River Sediment Transport in Serbia - page 01

Sediment Database

The periods for monitoring hydrological parameters in Serbia vary, as a result of historical circumstances. The gauging stations on the largest river, the Danube, feature the longest time series, given that observations of that river began in the early 20th century. Monitoring of other rivers started much later, after World War II. However, there are no significant differences in sediment monitoring, given that nearly all the sediment data were collected after the year 1945. In central Serbia, the longest time series are provided for the Velika Morava, the Južna Morava and the Zapadna Morava Rivers. Over time, the monitoring network of the National Hydrometeorological Service of Serbia (RHMZ) was expanded to include more rivers. Unfortunately, sediment monitoring has not been continuous at all stations, so the time series are not chronologically the same. Additionally, sediment monitoring data are almost exclusively related to suspended sediment, except for short-term bedload measurements on the Velika Morava and the Drina.

The suspended-sediment monitoring method applied is comprised of suspension sampling from the main stream and laboratory analysis of the samples, to determine the concentration of the solid phase in the water. In Serbia, samples are generally collected once a day (in the morning), and then taken to the lab. It should be noted that the suspended-sediment monitoring procedure followed at all RHMZ gauging stations has not been adequate for quite some time. Namely, suspension samples are always collected from the same point in the hydrometric cross-section, in an insufficiently standardized manner—using one-liter bottles.

Also, the samples are filtered and measurements made in makeshift conditions, which limit accuracy.

River sediment sampling once a day, at a particular time, cannot provide realistic and representative information about suspended-sediment concentrations and transport, especially in the case of small rivers. It is well-known that the largest amounts of sediment are conveyed by flood waves and that in most cases flood wave peaks tend to occur between the scheduled sampling times. As a result, daily sediment concentrations are recorded, which are much lower than the maximum values during the day (e.g. at the time of a flood wave peak). The ultimate consequence of this inadequate suspended-sediment monitoring approach is an unrealistically low level of the summary annual rate of sediment transport along the considered river.

When suspended-sediment monitoring data collected in the above manner are analyzed, the size of the river and the hydrological regime also need to be considered. Hydrological and sediment-related changes in large alluvial rivers tend to be relatively slow, such that same-time monitoring once a day will generally provide representative data. However, the situation is very different in the case of small rivers, especially those whose hydrological regime is torrential in nature. It is often the case that recorded daily sediment concentrations are much lower than the maximum values at the time of flood wave peaks. As such, the results need to be corrected on the basis of available general knowledge of the hydrological and sediment regimes of the river in question, as well as through analogy with other rivers of similar characteristics.

Fig01
Figure 1: Serbian river network.
 

Review of Sediment Transport Data on Some Rivers in Serbia

Given the extensive suspended-sediment monitoring data on Serbian rivers, this paper addresses only some watercourses, being specific from a sediment transport perspective and representative of different river categories: the Velika Morava is a large river, the Lim is medium-sized, and the Vlasina and the Toplica (tributaries of the Južna Morava) represent rivers with a catchment area of up to 1000 km2.

 

Velika Morava River

The Velika Morava is the largest river in central Serbia, having a catchment area of some 37,000 km2.

This river is sometimes referred to as "the muddy river", because of considerable amounts of suspended sediment resulting from the substantial erosion potential of its drainage area. There are several distinct erosion-prone areas in the Morava River Basin, particularly in the catchment area of the Južna Morava. On the other hand, the suspended-sediment transport capacity of the Velika Morava is considerable, such that most of the sediment delivered by its tributaries reaches the confluence of the Velika Morava and the Danube.

As in the case of all natural watercourses, the suspended-sediment transport regime of the Velika Morava varies to a large extent during the year. Since sediment transport depends, above all, on the hydrological conditions of the river, there is a certain general (but not strong) correlation between river discharge and suspended sediment transport.

Suspended sediment concentrations (C) in the Velika Morava span a broad range, between 10-2 kg/m3 and 10 kg/m3. In other words, the ratio of maximum to minimum concentrations of suspended sediment is of the order of 103. However, the most frequent suspended sediment concentrations in the Velika Morava are 0.01 kg/m3.

Figure 2 gives the average annual river discharge (Qav) and the annual rate of suspended sediment transport (Ps) at the Velika Morava River, between 1961 and 2003. The values of Qav range from 100 to 350 m3/s, while those of Ps are from 0.40 to 13.00 million tons/year. The chronological chart exhibits a distinct downward Ps trend during the study period (1961-2003). At the beginning of that period (from 1961 to 1968), the values were Ps ≥ 5 million tons, from 1971 to 1981 2.5 < Ps < 4.5 million tons, and in recent times (1981-2003) 0.5 < Ps < 3.8 million tons. Several hypotheses can expain this significant decline in the Ps trend:

Initially (before the year 1970), the sediment measurement approach was not sufficiently accurate or reliable, resulting in higher-than-real suspended sediment concentrations. Consequently, only the period from 1971 to 2004 can be deemed representative for sediment transport assessments of the Velika Morava.

Erosion control and hydraulic engineering projects in the Velika Morava River Basin have reduced sediment production and retained a significant proportion of the sediment within the drainage areas of tributaries, thus decreasing the amounts of sediment delivered to the Velika Morava.

 

Fig02
Figure 2: Chronological bar chart of annual mean river discharge (Qav) and annual suspended sediment transport (Ps) of the Velika Morava, Ljubičevski most, 1961-2003.