CHAPTER 13 Shorelines
332
C H A P T E R
T H I R T E E N
Shorelines
in Review
The shore is the area extending between the lowest tide level and
the highest elevation on land that is affected by storm waves. The coast
extends inland from the shore as far as ocean-related features can be
found. The shore is divided into the foreshore and backshore. Seaward
of the foreshore are the nearshore and offshore zones.
A beach is an accumulation of sediment found along the landward
margin of the ocean or a lake. Among its parts are one or more berms
and the beach face. Beaches are composed of whatever material is
locally abundant and should be thought of as material in transit along
the shore.
Waves are moving energy and most ocean waves are initiated by the
wind. The three factors that influence the height, wavelength, and period
of a wave are (1) wind speed, (2) length of time the wind has blown, and
(3) fetch, the distance the wind has traveled across open water. Once
waves leave a storm area, they are termed swells, which are symmetrical, longer-wavelength waves.
As waves travel, water particles transmit energy by circular orbital
motion, which extends to a depth equal to one half the wavelength.
When a wave travels into shallow water, it experiences physical
changes that can cause the wave to collapse, or break, and form surf.
Wave erosion is caused by wave impact pressure and abrasion (the
sawing and grinding action of water armed with rock fragments). The
bending of waves is called wave refraction. Owing to refraction, wave
impact is concentrated against the sides and ends of headlands.
Most waves reach the shore at an angle. The uprush (swash) and
backwash of water from each breaking wave moves the sediment in a
zigzag pattern along the beach. This movement, called beach drift, can
transport sand hundreds or even thousands of meters each day.
Oblique waves also produce longshore currents within the surf zone
that flow parallel to the shore.
Features produced by shoreline erosion include wave-cut cliffs (which
originate from the cutting action of the surf against the base of coastal
land), wave-cut platforms (relatively flat, benchlike surfaces left behind
by receding cliffs), sea arches (formed when a headland is eroded and
two caves from opposite sides unite), and sea stacks (formed when the
roof of a sea arch collapses).
Some of the depositional features formed when sediment is moved
by beach drift and longshore currents are spits (elongated ridges of
sand that project from the land into the mouth of an adjacent bay),
baymouth bars (sandbars that completely cross a bay), and tombolos
(ridges of sand that connect an island to the mainland or to another
island). Along the Atlantic and Gulf coastal plains, the shore zone is
characterized by barrier islands, low ridges of sand that parallel the
coast at distances of 3 to 30 kilometers offshore.
Local factors that influence shoreline erosion are (1) the proximity of
a coast to sediment-laden rivers, (2) the degree of tectonic activity, (3) the
topography and composition of the land, (4) prevailing winds and weather
patterns, and (5) the configuration of the coastline and nearshore areas.
Hard stabilization involves building solid, massive structures in an
attempt to protect a coast from erosion or prevent the movement of
sand along the beach. Hard stabilization includes groins (short walls
constructed at a right angle to the shore to trap moving sand),
breakwaters (structures built parallel to the shore to protect it from the
force of large breaking waves), and seawalls (armoring the coast to prevent waves from reaching the area behind the wall). Alternatives to hard
stabilization include beach nourishment, which involves the addition of
sand to replenish eroding beaches, and relocation of damaged or threatened buildings.
Because of basic geological differences, the nature of shoreline erosion
problems along America’s Pacific and Atlantic coasts is very different. Much
of the development along the Atlantic and Gulf coasts has occurred on
barrier islands, which receive the full force of major storms. Much of
the Pacific Coast is characterized by narrow beaches backed by steep
cliffs and mountain ranges. A major problem facing the Pacific shoreline is a narrowing of beaches caused in part by the natural flow of
materials to the coast being interrupted by dams built for irrigation
and flood control.
Although damages caused by a hurricane depend on several factors,
including the size and population density of the area affected and the
nearshore bottom configuration, the most significant factor is the
strength of the storm itself. The Saffir–Simpson scale ranks the relative
intensities of hurricanes. A category 5 storm is most severe and a
category 1 storm is least severe. Damage caused by hurricanes can be
divided into three categories: (1) storm surge, which is most intense on
the right side of the eye where winds are blowing toward the shore,
occurs when a dome of water sweeps across the coast near the point
where the eye makes landfall; (2) wind damage; and (3) inland flooding,
which is caused by torrential rains that accompany most hurricanes.
One common classification of coasts is based upon changes that
have occurred with respect to sea level. Emergent coasts, often with
wave-cut cliffs and wave-cut platforms above sea level, develop either
because an area experiences uplift or as a result of a drop in sea level.
Conversely, submergent coasts, with their drowned river mouths,
called estuaries, are created when sea level rises or the land adjacent
to the sea subsides.
Tides, the daily rise and fall in the elevation of the ocean surface,
are caused by the gravitational attraction of the Moon and, to a lesser
extent, by the Sun. Near the times of new and full moons, the Sun and
Moon are aligned, and their gravitational forces are added together to
produce especially high and low tides. These are called the spring tides.
Conversely, at about the times of the first and third quarters of the
Moon, when the gravitational forces of the Moon and Sun are at right
angles, the daily tidal range is less. These are called neap tides.
Tidal currents are horizontal movements of water that accompany
the rise and fall of tides. Tidal flats are the areas that are affected by the
advancing and retreating tidal currents. When tidal currents slow after
emerging from narrow inlets, they deposit sediment that may eventually create tidal deltas.
