International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 4
Number 2
June 2010
199
1 INTRODUCTION
The mean sea level in the Baltic Sea has visibly in-
creased during the last century. The global genera-
tors of this increase may be supported or reduced by
local influences. Routinely measured level values
are the result of all the affecting causes. As some of
these impacts appear irregularly in time, the result-
ing sea level changes are irregular, too. Daily oscil-
lations of the levels at the southern coast may reach
even 2,5 – 3 m in extreme cases.
Very low water levels may cause small harbours
to fall dry and may cause obstructions to navigation.
To ensure the safety of navigation in the difficult
waters of the Southern Baltic Sea, reliable data on
water levels, particularly low water levels, are highly
important.
Low sea levels are one of the most important fac-
tors of the navigation safety. The practical im-
portance of water level for shipping and harbour en-
gineering was recognized early. Representatives of
shipping are always deeply interested in safe passag-
es of their vessels through the Baltic Sea, where at
particular coasts significant differences in the water
level are known to occur, in spite of negligible tides,
nearly lacking in this sea.
Analyzed were the low sea level events at the
southern Baltic Sea coast, basing on 50 years long
series of sea level data from 1955 – 2005. The mare-
ographic records were obtained from the water
gauges in Wismar, Warnemünde, Sassnitz,
Świnoujście and Kołobrzeg. The investigation was
realized by Instytut Meteorologii i Gospodarki
Wodnej - Oddział Morski (IMGW OM) from the
Polish side and by Bundesamt für Seeschiffahrt und
Hydrographie (BSH) from the German side. Used
were the hydrological and meteorological data
stored in the BSH and the IMGW archives.
This study on low sea levels occurring at the
southern Baltic Sea coasts was realized as an inter-
nal IMGW and SEAMAN projects.
2 NATURAL CONDITIONS
The here considered section of the South-western
Baltic Sea coast comprises, going east, three German
water gauges, Wismar, Warnemünde and Sassnitz,
and two Polish ones, Świnoujście and Kołobrzeg.
The westernmost part of the southern Baltic coast
between Wismar, on the Mecklenburg Bay, and
Cape Arkona on the island of Rugia extends roughly
from southwest to northeast. This part of the coast
has a highly variable topography: it is shallow and
rich in creeks, shoals, and sandbanks. Also the
coastal section extending from the high cliffs of
Cape Arkona to Świnoujście and the Odra estuary,
oriented from northwest to southeast, has an intricate
topography: it is rich in small sandy coastal islets,
narrows, and sandbanks. In the adjacent Pomeranian
Low Sea Level Occurrence of the Southern
Baltic Sea Coast
I. Stanisławczyk, B. Kowalska & M. Mykita
Instytut Meteorologii i Gospodarki Wodnej, Oddział Morski, Gdynia, Poland
ABSTRACT: The level of 440 cm is defined as the upper limit of low sea level. This value is also accepted as
the warning level for navigation, according to the NAVTEX. The low sea levels along the southern Baltic Sea
coast were analyzed in the years 1955 – 2005. Lowest values recorded ranged from 309 cm in Wismar to 370
cm in Kołobrzeg. The phenomenon was chiefly generated by hurricane like offshore winds. Extremely low
levels were not frequent, their occurrence did not exceed more than 0,3% in Świnoujście and not more than
1% in Warnemünde. In summer months these phenomena occurred extremely seldom, they were more fre-
quent in the western, than in the eastern part of the coast. Long-term variation and statistical analysis was pre-
sented. Probability of low sea levels occurrence was calculated by Gumbel method and percentile distribution
for 4 gauge stations was analyzed. The calculations revealed that, for instance, in Warnemünde once in 20
years the minimum sea level can be as low as 358 cm.
200
Bay, the seabed also is highly variable, with shallow
depths below 10 m prevailing. A particularly wide
belt of shoals exists off the island of Uznam in the
waters close to Świnoujście and around the island of
Wolin. Farther to the east, between Wolin and
Kołobrzeg, the 10-m isobath runs as close as one
nautical mile from the shore. The coastline in this
area, running from west-southwest to east-northeast,
is rather smooth, and the isobath 50 m is not more
distant than about 15 NM off the shore.
