Building Reservoirs as Protection against Flash Floods and Flood Basins Management—The Case Study of the Stubo–Rovni Regional Water-Management System

Abstract

Global warming and climate change cause large temperature oscillations and uneven annual rainfall patterns. The rainy cycles characterized by frequent high-intensity rainfall in the area of the Stubo–Rovni water reservoir, which in 2014 peaked at 129 mm of water in 24 h (the City of Valjevo, the Republic of Serbia), caused major floods in the wider area. Such extremes negatively affect erosion processes, sediment production, and the occurrence of flash floods. The erosion coefficient before the construction of the water reservoir was Z_m = 0.40, while the specific sediment production was about 916.49 m³∙km⁻²∙year⁻¹. A hydrological study at the profile near the confluence of the Jadar and Obnica rivers, i.e., the beginning of the Kolubara river, the right tributary of the Sava (in the Danube river basin), indicates that the natural riverbed can accommodate flows with a 20% to 50% probability of occurrence (about 94 m³/s), while centennial flows of about 218 m³/s exceed the capacities of the natural riverbed of the Jadar river, causing flooding of the terrain and increasing risks to the safety of the population and property. The paper presents the impacts of the man-made Stubo–Rovni water reservoir on the catchment area and land use as the primary condition for preventing erosion processes (specific sediment production has decreased by about 20%, the forest cover increased by about 25%, and barren land decreased by 90%). Moreover, planned and controlled management of the Stubo–Rovni reservoir has significantly influenced the downstream flow, reducing the risks of flash floods.

1. Introduction

Climate changes in the last decade have brought about large fluctuations in the amount, distribution, and intensity of rainfall, air temperature, wind strength, etc., in the territory of the Republic of Serbia and globally [1]. The analysis of the total annual rainfall shows that it does not differ from the data from the previous several decades [2]. However, the distribution, intensity, and type of rainfall have changed significantly. The frequency of winters with a small number of days with snowfall and snow retention is increasing. Winters with little snow are followed by cycles of frequent [3] high-intensity rainfall with elements of storms accompanied by hail [4]. Moreover, there are very frequent cases of various extreme microclimatic phenomena in a small area [5].

The adaptation of vegetation to climate change depends precisely on the degree of changes and the ability of some plant species to adapt in short time intervals. The loss of vegetation on unfavorable relief terrains makes the soil conducive to erosion and sediment production [6,7].

The damaged soil structure has unfavorable infiltration characteristics, which results in surface runoff, torrential flows, and floods, especially in areas with specific relief and climate features [8,9]. The production, transport, and deposition of sediment reflect the extent of loss of land resources in the timeframes in which mankind faces population growth and permanent reductions of agricultural and forest land fit for agricultural production and forest cultivation [10,11,12].

Torrential floods and flows have become a daily occurrence in the last decade, particularly in rainy cycles in some streams in Serbia [13], accompanied by erosion, i.e., meandering of riverbeds [14]. The adaptation of the recipients to the change in the rainfall regime and the occurrence of high waters are also made possible by the construction of regulation structures and embankments to receive and conduct water in short time intervals [15].

The capacities of some regulation structures set up a decade or two ago in Serbia are not able to respond adequately to these new circumstances, i.e., they are not adapted to changes in rainfall and runoff regimes [16]. Also, there are growing construction activities in the areas of regulated and unregulated streams. Long-term non-maintenance, unplanned use, and management cause significant damage to the recipients, which results in a reduction of transverse profiles and, thus, in the capacity to receive large amounts of water. The unplanned construction of facilities in the immediate vicinity of riverbeds affects, in many respects, runoff regimes and the maintenance and management of rivers in the territory of Serbia, reducing the possibility of adequate responses to changes in climate characteristics, the occurrence of rain cycles, rainfall, and runoff regimes [17,18].

