SURGE: A “RISING” CONCERN AMONG COASTAL RESIDENTS
30, 2005 — As rescue teams searched the devastation along the Gulf
Coast, it was evident that Hurricane
Katrina claimed many victims with its storm
surge — something that hasn't happened with a mainland U.S.
hurricane in nearly half a century.
hurricane to cause similar storm surge damage in this area was Hurricane
Camille when it came ashore at Pass Christian, Miss., in August 1969.
Although much smaller in size than Katrina, Camille’s 190 mile per
hour winds generated a record storm surge measuring as much as 24.3 feet
along a large portion of the Mississippi coastline. In its wake, Camille
destroyed or seriously damaged more than 18,000 homes and 700 businesses
— killing 172 people, with surge accounting for most of those deaths.
of September 30, 2005, official storm
surge data for both Hurricanes Katrina and Rita were still pending,
but it’s likely that Katrina’s storm surge values will surpass
that of Camille’s record 24.3 foot storm surge. “Unfortunately,
many of the local area tide gauges were destroyed during Katrina, making
it difficult to get accurate storm surge estimates," said Stephen
Baig, NOAA hurricane and storm surge expert at the NOAA
National Hurricane Center in Miami, Fla. “Before Katrina made
landfall, however, NOAA’s storm surge model predicted that Katrina’s
storm surge could reach 18 to 22 feet above normal tide levels, locally
as high as 28 feet.”
coastal areas in and around the Gulf Coast have always been susceptible
to storm surge from hurricanes, but the situation has worsened over time
as protective coastal wetlands have disappeared due to land subsidence
and human intervention.
Surge over the Years
When one thinks of hurricanes and similar tropical storms howling winds
and torrential rain
typically first come to mind, but the surge
accompanying these storms is potentially the greatest killer. Prior to
the mid to late 1960s storm surge flooding was the major cause of hurricane-related
deaths, but has accounted for only about six percent of all hurricane
deaths from 1970 through 2002 — thanks to the evolution of NOAA’s
storm surge models, better hurricane forecasts
and coastal evacuations. These “post-Camille” developments
have assisted in planning coastal development and hurricane evacuation
routes and can be credited for having greatly increased storm surge awareness
among coastal residents. In fact, over the last few decades, inland flooding,
winds and tornadoes now account for a majority of hurricane-related deaths.
However, storm surge is still a potential killer and is a primary reason
coastal areas are evacuated when they are threatened by hurricanes.
hurricane-related deaths have decreased in recent decades, NOAA has always
stressed the deadly history of storm surge," said Edward Rappaport,
deputy director of the National Hurricane Center in Miami.
Storm Surge Model
NOAA’s storm surge model, known as SLOSH
for “Sea, Lake and Overland Surges from Hurricanes,” is used
by the NOAA National Hurricane Center and local National Weather Service
offices to forecast storm surge heights resulting from historical, hypothetical
or predicted hurricanes by taking into account a hurricane’s central
pressure, size and track. The model also incorporates bathymetry (water
depth) and topography (land elevations), including bay and river configurations,
roads, levees and other physical features that can modify the storm surge
flow pattern. Thirty-nine
computational domains, or SLOSH basins, cover the U.S. East and Gulf
coasts, Puerto Rico, the Virgin Islands, Guam and the Hawaiian Islands
of Oahu and Kauai. The graphical output from the model displays color
coded storm surge heights for a particular area in feet above the model's
reference level, the National
Geodetic Vertical Datum, which is still the elevation reference used
for most topographic maps. With the SLOSH model, the NOAA
National Weather Service can now generate accurate estimates of the
extent of storm surge inundation. The NOAA National Hurricane Center uses
this information when issuing its hurricane
advisories. Emergency managers and other officials use the results
of thousands of precomputed hypothetical storms to determine which coastal
areas should be evacuated due to potential storm surge threats.
surge along a shallow and steep sloping shoreline.
of Storm Surge
Storm surge is water that is pushed toward the shore by the force of the
winds swirling around a hurricane or other coastal storm. Howling winds
around a hurricane's eye create storm surge by piling water up in the
form of a dome. In the deep ocean, this dome of water sinks and harmlessly
flows away. But as a storm nears land, the rising sea floor blocks the
water's escape and it comes ashore as deadly storm surge. An intense hurricane
can send a dome of water many miles wide and more than 25 feet deep barreling
toward the shore as the storm hits land.
