Seasonal and long term variation 
23 
Table 3.3. and Fig. 3.3. b show Gumbel probabili 
ties for the recurrence interval T (years) at Ger 
man and Polish gauges. The values in italics 
have been extrapolated for the Gumbel statistics 
because at some of the gauge stations, e. g. 
Swinoujscie, the time series covers only a 50- 
year period. The other values are based on ob 
servations covering at least 100 years. From 
these the interval at which a particular water level 
is likely to occur on average can be derived. In 
Warnemünde, for example, a low water level of 
1.47 m occurs every 20 years on average. One 
clearly sees that the probability of occurrence of 
low sea level events decreases from west to 
east. One of the reasons for this is the “bay ef 
fect” described in chapter 1.3. The lowest levels 
occur in bights that are open to the northeast. 
The values at the gauges of Swinoujscie and 
Sassnitz are in the same range, which is due to 
their very close geographical position and 
bathymetry. 
This can also be seen in the percentile distribu 
tion of monthly lowest sea level at the 5 gauge 
stations (Fig. 3.3. c). A percentile is the value of a 
variable below which a certain percent of obser 
vations fall, i. e. the 20 th percentile is the value (or 
score) below which 20 percent of all observations 
fall. The 20 th percentile in Wismar, for example, is 
365 cm, in Warnemünde 389 cm, in Sassnitz 
406 cm, in Swinoujscie 406 cm, and in Kotobrzeg 
412 cm. This means that 20 percent of all obser 
vations (in this case the monthly low sea level 
observations) are lower than these values. At 
Warnemünde 75% of all values are below 
414 cm (or 86 cm below mean sea level), 50% of 
all values are below 403 cm (or 97 cm below 
mean sea level) and 25 % of all values are below 
392 cm (or 108 cm below mean sea level). 
A very long time series of annual low water 
events has been recorded at the Warnemünde 
gauge station. Fig. 3.3. d shows annual low sea 
level occurrences from 1910 to 2005 at the 
Warnemünde gauge. No data are available for 
the years 1941/42 and 1945/46. 
The orange coloured line indicates the water 
level of 440 cm (60 cm below mean), which is the 
definition of low water in this monograph. The 
red line marks the German alert level (400 cm). 
The 3 blue lines show percentiles from Fig. 3.3. c. 
The blue solid line represents 75 % of all values, 
the dashed blue line represents 50 % and the 
dotted blue line 25 % of all values. Levels below 
440 cm have been recorded in all years. Levels 
which are more than 100 cm below mean water 
level have not been recorded every year. Extreme 
values of 140 cm below mean or less are very 
rare and occurred only twice: in the years 1967 
and 1999. 
—"' Wismar ““Warnemünde — Sassnitz Swinoujscie Kotobrzeg 
Fig. 3.3. b Low sea level in m as a function of statistical recurrence