Torrent Flood Defense and Protection of Water Management Facilities from Sedimentation in the Republic of Serbia

Slobodan Petković1

 

1 Institute for the Development of Water Resources "Jaroslav Černi", Jaroslava Černog 80, 11226 Belgrade, Serbia

 

Abstract

This paper presents an overview of the present state of torrent flood and erosion protection in Serbia, with an overview of the main settlements and transport corridors which are endangered by erosion and torrent floods. Particular emphasis is placed on the importance of establishing appropriate systems for flood and erosion control on small and medium streams. Examples of reservoir sedimentation are given/presented and recommendations for sedimentation reduction in reservoirs are provided.

Keywords: erosion, sediment accumulation, reservoir, torrent.

Introduction

 

The magnitude of erosion and the areas endangered by torrential floods depend on geo-morphological characteristics of the area, and they cover most of the territory of Serbia. Hilly-mountainous areas, which cover three quarters of the territory of Serbia, have a natural predisposition for the development of erosion and torrential processes. Almost all these areas are, to some degree, affected by erosion with more intense erosion process occurring in half of the total area (Petković et al. 1995). Also, torrential processes are conditioned by the configuration and characteristics of the hydrographic network in hilly-mountainous regions. Given the distribution of these regions in Serbia, the control of torrential erosion processes is of great economic and social significance for the country (Petković and Stefanović, 2008).

A significant part of the territory of central Serbia is threatened by torrential erosion processes. These processes endanger multiple industries, infrastructure, and social activities (Petković et al. 1995). The effects of torrential erosion processes can be systematized as follows:

  • Increased hazard of settlement flooding by torrential floods,
  • Increased hazard to transport infrastructure from torrential flooding and the damage caused by land erosion and debris deposition,
  • Increased hazard of agricultural losses from torrential flooding,
  • Decreased functionality or damage of water management facilities due to torrential floods and deposition of debris and sediments.

The effects of torrential erosion processes are discussed in the following chapters. In this framework, the distribution of torrential floods in the territory of Serbia is shown first.

 

The prevalence and Characteristics of Torrential Floods

Watercourses with a heavy hydrological regime have specific characteristics, which are manifested by a rapid increase and the short duration of high intensity flows. The specific dynamics of torrential phenomena required an adequate approach to flood defense. Unlike large alluvial rivers, with gradual change of flows, in watercourses with torrential hydrological regimes, approaches to flood defenses can not be based on the ordinary three stages of control actions: preparation, regular and emergency flood defense. Instead, due to the rapid occurrence of high flows and torrential streams, only preparation and emergency flood defense is possible. It should be noted that preparatory measures are of utmost importance in torrential watersheds.

Torrential hydrological regime usually occurs in hilly - mountainous watercourses, with relatively small catchment areas (from several tens to several hundred square kilometers) and with specific geomorphologic characteristics (steep relief of the basin and large longitudinal slope of the riverbed). However, it is less known that moderate-sized watercourses, with a surface area of 1000 km2, may also have a torrential hydrological regime. These are, basically, basins with specific geomorphological characteristics which increase the dynamics of high flow generation. On the territory of Serbia, the torrential hydrological regime is evidenty on tributaries of the South Morava River (Vlasina, Veternica, Jablanica), the Western Morava River (Skrapež, Bjelica), the Great Morava River (Crnica, Lugomir, Lepenica), the Danube River (Mlava, Pek, Timok) and the Drina River (Jadar), (Petković et al. 1995, Petković 1996, Petković 2007, Petković 2014).

Hydrographic network in Serbia is shown in Figure 1, with marked total three categories of watercourses:

  • Large alluvial watercourses,
  • Small or medium watercourses with torrential hydrological regime,
  • Torrential water courses (small streams).

