5. Flood protection

The first basic stage of the Danube regulation and flood control development was realised between 1759 and 1914. The main channel, which has been in use ever since, was created by regulation, started in 1831 and completed in the last years of the past century. Floods through breaches in the protected and reinforced levees have devastated large parts of the Hungarian Szigetköz area in 1954, and of Slovakia at Žitný ostrov in 1965. To appreciate the proportions of the disaster, it should be noted that in 1954 one-half of the Szigetköz area was flooded and the water rose to the second floor windows in the Bács district of Győr. The area flooded in 1965 was much larger in the Slovak Žitný ostrov territory (Fig. 5.1).

5.1. Area between Bratislava and Gabčíkovo, protection of Szigetköz area

The protection of the area downstream from Bratislava was incorporated into the Gabčíkovo/Nagymaros project which, based on a great deal of study and interpretation of previous flood events, provided further for the dissipation of flood waters through a precise water regulation system and the construction of dikes with underwater sealing screens to prevent the seepage.

In the section from Bratislava downstream to Gabčíkovo the flood water will be divided between the bypass canal, the Danube riverbed and the side arm system, thus allowing some dissipation of flood waters. In addition, appropriate safety margins were incorporated into the design, of Gabčíkovo and Čunovo structures so that water levels could always remain comfortably below retaining levels of the various structures. The Gabčíkovo structures protect mainly the Szigetköz area, flooded for example, in 1954.

5.2. Area downstream Gabčíkovo, protection of Žitný ostrov area

In the reach downstream from the Gabčíkovo power station (reach of the Nagymaros part of the Project) the risk of flooding is greatest, Slovakia is particularly vulnerable. Therefore the protective lines on the Slovak territory were reconstructed so as to meet the conditions issuing from the operation of the planned project at Nagymaros. In the reach between Gabčíkovo and Kližská Nemá the projected Danube riverbed deepening was not realised. This was one of the pre-conditions for the increase of flood control of the adjacent territory. Thus, the protective sealing walls were constructed additionally at Sap, but it is still necessary to strengthen the protective measures on the Slovak side in this reach of the Danube. The best way to protect this region, flooded extensively in 1965, would be the construction of a downstream dam (for example Nagymaros). This would allow the realisation of the projected deepening of the riverbed and thus an enlargement the Danube flow cross-section. The adjacent section on the Hungarian territory downstream of Győr is formed by the higher ground of the nearby foothills and there is therefore no need for dikes since flood waters are contained by the natural terrain. The higher ground and the hills bordering the Hungarian side of the Danube provide considerable protection against flooding, whereas the low-lying flood-plain on the Slovak side, by contrast, forms a natural outlet into which the ground water seepage and surface flood water can escape.

Downstream of Gabčíkovo and Sap, where the deepening of the river bed was not realised, protective measures against the seepage during the flood were implemented. The most dangerous situation occurs where the ground water flow direction is from the depth towards surface. Such situation usually occurs during the high water level in the Danube at places behind the protective dikes. Suffusion of fine sand particles upwards can result in the inner erosion and undermining and falling down the protective dikes.

The main measure is to decrease the vertical ground water flow velocity to the state, that sand particles are not washed out. This is usually done by deep wells which decrease the hydraulic pressure in the deeper part of the aquifer behind the protective dikes, or by construction of deep impermeable concrete or clay-cement wall and water-proof insulation between the wall and the top of the protective dike. Downstream from Sap both systems are used. The construction of an impermeable wall some 20 metres deep that is connected with the water-proof insulation between the wall and dike is shown on Fig. 5.2.