Storm Surge

"Storm surge" refers to a rapid rise of sea level that occurs as a storm approaches a coastline. This is in addition to changes in variations in sea level due to tides. Thus, a storm surge causes greatest inundation at high tide. A very strong hurricane may produce a storm surge of 20 ft (6 m), of which about 3 ft (1 m) is due to the lower atmospheric pressure at the center of a hurricane. The remaining storm surge is due to: (i) the piling up of water at the coast, generated by the strong onshore winds and (ii) a decreased ocean depth near the coast, which steepens the surge. A common misconception is that the lower pressure at the center of a storm is the primary cause of the storm surge.

At landfall, storm surge is highest in the front right quadrant of a westward-moving tropical cyclone (in the Northern Hemisphere), where the onshore winds are the strongest. It is also large where ocean bottom bathymetry focuses the wave energy (e.g., as in a narrowing embayment). Peak storm surge from a landfalling cyclone increases with greater wind speeds and the areal extent of the storm's maximum winds, out to about 30 miles (48 km).

Storm surge also occurs when a storm parallels the coast without making landfall. The storm surge will precede the passage of the storm's center when winds blow onshore preceding passage of the eye. Similarly, the surge will lag the storm's center when the hurricane is moving such that onshore winds follow the passage of the eye.

Offshore winds that are associated with a storm can produce a negative surge, as the sea level is lowered by the strong winds blowing out from the coast.

Storm surge is estimated to generally diminish in depth by 1 to 2 ft (0.3 to 0.6 m) for every mile (1.6 km) that it moves inland. Even if the inland elevation were only 4 to 6ft (1.2 to 1.8 m) above mean sea level, a storm surge of 20 ft (6 m) might typically reach no more than 7 to 10 miles (11 to 16 km) inland. Thus, the most destructive effect of the storm surge hazard is on beaches and offshore islands.

Storm Surge Hazards. A storm surge can be deadly. In 1900, up to 12,000 deaths occurred in Galveston, Texas, primarily as a result of the storm surge that was associated with a Gulf of Mexico hurricane. In 1957, a storm surge was the major cause of death for 390 people in Louisiana. The storm surge, associated with hurricane Audrey, was over 12 ft (3.5 m) in depth and extended as far inland as 25 miles (40 km) in this particularly low-lying region. In September 1928, the waters of Lake Okeechobee, FL driven by hurricane winds, overflowed the banks of the lake and were the main cause of more than 1800 deaths.

Areas to be evacuated due to storm surge in the case of hurricane landfall are determined through a model developed by the National Weather Service (NWS) called SLOSH (sea, lake, and overland surges from hurricanes; Jarvinen and Lawrence, 1985). The SLOSH model is used to define flood-prone areas in 31 "SLOSH basins" along the U.S. Gulf of Mexico and Atlantic coasts (Fig. 2). Determination of storm surge vulnerabilities is the result of an interagency and intergovernmental process funded by the National Oceanic and Atmospheric Administration (NOAA), the Federal Emergency Management Agency (FEMA), Army Corps of Engineers, and various state and local governments (BTFFDR, 1995). From development through application the SLOSH process for a particular location takes about 2 years. Because coastlines are constantly changing due to human and natural forces, the SLOSH process is an ongoing challenge.

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