A surge refers to the change of sea level in coastal waters caused by winds and air pressures acting on the sea surface. A storm surge event or a storm tide event is an occasion with particularly high surges (Pugh and Woodworth 2014).

For the German North Sea and Baltic Sea coasts the standard DIN 4049 defines a storm surge as an event in which regionally defined thresholds are exceeded. These thresholds are defined such that an event with given severity can be expected on average once for a given period (e.g. the once in a 20 year event). For their forecasts, the German Federal Maritime and Hydrographic Agency (BSH) uses uniform exceedance thresholds for the North- and the Baltic Sea.

At the German North Sea coast, in Emden, Bremen and Hamburg, three levels of storm surge severity are distinguished:

1. Storm surge: 1.5 to 2.5 m above mean high water (MHW)
2. severe storm surge: 2.5 to 3.5 m above MHW
3. Very severe storm surge: more than 3.5 m above MHW

For the German Baltic Sea coast four levels are used:

1. Storm surge: 1.00 to 1.25 m above mean water level (MW)
2. medium storm surge: 1.25 to 1.50 m above MW
3. severe storm surge: 1.50 to 2.00 m above MW
4. very severe storm surge: more than 2.00 m above MW

References:
Pugh, D. and Woodworth, P. 2014: Sea-Level Science. Cambridge Univ. Press
BSH Storm surges

The MHW and MW (in m relative to NHN) of the gauges are calculated over the reference period 1961-1990 (see the Table below).

According to the German standard DIN 4049 storm surges and their severity are defined based on their average return periods. Due to regional differences, such as for example in wind climate, exceedance thresholds for given return periods differ. The German Federal Maritime and Hydrographic Agency (BSH) accounts for such differences and uses different thresholds for the German North Sea and Baltic Sea coasts. The StormSurgeMonitor adopts the values used by the BSH.

At the German North Sea and Baltic Sea coasts storms and storm surges occur mostly during the cold season; that is, from fall to spring next year. To capture all storm surges occurring during winter in one season we decided to shift the period and calculate all yearly statistics based on the period from July to June the following year.

Given defined exceedance thresholds, rising sea levels will contribute to more frequent and higher extremes even if there is no change in wind climate. The situation is comparable to a child sitting in a bathtub trying to get the surrounding ground floor wet. The more water there is in the tub (sea level) the less active the child needs to be (storm activity) to get the same effect (wet floor, storm surge).