Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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- 178 - 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
Figure 1. The area of Gdańsk subject to flood in July 2001 than 5000 people received special calamity status, which deserve social assistance. 5. The rescue action It was first necessary to rescue people and their property from complete damage and destruction. The next step was to repair the embankment of RCh to prevent further flooding of downstream areas. Simultaneously, pumping was carried out to drain the flooded areas. A difficult and slowly proceeding action was to repair the streets and remove sediments brought-in by the flowing water. Basements of numerous houses were flooded and required draining and drying. After some days, the risk of diseases caused by damaged household facilities was appearing. 6. Planning for the future The following steps were undertaken. – The repair of the embankment of RCh and increase of the conveyance of the channel; this is a difficult task, as numerous ecological organizations were against cutting off the trees which grew naturally on the embankment. – Development of a comprehensive mathematical model for the calculation of hydraulics in the - 179 - complicated system of channels, streams and artificial reservoirs, the model was based on 1D MIKE 11 unsteady flow. – Exact measurements of all rivers and channels in order to estimate their geometry and conveyance. Analysis of the existing spatial situation of the city indicates that present infrastructure prevents from making any general change in the network of channels. It was found out that there is no possibility to increase significantly the conveyance of RCh. According to hydraulic calculations it was decided to construct many small artificial impounding reservoirs on all streams discharging to the RCh, in order to decrease the inflow to the channel in cases of intensive precipitation. Simultaneously it was proposed to construct several bypasses from RCh to the Radunia and Motława Rivers to diminish the flow in the channel in case of intensive precipitation. It is also necessary to install several precipitation and water level gauges in the system of channels and this way to create flood warning system. All these measures are very costly and construction of them will take a long time but at present there is no other solution.
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Fourth Stu
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Preface The 4 th Study</str
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- I - Table of Abstracts Title Auth
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- III - Sensitivity in Calculation
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- V - Parameter Estimation of the S
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- VII - The Realism of the ECHAM5.2
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Adam, W. K. .......................
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- 1 - Activities of the GEWEX Hydro
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eference site archive (Cabauw, Neth
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- 5 - Remote Sensing of Atmospheric
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minute averages around the satellit
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- 9 - Precipitation Type Statistics
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- 11 - Assimilation of New Land Sur
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Multichannel Microwave Radiometer f
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output has been processed in an equ
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height dependence of the Z-R-relati
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- 19 - CERES and Surface Radiation
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- 21 - Coastal Wind Mapping from Sa
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- 23 - Clouds and Water Vapor over
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- 25 - Broadband Cloud Albedo from
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- 27 - Observation of Clouds and Wa
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shows a ground-track of the vessel
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- 31 - Determination and Comparison
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- 33 - Spatial Variability of Snow
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- 35 - EVA-GRIPS: Regional Evaporat
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- 37 - LITFASS-2003 - A Land Surfac
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-39- Calibrated Surface Temperature
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- 41 - The Marine Boundary Layer -
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- 43 - Characteristics of the Atmos
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Figure 2. Surface stress from two d
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During the experiment the water was
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While the number of smallest drops
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SCANDIA will not be supported any l
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- 53 - Relationships Between Precip
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- 55 - Analysis of the Role of Atmo
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- 57 - Meteorological Peculiarities
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Baltic Sea Inflow Events Jan Piechu
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- 61 - The Different Baltic Inflows
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- 63 - Observations of Turbulent Ki
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- 65 - The Influence of Synoptic Si
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-67- Improved Method for the Determ
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-69- The Helicopter-Borne Turbulenc
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- 71 - CEOP Reference Site Data fro
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- 73 - The BALTEX Hydrological Data
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- 75 - Hydrological and Hydrochemic
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-77- Sea-level Monitoring at MARNET
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1 0.8 0.6 0.4 0.2 GO(CLD-M) ERA40 S
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Figure 2. Monhly mean values of lat
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In the case of an ideal fit, the co
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- 85 - Influence of Atmospheric For
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The largest inter-annual variabilit
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References Andrejev, O, Sokolov, A.
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On the other hand, if the stratific
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Cyberska 1989, Cyberska and Krzymin
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- 95 - Continental-Scale Water-Bala
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- 97 - ICTS (Inter-CSE Transferabil
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The subsurface flow calculation is
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functions only data with higher qua
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- 103 - Modelling the Impact of Ine
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- 105 - Objective Calibration of th
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- 107 - Validation of Boundary Laye
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- 109 - Simulated Dynamical Process
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spreads along isobaths (fig.2). As
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than the precipitation pattern of J
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Figure 2. Long-term variability in
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obvious from temperature charts. Ho
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shortening of the winter seasons by
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Maximum 5 day precipitation (mm) Nu
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scale parameters the correlation be
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similar: the locations of main mini
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Page 145 and 146: - 129 - Trends in Wind Speed over t
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Page 151 and 152: Figure 3. Cross section of salinity
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Page 155 and 156: Figure 2. Time series of Mean Sea L
Page 157 and 158: - 141 - Calculation and Forecast of
Page 159 and 160: - 143 - An Overview of Long-Term Ti
Page 161 and 162: - 145 - Baltic Sea Saltwater Inflow
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Page 165 and 166: Lindström, G., Johansson, B., Pers
Page 167 and 168: - 151 - Air-Sea Fluxes Including Mo
Page 169 and 170: - 153 - The Realism of the ECHAM5.2
Page 171 and 172: - 155 - Figure 3. Accumulated total
Page 173 and 174: Figure 2. Example for Fourier decom
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Page 177 and 178: conditions even more challenging th
Page 179 and 180: surface heat fluxes close to zero.
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Page 183 and 184: - 167 - Present-Day and Future Prec
Page 185 and 186: - 169 - Extreme Precipitation on a
Page 187 and 188: at the stations Pärnu (Estonia) an
Page 189 and 190: 6. Results There are many possible
Page 191 and 192: and vegetation, and to derive the h
Page 193: The structure of water bodies cadas
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Page 199 and 200: - 183 - Generating Synthetic Daily
Page 201 and 202: The evapotranspiration is affected
Page 203 and 204: Model parameters and coefficients:
Page 205: 3. Hydrodynamic model of nutrient l
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Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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