In particular, it should be noted that most floods in the recent period in Serbia have occurred on smaller and medium watercourses, with a torrential hydrological regime. Fro example. the Jadar River has a high frequency of flood events and the rivers Vlasina, Jasenica and Pčinja have a high frequency of catastrophic flood events. Problems related to floods on small and medium watercourses arise from the fact that flood protection measures on these watercourses are either not implemented, or are implemented only in part. In the past, flood protection investments in Serbia were directed primarily directed towards larger watercourses (Danube, Sava, Tisa, Velika, Južna and Zapadna Morava), while investments into systems in watersheds of small and medium sized watercourses were much lower. Therefore, priority needs be given to finding flood protection solutions for small and medium sized watercourses in Serbia.

 

fig01
Figure 1: Hydrographic network of the Republic of Serbia.

 

Settlements and Agricultural Areas Endangered from Torrential Floods

All settlements, including industrial zones and traffic infrastructure in the urban areas, located along watercourses with a torrential hydrological regime are potentially threatened by torrential floods. On the map of Serbia (Figure 2), urban settlements potentially threatened by torrential floods are marked. In most cases, they are parts of urban settlements, near the watercourse, such as parts of city of Belgrade (suburban settlements, endangered by the Topčiderska River), Niš (tributaries of Nišava River), Zaječar (Beli Timok River), Uzice (Đetinja River), Jagodina (Lugomir River), Leskovac (the rivers Veternica and Jablanica), Vlasotince (Vlasina River), Osečina (Jadar River), Kučevo (Pek River) and Trgovište (Pčinja River). The mentioned watercourses should be given priority in finding flood protection solutions on watercourses with heavy hydrological regimes.

Agricultural areas are predominantly located in valley watercourses with a torrential hydrological regime,. Flooding of fertile flood plains causes great damage to the crops. Although these damages are less severe than in cases of floods in urban areas, they still cause significant losses, especially frequently occurring floods. In Figure 2, significant agricultural areas potentially threatened by torrential floods are indicated. In addition to land along the Jadar River, large agricultural areas are at high flood risk in the valleys of Kolubara, Jasenica, Lepenica, Resava, Mlava, Timoka, Lugomir, Vlasina and other rivers. The construction of defensive embankments along these watercourses (or their reconstruction in cases of insufficient protection height), would greatly reduce the frequency and damage from floods.

In agricultural areas, there is a hazard of torrential erosion processes in the valleys of major rivers, through which torrential tributaries pass. These processes can be manifested not only by the flooding of agricultural land, but also by deposition of eroded material on land surfaces. This problem has two aspects - mechanical and chemical. The mechanical aspect refers to the formation of low-fertile sediment layers covering fertile fields, while the chemical aspect is related to the process of washing off of the surface layer of the soil in the upstream sectors which can contain herbicides and pesticides, transporting it and depositing it on agricultural land downstream, potentially endangering agricultural production.

 

fig02
Figure 2: Settlements and agricultural areas threatened by torrential floods in Serbia.

 

Endangerment of Road Traffic from Torrential Erosion Processes

It is well known that river valleys represent natural corridors for traffic infrastructure. A large number of roads and railroads in Serbia are passing by river valleys and crossing watercourses. Hence, it is logical that torrential erosion processes can cause problems for the traffic infrastructure. On the map of Serbia given in Figure 3, main roads which are potentially threatened by erosion and torrents are marked. Road corridors in narrow river valleys (Grdelička Gorge, Ibarska Gorge, Ovčarsko-Kablarska Gorge, etc.) are at the highest risk for flooding, where torrential tributaries during occurence of high flow waves, with high deposition rates, often cause traffic disruptions. The best known case is the Grdelica Gorge, with a dense hydrographic network of torrential tributaries of the South Morava. Prior to the commencement of major works on erosion and torrent control (commenced in 1950s), traffic on the main international road and railroad corridors in the gorge were frequently disrupted due to torrential floods. After implementation of anti erosion measures in this part of the South Morava River Basin, these traffic corridors are adequately protected from erosion and torrential processes. However, in recent years, the problem of insufficient, or even the absence, of regular maintenance of erosion and torrent control structures is evident, which may lead to increased hazards of traffic disruption by torrential floods.

