Ice Regime Variation in the 20th Century Along the Serbian Sector of the Danube and Assessment of Global Climate Change Impact - page 02

Effect of Hydraulic and Morphological Changes

The hydraulic and morphological conditions of a river, along with climate parameters, constitute the main drivers of a river's ice regime. Especially important hydraulic/morphological parameters are mean flow velocity (V) and width (B). A specific parameter of the ice regime – the ice conveyance capacity of the river channel (V·B) – was defined based on analyses of ice flow dynamics and continuity. Using plots of variations in this parameter along the river at a certain discharge, it was possible to determine critical zones from the ice regime perspective. Minimal values of (V·B) indicated ice flow "bottlenecks".

Following damming and impoundment for the purposes of the Iron Gate 1 HPP (at rkm 943), the hydraulic conditions along the reservoir, which extends all the way to rkm 1255, became radically altered. As the river stages rose, the flow velocity of the Danube (V) declined. This had the greatest impact on (V·B), which was reduced, while the increased width of the river channel (B) along the reservoir had no significant effect. Figures 5 and 6 show flow velocity and (V·B) as a function of discharge (Q) along several reaches of the reservoir, for both natural and impounded flow regimes of the Danube.

 

Fig05
Figure 5: Change of flow velocities after the impoundment of the Iron Gate 1 reservoir.

Fig06
Figure 6: Change of ice conveyance capacity after the impoundment of the Iron Gate 1 reservoir.

 

It is obvious in Figure 5 that the effect of damming on hydraulic parameters declines as the distance from the Iron Gate 1 HPP increases. The reach from rkm 1010 to 1020 exhibits the largest difference in flow velocity between the natural and impounded flow regimes: at 8000 m3/s, the impounded flow velocities were found to be about 50% lower than in the natural regime, along the reach from rkm 1060 to 1070 they were about 40% lower, and along the reach from rkm 1110 to 1120 the difference was about 30%.

The ice conveyance capacities of the river channel (V·B) are not changed in a similar way, due to increased river channel width as a result of elevated water levels.

All the above led to the conclusion that the Iron Gate 1 HPP has had a fundamental effect on the ice regime in the Serbian sector of the Danube. Above all, the dam totally changed the ice flow boundary conditions in two ways: it acts as a physical barrier to the incoming ice (due to problematic evacuation of ice over the dam spillway), and as a hydraulic barrier (due to low flow velocity and ice discharge capacity (V·B) immediately upstream from the dam. Additionally, the hydraulic and morphological conditions of the ice regime along the lengthy reservoir (more than 300 km from the dam) were also altered to a large extent. The lowered ice conveyance capacity of the river channel (compared to the Danube's natural regime) tends to hinder ice travel from the upstream sector of the Danube.

 

Effect of Thermal Pollution on River Cooling

The river cooling process begins in the autumn and is accelerated with the onset of sub-zero air temperatures. The rate of the river cooling down to 0°C, or to the time the water begins to change its physical state, depends on weather conditions and the downward mean daily air temperature gradient. In the case of a sudden cold wave and several days of low average temperatures, the time interval of water temperature decline from 3-5°C (most frequent before the onset of sub-zero mean daily air temperatures) to 0°C takes 5-15 days.

The river cooling phenomenon is illustrated in Figure 7 by means of chronological plots for severe winters in P1 (1955/56) and P2 (1984/85 and 1986/87) at the hydrological station of Bezdan.

For comparison with other ice regime parameters, Figure 7 also shows sums of negative air temperatures (SNAT) and river discharges. It is apparent from the plots that a certain general rule governs the river water cooling process, from the time of occurrence of sub-zero mean daily air temperatures to the time the water temperature drops to 0°C (T). Two important factors drive the rate of the process and the duration of this period. They are the initial condition (water temperature at the time of onset of sub-zero air temperatures) and the downward air temperature gradient, geometrically represented by the slope of the SNAT curve. Although the initial water temperature in the studied winters was similar (3-4°C), the duration of the cooling processes differed considerably: from 6 days in 1955/56 (P1) to 14 days in both winters of P2.

A significant difference was noted in the river cooling parameters between P1 and P2. The cooling interval was shorter by a factor of 2-3 in P1 than in P2, although the hydrographs were similar. Given that the differences in the Danube's thermal regime between P1 and P2 could not be correlated with hydrological drivers, the conclusion was that the cause was anthropogenic in nature (i.e. thermal and chemical pollution).

"Thermal pollution" means the discharge of warm water (whose temperature is higher than that of the river water), and "chemical pollution" is the discharge of wastewater into the river. The hydrological station of Bezdan is located at the point of the Danube's entry into Serbia. Upstream from the Serbian-Hungarian border, the Danube flows through highly industrialized areas in Austria, Slovakia and Hungary. The largest thermal polluters are the Hungarian nuclear power plant at Paks and district heating plants along the Danube. In the 1980's, the Danube entered Serbia with a high level of chemical pollution, which affected the ice regime.

It should be noted that the thermal regime of the Danube downstream from the mouth of the Sava is potentially affected by another two factors. Namely, the Sava is warmer than the Danube, such that ice formation on the Sava is of a much lower intensity than on the Danube. The cause of this phenomenon is certainly the Nikola Tesla Thermoelectric Power Plant, and another possible cause is the thermal effect of groundwater in the riparian lands of the Sava. Although the effect of the Kostolac thermoelectric power plants on the thermal regime of the Danube has not been studied, they do impact the ice regime near the mouth of the Mlava River.