BATHYMETRY OF THE OFFSHORE
BOTTOM
Bathymetry is a term used to describe the depth
of the sea at a given location. The sea bottom has a topography, just as
the land has a topography. This topography greatly influences the strength
of the waves that strike a coastal area. If the bathymetry indicates shallow
water, waves will have less energy when they hit the shore. Deep water
close to shore allows strong waves to approach the coast, potentially doing
more damage to the beach area. For more bathymetry information see the
Army Corps of Engineering
Web Site.
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Beach nourishment (replenishment) is the process of moving sand from the off shore continental shelf and directing it onto the beach. Sand is dredged from the off shore shelf, about five miles from the shore and is loaded onto a barge which carries it close to the shore. The sand is sprayed onto a beach with the intent of widening the beach and increasing its height. The process of beach nourishment is very expensive, and the results of the effort are usually short lived in that winter storms usually remove the sand added by the nourishment process after about three years.
CONSTRUCTION OF SEDIMENT TRAPPING UPLAND DAMS
Upland dams are used to control the flow of water along
a river, on its route to the ocean. The sediments that are moved by the
water are trapped behind the dam, even though the water is allowed to flow
through in a controlled fashion. The trapping of this sediment deprives
the beach of material that could be used to build it up through wave action.
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CONSTRUCTION OF HARBORS IN NEAR SHORE WATERS
The presence of harbors in near shore waters interferes
with the long shore currents that carry sand from one place to another.
In addition, the construction of a harbor changes the normal wave patterns.
This could lead to undesirable effects on the coastal area on either side
of the harbor.
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CONSTRUCTION OF GROINS AND JETTIES
Groins and jetties are walls built perpendicular
to the shoreline. They are designed to trap sand that is moving along the
shore due to the long shore current. A groin usually extends to the end
of the surf zone while a jetty extends further into an inlet to stabilize
a navigation channel. The construction of both groins and jetties severely
affects the flow of sand moved by the long shore currents, depriving down
stream beaches of the sand needed for replenishment.
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Tidal entrances to harbors become shallower because
of the accumulation of sediments that are dropped when water entering the
harbor from the shore slows down. In order to maintain sufficient depth
of water for shipping traffic to freely pass through the harbor, dredging
must be done. Dredging removes sediments from the harbor floor and moves
them to another location.
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EFFECTS OF WAVES, CURRENTS, TIDES AND WAVES
Waves result from the movement of water caused by the rotation of the Earth and the winds that blow over the surface of the water. Waves lose energy when they come closer to shore, hit the ocean bottom and then reach the beach.
Currents move bodies of water from one location to another, like a river flowing within the ocean. An example of a current is the Gulf Stream which brings water from Florida, along the east coast of the United States, and then off the Cape Hatteras coast en route to Europe. {currents picture}
Tides are the daily cyclic movement of water towards and away from the coast caused by gravitational forces exerted by the sun and moon.
GEOLOGICAL CHARACTERISTICS OF THE SHORE
This includes factors such as rock type, geologic history,
and topology of the area. These factors influence the rate of weathering
and the effectiveness of erosional factors such as moving water and winds.
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HARDENING OF SHORELINES WITH SEAWALLS OR REVETMENTS
A seawall is a structure built on a beach, parallel
to the shoreline, designed to protect buildings from the action of waves.
A revetment is sometimes referred to as shore armor since it involves the
placement of a seawall directly on the side of a sand dune that is exposed
to wave action. A revetment is often made of large rocks.
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A significant factor in the decision about where to build a house, business or other structure is the availability of property insurance. With the onset of the National Flood Insurance Plans (NFIP) (http://www.fema.gov/fema/nfip.htm) in 1968, people began to build on property that had been thought to be uninsurable because of the high premiums charged by private insurers. With low cost insurance available, people built on property that had a higher probability of flood damage. Through the Federal Emergency Management Agency (FEMA), FIRM maps (flood insurance rate map) were created by NFIP for areas where there was any risk of flooding, even it might be classified as a "hundred year flood" (one chance of a flood in 100 years). New construction on one of the flood-plains had to meet a set of criteria before insurance could be purchased.