Endangerment of road traffic from torrential erosion processes also occurs in many other areas in Serbia. However, the threat is less pronounced on regional and local roads and consequently does not receive adequate attention. Hence, frequent traffic disruptions occur on the sections of the roads that pass along the torrential water streams. Therefore, proactive erosion and torrent control measures are needed in the areas that gravitate towards transport infrastructure.

 

The Influence of Erosion and Torrential Processes on Water Management Facilities

Water management facilities are particularly vulnerable to erosion and sedimentation. The erosion processes, which result in transport of large amounts of deposits through the hydrographic network of the basin result in sedimentation in downstream sections of rivers and especially in reservoirs.. The accumulation of deposits in reservoirs is a common problem of water management and require implementation of control measures in upstream areas of the watershed. Sedimentation also occurs, to a lesser degree, in downstream river sections which are usually regulated (with flood defense systems in place). However, it should be noted that accumulation of sediments on regulated watercourse sections can significantly reduce flow capacity of river channel and therefore substantially reduce designed level of flood protection.

There are about 30 large reservoirs (with volumes exceeding 10 million m3) and around 100 small reservoirs in Serbia. All of these reservoirs are to some degree endangered by sedimentation of eroded materials. On the map of Serbia given in Figure 3, some significant accumulations are marked, with corresponding catchment areas (it should be noted that the catchment area of the Iron Gate Reservoir is not shown since it far exceeds the borders of Serbia). It should be noted that existing reservoirs have different levels of sedimentation. The most endangered reservoirs were built some 50 years ago (reservoirs Zvornik, Merjuvršje, Ovčar Banja, etc.), when sufficient knowledge was lacking on the significance of erosion and sediment control in reservoirs. In recent years, this problem is addressed either by the construction of appropriate evacuation devices or by implementing anti erosion measures in the catchment area. However, the sediment accumulation in reservoirs remains one of biggest challenges in operation and maintenance of water infrastructure in Serbia. Hence, in all existing reservoirs it is necessary to monitor sediment accumulation by implementing appropriate geodetic methods. In the case of new reservoirs, in addition to the timely execution of anti erosion works in catchment area, the application of contemporary sediment management models is also preferred, at they should be included in design phase (Babić Mladenović et al. 2014).

The dynamics of sediment accumulation depends on several factors. The basic factors relate to the hydraulic and morphology characteristics of the reservoir (ratio between the reservoir volume and water inflow, shape of the reservoir, type and elevation of discharge, mode of operation). In addition, the dynamics are influenced by the characteristics of the sediment which enters the reservoir (particle size distribution, particle shape, solids concentration), water characteristics (temperature, chemical composition), stream patterns, etc. In order to illustrate the problem sediment accumulation, two examples of reservoirs on the Drina River are presented below.

 

fig03
Figure 3: Road traffic congestion and accumulation of sediments from erosion and torrents.

 

Reservoir Zvornik

The oldest reservoir on Drina River was formed by the construction of a dam and hydro power plant (HPP) Zvornik in 1955 (shared between Serbia and Bosnia & Herzegovina). Over a period of approximately 50 years, regular monitoring of sediment accumulation in the reservoir was carried out. Figure 4 shows volume - stage curves of the reservoir (water volume only), for the period 1955 - 2005.

 

fig04
Figure 4: Water volume - stage (in masl) curves for the Zvornik Reservoir on the Drina River for the period 1955-2005.

 

Figure 4 clearly indicates a significant reduction in reservoir volume in the observed period due to sedimentation. The highest rate of sedimentation (volume decrease) can be observed in the initial period of reservoir operation, from 1955 to 1961, while it gradually declines over time. The observed process is fully in line with the basic laws of river hydraulics, which relates to the conditions of river sediment transport through a reservoir. The total reduction in reservoir storage decreased approximately 50% over the 50-year period.

