September 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.

The last 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.

As 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.”

Low lying 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.

Storm 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.

"Although 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.

NOAA’s 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.

Storm surge along a shallow and steep sloping shoreline.

Mechanics 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.

The 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.

Section of coastline before and after Hurricane Hugo's storm surge hit Folly Beach, SC.

Impacts 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.

Storm 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 wetlands.

Public 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 seriously.

Relevant Web Sites
NOAA Hurricane Preparedness

NHC Hurricane Awareness Web site: Storm Surge Information

Hurricane KATRINA Advisory Archive

NOAA Hurricane Katrina Overview

NOAA Hurricane Research Division: Hurricane Katrina

NOAA MOBILIZES RESOURCES TO AID IN RECOVERY FROM HURRICANE KATRINA

NOAA CONDUCTS AERIAL SURVEY OF REGIONS RAVAGED BY HURRICANE KATRINA

NOAA HURRICANE HUNTER PILOT CAPTURES KATRINA AT HER MEANEST

NOAA HURRICANE KATRINA SUPPORT ACTIVITIES: Aerial Photography Flights Yield Thousands of Images

NOAA AERIAL MAPPING ASSISTING U.S. COAST GUARD, FEMA SHOWS FLOOD WATERS RECEDING IN REGIONS AFFECTED BY HURRICANE KATRINA

NOAA Support for the Gulf of Mexico Regional Partnership

Media Contacts:
NOAA Weather Service Public Affairs, (301) 713-0622 or Frank Lepore, NOAA National Hurricane Center, (305) 229-4404