Near the geographical centre of Europe, the Tisza drains an area of 157,218 km2; the Tisza River Basin (TRB) has a population of 14.4 million. On its route from the Ukrainian Carpathian Mountains to the confluence with the Danube in Serbia, the Tisza flows mainly through Hungary’s Great Pannonia plain. The topography of the TRB is characterized by high, narrow chains of mountains surrounding expansive, flat lowlands. With a length of 966 km and an average discharge of 794 m³/s the Tisza is the Danube’s longest and second largest tributary. Most discharge is generated directly from rainfall but there is a contribution from both snowmelt and subsurface soil water. Serious floods can originate from the mountains when rainwater flows quickly down the slopes and accumulates in lowland areas. This problem has become more and more serious over time as deforestation and soil sealing had progressed and precipitation patterns changed.
The river and its tributaries were regulated in the second half of the 19th century. The main purpose of this regulation was to increase the extent of agricultural land, in place of wetlands, marshes and areas at risk of regular flooding (Borsos et al., 2010). The length of the river was reduced by over 400 km as the meandering sections were cut through, while the size of the floodplain area decreased by over 90% as dikes were raised to protect against floods. As a consequence, peak flood water levels increased. As a result of the impacts of additional factors, especially deforestation in the river basin, related increased sedimentation in slow flowing river sections and changing climate, as well as the fall back of agricultural activity in floodplains, peak water levels measured during floods continue to rise. Taking into account a few historical recorded flood events, peak water level was 753 cm in 1876, 909 cm in 1970 and 1040 cm in 2000 (Sendzimir and Magnuszewski, 2009).
Flood defence is based on the runoff capacity of the flood-water riverbed (the permeable capacity of the cross section between the dikes). This capacity has been increased step-by-step since the river regulation works of the 19th century to cope with the increasing peak levels of floods. Today, the length of the flood defence dikes along the Tisza and its tributaries in Hungary is 2850 km. The size of the protected territory is 16,000 km2, out of the total catchment area of the Tisza in Hungary of 47,000 km2. As the peak level of floods continued to increase during the last century and a half, so did the height of the dikes. A further rise of peak flood levels is projected for the 21st century as a consequence of climate change, and it is clear that the current level of flood embankments will not be sufficient to provide adequate protection. Flood defence exclusively as a result of enlarging and strengthening of embankments is estimated to be excessively expensive. In 1999 a World Bank financed research project estimated the cost of the remaining upgrade works to be HUF 175 billion, equivalent to EUR 700 million at a 1999 exchange rate (Szlávik, 2001).
In the period 1998-2001 four serious flood events took place on the river Tisza with peak water levels exceeding all historical values. One of the events (2001) included the rupture of a dike, flooding areas that were supposed to be protected, thereby pointing out that neither the height of the dikes, nor their strength are adequate. Afterwards, a 4-year-long project was launched to investigate the validity of the flood risk projections used at the time (VITUKI, 2006). Besides applying novel methods of time series simulation processes and a revised historical hydrological database, the project considered the impact of changes (in forest cover, reservoirs, and flood embankments) within the different sections of the rivers passing through Hungary, including also climate change (Haase et al., 2006). The key conclusion of the project was that compared to previous projections there is increased uncertainty and higher expected water levels during floods. The land use change in the basin caused an increased surface runoff, while the modifications of the river banks caused an increase of the water speed in some parts of the river; these factors impacted on the height of the water level during extreme events. During an extreme precipitation event, a 100 years return period (an engineering measurement of probability to have an event of similar magnitude in a time period; in this case it indicates a probability of occurrence of one in a century) streamflow provoked an increase of the water level of around 70 cm, in comparison to earlier values of between 900 and 1000 cm. These values are expected to further increase as consequence of climate change. Koncsos (2006) also points to the role of mud accumulation in the river bed which considerably raises the level of water in a long time horizon measured in decades (average value of 0.77 cm/year) (Schweitzer, 2001).