 

Reservoir Višegrad

Another example of the sediment accumulation process in reservoirs is the newest reservoir in the Drina River basin - the Višegrad HPP Reservoir (Bosnia & Herzegovina), located upstream from the Zvornik Reservoir. Figure 5 shows the volume - stage curves of the reservoir (water volume only), for the period from 1991 (commissioning of the reservoir and HPP) to 2005.

 

fig05
Figure 5: Water volume - stage (in masl) curves for the Višegrad Reservoir on the Drina River for the period 1991-2005.

 

It can be noted that the dynamic of reservoir sedimentation is considerably slower than in the case of the Zvornik Reservoir. This difference can be explained by the following facts:

  • The Zvornik dam and reservoir were built at a time when activities related to the control of erosion, transport of deposition processes in the Drina River basin had not yet begun. The production and transport of deposits in the basin, under natural conditions, caused significant reservoir sedimentation.
  • After completion of the Zvornik HPP Reservoir, several reservoirs were built in the Drina River Basin, which significantly influenced the transport of deposits along the Drina River. In that regard, the construction of the dam and HPP "Potpeć" on the Lim River upstream HPP Zvornik (right main tributary of the Drina River) significantly reduced sediment inflow into the Višegrad Reservoir.
  • Another very important factor influencing sediment accumulation in the Višegrad Reservoir refers to the dam's outlet design. The Višegrad Dam is characterized by outlets located at the bottom of the dam (reservoir), with a very high capacity (maximum capacity of about 5000 m3/s), which provides the possibility of flushing significant amounts of sediments through the dam. In the period 1991 - 2005 occasional operation of outlets resulted in the discharge of the most of the accumulated deposits that entered the Visegrad Reservoir downstream.

 

Conclusions

  • Watercourses with a torrential hydrological regime have specific characteristics, which are manifested by rapid increases and short durations of high flows. The specific dynamics of torrential phenomena require an adequate flood defense approach. Because of the rapid occurrence of high flows, only emergency flood protection is possible, while implementation of preventive measures must remain a priority.
  • In recent years, the most severe floods occurred in smaller and medium watercourses, with a torrential hydrological regime. The problems of floods on small and medium waterways arise usually from the fact that there are no adequate flood defense systems on these watercourses, or these systems have not yet been completed. Therefore, in the coming period, small and medium sized watercourses should be given priority in solving the problem of water protection on the territory of Serbia.
  • The previous experience with reservoirs in Serbia can be summed up by concluding that the problem of sediment accumulation in reservoirs has not been addressed with the necessary attention. More recent research and lessons learned suggest that reservoir sedimentation processes must be considered even at the design stage, which would allow for timely observation of the erosion/deposition/sedimentation processes and planning of adequate measures. During this stage, in addition to erosion prevention and sedimentation control measures in the reservoir basin, adequate dam outlet designs should be considered a priority.

 

References

Babić Mladenović M., Bekić D., Grošelj S., Mikoš M., Kupusović T., Oskoruš D. and S. Petković (2014). Towards sediment management in the Sava river basin. Water research and management, Vol. 4, No 1, 3-13

Petković, S and S. Stefanović (2008). Study on the production and transport of eroded sediment in Serbia (in Serbian), report, Jaroslav Černi Institute, Belgrade

Petković, S. (2007). River sediment yield and sediment budget in the river network of Serbia. 10th International symposium on river sedimentation, IISD, 1-4 August, Moscow, Russian Federation

Petković, S. (2014). River sediment transport in Serbia. Water research and management, Vol 4. No 4, 25-38

Petković, S. Dragović N. and S.Marković (1995). Erosion and sedimentation problems in Serbia. Hydrological Sciences Journal.

Petković, S.(1996). Origin and transport of sediment in the catchment area of the Južna Morava river (in Serbian). University of Belgrade, Faculty of Forestry.