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| Enlarge ImageBreaching the beach: The shoreline of Chatham, Mass., has been battered and reshaped by potent Atlantic winds and waves for centuries. This series of photos shows the barrier beach in 1985 (top), 1986 (middle), and 1995 (bottom), before and after a winter nor’easter
created a new inlet. Improved understanding of how
shorelines change over time can help coastal managers to better plan development and respond to recurrent or episodic threats. (Top photos courtesy of Duncan Fitzgerald, Boston University. Bottom photo by Joseph R. Melanson of skypic.com) |
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| Enlarge ImageScientists from the WHOI Department of Geology and Geophysics (above, and two following photos) are working with colleagues around the world to apply novel techniques to understanding how the shoreline is changing in response to rising sea level. Assistant Scientist Liviu Giosan (tan shirt) and Graduate Student Jonathan Woodruff extract sediments from the beach using a vibracorer. (Tom Kleindinst, WHOI Graphic Services) |
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| Enlarge ImageAssistant Scientist Jeff Donnelly holds a core of mud and sediment pulled up from a marsh. (Tom Kleindinst, WHOI Graphic Services) |
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| Enlarge ImageLiviu Giosan marks and prepares sediments from a split core for laboratory study. (Tom Kleindinst, WHOI Graphic Services) |
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| Enlarge ImageWHOI Associate Scientist Rob Evans (left) works with Engineering Assistant Matthew Gould to test a seafloor electromagnetic surveying instrument. This system is one of many new technologies developed to better map and monitor the coastal system. (Tom Kleindinst, WHOI Graphic Services) |
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| Enlarge ImageResearchers used LIDAR instruments to generate this seafloor map of the Piscataqua River inlet between Kittery Point and New Castle Island on the border between New Hampshire and Maine. New imaging techniques are allowing coastal scientists to visualize the geologic framework of the coastline, track major movements of sediment, and project how the shoreline might change with time. (Larry Mayer, University of New Hampshire) |
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| Storm surges from nor'easters and hurricanes carry sand from the ocean floor and beach face (left) over the dunes and into marshes and lagoons behind the beach (right). Between major storms, the beach builds up while marsh muds and peat slowly cover up the sand layers. Researchers have found that when they take sediment cores from the marsh, they can use the sand overwash layers to find and date intense storms that are not necessarily recorded in history books. (Animation courtesy of Geological Sciences at Brown University) |
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| Enlarge ImageTrouble in Paradise: This sequence (top, August 12, 1997) shows how the seas advanced and property was destroyed in Floralton Beach, Fla. Vegetation and dune lines were completely wiped away after Hurricane Frances (middle photo, September 8, 2004), leaving shoreline properties directly exposed to coastal surges from Hurricane Jeanne (bottom photo, September 29, 2004). (U.S. Geological Survey) |
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Rob L. Evans,
Associate Scientist
Geology and Geophysics Department
Woods Hole Oceanographic Institution Nae man can tether time or tide.
— Robert Burns
For the past century, the pace and density of development near the
ocean has been unprecedented, and much of it is incompatible with the
dynamic nature of the shoreline. More than $3 trillion are invested in
dwellings, resorts, infrastructure, and other real estate along the
Atlantic and Gulf coasts of the United States, and more than 155
million people live in coastal counties. The coastal population is
estimated to rise by 3,500 people per day.
Yet, as the
devastating hurricane season of 2004 showed, there is a price to be
paid for living at sea level and building on sand. Even without extreme
storms, the shoreline naturally advances and retreats on scales ranging
from seconds to millennia.
As a growing population hugs the
coast, understanding the complex processes by which coastlines change
has never been more relevant and more important to our well-being.
A rising tide
Changes to the shoreline are inevitable and inescapable. Shoals and
sandbars become islands and then sandbars again. Ice sheets grow and
shrink, causing sea level to fall and rise as water moves from the
oceans to the ice caps and back to the oceans. Barrier islands rise
from the seafloor, are chopped by inlets, and retreat toward the
mainland. Even the calmest of seas are constantly moving water, sand,
and mud toward and away from the shore, and establishing new shorelines.