3 DEFINITION OF LOW SEA LEVEL
No obligatory definition of “low sea level” is in
force in the hydrological forecasting and warning
service and no obligatory warnings on expected oc-
currence of this phenomenon are issued. For this in-
vestigation the level 440 cm has been accepted for
the southern coast of the Baltic Sea as the warning
level for navigation, according to the NAVTEX.
4 LOW SEA LEVELS GENERATORS
Sea level changes along the coasts are generated by
several factors, mainly by the wind impact on the
sea surface and, to some extent, by the actual water
volume of the sea. Most spectacular deformation of
the water surface at the Baltic Sea shores can occur
due to the already mentioned off- or on-shore,
stormy, sometimes hurricane-like winds, veering or
backing, accordingly to the weather system actually
passing along or across the coast. The force of the
wind against the water surface causes deformations
which become the greater, the shallower is the area
of the sea.
Of essential influence are however, the local condi-
tions. In shallow areas, such as the Baltic Sea, the
wind exerts dominating influence on the water level.
The magnitude and the character of the sea level
changes depends thus on the coast line configura-
tion, on the exposition of particular coast part to the
actual wind, on the bathymetry of the adjacent sea
basin, the currents, etc. Sometimes, even in the same
time the opposed water level tendencies can be ob-
served along a comparatively short section of the
same coast. The grounds for this differences lie
mainly in the morphology and in the peculiar hydro-
graphic character of this coast. In some cases, how-
ever, these opposed tendencies are due to a rapid
changes of the storm direction within a limited area
of the wind field.
5 METEOROLOGICAL INFLUENCES
Strong wind is the dominant factor, which forces the
water surface oscillations in the Baltic Sea. An off-
shore wind depressing the water surface at the shore
is usually less vehement over the land than on the
sea, and, depending on the character of the coast
line, can have more or less deflected direction. Much
stronger effect than the wind measured at the shores
exert the storms, which accompany the low pressure
systems moving across the Baltic Sea, affecting con-
siderable areas of water. A rather seldom, though
noteworthy cause of low levels is a long lasting gale
connected with an anticyclone over Scandinavia and
Russia and influencing a vast area of the whole Bal-
tic basin. Each of the mentioned wind systems,
though transforming and very much influenced on
their way, develops in accordance with the actual
specific pressure pattern over Europe and the adja-
cent Atlantic Ocean.
The Norwegian and North Seas, the Scandinavia
and the Baltic Sea, are situated in the west wind
zone. They are an area over which the atmospheric
disturbances, mostly active depressions with the
fronts systems, move eastwards from over the Atlan-
tic Ocean. In the fore field of a depression winds of
a strong southern component prevail, behind the
fronts usually veering. The winds in the fore field of
a depression are offshore in relation to the southern
coasts of the North and Baltic Seas. The depressions
are common here.
Some depressions, on their way eastwards, when
entering Scandinavia, slow down, though continue
to deepen. The pressure gradient gets very steep and
the initially stormy wind grows to hurricane force.
At the southern coasts the offshore-wind-driven-
level-decrease begins and holds on, until the wind
calms, or veers. Typical reaction of the levels to
such wind forcing is, at first, a gradual decrease
along a big part of the coast, then, pretty often, a
sharply accelerated sinking when the storm grows to
maximum force and finally a prolonged minimum
which lasts as long as the hurricane force hold on,
without changing the direction. When the wind
veers, the levels begin a more or less rapid increase
(often supported by the now along or onshore wind).
