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What is a storm surge?

A storm surge is an abnormally high sea level produced by severe atmospheric conditions, lasting for a period ranging from a few minutes to a few days. Famous storm storm surges include the 1953 North Sea Flood, the 1970 Bangladesh floods and Hurricane Katrina in New Orleans.

Storm surges are particularly associated with deep atmospheric low pressure systems (either Tropical Cyclones, TC, or Extra-Tropical Cyclones, ETC), which have associated high winds which drive water onto the shore. However smaller-scale storms can also induce storm surges.

The water level produced by a storm surge is the result of a complex set of interactions between the storm surge, local tidal characteristics, wind and swell waves, currents and local water flow. However the main factor is the state of the tide at the time that the storm hits the shore. When a storm hits a low lying shore at the same time as a high tide, water levels can rise considerably above their normal level, resulting in severe flooding.

“Negative Storm Surges”, when water is pushed away from the shore, can also occur, leading to abnormally low water levels. Though less destructive than normal storm surges, they can pose risks to water navigation, spread pollution and redefine the sea bottom topography.

Picture by SuperManu (Image:Surge big.jpg by Pierre cb) [Public domain], via Wikimedia Commons from Wikimedia Commons

A Surge Event (called an "SEV" within the eSurge project) is an event where a particular storm causes abnormal sea levels in a given location. A location could be an isolated point, or a more general area such as along a specific coastline. 

There are different ways in which a storm surge can be defined: for example as the Total Water Level Elevation (TWLE), or as the difference between the TWLE and the expected water level, as measured by the skew surge (see figure). Some criteria are also needed for what constitutes a significant event. (For example, TWLE that exceeds the height of a flood barrier, or a skew surge which is only seen once in 30 years.)

A surge event need not result in flooding. For example, an event with a high skew surge but which occurred at neap tide is still significant. False positives, in which a surge is predicted but does not in fact occur, are also important to study.

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