Coastal changes have accelerated in the past century. Although sea
level has been rising since the end of the last glaciation (nearly
11,000 years), the rate of sea-level rise has increased over the past
200 years as average temperatures have increased. Global warming has
added water to the oceans by melting ice in the polar regions. But the
greater contributor is thought to be thermal expansion of the oceans—a
rise in sea level due to rising water temperature. Sea level has risen
10 to 25 centimeters in the past 100 years, and it is predicted to rise
another 50 centimeters over the next century (with some estimates as
high as 90 centimeters). Whether or not human activities have
contributed to the change, the sea is definitely rising, and it
jeopardizes our rapidly growing coastal communities.
Coastal
erosion accelerates as sea level rises. Erosion decreases the value of
coastal properties because it decreases “the expected number of years
away from the shoreline,” as researchers and underwriters put it. This
quiet loss of U.S. property value amounts to $3 to $5 billion per year.
Then there is the actual loss of property, including structures, which
amounts to as much as $500 million a year.
Eroding
coastlines are also at greater risk from storm damage. Property damage
from hurricanes along the eastern U.S. is estimated to average $5
billion per year, with the cost in 2004 alone estimated at more than
$21 billion. Such calculations rarely account for the long-term costs
of flooding and erosion, damage to natural landforms or ecosystems, and
lost recreation and tourism opportunities.
There is
significant debate about how to best manage coastal resources to cope
with the changing shoreline. When and where will the coast change? And
what, if anything, should we do about it?
Billions of tax
dollars are being spent to restore and protect our wetlands, maintain
our beaches and waterways, and rebuild coastal infrastructure. For
example, the State of Louisiana is proposing to spend $14 billion over
the next 40 years to restore coastal barriers along the Mississippi
River delta. Despite these vast sums of money, very little is being
invested in basic research that can improve our ability to predict
shoreline change, inform managers in their decision-making, or provide
more accurate risk assessment.
More than just a beach problem
The coast is an incredibly complex system, of which beaches are only
one part. All aspects of the system—rivers, estuaries, dunes, marshes,
beaches, headlands, the surf zone, and the seafloor—influence and
respond to the others. But many parts of the system have yet to be
studied in sufficient detail to fully understand their role in
shoreline change.
Beach erosion threatens property near the
shoreline, but it also profoundly influences a critical part of our
coastal ecosystem: the marshes. Tidal marshes in estuaries and behind
barrier islands are the dominant habitat along the Atlantic Coast of
the U.S., and they are particularly vulnerable to rising sea level.
Marshes are ecologically and economically important because they
regulate the exchange of water, nutrients, and waste between dry land
and the open ocean. They filter and absorb nutrients and pollutants,
and buffer coastlines from wave stress and erosion. And tidal marshes
provide nursery grounds for countless species of fish and
invertebrates. They are among the most biologically productive
ecosystems in the world, producing more biomass per area than most
other ecosystems.
Whereas researchers have been studying the
fertility and biologic productivity of marshes for many years, they
have only recently started to determine how these coastal wetlands grow
and erode. As sea level rises, we need to know the threshold at which
marshes can no longer grow fast enough to keep pace with rising waters.
If the rate of sea-level rise doubles over the next 100 years—or
quadruples, as some more extr eme models project—tidal marshes and
coastal ecosystems will likely experience unprecedented changes. Some
may disappear altogether. Our coast may return to its condition at the
end of the last glaciation, 11,000 years ago, when sea level was rising
too fast for marshes to be established.
New toys for the sandbox
Although there has been progress in many areas of coastal geology, our
understanding of the fundamentals of shoreline change has been limited
by the lack of a broad and integrated scientific focus and a lack of
resources. In many locations, we cannot answer simple questions, such
as where sand goes after it is eroded from the beaches, or what role
underwater formations play in determining which areas of the coast will
erode and accrete.