Reservoirs as multi-purpose capital facilities have a great influence in controlling the runoff regime, i.e., their construction facilitates the acceptance of flood waves in rainy cycles on the one hand and supplying water to the population in dry periods [19], achieving the ecological minimum and preventing the drying up of rivers, on the other. The construction of the reservoir significantly changes the way of management and use of the space—the reservoir basin [20]. In this way, a certain influence is exerted on biodiversity and the environment in general [21,22,23,24,25], which is believed to be jeopardized in the territory of half of our planet, i.e., half of the world’s ecosystems [26]. Some measures, restrictions, and prohibitions have a great impact on land resources, which results in an increase in forest areas, planned management of agricultural land [27], prevention of facilities construction, prevention of activities that would lead to soil erosion, production of water pollution deposits, etc. [28].

The paper presents a concrete project of the multi-purpose Stubo–Rovni reservoir system in Serbia and the contributions it makes in protecting the area subject to erosion processes and flash floods, securing preconditions for protecting the population, facilities, biodiversity, and land infrastructure by means of which the said benefits cover all the aspects of space and the environment. The applicability of scientific knowledge is supported by relevant calculations of high waters as the basic element of the hydrological study, based on which the flood occurrence probability is determined, and strategic responses are defined. The EU Floods Directive has been transposed into Serbian law (specifically, into the Serbian National Disaster Risk Management Plan), with the Ministry of Agriculture, Forestry, and Water Management being the competent authority in the area [29].

2. The Initial Position—A Case Study

The Stubo–Rovni regional water-management system (basin, dam, and reservoir) is in the western part of the Republic of Serbia, around 80 km southwest of the capital of Belgrade (Figure 1). It was constructed and put into operation in 2015. The dam and the reservoir are located on the Jablanica river, 12 km southwest of the city of Valjevo. The Jablanica river has a basin area of around 155 km², while the reservoir was formed with the construction of a 75 m high dam made of clay-core rockfill embankment. The presence of the mountains of 976 to 1270 m above sea level characterizes the basin area, influencing the formation of climatological and hydrological specificities of the basin area. The basin of the Stubo–Rovni reservoir extends over the hilly and mountainous terrains of Povlen and Jablanik and sub-mountainous surroundings in altitudinal zones of 280 m above sea level in the Stubo–Rovni dam reservoir zone (the lowest basin point), to the top of Mali Povlen at 1347 m above sea level (the highest basin point). The average altitude is between 500 and 600 m above sea level. The area of the Stubo–Rovni reservoir basin is 116 km². The surface area of the Stubo–Rovni reservoir, i.e., the volume of the reservoir area, is 51.5 × 10⁶ m³, of which the useful volume is 49.5 × 10⁶ m³ at normal deceleration. The elevation of the minimum working level is 310 m above sea level, the elevation of the dam crest is 363.5 m above sea level, and the elevation of the normal working level is 360 m above sea level [30].

The elevation of land in the Stubo–Rovni reservoir (Figure 2) ranges from 400 m to 1300 m, with the highest percentage of land (66%) ranging between 400 m and 800 m.

Arable land is primarily at heights up to 500 m above sea level, except for steep slopes and river courses that are under the forest. Areas up to 700 m above sea level are under meadows and pastures, while the higher parts of the mountain massifs are mainly under deciduous forests (Figure 3).

Of the total reservoir territory, 70% pertains to slopes greater than 10° and unsuitable for agricultural production, with the possibility of use for fruit plantations if the slope does not exceed 16° or for pastures on slopes up to 24°. The terrains with slopes greater than 30° make up 10% of the total territory, and they are only suitable for growing forests with the possibility of growing grassland if the slope does not exceed 31°, with special restrictions (Figure 4) [30].

In the territory of the reservoir basin, agricultural land is dominant, with a share of around 55% of the total territory. In this regard, it is important to note that for the needs of fields and gardens, the share is around 20%, and for orchards, around 6%. This implies that around 30% of the total agricultural land is used for meadows and pastures. The analysis of land capability classes showed a high percentage of poor soils, unfavorable for agricultural production, i.e., cultivation of crops on arable land. More than half of the arable land belongs to the 7th and 8th cadastral class—it can be considered shallow and erodible soil and is, in principle, more suitable for establishing meadows and pastures, i.e., for afforestation, rather than for growing field crops [30].

