Flood protection in the Upper Vistula river basin: grey and green measures implemented in the Sandomierz area

The main challenge in the Upper Vistula region is the high occurrence of floods. Precipitation and run-off in the area are respectively 15% and 50% greater than Poland’s average. Other elements determining high flood vulnerability are: a dense hydrographic network, steep slopes, the oval shape of the catchment area, high road intensity and high population density in the catchment area. Floods are of diverse origin: flash floods caused by intense and short-lived downpours, freshets caused by melting snow, and flood caused by ice dams. Several areas of the Upper Vistula region are vulnerable to floods. The highest flood risk is located on the right bank of the Vistula, upstream the town of Sandomierz which is home to about 24,700 people. The floods of May and June 2010 were caused by heavy rainfall and have resulted in collapsed levees in the neighboring area of Sandomierz, affecting this town and the neighboring towns of Tarnobrzeg and Gorzyce. In Sandomierz, an area of 1,154 ha was flooded, corresponding to 40% of its total extension. The area was flooded for about two weeks and the region had a standstill of several months. The flood losses were estimated at approximately EUR 100 million, while 3,000 people were evacuated and 4 died. Then, in July 2011, heavy rainfall caused another flood event, damaging the old town in Sandomierz, road infrastructure, the sewerage network and recreational areas. Total losses amounted to almost EUR 2 million. Although current flood risk is already high, climate change models indicated that this risk will increase tremendously. In the region of Upper Vistula, the additional area prone to flooding can extend up to 1,751 km 2 in case of a 1 in 100 year return period flood by 2050, including an additional urban area of 89 km 2 .

The main objective of the adaptation measures implemented in the Sandomierz area is to increase the retention capacity and to reduce flood risk in and around this town, also in light of future climate change. Other objectives that were considered in the design of the solutions are: protection of public health and safety, biodiversity conservation, environmental objectives with regard to water quality, protection of soil, landscape and cultural heritage protection, and economic prosperity.

The adopted solution to reduce flood risk and to improve flood protection in the area of Sandomierz is a hybrid and integrated approach of green and grey infrastructure measures, including some institutional measures as well. These solutions were designed within the scope of the Vistula River Management Basin Plans, which are part of the implementation of the European Water Framework Directive and the European Flood Directive. Renaturalization of reservoirs and wetland restauration are considered green measures to reduce exposure of people to flooding. These adaptation measures are implemented in 15 locations along the Upper Vistula River and aim to increase the controlled water retention capacity of the area. Indeed, naturalized reservoirs and restored wetlands are used to capture the surplus of river water and therefore to reduce the risk of flooding. These green measures are combined with grey ones: expansion, reconstruction and modernization of river embankments, specifically aimed to further protect urbanized areas from flooding reducing the risk of water overflow. The existing embanked area is enlarged to include the entire area that can be flooded during high water levels. Moreover embankments are raised up to a maximum elevation of 1.4 m, ensuring that embankment crowns along both river banks are adapted to the same safety level. Modernization intervention includes also the (re)construction of water pump stations and water discharge channels, that help to discharge the water in excess.

The total cost of the adaptation measures implemented in the Sandomierz area is about EUR 217 million. This includes costs incurred to implement the expansion, reconstruction and modernisation of the river embankments (grey measures) as well as the renaturalisation of reservoirs and wetland restauration (green measures), including the needed land purchases. Funds for implementing these measures were requested and received from the World Bank. The avoided flood damage to buildings (5,632 buildings in total) was estimated to be about EUR 445 million in constant values. Taking into account only this estimate, and therefore not considering impacts on people, the benefit-cost ratio is 2.05, implying that expected benefits are at least approximately twice larger than costs (DHI Polska, 2013, Raport z opracowania programu działań dla Regionu Wodnego Górnej Wisły, Report KZGW/ DpiZE-op/POPT/1/2013, DHI Polska, Warsaw, Poland; EEA, 2017 ).

One of the major drivers for the implementation of the flood protection measures was the direct experience of local communities being affected by recent floods (2010 and 2011). This has contributed to increase the sense of urgency for the implementation of measures contributing to cope with the flooding challenges. The EU Water Framework and the Flood Directives also played an important role in initiating the integrated process aiming to reduce the flood risk. As alternative to traditional measures, these Directives have been also relevant in promoting green measures aiming to increase the retention capacity of the area while respecting their natural values. During construction the natural value of the area had to be maintained. Therefore, the impact on natural habitats and habitats of species for community interest, according to Natura 2000 policies, had to be monitored. Another factor affecting the measure implementation is changes in land entitlement that had to be organised in order to expand the embankments.