Recent advances in technology make this an
ideal time to tackle some of these science problems. Our ability to
map, measure, model, and understand the fundamental processes shaping
the shoreline has never been better. We can gather a more precise
record of long-term trends in shoreline motion, which were previously
identifiable only through historical records, such as by comparing old
nautical charts with modern ones.
Several instruments have
allowed us to make dramatic improvements in our ability to map the
beach and seafloor, and what lies beneath.
Light detection
and ranging (LIDAR) allows researchers to use radar-like pulses of
light to map beaches and the bottom of clear, shallow waters. It
provides maps that are precise to within 10 centimeters.
Global Positioning System receivers and monuments use satellites to
track the movement of shoreline features from day to day in three
dimensions. These devices allow positions to be obtained accurately
within a few centimeters. High-resolution seismic imaging,
ground-penetrating radar, and electromagnetic resistivity instruments
employ sound and electrical signals and the properties of rocks and
sediments to “see” the layers beneath the beach surface and seafloor.
They can probe to depths of tens of meters. The processes that
shape our coasts occur on a variety of time and space scales. Linking
these diverse processes is a challenge that requires a system-wide,
multidisciplinary approach. It also requires the willingness of
policymakers and coastal managers to support basic research and to pay
more attention to its results. There are several clear, process-based
science problems that need to be addressed before we can accurately
predict shoreline change.
How is the shoreline changing with time and geography?
Many studies of nearshore processes have been conducted on long,
straight shorelines, and scientists have made some progress in
understanding how waves, sandbars, and currents interact in simplified
situations. But the mechanisms driving shoreline change are not well
understood in regions where the nearshore region has complicated
seafloor topography, inlets, or headlands—which means most beaches.
Waves traveling across the continental shelf are reflected, refracted,
amplified, and scattered by underwater topography, and research has
suggested that erosional hotspots along the coast are often the result
of these seafloor formations. Banks, shoals, canyons, and even
different types of sediment cause waves to decay and break differently.
Wave-induced currents cause sediments to erode and accrete and reshape
the seafloor near the coast, changing how future waves will evolve.
The complex dynamics between waves
and seafloor evolution need to be unraveled before we can make
predictions about changes to the shoreline. We need to build a network
of wave-measuring instruments along different coastlines and feed those
measurements into computer models
of how the shoreline reacts to waves and currents. These models will
help us make predictions about how water might circulate and how
sediment might move in response to those different underwater
formations.
How will barrier islands respond to sea level rise?
Barrier islands account for approximately 15 percent of the world’s
shoreline, and they dominate the Atlantic and Gulf coasts of the United
States. Built by the action of waves and currents, these narrow ridges
of sand usually run parallel to the mainland, protecting the coast from
erosion. These natural barriers are bisected by tidal inlets and
channels, and they shelter back-barrier salt marshes, tidal flats and
deltas, and mangroves. Though usually no more than a few meters above
sea level, these islands are often covered with human developments.
The long-term fate of today’s barrier islands is dependent on future
sea-level rise. The latest report of the Intergovernmental Panel on
Climate Change predicts that global warming will cause sea level to
rise by 50 to 90 centimeters in the next 100 years. At the higher end
of these estimates, many back-barrier marshes will struggle to keep up
with the inundation.
Sand will move from barrier beaches to
the nearshore underwater regions in order to re-establish equilibrium
between the slope of the beach and the higher tides and waves. The
water levels and topography behind these barriers could gradually or
catastrophically change. Inlets will become more dynamic, while deltas
will enlarge. Whole marshlands might disappear, being converted to
tidal lagoons or bays. Catastrophic amounts of sand could be lost from
some beaches.
To properly protect barrier beaches—or learn
when to abandon them—we need to map and monitor them regularly. We also
need to dig into the sediments of the coast to piece together the
history of past changes. Such efforts will allow us to model how tidal
systems are likely to respond to rising ocean waters.
What is the impact of storms?
