15 
3.3 Duration of storm surges at Swinoujscie 
In order to estimate the duration of a storm surge, two points in time have to be determined: 
its beginning and end. For example, the inflexion point of the tide curve may be used as the 
beginning of a storm surge. In the forecasting routine in the Baltic Sea, however, this point 
cannot be predicted in advance (in real time). Therefore, for practical purposes, a daily 
reference sea level indicator parameter is applied to calculate the duration of a storm surge. 
The hour at which the sea level rises above the daily reference sea level (before reaching the 
maximum value) has been defined as the beginning of a storm surge, and the hour at which 
it drops below the daily reference sea level (after having reached the maximum value) as its 
end. 
At Swinoujscie, surges lasting 21 - 30 hours and 31 - 40 hours had the highest frequency 
(26% and 25%, respectively), which amounted to about half of all surges. 19.2% of all surges 
lasted 11-20 hours, and about 12% of surges were in the 41 - 50 hour range. Only 10.9% 
of the surges lasted from 51 to 70 hours, and 6.9% more than 70 hours. The surges with the 
longest duration on record at Swinoujscie in the second half of the 20 th century exceeded 80 
hours and occurred in April 1997 (117 hours), February 1979 (85 hours), November 1981 (93 
hours), and January 1987 (82 hours). 
Fig. 3.5. Storm surge duration at Swinoujscie - frequency distribution 
3.4 Warning and alarm levels 
The warning and alarm levels for a particular section of the coast are determined taking into 
account the height and frequency of occurrence of dangerous surge levels, the configuration 
of the coastline, and existing coastal defence structures such as dikes etc. In case of an 
expected storm surge, the hydrological forecasting services are required to inform the 
responsible authorities of the expected duration of the dangerous surge level above the 
warning levels, and particularly above the alarm levels. This information is essential for the 
organisation of protection measures. 
At the coasts around Swinoujscie, the warning and alarm levels have been set at 560 cm 
and 580 cm, respectively. Higher levels have been set at the coasts between Sassnitz and 
Wismar, at 600 cm. 
The period during which emergency levels were exceeded is indicated in hours as an annual 
total of each stormy season (Table 3.4.).