Another type of low sea level variation can be ob-
served when a strong, stationary anticyclone covers,
or oscillates over the Fennoscandia and the north-
western parts of Russia. In such atmospheric situa-
tion two main factors strive to diminish the water
levels. The first one is the considerably increased
hydrostatic pressure in the powerful high. The other
one is the wind system, which develops at the south-
western outskirts of the high: over the northernmost
areas of the Baltic Sea prevail light to moderate
winds of a high northern component; going south the
201
winds veer to easterly and south-easterly and grow
in force, sometimes to storm in places. In the west-
ernmost part of the Baltic Sea, in the Sound and in
the south-east of the North Sea dominate the south-
easterly and southerly winds. If such configuration
of pressure systems persists for long enough – a
week, two or even more, the surface water not only
is pushed away from the shores, but is also forced
out of the sea basin through the Belts and the Sound.
All the coastal water gauges in the Baltic Sea (ex-
cept, perhaps, those in the Sounds) should then rec-
ord decreased levels.
6 SOME PARAMETERS OF THE DECREASING
LEVELS
The process of sea level decrease begins, as already
said, forced by the impact of a strong wind. Depend-
ing on the wind character, the decrease can be rapid
and short lasting, or smooth and need a longer time
to develope. The acting force (wind) can cover the
whole area in the same time or, what is more fre-
quent, can progress from west to east, only in rare
cases - from east to west. Sometimes it can influence
only a part of the coast. It must be also kept in mind
that a decrease sea level is only a phase of the sea
surface oscillation, followed (or preceded) by a rise
of sea level – the other phase of this wind driven os-
cillation. An illustration of a sea level decrease can
give the variation curves of the falling sea surface at
the southwestern coast of the Baltic Sea.
In the afternoon and night of 17 October, a deep
meteorological depression was almost stationary
over the Kattegat and southern Sweden. A very
strong westerly to southwesterly storm of 8-9 Bft,
and of 10 Bft in places, developed over the eastern
North Sea and the southwestern basins of the Baltic
Sea. Behind the occluded front, the storm veered
northwest in the early hours of 18 October, without
calming during the next several hours.
On 17 October 1967, sea levels on the southwest-
ern Baltic Sea coast oscillated slightly above the
mean value. Around noon, they dropped first in the
Wismar Bay, which is the area most sensitive to the
impact of gale-force offshore winds. Water levels
began to fall steadily, initially at a rate of about 10
cm / hour, later 10-15 cm/hour. A less regular rate of
decrease was recorded at the other water level gaug-
es. Kołobrzeg, the easternmost of the considered wa-
ter gauges, was the last station to record falling wa-
ter levels on this part of the coast. The values there
remained above 500 cm until the westerly (along-
shore) winds had backed SW, partly S, at about 21
UTC. This forced a rapid drop of sea levels in this
area. Minimum levels were recorded just after mid-
night on 18 October, between 01 and 04 UTC. The
storm however, still came from southerly directions,
causing water levels to drop particularly rapidly
(Fig.1).
Rates of decrease were as high as about 40 cm/h
in Sassnitz, and about 50 cm/h in Kołobrzeg and
Świnoujście. The lowest minima were as follows:
Warnemünde 332 cm, Wismar 334 cm, Świnoujście
362 cm, Sassnitz 381 cm, and Kołobrzeg 435 cm.
During that storm levels below 440 cm remained for
17 hour in Wismar, while in Kołobrzeg only for 2
hours. The severity of the storm, which veered NW -
N, caused the sea levels to start rise again, immedi-
ately at high rates, to compensate the difference of
more than 1.5 m in 5-7 hours.
Figure 1 Fragment of sea level decrease during the storm on
17-19
th
Oct, 1967
During a series of another three successive
storms, between 29 November and 7 December
1999, one of them caused a particular deep sinking
of the sea levels, particularly on 4 December. The
intensity of the storm raging in the whole southwest-
ern area of the Baltic Sea, from the Sounds to the
coasts of Kołobrzeg, reached and at times exceeded
9 to 10 Bft. The highest rates of decrease oscillated
about 25-15 cm / hour, while the absolute minima
were as low as 309 cm in Wismar and 333 cm in
Warnemünde. In Wismar the water stayed below
440 cm for 19 hours.
