River Sediment Transport in Serbia - page 03

Fig05
Figure 5: 1986 annual hydrograph and sediment load graph, Lim River at Prijepolje.

 

Fig06
Figure 6: Hydrograph and sediment load graph of flash-flood wave in February 1986, Lim River at Prijepolje.

 

Fig07
Figure 7: Water discharge duration curve, Lim River at Prijepolje.

 

Fig08
Figure 8: Sediment transport duration curve, Lim River at Prijepolje.

 

Integration of the function P(t) resulted in the annual suspended sediment discharge on the Lim River: P = 1,123,000 t/year.

In the case of the Lim River it was possible to check whether the estimated annual rate of sediment transport was realistic. Namely, the Potpeć Reservoir retains most of the sediment load of the Lim River. The reservoir sedimentation was surveyed a few times between 1967 and 1999 (Table 2).

 

Tab02

 

Table 2 shows that the Potpeć Reservoir had a serious sedimentation problem, given that more than 40% of its storage volume was lost between 1967 and 1999. On the other hand, it is also evident that the sedimentation rate has slowed down over the lifetime of the reservoir. Initially, the sedimentation rate was 0.9 million m3/year, but subsequently declined to 0.35 million m3/year.

The reservoir sedimentation data could also be assessed in the context of sediment discharge estimation. If the annual suspended sediment discharge is enlarged by 10% (usual proportion of bed-load in total sediment load), the total sediment load is 1,200,000 tons/year or 1,000,000 m3 (density of sediment deposits is 1.2 t/m3). The comparison of these figures with the average annual sedimentation rate of the Potpeć Reservoir in the initial period (890,000 m3) demonstrated that the estimates were realistic, keeping in mind that the reservoir could not retain more than 80% the total sediment load (or some 800,000 m3) since a part went through the power plant turbines and over the dam.

 

Vlasina and Toplica rivers, tributaries of the Južna Morava

The Vlasina River drains an area of 880 km2. The suspended sediment database covers the period from 1962 to 2002, and reveals that the highest suspended sediment concentrations (C) were recorded in the spring, from March to May, and the lowest from September to November. The extreme daily values of C were: minimum of the order of 10-4 g/l, and maximum 101 g/l, which is a broad range.

Variations of river discharge and sediment concentration in 1976, which was characterized by high suspended sediment loads, are presented in Figures 9 (entire year) and 10 (flood wave in June 1976, Cmax= 23 g/l).

The Toplica River, up to the gauging station Pepeljevac, where sediment is monitored, drains an area of some 1000 km2. The suspended sediment database covers the period from 1973 to 2002. The highest sediment concentrations (C) were recorded from February to May, and the lowest from September to November. The range of values is similar to those of the Vlasina River.

Figure 11 presents the variations in river discharge and suspended sediment concentration in 1979, during which very high suspended sediment loads were recorded. Similar data are presented on Figure 12 for a flood wave in November 1979 (Cmax = 35 g/l).

The total annual suspended-sediment loads transported by the Vlasina and the Toplica were determined from sediment transport duration curves P(t), prepared from the P(Q) relations (shown on Figs. 13 and 14) and taking into account the duration of river discharges (given for both rivers on Fig. 15).

River discharge duration curves of the Vlasina and the Toplica (Figure 15) are very similar, due to their comparable size and geomorphology. However, the Vlasina is of a more torrential nature, with a wider span between low and high flows (e.g. major flood in 1988 had peak discharge as high as 500 m3/s or more). The Toplica features a broader range of low and medium flows, but a narrower range of high flows.

 

Fig09
Figure 9: Variation in river discharge and suspended sediment concentration in 1976, Vlasina River at Vlasotince.