Storm surge is an abnormal rise in water caused by a storm.
It is produced by water being pushed toward the shore by the force of the winds moving around the storm. The impact on surge of the low pressure associated with intense storms is minimal in comparison to the water being forced toward the shore by the wind.
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.
Surge can be measured directly at coastal tidal stations as the difference between the forecast tide and the observed rise of water. Surge can also be measured by pressure transducers that are placed along the coastline just before a tropical cyclone. These types of sensors can be placed in locations that will be submerged, and can accurately measure the height of water above them.
The National Hurricane Center in the US, forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes. The model is accurate to within 20 percent.
SLOSH inputs include the central pressure of a tropical cyclone, storm size, the cyclone’s forward motion, its track and maximum sustained winds. Local topography, bay and river orientation, depth of the sea bottom, astronomical tides, as well as other physical features are taken into account, in a predefined grid referred to as a SLOSH basin.
The storm surge, combined with the normal tide, creates what is called hurricane storm tide. This can increase the water level to heights that can impact roads, homes, other critical infrastructure and cause beach erosion. In addition, this can cause severe flooding in coastal areas.
Storm surges are estimated to be responsible for 90 percent of all hurricane related deaths.
In the United States, one of the greatest recorded storm surges was generated by 2005’s Hurricane Katrina, which produced a maximum storm surge of more than 25 ft (8 meters) in the communities of Waveland, Bay St. Louis, Diamondhead, and Pass Christian in Mississippi, with a storm surge height of 27.8 ft (8.5 m) in Pass Christian.