332
C H A P T E R
T H I R T E E N
Shorelines
in Review
The shore is the area extending between the lowest tide level and
the highest elevation on land that is affected by storm waves. The coast
extends inland from the shore as far as ocean-related features can be
found. The shore is divided into the foreshore and backshore. Seaward
of the foreshore are the nearshore and offshore zones.
A beach is an accumulation of sediment found along the landward
margin of the ocean or a lake. Among its parts are one or more berms
and the beach face. Beaches are composed of whatever material is
locally abundant and should be thought of as material in transit along
the shore.
Waves are moving energy and most ocean waves are initiated by the
wind. The three factors that influence the height, wavelength, and period
of a wave are (1) wind speed, (2) length of time the wind has blown, and
(3) fetch, the distance the wind has traveled across open water. Once
waves leave a storm area, they are termed swells, which are symmetrical, longer-wavelength waves.
As waves travel, water particles transmit energy by circular orbital
motion, which extends to a depth equal to one half the wavelength.
When a wave travels into shallow water, it experiences physical
changes that can cause the wave to collapse, or break, and form surf.
Wave erosion is caused by wave impact pressure and abrasion (the
sawing and grinding action of water armed with rock fragments). The
bending of waves is called wave refraction. Owing to refraction, wave
impact is concentrated against the sides and ends of headlands.
Most waves reach the shore at an angle. The uprush (swash) and
backwash of water from each breaking wave moves the sediment in a
zigzag pattern along the beach. This movement, called beach drift, can
transport sand hundreds or even thousands of meters each day.
Oblique waves also produce longshore currents within the surf zone
that flow parallel to the shore.
Features produced by shoreline erosion include wave-cut cliffs (which
originate from the cutting action of the surf against the base of coastal
land), wave-cut platforms (relatively flat, benchlike surfaces left behind
by receding cliffs), sea arches (formed when a headland is eroded and
two caves from opposite sides unite), and sea stacks (formed when the
roof of a sea arch collapses).
Some of the depositional features formed when sediment is moved
by beach drift and longshore currents are spits (elongated ridges of
sand that project from the land into the mouth of an adjacent bay),
baymouth bars (sandbars that completely cross a bay), and tombolos
(ridges of sand that connect an island to the mainland or to another
island). Along the Atlantic and Gulf coastal plains, the shore zone is
characterized by barrier islands, low ridges of sand that parallel the
coast at distances of 3 to 30 kilometers offshore.
Local factors that influence shoreline erosion are (1) the proximity of
a coast to sediment-laden rivers, (2) the degree of tectonic activity, (3) the
topography and composition of the land, (4) prevailing winds and weather
patterns, and (5) the configuration of the coastline and nearshore areas.
Hard stabilization involves building solid, massive structures in an
attempt to protect a coast from erosion or prevent the movement of
sand along the beach. Hard stabilization includes groins (short walls
constructed at a right angle to the shore to trap moving sand),
breakwaters (structures built parallel to the shore to protect it from the
force of large breaking waves), and seawalls (armoring the coast to prevent waves from reaching the area behind the wall). Alternatives to hard
stabilization include beach nourishment, which involves the addition of
sand to replenish eroding beaches, and relocation of damaged or threatened buildings.
Because of basic geological differences, the nature of shoreline erosion
problems along America’s Pacific and Atlantic coasts is very different. Much
of the development along the Atlantic and Gulf coasts has occurred on
barrier islands, which receive the full force of major storms. Much of
the Pacific Coast is characterized by narrow beaches backed by steep
cliffs and mountain ranges. A major problem facing the Pacific shoreline is a narrowing of beaches caused in part by the natural flow of
materials to the coast being interrupted by dams built for irrigation
and flood control.
Although damages caused by a hurricane depend on several factors,
including the size and population density of the area affected and the
nearshore bottom configuration, the most significant factor is the
strength of the storm itself. The Saffir–Simpson scale ranks the relative
intensities of hurricanes. A category 5 storm is most severe and a
category 1 storm is least severe. Damage caused by hurricanes can be
divided into three categories: (1) storm surge, which is most intense on
the right side of the eye where winds are blowing toward the shore,
occurs when a dome of water sweeps across the coast near the point
where the eye makes landfall; (2) wind damage; and (3) inland flooding,
which is caused by torrential rains that accompany most hurricanes.
One common classification of coasts is based upon changes that
have occurred with respect to sea level. Emergent coasts, often with
wave-cut cliffs and wave-cut platforms above sea level, develop either
because an area experiences uplift or as a result of a drop in sea level.
Conversely, submergent coasts, with their drowned river mouths,
called estuaries, are created when sea level rises or the land adjacent
to the sea subsides.
Tides, the daily rise and fall in the elevation of the ocean surface,
are caused by the gravitational attraction of the Moon and, to a lesser
extent, by the Sun. Near the times of new and full moons, the Sun and
Moon are aligned, and their gravitational forces are added together to
produce especially high and low tides. These are called the spring tides.
Conversely, at about the times of the first and third quarters of the
Moon, when the gravitational forces of the Moon and Sun are at right
angles, the daily tidal range is less. These are called neap tides.
Tidal currents are horizontal movements of water that accompany
the rise and fall of tides. Tidal flats are the areas that are affected by the
advancing and retreating tidal currents. When tidal currents slow after
emerging from narrow inlets, they deposit sediment that may eventually create tidal deltas.