Thursday, June 29, 2006

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, but a higher and more powerful wave. This situation is seen along the southeast coast of Florida. The edge of the Floridian Plateau, where the water depths reach 91 meters (300 feet), lies just 3 km offshore of Palm Beach, Florida; just 7 km offshore, the depth plunges to over 180 meters (Lane, 1980). 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.

Conversely, coastlines such as those along the Gulf of Mexico coast from Texas to Florida, have long, gently sloping shelves and shallow water depths. On the Gulf side of Florida, the edge of the Floridian Plateau lies more than 160 km offshore of Marco Island in Collier County. Florida Bay, lying between the Florida Keys and the mainland, is also very shallow; depths typically vary between 0.3 and 2 meters (Lane, 1981). These areas are subject to higher storm surges, but smaller waves.

This difference is because in deeper water, a surge can be dispersed down and away from the hurricane. However, upon entering a 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 another important element in storm surge extent. Areas where the land lies less than a few meters above sea level are at particular risk from storm surge inundation.


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