Storm surge refers to abnormally high water levels generated by severe storms such as hurricanes, cyclones, and nor'easters. A surge forms when strong winds over the ocean combine with low pressure to drive water onshore. Storm surges can produce sea levels much higher than normal high tide, resulting in extreme coastal and inland flooding. Sometimes called "storm tides," storm surges can cause tremendous damage; if they coincide with high tide, they can raise water levels by 20 feet or more above mean sea level. As a result of global sea level rise, storm surges that occur today are eight inches higher than they would have been in 1900. By 2100, storm surges will happen on top of an addtional 8 inches to 6.6 feet of global sea level rise.
Water weighs about 1,700 pounds per cubic yard, so extended pounding by wind- and tide-driven waves moving at 10 to 15 mph can damage or destroy any structure not built to withstand such forces. Storm surges driven by strong hurricanes or extra-tropical storms can cause deaths and extensive property loss, including erosion of beaches, damage to coastal habitats, and undermining the foundations of vital infrastructure like roads, railroads, bridges, buildings, and pipelines. Hurricane Katrina (2005) is a prime example of the damage and devastation that is possible. At least 1,500 people lost their lives during Katrina, and many of those deaths occurred either directly or indirectly because of storm surge. Katrina also caused well over $100 billion in damage from its surge and winds.
The width and slope of the continental shelf where a storm comes ashore are key factors in determining whether and how severely a storm surge will impact the coast. Shallow sloped lands will likely experience a greater storm surge than steep sloped lands (to view animated clips, click here to see storm surge on shallow sloped lands and click here to see storm surge on steep sloped lands).
In order to clearly communicate the potential impacts of flooding from storm surge, National Weather Service predictions for inundation from storm surge are described in terms of height above ground level. For example, a storm surge prediction of 20 feet above ground level for a particular area means that forecasters expect 20 feet of water to cover that area. The depth of the flooding above ground level will gradually decrease as it moves further inland. However, as some coastal regions have vast areas of low-lying land inland from coast, it's important for residents and visitors to be aware of local conditions, and know how far inland local authorities consider lands to be vulnerable from various heights of storm surge.
Winds from coastal storms generate waves in the ocean. The stronger the wind, the larger the waves. As these waves move toward the shore, wave heights increase due to a process called “shoaling” as they interact with the ocean bottom. As the wave heights increase, the waves eventually break and impart their momentum to the water, causing onshore flow near the surface and offshore flow or “undertow” near the bottom, and an overall increase in elevation in water level at the coast. Two processes add to this elevation increase: “wave setup,” or the rise in the water surface caused by breaking waves, and “wave runup,” which is the rush of wave water up a slope or structure. Wave run-up/setup is often modeled by coastal engineers for project design to make sure structures are built to withstand strong waves, and that overtopping will not occur. Setup and run-up are also major causes of beach erosion and overtopping of man-made and natural coastal structures (like dunes).