River Sediment Transport in Serbia - page 05

Overview of Suspended-sediment Transport Along Small and Large Watercourses

Long-term suspended sediment monitoring has been conducted on ten rivers in Serbia, such that there is a relevant sediment database. The monitoring network covers a broad range of catchment sizes, from 145 to 37,320 km2. This has facilitated assessments of the effect of catchment size on sediment transport processes and provided a general picture of the processes.

Figure 17 shows the correlations between unit sediment transport (g) and catchment size (Аb) for all the studied rivers. The g(Аb) envelopes for Serbia, determined on the basis of the sediment transport and reservoir aggradation data, are also indicated. It is obvious that most of the studied rivers are in the middle of the envelopes of extreme values of g(Аb). It is also evident that the span between two envelopes includes all the (g, Аb) pairs, meaning that the span reflects the characteristics of sediment formation and transport within the territory of Serbia.

 

Approximated Sediment Budget for Serbian Territory

Serbia's sediment monitoring network is not all-encompassing, such that not all the rivers which are significant contributors to the sediment budget are covered. However, it was possible to approximate Serbia's sediment budget based on the existing sediment database, along with the erosion map and the hydrological characteristics of major watercourses.

The largest watercourses in Serbia are transboundary rivers: the Danube, the Sava and the Tisa. Extensive databases are available, mostly relating to the period after the Danube was impounded and a dam built for the Iron Gate Hydroelectric Power Plant (one of the largest in Europe).

The sediment budget along the Danube through Serbia was determined from suspended sediment data on the Danube, the Sava and the Tisa, taking into account the morphological changes within the impoundment. As it enters Serbia, the Danube annually delivers 6.7 million tons of sediment on average. The Sava conveys 3.0 million tons and the Tisa 4.4 million tons. Downstream from Belgrade, after the mouth of the Velika Morava, the Danube discharges 18.5 million tons of suspended sediment, most of which remains within the Iron Gate Reservoir. Only about 3 million tons of suspended sediment passes the dam.

Apart from the transboundary rivers (the Danube, Sava and Tisa), the largest domestic river is the Velika Morava. The available sediment database on its river basin allowed an assessment of the summary suspended-sediment budget. The annual averages, related to the long - term period, were found to be:

  • Južna Morava 2,500,000 tons
  • Zapadna Morava 1,500,000 tons
  • Velika Morava 4,000,000 tons

It became apparent that the Južna Morava transports more sediment than the Zapadna Morava, even though the difference between the sizes of the two drainage areas is only 4% in favor of the Južna Morava. This means that much more eroded sediment is produced in the drainage area of the Južna Morava, compared to the Zapadna Morava. It should also be kept in mind that suspended sediment is conveyed down the river, such that there is a certain continuity to sediment transport. Of course, that does not mean that the sediment does not become deposited along the way (in places where the transport capacity is low), or that there are no local increases (due to fluvial erosion), but such occurrences have no major impact on the annual sediment budget.

Fig17
Figure 17: Unit sediment transport as a function of catchment size.

 

Another major watercourse within Serbia's river network is the Drina, which forms a large part of the border between Serbia and Bosnia and Herzegovina. The sediment database for this river is not sufficiently representative, but in its case another fact is of particular importance, as this watercourse is a major contributor to the approximated sediment budget. Namely, there are many impoundments in the Drina River Basin, where sedimentation is monitored and allows the continuity of sediment transport to be assessed. As a result, it was determined that the average annual rate of sediment transport in the Drina River Basin declined considerably following the formation of a system of reservoirs (from 2.5 million tons in the middle course to 0.5 million tons in the lower course).

For smaller rivers within the territory of Serbia, apart from existing sediment data, information on the production of eroded sediment and the hydrological characteristics of the drainage areas were also used.

Serbia's river sediment budget was approximated on the basis of the existing sediment database, taking into account the erosion map and hydrological characteristics of significant watercourses, as interpreted in Fig. 18.

 

Fig18
Figure 18: Sediment budget of Serbia’s river network.