7 FREQUENCIES
Extremely low sea level values recorded along the
southern coast of the Baltic were as high as 370 cm
in Kołobrzeg (1979-11-04), 366 cm in Świnoujście
(1967-10-18), 357 cm in Sassnitz (1939-12-22), 332
cm in Warnemünde (1967-10-18) and 309 cm in
Wismar (1999-12-04).
Figure 2. Frequency distribution of sea level values,
Świnoujście, 1955-2005.
OCTOBER 1967
300
320
340
360
380
400
420
440
460
480
500
520
540
15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12
water level (cm)
Wis mar Warnemünde Sassnitz Świnoujscie Kołobrzeg MSL
1967-10-17 1967-10-18
0
5
10
15
20
25
30
35
40
45
360-380
380-400
400-420
420-440
440-460
460-480
480-500
500-520
520-540
540-560
560-580
580-600
600-620
620-640
640-660
660-680
Sea level (cm)
Frequency (%)
202
In Świnoujście, for example, most frequent val-
ues of the levels were closest to the mean sea level,
that means to H= 500 cm (Fig.2). About 90 % of the
measured levels were included in the intervals be-
tween 520 cm to 480 cm only about 0,3 % of the
levels were lower than 440 cm and in the months
from May to August such low values were not rec-
orded at all. In the same years in Warnemünde the
frequencies of particular sea level values were as
follows: between 520 cm and 480 cm included were
about 70 % of the values. Only scarcely smaller than
1 % were the levels below 440 cm.
In one month of the year namely in June no levels
lower than 440 cm occurred.
8 LONG-TERM VARIATION OF LOW SEA
LEVEL OCCURRENCES
The long-term variation of low sea level occurrence
and annual frequency distribution provides im-
portant information on this hydrological effect.
Very low sea levels in the Baltic Sea occur very
irregularly and are extremely rare in summer. At the
declared low level limit of 440 cm they are a mar-
ginal phenomenon. At the southern coast their fre-
quency and deviation of magnitude decrease, when
moving east. For instance, while in Wismar and
Warnemünde, in each of the years between 1955 and
2005 the values of absolute minima fell lower than
440 cm or even lower than 420 cm, so already in
Sassnitz and Świnoujście they remained above the
threshold value in three of these years (1989, 1998
and 2000), and in Kołobrzeg ten of the absolute an-
nual minima in this 50 years stayed above 440 cm.
The range of variability of the absolute annual
minima reached to about 1 m in those 50 years: in
Wismar from 420 cm to 309 cm, in Warnemünde
from 431 to 331cm, in Sassnitz from 444 to 364 cm,
in Świnoujście from 445 to 366 cm and in
The 5 extremely deep minima of the time consid-
ered (1956, 1967, 1981, 1989, 1999) were the effect
of the same hurricane like storms. So was also the
case in the year 1979, when the decrease was excep-
tionally deep in Kołobrzeg, where the water fell
nearly as low as in Wismar and Warnemünde, what
usually is not the case (Fig.3). Those events were
due to very deep depressions with accompanying
hurricane like winds, passing across the Baltic Sea.
The long term variability of low sea level (H≤440)
shows much more low levels occurrences in the
western part of the coast than in the east. However,
in some years low levels were not recorded at all,
e.g. in 1989, 1998 in Sassnitz, Świnoujście,
Kołobrzeg (Figs 4a, b). Maximum number of low
sea level events (H≤440) had the year 1993 in the
whole southern cost, from Wismar to Kołobrzeg,
similarly was in 1979 and 1959. The number of low
level events varies from year to year, changing by as
much as two or three times. In general an increase of
low level events was observed at the turn of nineteen
fifties to sixties, in the seventies to mid-eighties and
also in the nineteen nineties. The behavior of the
general low sea level variation is given in Figures 4a
and 4b, by bars and by curves of 8 years moving av-
erages, calculated for Wismar and Świnoujście.
9 PROBABILITY
The Gumbel method is well suitable for evaluating
the probability of low sea levels. In Gumbel distribu-
tion, the extreme-value events are distributed asym-
metrically.
