2. Danube at Bratislava

For over 300 years, the Danube has served as an avenue for commerce and as the basis of the economic development of its riparian States. Its waters have been managed and extensively utilised by these States. The region of the Danube downstream Bratislava, along the Slovak-Hungarian boundary has also become an increasingly developed area. It is high urbanised, intensively farmed, and the forests between Bratislava and Budapest have long been managed so as to produce industrially useful wood, leading to the gradual replacement of the original species. This, along with the extensive navigation and flood protection works, both in the region downstream Bratislava and also in upstream States, has created specific environmental effects.

The Danube water flow velocity just over the granite threshold between Alps and Carpathians is very high, it reaches 2 ? 5 Fig. 2.1. Downstream from this threshold, as a result of construction of dams and dredging of the Danube upstream Bratislava, erosion of the river bed prevailed during pre-dam conditions. At present the slope of the water level and flow velocities are lowered by the Gabčíkovo structure?s impoundment and equilibrium is reached between erosion and sedimentation in the Bratislava stretch of the Danube, or sedimentation prevailed on some stretches.

2.1. Some characteristics of the Danube

The Danube is 2850 km long and it passes through the central part of the Europe.

The Danube?s spring source is to be found in the lower foothills of Baden-Wü rttemburg, in the middle of the Black Forest (Schwartzwald), at an altitude of 1078 m.

On the long way down, the Danube passes through several countries, important regions and towns, and connects some 70 million people.

The Danube also creates a natural border between states along several parts of its flow, including the Slovak-Hungarian border, between Čunovo-Rajka and Štúrovo-Estergom.

The average flow volume is 6500 m³/s at the estuary of the Danube, 2340 m³/s at Budapest, 2025 m³/s at Bratislava and 1920 at Vienna. For comparison, the flow in the Rhine river, downstream of Basel, is 1000 m³/s. The minimum flow volume measured at Bratislava is 570 m³/s and maximum 10 700 m³/s.

The largest estimated Danube discharge in Vienna was some 14 000 m³/s at the time of the flood of 1501 when the water level was some 1.8 m above the water level at the time of the 1954 flood.

In 1954 the river flooded some 33000 hectares in the Szigetköz area and in 1965 it flooded some 114 000 hectares (peak discharge of 9170 m³/s) in Slovakia.

The first big dikes, designed to protect agricultural land and settlements from floods, were constructed under the rule of Béla IV. in 1235-1270, downstream from Bratislava, at Gö nyü .

The Iron Gate hydropower project is a joint Romanian and Yugoslav project, and it is also the largest hydroelectric power station in Europe with an installed capacity of 2050 MW.

Systematic observation of water stages and discharges started in Bratislava in 1823, and records are available dating from 1876.

2.2. Danube and Drinking water Supplies

The central depression of the Danube Lowland is made of water bearing sediments, gravel and sands. These sediments constitute one of the most important aquifer complexes in Central Europe. In terms of recharge of this aquifer, the dominant factor is the Danube. It influences the intensity of aquifer recharge (by infiltration, speed and direction of ground water flow and also the chemical composition of the water in the aquifer. Thus, in terms of ground water quantity and quality in the aquifer, water level and water quality in the Danube are the major factors.

Typical natural Danube riverbank exists, for example, in front of the waterworks well fields of Pečniansky les, supplying Bratislava by some 600 l/s of drinking water (Fig. 2.2). Wells are situated, on average, 120 m from the Danube. Situation of municipal waterworks using the aquifer recharged by the Danube water is on Fig. 2.7.

2.3. Petržalka suburb

The technological skills related to dam construction, and the deeper knowledge of the hydrology conditions, have been combined with the experiences from many flood disasters. In the course of the 20^th century, there were large floods in 1929, 1947, 1954, 1959, 1965, 1975, and 1991 which all provided opportunities to study the changes of the hydrological conditions in the upper part of the central Danube. During floods, but also during normal high water levels in the Danube, the water masses quite often penetrated through the dikes on the right side Danube area at Bratislava, mainly the area of the village and later a suburb of Bratislava, Petržalka. The previously small village has grown into the suburb of Petržalka having some 150 000 inhabitants. In connection with the development of the Petržalka, the objectives to river training were significantly changing. The first priority is given to the flood protection.

Because the river bed between Bratislava and Petržalka is narrow, only a few measures were available. On the Bratislava Danube?s left bank quay under the castle have been increased in height by 1 to 2 metres and river banks have been additionally strengthened. On the right Danube side, in Petržalka, the protective dikes have been sealed and reconstructed to the same level (Fig. 2.4). But this was not sufficient. The only possibility was to increase the Danube water flow cross-section by means of deepening the river bed. This has decreased the Danube water levels, decreased passage possibilities for ships into the harbour, decreased discharges into the Malý Danube and increased the possibility of erosion of the Tertiary sands situated beneath the quaternary gravel. The situation was partly solved using the technical possibilities given by the construction of the Gabčíkovo part of the Project.

The present flood protection of Bratislava and Petržalka, consisting of increasing and reconstruction of left and right side quays and enlarged the Danube water flow cross-section. By means of water impoundment by the Čunovo structures, the Danube water level corresponding to low Danube discharges have been increased to stages 1 to 2 metres higher than in pre-dam conditions, thus passage for ships into harbour is assured. Water levels up to 6000 m³/s are in Bratislava similar to natural water levels. During larger discharges of the Danube, characterising the flood situation, gates on Čunovo inundation weir are open and thus water levels in Bratislava are reduced in comparison with the long term pre-dam conditions (because the water flow cross-section in the river bed is enlarged).

In the Petržalka suburb the inundation by inland water during and after the flood event (seepage water by ground water level increase), is of special importance. The Petržalka is protected by new dike with impermeable wall, downstream the Old Bridge. The function of the Chorvátske rameno river arm (Fig. 2.3) was to take away surplus seepage water, during and after the flood. After decrease of ground water level in the pre-dam conditions the Chorvátske river arm was mostly dry or partially filled with stagnant stinking water. After putting the Gabčíkovo system into operation ground water level increases and water flow in Chorvátske rameno river arm was re-established. Because the area around the river arm was developed, works are prepared to increase the minimum flow and to re-naturalise further the area along this river arm.

2.4. Bratislava Harbour

The scope of problem of navigation may be seen in the fact that in the Bratislava section of the river, the minimum navigation depth of 2.5 m was guaranteed in terms of navigable days for just 51 % of 1984 and just 40% of 1991. The percentage availability for each year from 1980 to 1991 is shown in the following table

Tab. 2.1. Percentage of days with full navigation possibility at Bratislava

Year Navigation possibility % 1980 64 1981 88 1982 73 1983 61 1984 51 1985 65

Year Navigation possibility % 1986 54 1987 66 1988 62 1989 50 1990 46 1991 40

The technical experts of the Danube Commission, on the meeting held 7 ? 15 December 1992 stated: "During period of low water levels in a series of ford sections (including in the Bratislava ? Nagymaros sector), the minimum depths were down to 1.3

The most significant positive impact of the impoundment of the Danube water level is the navigability of the Danube in any discharge conditions and the avoidance of a gradual failure of the harbour due to erosion of the riverbed, which would lead to a significant investment to remedy. By raising the water level in the Čunovo section to 131.1 m a. s. l. elevation, the effectiveness of the harbour will be ensured during the whole year (Figs. 2.5 and 2.6).