Monitoring of the Iron Gate Hydropower and Navigation System on the Danube River - page 7

The Jaroslav Cerni Institute did the analyses of morphological changes within the riverbed of the Danube and its tributaries after all reservoir surveys. Riverbed changes determined by comparing two reference surveys represent the summary impact of morphological changes due to the realized water and sediment regime (sediment deposition and riverbed and riverbank erosion) and human intervention within the riverbed (dredging and river training).

The conclusion of this monitoring program is that the sedimentation of the IG1 reservoir is considerable and has substantial environmental and water management impacts, both within its range and on downstream reaches of the Danube. This conclusion is based on the following monitoring results:

  • Between 1972 and 2009 the input of suspended load into the IG1 reservoir by the Danube and its major tributaries was 619.3 million tons or approximately 16.3 million tons per year. The Danube River contributed with about 39% of suspended sediment, while the Tisza, the Sava and the Morava Rivers brought 26%, 21% and 14%, respectively;
  • Some 103.5 million tons of sediment (or 2.7 million tons per year) have passed through the turbines and over the spillways of the IG1 and IG2 dams;
  • The IG1 reservoir retained approximately 452.2 million tons of sediment (or 12.6 million tons per year on average). As expected, the greatest volumes of sediment were deposited within the reservoir during the first three years. In the following years, the ratio of the total deposited sediment to the total sediment inflow into the reservoir (trap efficiency) varied between 66% (1981) and 86% (1990), depending on water inflow from the catchment area and the HPP operating mode. During the entire period of operation, approximately 77% of incoming sediment was deposited within the IG1 reservoir. Along the most downstream 160 km-long stretch of the IG1 reservoir, some 434.9 million tons have been deposited. No significant siltation trend was noted in the upstream sectors of the Danube and its tributaries (on average, 500 000 tons of sediment were retained every year);
  • In the "immediate IG1 reservoir" (between the IG1 dam and km 1075 of the Danube), the Danube's natural regime was completely altered as soon as the reservoir was filled, and an intensive sediment deposition started. Riverbed surveys indicated that in the period 1976-1997 the highest sediment deposits were formed between km 970 and km 1003 of the Danube River (the Donji Milanovac zone, Fig. 14). In a channel expansion within the gorge, which was acting as a sediment trap, up to 15 m high sediment deposits were formed (Fig. 15). However, recent surveys of the riverbed (conducted after the 2006 flood and in 2010) indicate that the sedimentation zone is now shifting further downstream (between the dam and km 970). Obviously, the area between km 970 and km 1003 cannot retain sediment anymore, because it has reached a new equilibrium.
  • The IG2 reservoir is considerably smaller than the upstream one, and almost without impounding effects. The variable regime of the water flow causes movement of small particles and erosion of the river bed (Babic Mladenovic, 2006);

 

Fig14
Fig. 14: Changes of the Danube channel area (DF), km 943-1075

 

Fig15
Fig.15: Development of sediment deposits on a cross section of the Danube (km 991)

 

The most important effect of sediment deposition is an additional increase of high water levels, when compared with the flood levels immediately after impoundment. This impact is of crucial importance for the protection of riparian lowlands against flooding, which host a large number of communities, industries, and infrastructure.

The reconstruction of flood protection structures started recently, when hydraulic analyses exposed that the levees immediately upstream of the Gorge (starting from km 1042) are endangered by the additional water level increase approaching the freeboard values.