The following probability of non-exceedance
W(x) has been used as design basis for the occur-
rence of a low sea water event with the value x:
y
e
exW
−
−
=)(
and:
)( bxay −=
where: W(x) = probability of non-exceedance;
x = annual water level minima
from where it follows:
)bx(a
e
e)x(W
−−
−
=
Probabilities according to the Gumbel method are
computed by means of the above conditional equa-
tions using annual extreme water levels.
They are updated annually.
The recurrence intervals were calculated using
the absolute annual sea level minima in Wismar,
Warnemünde, Świnoujście and Kołobrzeg from the
years 1955-2005 and the above mentioned Gumbel
probability method. From the calculations it follows
that, for instance every 50 years a sea level slightly
below 330cm would occur in Wismar, which in
Kołobrzeg a 50 years value is as low as 390 cm.
A 20 years value in Wismar is as low as about
335 cm , while in Kołobrzeg is scarcely falls below
400 cm (Fig.5). The probability of occurrence of low
sea level on the southern coast decreases from west
to east.
The percentile is a measure which gives infor-
mation on the per cent of observations found below
a definite value. Figure 6 shows the percentile distri-
bution of monthly minimum sea levels for the south-
ern coast of the Baltic Sea.
203
Figure 4a. Long term variation of low sea level events H≤440 cm in Warnemünde, Sassnitz and Wismar, 1955-2005.
Figure 4b. Long term variation of low sea level events H≤440 cm in Świnoujście, Kołobrzeg, 1955-2005.
Figure 5. Low sea level as a function of statistical recurrence
from 4 gauge stations, 1955-2005.
25
th
percentile (lower quartile) is the value, below
which 25 percent of all observed water levels fall. In
Wismar 25% of all values was below 367 cm, in
Warnemünde below 392 cm, in Świnoujście below
407 cm and in Kołobrzeg below 414 cm. Upper
quartile (75
th
percentile), often used in statistical
characteristics, is the value of variable below which
there is 75% of all analyzed values. In the case of all
here considered stations, the maximum value of 75
th
percentile was 439 cm in Kołobrzeg and 427 cm in
Świnoujście.
0
10
20
30
40
50
60
1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 1995 1999 2003
Warnemünde
Sassnitz
Wismar 8 years moving average
(n)
0
10
20
30
40
50
60
1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 1995 1999 2003
Świnoujście Kołobrzeg 8 years moving average
(n)
300
330
360
390
420
450
480
0 25 50 75 100 125 150 175 200
time (years)
level (cm)
Kołobrzeg Świnoujś cie Warnemunde W is mar
Figure 3. Long term variation of absolute annual sea level minima in Warnemünde, Wismar Sassnitz, Świnoujście and Kołobrzeg,
1955-2005.
204
Figure 6. Percentile distribution of monthly lowest sea level at
the 4 gauge stations.
This means that in Świnoujście 75% of observa-
tions was below 427 cm. In Warnemünde 75% of
observations of monthly lowest sea level was below
414 cm and below 399 cm in Wismar. The 50
th
per-
centile is an equivalent of the median. In case of the
analysed stations the median amounts to: 431 cm in
Kołobrzeg, 419 cm in Świnoujście, 403 cm in
Warneműnde and 389 cm in Wismar.
10 CONCLUSIONS
Considered was the low sea level at the southern
coast of the Baltic Sea as a factor influencing the
navigation safety, the off shore engineering and in
general the maritime management. The range of the
actually met sea level oscillations may grow in face
of the expected global warming.
Extreme values of the lowest sea level met in the
presented paper, range from 309 cm in Wismar, in
the western area, to 370 cm in Kołobrzeg - central
part of the southern coast (that means 191 cm below
the mean sea level and 131 cm below the NAVTEX
warning level). Extreme rates of decrease can reach
50 cm / hour (or more).
Such conclusions point at the importance of the
warnings against very low level. This would be a
significant information when the under keel clear-
ance could be critical.
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500
0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10
percentile
sea level [cm]
Świnoujś cie Kolobrzeg W is mar Warnemunde