Forests in the basin area consist of the mesophilic beech (Fagus); xerothermophilic Hungarian oak (Quercus frainetto) and Austrian oak (Quercus cerris); and xeromesophilic sessile oak (Quercus petraea), Austrian oak (Quercus cerris), and common hornbeam (Carpinus betulus). The most prevalent deciduous species is beech (Fagus), making up around 95.5% of the forest area. Other deciduous species are present to a negligible degree, making up less than 5% of the territory. They include sessile oak (Quercus petraea), Hungarian oak (Quercus frainetto), Austrian oak (Quercus cerris), maple (Acer), ash (Fraxinus), common hornbeam (Carpinus betulus) and black locust (Robinia pseudoacacia). The basic tree species play a dominant role in these types of forest ecosystems [30].

2.1. Basic Climate Features of the Stubo–Rovni (Jablanica River) Reservoir Basin

The area of the Stubo–Rovni reservoir basin is characterized by a moderate–continental climate in the lower and a mountain climate in the higher parts of the basin. The proximity of the Pannonian Basin and the transition from plain to hilly–mountainous areas have a strong influence on the climatic conditions of the Valjevo region. Annual temperatures range from −0.2 °C in the coldest month of January to 21.4 °C in July. Extreme temperatures range from 42.5 °C (the highest measured temperature) to −29.6 °C (the lowest measured temperature). The average annual temperature is 11 °C [33]. Rainfall in the basin area has the characteristics of the Central European Danube regime of annual distribution. The average annual precipitation was around 815 mm for the period from 1970 to 2015 or 956 mm for the period from 1951 to 1970. The maximum annual precipitation from 1951 to 1970 was 1439 mm (for 1955) (the spatial plan for the special purpose area of the Stubo–Rovni reservoir, Republic Hydrometeorological Service of Serbia). The wettest month is June, with 110.2 mm, and the driest is February, with 44.6 mm [34]. Average annual precipitation increases with an increase in altitude from 900 mm to over 1000 mm. Snow retention ranges from 30.9 days to over 120 days on the highest terrains of Povlen. Data on the maximum 24-h rainfall on an annual basis for the 1961–2015 period [35], i.e., the completion of the construction and launch of the Stubo–Rovni reservoir, show 129 mm in 2014 (the year of great floods in Serbia). Air masses of different origins pass over the Republic of Serbia, and the effect of some centers of high and low air pressure, whether they move from the Mediterranean Basin to the north or from the Eurasian areas in symbiosis with the relief of the reservoir area and the wider area, create microclimates with variabilities of weather changes and climate conditions, which results in the occurrence of rain cycles and high-intensity rainfall [4].

2.2. Erosion Processes in the Jablanica River Basin

There were eight very slow-moving and two potential landslides in the territory of the reservoir basin, i.e., they were located along the right valley side of the Jablanica, the mouth of the Sušica. The 4 m deep diluvium, of consequential type, passes through contacts of decomposed and fresh rocks with an estimated volume of 0.57 × 10⁶ m³, while potential landslides occupy a volume of 2.02 × 10⁶ m³. Denudation is also present, as well as occasional occurrence of ravines around 5 m deep.

The erosion of the Jablanica riverbed, i.e., linear erosion, was present in the valleys, especially in places with a carbonate substrate. The foundations built from the lower Triassic and Anisian strata, i.e., the smaller and occasional streams on them, have deeply cut bottoms and steep valley sides. The common characteristic of all streams is torrential character and meandering as a result of lateral erosion and bed erosion, depending on the resistance of the rocks. It is reflected in the processes of denudation and the creation of ravines, streams, and smaller rivers.

