Storm surge
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A storm surge is an onshore rush of water associated with a low pressure weather system, typically a tropical cyclone. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry of the body of water. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems.
Storm surges are particularly damaging when they occur at the time of a high tide, combining the effects of the surge and the tide. This increases the difficulty of predicting the magnitude of a storm surge since it requires weather forecasts to be accurate to within a few hours.
The most extreme storm surge events occur as a result of extreme weather systems, such as tropical cyclones, but storm surges can also be produced by less powerful storms.
The highest storm surge ever recorded was produced by the 1899 Bathurst Bay Hurricane, which caused a 13 meters (43 feet) storm surge at Bathurst Bay, Australia. In the US, the greatest recorded storm surge was generated by Hurricane Camille, which produced a storm surge exceeded 5 meters along 50 km of the Mississippi Gulf Coastline, with peak surge over 7 meters in the area of Gulfport, Mississippi. (Jelesnianski, 1992).
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Hurricane Storm Surge; Potential For Disaster
Nine out of ten people who die in hurricanes are killed by storm surge. The category 4 hurricane that struck Galveston, Texas, on 8 September, 1900 drove a devastating surge ashore; 6,000 lives were lost in this, the second greatest natural disaster ever to strike the United States (Hebert, 1990). Only in the 1889 Johnstown Flood in Pennsylvania has that number been exceeded, leaving 2,200 dead. The third greatest natural disaster in the U.S. in terms of lives lost was the storm surge from Lake Okeechobee in the Great Miami Hurricane of 1928 which swept up the middle of the Florida Peninsula during the night on September 16. The lake surged over its southern bank, virtually wiping out the settlements on its south shore. The estimated death toll was 1,850 to 2,000, yet many of the bodies were never found. Only two years earlier, storm surge from the September 1926 hurricane broke through the small earthen dike rimming the lake's western shore, killing 150 people at Moore Haven (Will, 1978).
These tragedies in the United States, grim as they are, are overshadowed by the tremendous losses of life suffered in other regions of the world. In the Bay of Bengal area, the "storm surge capital of the world", 142 moderate to severe storm surge events are on record from 1582 to 1991. These surges, some in excess of eight meters, have annihilated hundreds of thousands of people, primarily in Bangladesh (Murty and Flather, 1994). The Caribbean Islands have endured many devastating surges as well.
Mechanics of the storm surge
At least five processes can be involved in altering tide levels during storms. These include the pressure effect, the direct wind effect, the effect of the earth's rotation, the effect of waves, and the rainfall effect (Harris, 1963). The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low pressure and fall in regions of high pressure. Wind stresses cause a phenomenon referred to as "wind set-up", which is the tendency for water levels to increase at the downwind shore, and to decrease at the upwind shore. This effect is inversely proportional to depth (Harris, 1963). Wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.
Surge and wave heights on shore are affected by the configuration and bathymetry of the ocean bottom. A narrow shelf, or one that drops steeply from the shoreline and subsequently produces deep water in close proximity to the shoreline tends to produce a lower surge, yet a higher and more powerful wave. This situation is seen along the southeast cost of Florida. The edge of the Floridan Plateau, where the water depths equal 91 meters (300 feet), lies just 3 km off shore of Palm Beach, Florida; just 7 km off shore, the depth plunges to over 180 meters (Lane, 1980).
Conversely, coastlines such as those along Florida's Gulf coast and the Florida Keys have long, gently sloping shelves and shallow water depths. The 180 meter (600-foot) depth contour followed southward from Palm Beach County lies more than 30 km to the east of the upper Keys. On the Gulf side of Florida, the edge of the Floridan Plateau (91 meter depth) lies more than 160 km offshore of Marco Island in Collier County. Florida Bay, lying between the Florida Keys and the mainland, is very shallow; depths typically vary between 0.3 and 2 meters (Lane, 1981). These areas are subject to higher storm surge, but smaller waves.
This difference is because in deeper water, a surge can be dispersed down and away from the hurricane. However, upon entering the shallow, gently sloping shelf, the surge can not be dispersed away, but is driven ashore by the wind stresses of the hurricane. Topography of the land surface is an important element in storm surge extent. Areas such as those seen in southern Florida where the land lies less than a few meters above sea level are at particular risk from storm surge inundation.
Public Awareness - a Problem
Unfortunately, most people currently living in hurricane-prone areas do not understand the threat of storm surge and have never experienced a direct hit by a major hurricane. In South Florida, many people, houses and animals were virtually swept away by storm surge in the great hurricanes of the early part of this century. Yet the collective memory no longer holds these recollections . Instead, mental images of hurricane destruction are likely be those of Hurricane Andrew in 1992. Andrew's peak storm surge, at 5.15 m (16.9 feet) NGVD (NOAA, 1993b) occurred in a very localized area. The vast majority of destruction was caused by winds, not storm surge. Consequently, discussions of future hurricanes typically revolve around the potential wind hazards.
The amount of money spent on earthquake disaster preparation and awareness greatly exceeds that spent on hurricane preparedness, even though many more lives have been lost in hurricanes than earthquakes.
Saving Lives through Storm Forecasting and Prediction of Surge
It is important to keep in mind that most people expect that the government is taking care of what needs to be done, in terms of growth planning and hurricane evacuation procedures. This may be true to some extent, as growth and emergency planners strive to devise plans and procedures to minimize loss of life in natural disasters. However, if public awareness of natural hazards falls grossly behind the actual danger, as it has in the area of coastal hazards, then what we see is uncontrolled building, construction and growth, coupled with a false sense of security. The muffled warnings of scientists, engineers and government officials are not heard.
Nevertheless, advances in weather forecasting do allow advance warnings of tropical cyclones. Evacuation can be started in time to prevent great losses of life. Hebert, 1990, points out that a large death toll in a U.S. hurricane is still possible; decreased death tolls in recent years may be as much a result of lack of major hurricanes striking the most vulnerable areas as they are of any fail-proof forecasting and warning systems. Hebert maintains that hurricane preparedness and response to warnings can reduce the death tolls, although huge property losses are inevitable. The National Weather Service is now capable of surprisingly accurate estimates of the extent of hurricane storm surge inundation, thanks to numerical modeling and modern computers. Predictions of storm surge inundation are now made with the use of a numerical model called SLOSH, which has great value in devising hurricane evacuation zones, as well as its potential role in planning and zoning of coastal development
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