A storm surge is a flood of high water, caused by wind and low pressure pushing water on shore from the ocean. The most spectacular and damaging examples of this occur during storm events such as hurricanes or cyclones. It is the storm surge that often causes most of the damage during a big storm.
The low pressure that initially causes the storm causes a dome of water inside the eye of the storm that is carried along by the forward motion of the storm. More water is pushed forward by the storm winds, adding to the volume of water in a wave following the low pressure dome. On reaching land this dome of water is pushed onshore leading to flooding, followed by the wave. The low pressure technically causes the storm surge, and a following wave is caused by the wind. Practically the effects come together to form one mass of water.
The highest-ever recorded storm surge was forty-three feet in Australia in 1899; the storm surge of hurricane Katrina by comparison was thirty feet. A storm surge will be higher if a high tide is expected at the same time.
The severity of a storm surge is affected by many factors. The lower the pressure in the storm and the faster the winds the more water will be collected. The shape and depth of the seabed in the area can cause changes in how the water comes ashore, and the geography of the land that is hit will also change the effects. The storm surge accompanying Hurricane Katrina traveled twelve miles inland.
A steep drop off into the ocean close to shore will produce a lower surge, with a high following wave. The wave may not cause much damage if the land slopes steeply, breaking as it will near the shore. Longer, shallower shorelines allow a greater development of the storm surge, with a shallower following wave. These surges will often produce more damage because the water can travel further inland.
The angle at which the storm surge hits the shoreline will also change its results, a sharp approach angle will break some of the wave, but may also funnel damage to a particular area. If the wave breaks straight on to the shore none of its force is dissipated.
An example of this is the storm surge that accompanied The 1938 Long Island expressway hurricane. This surge was funneled up along the shore of Long Island causing massive coastal damage and flooding. The massive forces of water deposited in the storm surge changed the entire geography of this part of the coastline.
It is a mistake to think that storm surges can only occur at the seashore. In 1928 The Okeechobee hurricane caused a storm surge on Lake Okeechobee that hit the southern shore of the lake causing an estimated two thousand deaths.
These effects are modeled in the SLOSH model (Sea Lake and Over Land Hydrography), in an attempt to predict the damage that may be caused. Warnings are issued to low lying areas on the basis of the predictions of this model when a hurricane is expected to make landfall.
These warnings are one of the greatest lifesavers in a hurricane. Even today most deaths in hurricanes are not caused by the winds, which people shelter from, but by the flooding following the storm surge and rain the hurricane brings with it
The deadliest natural disaster ever to hit the United States in terms of human deaths was the 1900 Galveston Hurricane, which caused over six thousand deaths. These deaths were not caused by the winds of the storm, but by the storm surge and the massive flooding that it caused.