Before the construction of the reservoir, two landfill partitions made of stone in cement mortar were built on the Jablanica river downstream of the reservoir to prevent sediment from reaching the Kolubara river. As part of the Stubo–Rovni system, i.e., during the construction of the dam, 25 facilities were built to prevent sediment transport on the main stream upstream of the dam, i.e., the reservoir lake and on the tributaries. Four partitions were built on the Jablanica, one of which was submerged. On the Žitkovica tributary, five barriers and one threshold were built, while on the Simić stream, six thresholds and two landfill barriers were constructed. The Rebeljska river has three built barriers. The Sušica has one that was submerged, while the Ledenjak has three landfill barriers. All facilities were built to retain sediment in the basin, i.e., to prevent sediment from reaching the Stubo–Rovni reservoir. As part of the measures defined during planning and design and to prevent the occurrence of erosion processes related to afforestation, until now, there has been no organized afforestation as only sporadic afforestation was carried out as part of regular stewardship by the “Boranja” forest estate. The negative trend of migration of the rural population to cities resulted in the natural renewal and expansion of forest resources in the area of the reservoir basin.

One of the basic indicators of erosion processes is the production and transport of sediment, and if one observes the inflow facility at the conflux of the two rivers and the source of the Kolubara river, the arrival of sediment on the Jablanica profile is negligible. This does not mean that there are no erosion processes in the basin, but it means that the arrival of sediment was prevented due to the construction of landfill barriers on the one hand and the implementation of defined measures and works on the basin, which significantly influenced the reduction of erosion processes, on the other.

2.3. Geological and Pedological Characteristics of the Jablanica River Basin

Present in the territory of the Stubo–Rovni reservoir basin are Triassic sediments and volcanites (48.6%), Jurassic sediments and magmatites (27.1%), Cretaceous sediments (22.6%), alluvial sediments (0.9%) and Permian sediments (0.8%). The petrographic composition consists of Permian, Triassic and Cretaceous limestones, Triassic dolomites, Cretaceous marls, Triassic, Jurassic and Cretaceous sandstones, Jurassic cherts and conglomerates, diabases, and porphyrites.

In the conditions of a moderate–continental climate, modified by mountain-ravine influences, several types of soil with a shallow humus horizon are formed, with limitations in agricultural use. Leptosols and regosols have a very weak potential for biomass production. Brown soils (Leptic Cambisol), rendzina (Rendzic Leptosols), and red soils (Rhodic Cambisols) are located on higher slopes, which is the main limitation for agricultural use. Vertisols, pseudogleys (Stagnic Gleysols), alluviums (Calcaric Fluvisols), and diluviums (Fluvisols) are located on level terrains, where the skeleton content, uneven depth of the humus layer, and exposure to torrents appear as limitations [36,37]. Uncontrolled forest cutting and unplanned land use increase the risks of disturbing the balance between the creation and degradation of the pedological layer that has been exposed to various types of water erosion.

After the completion of the reservoir, works are carried out on the protection of the basin and downstream area, which has the following positive effects: prevention of flash floods and droughts, prevention of erosion processes and sediment production, increase in surfaces, and improvement of forest quality, development of agriculture, tourism, and economy.

3. Method

The research of the Jablanica river basin where the Stubo–Rovni reservoir was constructed (of the first level of importance) aimed to present the positive effects of the construction of the Stubo–Rovni dam from the aspect of protection against flash floods and soil erosion and the protection of the city of Valjevo and the downstream area from high waters. The data used were collected through the analysis of planning, design, and other documentation by means of fieldwork in the period before and after the construction of the reservoir. Given that the reservoir is filled, i.e., the management and use of this capital water-management complex have begun, there are clearly visible changes in hydrology, erosion processes, the occurrence of torrents, and flash floods compared to the period before the construction of the reservoir.

