Storm surge

A storm surge is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, and the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges.

The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm's center.

The deadliest storm surge on record was the 1970 Bhola cyclone, which killed up to 500,000 people in the area of the Bay of Bengal. The low-lying coast of the Bay of Bengal is particularly vulnerable to surges caused by tropical cyclones. The deadliest storm surge in the twenty-first century was caused by the Cyclone Nargis, which killed more than 138,000 people in Myanmar in May 2008. The next deadliest in this century was caused by the Typhoon Haiyan (Yolanda), which killed more than 6,000 people in the central Philippines in 2013 and resulted in economic losses estimated at $14 billion (USD).

The Galveston Hurricane of 1900, a Category 4 hurricane that struck Galveston, Texas, drove a devastating surge ashore; between 6,000 and 12,000 lives were lost, making it the deadliest natural disaster ever to strike the United States

Strong surface winds cause surface currents at a 45° angle 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

Similar to tropical cyclones, extra-tropical storms cause an offshore rise of water. However, unlike most tropical cyclone storm surge, extra-tropical storms can cause higher water levels across a large area for longer periods of time, depending on the system. This is due to many factors, such as storm size and different steering winds, which could keep a system in a storm-surge prone area for longer periods of time.

Another component of extra-tropical storm surge is the phenomenon of negative water levels. If strong winds are blowing offshore, situations can arise where mean water levels in a bay fall significantly, which poses a serious threat for ships tied up at piers. If negative water levels are severe enough, ships tied up at docks can actually sit on the seafloor, preventing them from leaving port.

In North America, extra-tropical storm surges may occur on the Pacific and Alaska coasts, and north of 31°N on the Atlantic Coast. Extra-tropical storm surges may be possible for the Gulf coast mostly during the wintertime, when extra-tropical cyclones affect the coast, such as in the March 1993 Storm of the Century.

After surge from a cyclone has receded, teams of surveyors map high-water marks (HWM) on land, in a rigorous and detailed process that includes photos and written descriptions of the marks. HWMs denote the location and elevation of flood waters from a storm event. When HWMs are analyzed, if the various components of the water height can be broken out so that the portion attributable to surge can be identified, then that mark can be classified as storm surge. Otherwise, it is classified as storm tide. HWMs on land are referenced to a vertical datum (a reference coordinate system). During evaluation, HWMs are divided into four categories based on the confidence in the mark; only HWMs evaluated as "excellent" are used by NHC in post storm analysis of the surge.

Two different measures are used for storm tide and storm surge measurements. Storm tide is measured using a geodetic vertical datum (NGVD 29 or NAVD 88). Since storm surge is defined as the rise of water beyond what would be expected by the normal movement due to tides, storm surge is measured using tidal predictions, with the assumption that the tide prediction is well-known and only slowly varying in the region subject to the surge. Since tides are a localized phenomenon, storm surge can only be measured in relationship to a nearby tidal station. Tidal bench mark information at a station provides a translation from the geodetic vertical datum to mean sea level (MSL) at that location, then subtracting the tidal prediction yields a surge height above the normal water height.
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