Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Flood in Gdańsk in 2001, Reasons, Run, and Mitigation Measures
Wojciech Majewski
Institute of Hydroengineering, Polish Academy of Sciences, Kościerska 7, 80-953 Gdańsk, Poland
e-mail: wmaj@ibwpan.gda.pl
1. Introduction
Gdańsk is a city of more than a thousand years history.
It is situated on the Baltic coast at the mouth of the
Vistula River. Gdańsk has 450 thousand inhabitants and
is a very important economic, cultural, scientific and
industrial centre. The city is situated on the lowland area
and is one of the most flood endangered urban
agglomerations in Poland. Any flood in Gdańsk,
resulted in considerable economic losses. Its lowland
position causes that removal of floodwaters is very
difficult and takes a long time.
In the XVIIIth and XIXth centuries frequent floods
inundated Gdańsk. These were mainly winter and spring
floods caused by ice jams, which formed on the western
arm of the Vistula. Therefore in 1895 the final section of
the Vistula was transformed into a direct channel to the
sea protected on both sides by flood dykes to facilitate
the flow of ice to the sea. Since that time, there has been
no serious flood in Gdańsk. However, flood danger still
exists. It may come from a storm on the Baltic Sea,
which may cause an increase in water level in the
Martwa (Dead) Vistula branch. The second flood
possibility is from the main channel of the Vistula in
case of very high discharge and breach of flood dykes.
The third danger is from the moraine hills situated to the
south of the city. The first two possibilities were always
considered as most probable. However, recently engineers
pointed out that moraine hills might constitute a danger
of severe floods. The flood, which occurred in Gdańsk
in July 2001 was a typical urban flash flood caused by
intensive rainfalls and came from moraine hills.
2. Description of the hydraulic characteristics of the
area subject to flood
Expansion of the city of Gdańsk in recent years
stretched towards the moraine hills with their slope
directed towards the city. This area was used for new
housing developments with new streets and parking lots,
which caused a decrease in the natural water retention
capacity. At the foot of the moraine hills, there is an
artificial channel, the Radunia Channel (RCh) built in
the XIV century to supply the city with water. The RCh
has an outflow from the Radunia River in Pruszcz
Gdański, where a small hydraulic power plant operates.
Discharge in the channel is controlled by hydraulic
structures. The length of the RCh is 13.5 km and its
catchment totally on the left hand side amounts to
55 km 2 . The channel has an embankment on the righthand
side and runs parallel to the main road leading to
Gdańsk from the south. The area on the right bank of the
channel is occupied by the old urban part of Gdańsk
lying in a depression.
There are several small natural streams and outlets from
storm drainage networks. Their discharges in normal
conditions do not exceed one cubic meter per second.
The RCh thus plays the role of an artificial storage
reservoir for water coming from the left catchment area.
The total volume of the RCh is estimated in about 0.3
mln m 3 . The bottom width of the channel is about 8 m
and it has an almost rectangular cross-section. The slope
of the channel is 0.5‰. In 2001, the channel was in a
very bad state. Both shores were overgrown with bushes
and trees. There were several places with high silting.
The conveyance of the channel at the maximum depth of
2.7 m was estimated at about 20 m 3 /s. RCh has an
artificial outlet to the Motłava River. The right
embankment of the channel has a crest width of 3 to 5 m
and height of 4 to 5 m. The inner slope of the channel is
in many places protected by means of concrete slabs
supported on sheet piles. The layout of the area
subjected to the flood in Gdańsk in July 2001 is shown
in Fig. 1.
3. Precipitation regime
The average annual precipitation in Gdańsk is about 600
mm and, the July average is 68 mm. In recent decades, it
was observed that maximum daily precipitation for a
particular year occurred in July. Before 2001, the
maximum daily precipitation was recorded in July 1980
and amounted to 80 mm. Precipitation in Gdańsk is
highly non-uniform in space and time. There were
frequent intensive rainstorms, which, however, covered
only small area. There are only three stations for
measuring precipitations in Gdańsk, which are operated
by the National Meteorological Service. They are
unfortunately located on the city boundaries, and,
therefore, it is very difficult to estimate spatial
distribution of precipitation over the whole area of
Gdańsk.
On the 9th of July over a period of 4 hours (from 14,00
– 18,00) practically the whole catchment area of the
RCh (about 50 km 2 ) received 80 mm of precipitation.
The daily amount of precipitation on 9th July was
120 mm, this value was estimated to have a probability
of 0.5 to 0.3 % (once in 200 - 300 years).
Simplified calculations indicate that the side inflow to
the RCh from its left side catchment was about 100 m 3 /s
over 4 hours. The conveyance of RCh with depth of
flow 2.7 m was estimated for 20 m 3 /s. The total amount
of water discharged in 4 hours to RCh was 1.44 mln m 3 .
4. Flood in Gdańsk
As the result of intensive precipitation, the following
structures were destroyed or inundated:
– the embankment of RCh was breached in 5
places, which resulted in flooding of the area
of the city situated in the depression on the
side of the channel and the main road;
– two main roads approaching Gdańsk from the
West turned into torrential rivers;
– Gdańsk main railway station was flooded,
which caused one week’s break in traffic;
– the main embankment of the small reservoir
on the Strzyża Stream was breached, which
resulted in a severe flood along the main street
and flooding of the crossing on the road
between Gdańsk and Gdynia.
Losses in the city infrastructure were estimated to about
50 million USD. More than 300 families were affected
by the flood (damaged houses, loss of property). More