In the final phase of the research, the calculation of high waters was carried out using the combined SCS method of the procedure for separating the effective from gross rainfall and the theory of the synthetic unit hydrograph for determining the peak ordinate of the unit runoff and Janković’s model derived for the territory of Serbia for the transformation of the maximum 24-h rainfall. The model enables daily rainfall values to be reduced to hourly duration. It enables the maximum daily rain for any rain gauge station in Serbia to be reduced to a maximum rain duration of less than 24 h [38]. The calculation was made for the Jablanica river before the construction of the reservoir at the end of the stream, i.e., the place where it joins the Obnica river, i.e., the source of the Kolubara river, the right tributary of the Sava river. Based on data from orthophotos and cadastre, an analysis of the purpose and use of the land was carried out to calculate potential infiltration. The processing of orthophotos, cadastre, and other spatial data was carried out in the ArcMap 10.8.2 software package. Data on maximum annual 24-h rainfall were taken from the Republic Hydrometeorological Service of Serbia in Belgrade.

The basic hypothesis is that by building a large facility such as the Stubo–Rovni reservoir, water is provided as one of the most important resources today, one of the most important resources for the future, and as flood protection. Economic justification is reflected in the supply of water to the population and the economy and prevention of floods in the Kolubara region as a primary role, while the secondary role is reflected in the integral protection of the basin from erosion, torrents, flash floods, droughts, etc. [39].

The analysis of the Jablanica river basin identified erosion processes in the basin, riverbeds, and tributaries before the construction of the dam and the filling of the reservoir area, and as such, has become widely used worldwide, while the modified version can be applied in colder climate as well [40]. The Erosion Potential Method (EPM) is an empirical method for assessing soil loss, erosion production, and sediment transport in the basin or area. The method was developed after many years of field research, observations, and measurements performed in torrential basins [41,42]. In addition to the calculation of sediment production and transport, the method was also created for the purpose of mapping erosion processes, erosion areas, and quantitative classification of torrential flows [42,43,44]. The EPM is the most important tool for developing the erosion map, calculating erosion production and sediment transport, and its use in engineering, design, and spatial planning practice. It is still used today in all the countries of the former Socialist Federal Republic of Yugoslavia (Montenegro, North Macedonia, Bosnia and Herzegovina, Slovenia, Croatia) [45,46,47,48,49], as well as in numerous countries across Asia, South America, Africa, and Europe [50,51,52,53,54,55,56,57,58]. The EPM is applied as a standard method and “instrument” for dealing with engineering issues related to the prevention of soil erosion and flash floods in the field of water management for the purposes of creating water-management foundations, studies, and projects, using the following formula:

$${\mathrm{W}}_{\mathrm{y}\mathrm{r}}=\mathrm{T}·{\mathrm{H}}_{\mathrm{y}\mathrm{r}}·\mathsf{\pi }·\sqrt{{\mathrm{Z}}^3}·\mathrm{F} \left[{\displaystyle \frac{{\mathrm{m}}^3}{\mathrm{y}\mathrm{r}}}\right]$$

(1)

Theorem 1.

T—temperature coefficient; H_yr—mean annual precipitation quantity (mm·yr⁻¹); π—Ludolph’s number (Archimedes’ constant) 3.1415; Z—erosion coefficient, Theorem 3, F—surface [km²] [44].

$$\mathrm{T}=\sqrt{{\displaystyle \frac{\mathrm{t}}{10}}+0.1}$$

(2)

Theorem 2.

The temperature coefficient is calculated via t—mean annual air temperature (°C) [44].

The method starts from the analytical processing of data on the factors that affect erosion. As erosion is a spatial phenomenon, it is shown on the map according to the classification based on the analytically calculated erosion coefficient (Z), which does not depend on climatic characteristics but on soil characteristics, vegetation cover, relief, and visible representation of erosion processes. According to the erosion coefficient Z, erosion processes are categorized according to Gavrilović (

Table 1. The values of the erosion coefficient Z [44].