Intense storms such as hurricanes, nor’easters, and typhoons often
result in substantial loss of life and resources, yet we know little
about the processes that govern their formation, intensity, and
movement. Nor do we know much about their history, due to the
relatively short history of reliable weather observations. With little
data on how coastal systems have responded to storms in the past, we
have been ill-equipped to model and project how climate and sea-level
change will affect future storm trends.
Geological
investigations of coastal environments can provide long-term records of
environmental change. Evidence of past storms can be found in
back-barrier sediments: When a storm washes sand over the dunes and
into back bays and marshes, it forms dateable layers in the muddy
sediments. Mapping regional occurrences of these “overwash” deposits
can allow researchers to estimate the storminess of years past and help
improve models of the probability of future storm strikes.
How is the shore linked to the shelf?
In the past, studies of the beach and surf zone were usually separated
and studied independently from what was happening further out on the
continental shelf. It has largely been a logistical problem, as the
region from 0 to 10 meters of water depth can present some of the most
difficult areas to sample. These areas are too shallow for most ships,
and too deep or turbulent for researchers on foot. However, the zone
from 10 meters above sea level to 10 meters below is perhaps the most
physically dynamic and ecologically vulnerable. If we are to fully
understand the coastal system, we have to eliminate the imaginary
barrier between the shallows and the deep.
What can the past tell us about the future of the shoreline?
Natural records from a variety of sources—deep-sea sediments, ice
sheets, corals, calcium carbonate formations in caves—show that abrupt
environmental changes are common in Earth’s history. Sea level rise
rates during the past 11,000 years have been uncharacteristically
steady, and may be ripe for change. That our coastlines have developed
such remarkable diversity during these stable times (environmental
stress usually promotes diversity; calm promotes homogeneity) suggests
the shape of the shore is affected by a lot more than sea level.
Coasts are complex, transitional environments that respond to changes
in both continental and deep ocean processes. The sediments on- and
offshore are great recorders of this variability, yet these archives
have yet to be systematically studied and compared with what we have
learned from inland and deep-sea environmental proxies for climate.
The high stakes of high water
Resource managers and civic leaders have a great responsibility for
managing the coast and human use of it, but they have not always had
the best information available to make scientifically sound decisions.
The link between sea-level rise and shoreline change, while undoubtedly
present, remains controversial.
For this reason, coastal
managers want more reliable data on sea-level rise. They need studies
that apply our knowledge of basic processes to more complex,
human-altered shorelines (seawalls, bulkheads, jetties, groins). They
need scientific analyses of the effects of adding and removing
sediments from the shoreline.
There is no doubt that sea level
is rising. It’s not the first time, and the rate at which it is
changing may or may not be unusual. What is different this time is that
humans have congregated along the shoreline without much awareness of
how much or how soon the sands might shift. We have the ability to make
better decisions about our lives along the coast. We just have to start
making the measurements that can provide the right answers.
This
article is the result of a workshop held at WHOI in April 2004. Many
colleagues who attended that meeting—-too numerous to list—contributed
to this article. I’d like to thank them. —R.L. Evans
A rising tide along the coastNever before has coastal research been more relevant
and more important to society’s well being. The
numbers are staggering:
More than 155 million people (53 percent of the
population) reside in U.S. coastal counties comprising
less than 11 percent of the land area of the lower
48 states.
- More than 155 million people (53 percent of the
population) reside in U.S. coastal counties comprising
less than 11 percent of the land area of the lower
48 states.
- Roughly 1,500 homes are lost to erosion each year.
- Nearly 180 million people visit the U.S. coast every
year, and coastal states account for 85 percent of
U.S. tourism revenues. The tourism industry is the
nation’s largest employer and second largest
contributor to gross domestic product.
- 71 percent of annual U.S. disaster losses are the
result of coastal storms.
- Close to 350,000 homes and buildings are located
within 150 meters of the ocean.Within 60 years, one
out of every four of those structures will be destroyed.
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Posted: November 16, 2004 [top] |
