sand in place
and the waves at bay than to admit that development was
improperly placed to begin with.*
Pacific Coast
In contrast to the broad, gently sloping
coastal plains of the Atlantic and Gulf
coasts, much of the Pacific Coast is characterized by relatively narrow beaches that
are backed by steep cliffs and mountain
ranges. Recall that America’ s western
margin is a more rugged and tectonically
active region than the eastern margin.
Because uplift continues, a rise in sea level
in the West is not so readily apparent.
Nevertheless, like the shoreline erosion
problems facing the East’ s barrier islands, West Coast
difficulties also stem largely from the alteration of a
natural system by people.
A major problem facing the Pacific shoreline, and
especially portions of Southern California, is a significant narrowing of many beaches. The bulk of the
sand on many of these beaches is supplied by rivers
that transport it from the mountainous regions to the
coast. Over the years this natural flow of material to
the coast has been interrupted by dams built for irrigation and flood control. The reservoirs effectively
trap the sand that would otherwise nourish the beach
environment. When the beaches were wider, they
served to protect the cliffs behind them from the force
of storm waves. Now, however, the waves move across
the narrowed beaches without losing much energy
and cause more rapid erosion of the sea cliffs.
Although the retreat of the cliffs provides material to replace some of the sand
impounded behind dams, it also endangers homes and roads built on the bluffs. In addition, development atop the cliffs aggravates the problem. Urbanization increases runoff,
which, if not carefully controlled, can result in serious bluff erosion. Watering lawns and
gardens adds significant quantities of water to the slope. This water percolates downward
toward the base of the cliff, where it may emerge in small seeps. This action reduces the
slope’ s stability and facilitates mass wasting.
Shoreline erosion along the Pacific Coast varies considerably from one year to the next,
largely because of the sporadic occurrence of storms. As a consequence, when the infrequent but serious episodes of erosion occur, the damage is often blamed on the unusual
storms and not on coastal development or the sediment-trapping dams that may be great
distances away. If, as predicted, sea level rises at an increasing rate in the years to come,
increased shoreline erosion and sea-cliff retreat should be expected along many parts of the
Pacific Coast. This issue is addressed in the section on “Sea-Level Rise” in Chapter 20,
p. 516.
C O N C E P T C H E C K 1 3 . 8
Briefly describe what happens when storm waves strike an undeveloped barrier island.
How might building a dam on a river that flows to the sea affect a coastal beach?
Hurricanes—The Ultimate Coastal Hazard
The whirling tropical cyclones that occasionally have wind speeds exceeding 300 kilometers (185 miles) per hour are known in the United States as hurricanes—the greatest storms
on Earth (FIGURE 13.23). In the western Pacific they are called typhoons, and in the Indian
Ocean they are simply called cyclones. No matter which name is used, these storms are
among the most destructive of natural disasters. When a hurricane reaches land, it is
capable of annihilating coastal areas and killing tens of thousands of people.
Fortunately, when Hurricane lke made landfall along the Texas coast near Galveston on
September 13, 2008, loss of life was relatively small. Because forecasts were accurate and
warnings were timely, tens of thousands were safely evacuated from the most vulnerable
areas before the storm arrived. Nevertheless, damages to the coastal zone and inland areas
were substantial and costly (FIGURE 13.24).
On the positive side, hurricanes provide essential rainfall over many areas they cross.
Consequently, a resort owner along the Florida coast may dread the coming of hurricane
season, whereas a farmer in Japan may welcome its arrival.
2
1
*Frank Lowenstein, Beaches or Bedrooms—The
Choice as Sea Level Rises, Oceanus 28 (No. 3, Fall
1985), 22.
D I D Y O U K N O W ?
Hurricane season is different in
different parts of the world.
People in the United States are
usually most interested in
Atlantic storms. The Atlantic
hurricane season officially
extends from June 1st through
November 30th. More than
97 percent of tropical activity in
that region occurs during this
six-month span. Statistically, the
“heart” of the season is August
through October and peak
activity is in early to midSeptember.
324
Atlantic and Gulf Coasts
Much of the development along the
Atlantic and Gulf coasts has occurred on
barrier islands. Typically, barrier islands
consist of a wide beach that is backed by
dunes and separated from the mainland by
marshy lagoons. The broad expanses of
sand and exposure to the ocean have made
barrier islands exceedingly attractive sites
for development. Unfortunately, development has taken place more rapidly than our
understanding of barrier island dynamics
has grown.