Height of the Storm Surge
Storm surge is usually estimated by subtracting the regular/astrological
tide level from the observed storm tide.
Typical storm surge heights range from several feet to more than 25 feet.
The exact height of the storm surge and which coastal areas will be flooded
depends upon many factors, including the strength, intensity and speed
of the hurricane; the direction it is moving relative to the shoreline;
how rapidly the sea floor is sloping along the shore; the shape of the
shoreline and the astronomical tide. In general, storm surge is most damaging
when it occurs along a shallow sloped shoreline, during high tide, in
a highly populated area with little or no natural buffers, such as barrier
islands, coral reefs and coastal vegetation. Storm surge is also most
damaging in the storm’s right front quadrant because the storm,
its winds and ocean waves are all moving in an onshore direction due to
the counter-clockwise rotation of hurricanes in the Northern Hemisphere.
In contrast, to the left of the eye, ocean waves and sea-level rise are
moving toward the shore, but the winds are blowing in an offshore direction,
thus counteracting or moderating some of the effects of the storm surge.
of coastline before and after Hurricane Hugo's storm surge hit Folly
of Storm Surge
Storm surge causes severe
flooding in coastal areas. Because much of the densely populated Atlantic
and Gulf Coast shorelines lie less than 10 feet above mean sea level,
the potential danger from storm surge is significant. Even if storm surge
does not destroy lives and property, the flooding it produces can cut
off escape routes, rush into underground structures such as basements
and subways, and flood subterranean utilities.
surge causes local sea levels to rise for a relatively short period of
time (four to eight hours, though some areas may take days or weeks to
recede to their pre-storm levels) — often resulting in extensive
coastal flooding that can weaken or destroy coastal structures. By itself,
storm surge rarely causes structural failure. However, by temporarily
raising sea level, storm surge permits "dangerous and battering waves"
and floating debris to access coastal areas and structures never conceived
of nor built to withstand the punishing effects of open ocean waves. It
is these battering waves that cause most beach erosion and extensive damage
to coastal structures (such as buildings, roadways, bridges, marinas,
piers, boardwalks and sea walls).
Storm Surge Mitigation
Although advanced hurricane forecasts and timely coastal evacuations can
save lives, the potential threat of hurricanes to property poses a much
greater challenge. Ironically, natural buffers like offshore barrier islands,
coral reef, mangrove forests and other coastal vegetation that are often
damaged or destroyed to make way for coastal development are also the
best defense against storm surge. NOAA is involved in a number of coastal
restoration activities, including rebuilding and preserving coastal
Awareness — a Problem
Despite the tremendous success of the NOAA’s storm surge activities,
as the nation’s coastal regions become more populated with both
visitors and permanent residents, storm surge remains a serious threat.
NOAA hurricane forecasts have improved over the years — allowing
more time for coastal areas to evacuate, however the population growth
in these areas will make evacuation more difficult, time consuming and
costly. Thus, a much greater percentage of the U.S. population is vulnerable
to storm surge than ever before. NOAA research indicates that the nation
has entered into a much more active
hurricane period, one that is likely to continue into 2020 or perhaps
beyond, so there is no better time than now to take hurricane warnings
Hurricane Awareness Web site: Storm Surge Information
KATRINA Advisory Archive
Hurricane Research Division: Hurricane Katrina
RESOURCES TO AID IN RECOVERY FROM HURRICANE KATRINA
AERIAL SURVEY OF REGIONS RAVAGED BY HURRICANE KATRINA
HUNTER PILOT CAPTURES KATRINA AT HER MEANEST
KATRINA SUPPORT ACTIVITIES: Aerial Photography Flights Yield Thousands
MAPPING ASSISTING U.S. COAST GUARD, FEMA SHOWS FLOOD WATERS RECEDING IN
REGIONS AFFECTED BY HURRICANE KATRINA
Support for the Gulf of Mexico Regional Partnership
NOAA Weather Service Public Affairs,
(301) 713-0622 or Frank
Lepore, NOAA National Hurricane
Center, (305) 229-4404