Storms, especially hurricane and typhoons, kill many people,
do great damage to coastal structures, and erode the coast. Great storms
in the middle ages removed tens of kilometers of Germany's coasts in
a few years. A storm
surge in the North Sea in 1953 flooded
a large area of the Netherlands and parts of eastern England,
causing great damage and 2100 deaths. Hurricanes in the Gulf of Mexico
move barrier islands, flood nearby land, and destroy coastal structures. "The greatest
tropical system disaster this century occurred in Bangladesh
in November 1970. Winds coupled with a storm surge killed between 300,000-
500,000 people." And the hurricane that struck Galveston Island
on the evening and night of 8 September 1900 killed more than 6,000.
Left: Panorama of Galveston after hurricane
hit the city on 8 September 1900. Right: Another view.
From Texas State Archives and http://www.1900storm.com/.
The Rosenberg Library in Galveston has many more photographs online.
hurricanes have caused severe damage to the coast of the United States.
Storm surges are caused by hurricanes, typhones,
and extra-tropical storms, The damage results from waves riding on a
rise of sea level, the storm
surge. Many processes influence the height of the surge.
- Wind speed. The force of the wind on the water is proportional to
wind speed squared.
- Wide shallow continental shelf offshore.The surge is directly proportional
to the width of the shallow water, and inversely proportional to the
depth. Essentially, hurricane force winds blowing onshore across tens
of kilometers of shallow water pile up water along coasts. The stronger
the wind, the shallower the water, and the greater the extent of shallow
water, the higher the pile of water. In some cases it can exceed 2-5
meters. In Galveston, Isaac Cline, the chief meteorologist, measured
a rise of sea level greater than 16 feet above mean sea level.
- Tides. If the surge occurs at high tide,
the tide adds to the height of the surge.
- Shape of the coast and winds at other areas along the coast. The
surge can travel along the coast, causing high waters away from the
- Barometric pressure. Sea level rises one centimeter per millibar
drop is air pressure. Strong storms can have 50 millibar drop in pressure,
causing a 50 cm rise in water levels on top of levels due to wind and
All these factors are included in numerical forecasts of storm surge
height produced by meteorological agencies such as the U.S. National
Oceanic and Atmospheric Administration NOAA.
The NOAA Storm
Surge Atlas for Florida shows how much of the land inland from
the beach will be flooded by storms of various strengths. Other maps
online. More information about storm surges is in the online textbook
Introduction to Physical Oceanography under storm
surges. NOAA has a web page that describes the different categories
of storms according to the Saffir-Simpson
To get a more graphic understanding of storm surges, look
at these simulated storm surge pictures for Brunswick,
Orleans, LA, Brooklyn,
Beach, NC and Manteo,
NC, all from NOAA
storm surge page. Here is a simulation
of the storm surge (348 KBytes) generated by a hurricane approaching
the Mississippi coast generated by the SLOSH (Sea, Lake, and Overland
Surges from Hurricanes) numerical forecasting model. Keep in mind, the
pictures are misleading. There are always large storm waves on top of
the surge, and the waves are missing from the photos. The make the surging
water much more dangerous. They are large, and because they can pound
coastal structures for hours, they can destroy buildings that might survive
a tsunami having the same change in water level.
Long-Term Changes in Frequency of Surges
The frequency of hurricanes and other severe storms depends
on oceanic conditions, especially sea-surface temperature. Temperature
over large oceanic areas, such as the north Atlantic or Pacific, changes
from decade to decade. The Atlantic
Multidecadal Oscillation AMO and
the Pacific Decadal Oscillation PDO
are two examples of long-term changes
Changes in the north Atlantic surface
temperature is linked to changes in hurricane frequency. Warm temperautres
are associated with many more hurricanes.
Paths of Atlantic hurricane during the two phases of the Atlantic Multidecadal
Top: Cold Phase. Bottom:
From From Atlantic Multidecadal
Oscillation web pages at the NOAA Atlantic Oceanographic and Meteorological
Storm surges are reduced by:
- Offshore barrier islands and coral reef. Barrier islands are "shock
absorbers" absorbing storm surges. The shallow water slows the
surge of water, reducing its amplitude at the mainland shore. Water
travels at a velocity of roughly square root (gH),
where g = gravity,
and H = depth of water. In shallow areas above reefs H is
small, and velocity is small.
- Mangrove forests at the shore. The tangle of branches slows the flow
- Forests just inland of the beach. Many areas have, of had, extensive
forests in the low flatlands just inshore of the
beach that slow the velocity of water and reduce the height of waves
and storm surge. Forests and mangrove areas also have many other uses:
They are nurseries for fish, shrimp, crabs, and other valuable marine
life; they provide wood and forest products; and they are a refuge
In April 1991 a cyclone ravaged the Chittagong
coastal belt [Bangladesh], located in the southern part of the country.
But people of Mirsarai and Sitakunda, two coastal locations, suffered
least casualties and property damage in the cyclone primarily due to
protection provided by a one-or-two kilometre wide belt of plantations
along the shore. This proved the effectiveness of coastal plantations
as protection against cyclone and storm surges. From Natural
Disasters, Forest and Environmental Security.
A view of Sundarban Mangrove forest of Bangladesh that has proven to
be effective in reducing storm surges.
- Lack of levees along rivers in coastal deltas. Flooding rivers deposit
sediments in the lowlands of their their deltas, keeping the areas
well above sea level.
Unfortunately, coastal development has removed
many of these barriers.
- Tsunami damage in Thailand in 2004 was much
worse onshore of areas where coral reefs had been mined for rock.
- Mangrove forests have been replaced by shrimp farms, and coastal
forests have been replaced by houses, commercial buildings, streets,
- The extensive system of levees along the Mississippi River has
prevented the flooding of delta areas, forcing the river to deposit
sediments in shallow offshore areas. As a result, the lowlands of the
delta gradually sank below sea level. New Orleans, originally above
sea level in the 19th century, was up to seven meters below sea level
by the beginning of the 21st century. Hurricane Katrina's storm surge
in August 2005 breached the levees protecting the city, leading to
disastrous flooding and loss of life.
- There is some evidence that the loss of coastal barriers
made the hurricane Katrina storm surge worse, leading to the flooding
of New Orleans in August 2005.
Extensive erosion of coastal lowlands of the
Mississippi delta in August 2005 by the storm surge generated by hurricane
Image from Wall Street Journal, 2 September 2005, page B1.
Isaac's Storm (1999) by Erik Larson,
Crown Publishers, gives a thorough account of the Galveston hurricane
of 1900, including first hand-accounts of those who survived a category
3 August, 2009