Because barrier islands face the open
ocean, they receive the full force of major
storms that strike the coast. When a storm
occurs, the barriers absorb the energy of
the waves primarily through the movement
of sand. This process and the dilemma that
results have been described as follows:
Waves may move sand from the beach to offshore areas
or, conversely, into the dunes; they may erode the dunes,
depositing sand onto the beach or carrying it out to sea;
or they may carry sand from the beach and the dunes into
the marshes behind the barrier, a process known as overwash. The common factor is movement. Just as a flexible
reed may survive a wind that destroys an oak tree, so the
barriers survive hurricanes and nor’easters not through
unyielding strength but by giving before the storm.
This picture changes when a barrier is developed for
homes or a resort. Storm waves that previously rushed
harmlessly through gaps between the dunes now encounter
buildings and roadways. Moreover, because the dynamic
nature of the barrier is readily perceived only during
storms, homeowners tend to attribute damage to a particular storm, rather than to the basic mobility of coastal
barriers. With their homes or investments at stake, local
residents are more likely to seek to hold the sand in place
and the waves at bay than to admit that development was
improperly placed to begin with.*
Pacific Coast
In contrast to the broad, gently sloping
coastal plains of the Atlantic and Gulf
coasts, much of the Pacific Coast is characterized by relatively narrow beaches that
are backed by steep cliffs and mountain
ranges. Recall that America’ s western
margin is a more rugged and tectonically
active region than the eastern margin.
Because uplift continues, a rise in sea level
in the West is not so readily apparent.
Nevertheless, like the shoreline erosion
problems facing the East’ s barrier islands, West Coast
difficulties also stem largely from the alteration of a
natural system by people.
A major problem facing the Pacific shoreline, and
especially portions of Southern California, is a significant narrowing of many beaches. The bulk of the
sand on many of these beaches is supplied by rivers
that transport it from the mountainous regions to the
coast. Over the years this natural flow of material to
the coast has been interrupted by dams built for irrigation and flood control. The reservoirs effectively
trap the sand that would otherwise nourish the beach
environment. When the beaches were wider, they
served to protect the cliffs behind them from the force
of storm waves. Now, however, the waves move across
the narrowed beaches without losing much energy
and cause more rapid erosion of the sea cliffs.
Although the retreat of the cliffs provides material to replace some of the sand
impounded behind dams, it also endangers homes and roads built on the bluffs. In addition, development atop the cliffs aggravates the problem. Urbanization increases runoff,
which, if not carefully controlled, can result in serious bluff erosion. Watering lawns and
gardens adds significant quantities of water to the slope. This water percolates downward
toward the base of the cliff, where it may emerge in small seeps. This action reduces the
slope’ s stability and facilitates mass wasting.
Shoreline erosion along the Pacific Coast varies considerably from one year to the next,
largely because of the sporadic occurrence of storms. As a consequence, when the infrequent but serious episodes of erosion occur, the damage is often blamed on the unusual
storms and not on coastal development or the sediment-trapping dams that may be great
distances away. If, as predicted, sea level rises at an increasing rate in the years to come,
increased shoreline erosion and sea-cliff retreat should be expected along many parts of the
Pacific Coast. This issue is addressed in the section on “Sea-Level Rise” in Chapter 20,
p. 516.
C O N C E P T C H E C K 1 3 . 8
Briefly describe what happens when storm waves strike an undeveloped barrier island.
How might building a dam on a river that flows to the sea affect a coastal beach?
Hurricanes—The Ultimate Coastal Hazard
The whirling tropical cyclones that occasionally have wind speeds exceeding 300 kilometers (185 miles) per hour are known in the United States as hurricanes—the greatest storms
on Earth (FIGURE 13.23). In the western Pacific they are called typhoons, and in the Indian
Ocean they are simply called cyclones. No matter which name is used, these storms are
among the most destructive of natural disasters. When a hurricane reaches land, it is
capable of annihilating coastal areas and killing tens of thousands of people.
Fortunately, when Hurricane lke made landfall along the Texas coast near Galveston on
September 13, 2008, loss of life was relatively small. Because forecasts were accurate and
warnings were timely, tens of thousands were safely evacuated from the most vulnerable
areas before the storm arrived. Nevertheless, damages to the coastal zone and inland areas
were substantial and costly (FIGURE 13.24).
On the positive side, hurricanes provide essential rainfall over many areas they cross.
Consequently, a resort owner along the Florida coast may dread the coming of hurricane
season, whereas a farmer in Japan may welcome its arrival.
2
1
*Frank Lowenstein, Beaches or Bedrooms—The
Choice as Sea Level Rises, Oceanus 28 (No. 3, Fall
1985), 22.
D I D Y O U K N O W ?
Hurricane season is different in
different parts of the world.
People in the United States are
usually most interested in
Atlantic storms. The Atlantic
hurricane season officially
extends from June 1st through
November 30th. More than
97 percent of tropical activity in
that region occurs during this
six-month span. Statistically, the
“heart” of the season is August
through October and peak
activity is in early to midSeptember.