Flood study could save Vistula delta
Flooding due to ice blockages would be catastrophic for the low-lying Vistula river delta region in Poland. As part of an EU project to prevent flooding, a study has been undertaken by BMT Cordah, a subsidiary of British Maritime Technology. Andrzej Kozakiewicz, managing director of BMT Cordah (Poland) and Marek Szmytkiewicz of the Institute of Hydro-Engineering of the Polish Academy of Sciences in Gdansk reveal their results.
The Vistula delta lowland is partly depressive at 0.5 - 1m below the mean sea level. It is a hydrographically complicated system, which has been evolving for thousands of years and is typical for non-tidal or micro-tidal regions.
In the 19th century, catastrophic floods in the Vistula delta gave rise to the decision to construct a new direct river outlet called Przekop Wisly, cutting off of all other branches in 1895. Since then no serious floods caused by ice at the Vistula mouth have occurred.
However, each winter the services responsible for keeping the Vistula estuary clear of ice are on standby. Minimisation of flood threats is directly related to the maintenance of the mouth layout to ensure free outflow of ice to the sea. Proper maintenance of the outlet requires breakwaters which have to be elongated continuously.
The complicated shape of the alluvial fan creates favourable conditions for ice jams in winter. In order to maintain the required navigable depth for fishing boats, dredging activities are carried out, but only rarely.
The detailed bathymetric map of this region is prepared every late autumn for the sake of icebreaking activities in winter. The measurements comprise water depths from at least 15m up to the shoreline and they include 500m long river segment.
There is a significant annual variation of water depths around the Vistula's main branch. This causes great concern for the Regional Board of Water Economy in Gdansk, the governmental institution responsible for the maintenance of the Vistula river mouth.
Data analysis has been transformation into a common geodetic system (several systems were used in the period 1894-2005) and assessment of errors as various methods of data acquisition were used.
Exemplary original bathymetric maps from different periods are shown in Figs. 3-8. The situation of 1894, just before construction of the new mouth, is presented in Fig. 3. Fig. 4 shows the bathymetric layout of September 1895, not more than half a year after appearance of the Przekop Wisly.
The marine alluvial fan started to develop rapidly and half a year later the alluvial fan was 2km wide and reached 1.2km offshore. The calculated volume of accumulated sand amounts to 13,116 m3 million.
The most recent echo-sounding of 2002/2003 is plotted in Fig. 8. It can be seen that the measured area is much larger than the area of the alluvial fan. The breakwaters are 2350m and 2251m long at the east and west side, respectively. The calculated volume of the sand accumulated at the river mouth equals to 126 million m3.
The fan volume is increasing gradually all the time. Assuming constant averaged hydro-meteorological conditions occurring for the entire over 100-year period since the cross cut has been constructed, one can represent the fan ac-cretion by a straight line.
One can argue, however, if the linear trend should be adopted to describe accretion rate, especially in the last 50 years. Shown also in Fig. 9, the approximation of the fan volumes for 1896-2003 by the second order polynomial clearly indicates gradual decrease of the accretion rate.
The computations by the DELFT3D model for the bathymetry and river dis-charges of 2002 carried out by specialists from the Institute of Hydro-engineering in Gdansk have yielded the annual volume of about 0.6 million m3, which is almost identical to the one estimated on the basis of the bathymetric data for recent 25 years.
Protection and maintenance
The track records related to maintenance of the mouth of Vistula river reveals that regulation works were started in 1897 and led to construction of two breakwaters (eastern and western), the main objective was to remove small islands and to displace the shoals offshore as far as possible; these shoals were the main reason for occurrence of ice jams, with ice cover frozen to the river/sea bed, being an obstacle for the river flow. The records also show that breakwaters were elongated several times as the bathymetric system became unfavourable in front of the river mouth some time later.
To date, lengthening the breakwaters has been seen as the most efficient way to use river current to erode a channel in the near-shore shoals developed as a result of sedimentation. The effectiveness of this method has been reviewed often. In particular, it is of great importance to know how the breakwaters should be elongated, how this elongation will improve the bathymetry system and for how long the elongation will be effective.
On completion of the wider EU Operational Radar and Optical Mapping (OROMA) project, the database and web enabled tools developed within the project to analyse data have been passed to regional authorities responsible for flood protection in Gdansk area. The system has been installed at headquarters of the Regional Water Board in Gdansk and will be maintained by IT department.
Access to gathered data is given to all involved in the maintenance activities related to the area of Vistula river mouth. All the available historical documents have been gathered, analysed, processed into digital format and put into the database to allow for a thorough analysis.