12 
Negative Surges in the Southern Baltic Sea 
Difference hourly-4 hourly data in hours 
Fig. 2.1. Difference between hourly data and resampled 4-hourly data at Warnemünde 
2.2. Definition and general charac 
teristics of negative surges 
According to the definition of the German Insti 
tute for Standardization (DIN 4049-3), a negative 
surge is a state of the surface water in which the 
water level or flow level has fallen to or below a 
certain value. Depending on the approach used, 
different limits may be defined. 
With respect to the German coast, negative surges 
have been defined as water levels that fall at least 
1 m below the generalised mean sea level. In terms 
of tide gauge data, this means to 400 cm or less. 
The official German alarm and warning levels are: 
- 425 cm, or 75 cm below mean sea level: 
surge information is issued 
- 400 cm, or 100 cm below mean sea level: 
surge warning is issued 
Navigational warnings (NAVTEX) are additionally 
issued at water levels below 440 cm, in line with 
international agreements. 
In Poland, negative surge levels have not been 
officially defined. Depending on the type of activ 
ity threatened by negative surges, the threshold 
values for negative surge warnings differ for par 
ticular users, e.g. shipping, hydraulic engineer 
ing, port activities, cargo services, or coastguard. 
For practical purposes, both in the daily hydro- 
logical forecasting routine and in scientific stud 
ies, a negative surge has been defined as a hydro- 
logical situation causing the water level to fall to 
440 cm or lower. This value is based on IMGW’s 
thorough analysis of the probability of occur 
rence of surges in the central part of the southern 
Baltic Sea coast. 
The value of 440 cm has been used for the pur 
poses of this monograph. Table 2.2. a shows all 
negative surge events between 1958 and 2005 
during which at least 4 of the 5 gauges included 
in the study (Wismar, Warnemünde, Sassnitz, 
Swinoujscie and Kotobrzeg) recorded values 
below 440 cm. The total number of such cases is 
107, and in 46 cases all of the 5 gauges fell below 
the limit. The longest duration of a single nega 
tive surge event was 69 hours in December 1959.