What was the impact of the availability of this insurance on the communities at risk for flooding? In the ten years before the availability of federal flood insurance, 186 deaths were reported from floods, with $2.2 billion in damages. In the ten years following the start of the federal flood insurance program there were 411 deaths reported from floods, with $4.7 billion in damages. (Savadove, p.178) What conclusion can be drawn from such a statistic? Discussion of this has lead to the understanding that the availability of flood insurance has indirectly encouraged people to build homes and businesses in areas that should not be used for those purposes.
Similar concerns about insurability of areas at risk from hurricanes and earthquakes have been raised. If a natural catastrophe hits an unpopulated area, there is a feeling that no real harm has been done, nature is at work as it has been forever. However, if a natural disaster hits a community and there is injury, death and property damage, it had to have been preceded by human decisions to develop in that hazard prone area. (Godschalk, p. 5)
Today, the federal government is trying to spread
catastrophic insurance risk. This means that private insurers are once
more becoming part of the flood insurance picture. Reinsurance policies,
private insurance pools, off-shore insurance purchases and trading options
on a catastrophe index at the Chicago Board of Trade are some of the different
approaches being taken to spread out the expense associated with paying
for catastrophic insurance for floods and other natural disasters. (see
Insurance Services Office, Inc. for more
information.)
RISING SEA LEVEL (from Global Environmental Change-Past, Present and Future, Karl Turekian, Prentice Hall, NJ, 1996)
To assess the scope of changes in sea level, scientists have developed methods to interpret the geologic record left by prehistoric events. One of the most useful markers for sea level studies has been the growth of coral found around islands such as Barbados, in the Caribbean Sea. The coral, Acropora palmata, is restricted to a growth zone within 2 meters of the sea surface. By observing growth patterns of this coral, direct information can be learned about sea level change over time. This approach is similar to the study of tree rings which is used to learn about the age of a tree and the environmental factors present during the tree's life. Based on studies with coral, it is known that sea level has changed drastically over the last 11,000 years.
Factors that have influenced sea level include:
It is known that about 18,000 years ago the sea level had been 120 meters (400 feet) lower than it is today. The coasts had very different shapes. Around 11,000 years ago there was a large melt of glacial ice which led to a rapid rise in sea level. The flooding that occurred led to the creation of islands (such as Long Island, NY and Nantucket Island, MA.) and the covering of many river valleys which became ocean canyons. Sea level has risen about 40 cm (16 inches) in the past century and is projected to rise another 60 cm (24 inches) in the next century.
The impact of the flooding of coastal communities
that will result from this change in sea level serves as the basis for
the following discussion of risk assessment.
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SAND-SHARING SYSTEMS OF BEACHES, DUNES AND OFFSHORE BARS
Sand that is removed from a beach can usually
be traced to offshore sand bars or to inland sand dunes. There is movement
of sand between these three areas of the beach environment. Transfer of
sand between these areas does not represent sand loss. If sand is moved
beyond the depth of about 15 meters (45 feet) it is said to be lost to
the system.
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Beaches with larger sand particles tend to erode at a slower rate than fine sandy beaches because they are permeable which leads to less backwash to carry sand back to the water. Sandy beaches tend to have more erosion due to wave action because they are less permeable to water and the larger backwash carries away as much sand as it delivers.
The shape of the sand particles correlates with the permeability of the beach. Irregularly shaped particles can pack together more tightly, making a less permeable beach. Particles with regular shapes pack together leaving uniform pore spaces which serve to increase permeability.
Particle density affects the rate at which the particles
settle onto the continental shelf. The more dense the particle, the closer
to shore it will be when it sinks to the bottom. The less dense particles
can travel further from the shore before settling out.
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Beach material can vary in size from very fine sand
(0.005 cm) to small pebbles (1.5 cm). Sand is brought to the shore from
the continental shelf, rivers and eroding cliffs, sand dunes, as well as
from other beaches through the action of long shore currents. (Pilkey,
1996, p. 24)
Sinks for the sand include continental shelf accumulations
of sand that are in water at depths greater than 30 meters (100 feet)
and sand that is carried into deep ocean canyons. This sand is below the
"reach" of the waves and cannot be moved and returned to the beach. In
addition, sand that is blown inland is also lost from the beach. (Psuty,
1997)
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