Storm Surge - Mechanics

Mechanics

At least five processes can be involved in altering tide levels during storms: the pressure effect, the direct wind effect, the effect of the Earth's rotation, the effect of waves, and the rainfall effect. The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low atmospheric pressure and fall in regions of high atmospheric pressure. The rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. This effect is estimated at a 10 mm (0.39 in) increase in sea level for every millibar drop in atmospheric pressure.

Strong surface winds cause surface currents perpendicular to the wind direction, by an effect known as the Ekman Spiral. 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. Intuitively, this is caused by the storm simply blowing the water towards one side of the basin in the direction of its winds. Because the Ekman Spiral effects spread vertically through the water, the effect is inversely proportional to depth. The pressure effect and the wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.

The Earth's rotation causes the Coriolis effect, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When this bend brings the currents into more perpendicular contact with the shore it can amplify the surge, and when it bends the current away from the shore it has the effect of lessening the surge.

The effect of waves, while directly powered by the wind, is distinct from a storm's wind-powered currents. Powerful wind whips up large, strong waves in the direction of its movement. Although these surface waves are responsible for very little water transport in open water, they may be responsible for significant transport near the shore. When waves are breaking on a line more or less parallel to the beach they carry considerable water shoreward. As they break, the water particles moving toward the shore have considerable momentum and may run up a sloping beach to an elevation above the mean water line which may exceed twice the wave height before breaking.

The rainfall effect is experienced predominantly in estuaries. Hurricanes may dump as much as 12 in (300 mm) of rainfall in 24 hours over large areas, and higher rainfall densities in localized areas. As a result, watersheds can quickly surge water into the rivers that drain them. This can increase the water level near the head of tidal estuaries as storm-driven waters surging in from the ocean meet rainfall flowing from the estuary.

Surge and wave heights on shore are affected by the configuration and bathymetry of the ocean bottom. A narrow shelf, or one that has a steep drop 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 well exemplified by the southeast coast of Florida. The edge of the Floridian Plateau, where the water depths reach 91 metres (299 ft), lies just 3,000 m (9,800 ft) offshore of Palm Beach, Florida; just 7,000 m (23,000 ft) offshore, the depth increases to over 180 m (590 ft). The 180 m (590 ft) depth contour followed southward from Palm Beach County lies more than 30,000 m (98,000 ft) to the east of the upper Keys.

Conversely, coastlines along North America such as those along the Gulf of Mexico coast from Texas to Florida, and Asia such as the Bay of Bengal, 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 kilometres (99 mi) 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 m (0.98 ft) and 2 m (6.6 ft). 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 cannot be dispersed, 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.

For a given topography and bathymetry the surge height is not solely affected by peak wind speed; the size of the storm also affects the peak surge. With any storm the piled up water has an exit path to the sides and this escape mechanism is reduced in proportion to the surge force (for the same peak wind speed) as the storm covers more area.

In the Asian region, Metro Manila or Manila is one of the most affected by storm surges of typhoons because the Philippines lie in the path of typhoons heading toward Japan, Taiwan, China, Vietnam, and Cambodia. Damage and casualties are to be expected. Even though these occurrences are normal, loss of life is inevitable. Global warming also affects the typhoon formation, strength, intensity, and speed.