Erosion Category	Severity of Erosion Processes in the Basin	Erosion Coefficient Z	Mean Value of Erosion Coefficient Z
I	Excessive erosion	≥1.01	1.25
II	Heavy erosion	0.71–1.00	0.85
III	Medium erosion	0.41–0.70	0.55
IV	Slight erosion	0.2–0.40	0.30
V	Very slight erosion	≤0.19	0.1

). The values usually range from 0.1 to 1.5 and more, i.e., from preserved basins and areas lightly affected by erosion to extremely degraded basins. The Z values can be above and below the specified limits only in exceptional cases [42]. According to the type of prevailing erosion and the values of the erosion coefficient Z, 13 categories were determined. The erosion coefficient Z is obtained from the formula no. 3 [44].

$$\mathrm{Z}=\mathrm{Y}·\mathrm{X}·\mathsf{\alpha }·\left(\mathsf{\phi }·\sqrt{{\mathrm{I}}_{\mathrm{m}}}\right)$$

(3)

Theorem 3.

Y—the soil resistivity coefficient; it represents the reciprocal value of the erosion resistance value and depends on the geological material, climate, and pedology; X·α—the basin protection coefficient; it represents the coefficient relating to the protection of soil from atmospheric effects and erosion forces in natural conditions X, or artificially created conditions, anti-erosion, technological, or biological works α; φ—the number equivalent of the visible erosion processes in the basin or the surrounding area; I_m—the mean basin dip [%] [44].

In relation to the defined methodological procedures, study, and data collection in the field, the levels of soil degradation were determined. The intensity of degradation is expressed by the erosion coefficient Z, as well as the values of production of erosion material in the researched area. Using the EPM, the erosion map was created, which shows the spatial distribution of erosion processes in the researched area. The erosion coefficient Z is obtained as a result of the raster database and is based on pixels, where each pixel shows the value of the erosion coefficient Z. The erosion map offers an insight into the state of erosion processes, different in intensity and character. The calculation of the erosion coefficient Z, as well as the production of erosive material, were analyzed only on surfaces exposed to intensive erosion processes (agricultural areas, forests, meadows, bushy and low vegetation, semi-natural habitats, etc.). Urbanized areas with a pronounced share of non-porous surfaces, rivers, lakes, wetlands, and the like were excluded from the analysis.

The following text shows the method’s basic parameters of the basin and the data necessary for the calculation of flow rate with the probability of occurrence of 50%, 20%, 10%, 2%, 5%, 1%, 0.5% for the Jablanica river before the construction of the water reservoir:

• Basin area (up to the conflux with the Obnica)—F = 155.45 km²;
• Length of the main stream—L = 27.97 km;
• Length of the stream from the basin center to the mouth—L_c = 16.11 km;
• Balanced stream slope—I_bs = 1.17%;
• Basin lag time—t_p = 5.708;
• Waveform coefficient—k = 1.7.

$${\mathrm{t}}_{\mathrm{p}}=0.751·{\left({\displaystyle \frac{\mathrm{L}·{\mathrm{L}}_{\mathrm{c}}}{\sqrt{{\mathrm{I}}_{\mathrm{b}\mathrm{s}}}}}\right)}^{0.336}$$

(4)

Theorem 4.

t_p is the basin lag time. L is the length of the main stream (km); L_c is the length of the stream from the basin center to the mouth (km); Iu is the balanced stream slope (%) [38].

The hydrograph shape coefficient k is a function of the basin surface F (km²) and is obtained from a diagram that is often used in Serbian hydrological practice [38].

In the following text, a series of maximum annual 24 h precipitation is shown, together with the transformation of a series of specified values into a series of adequate logarithmic values, i.e., Log Pearson Type III distribution (LPT III) calculation of the flow rate for the Jablanica river [38].

X_i = logP_i

(5)

Theorem 5.

X_i is the logarithmic value. P_i is the Maximum 24-h annual rainfall from 1961 to 2015, meteorological station of Poćuta, reference for the Jablanica river basin, i.e., the Stubo–Rovni reservoir (mm) [38].

• Mean of series of members—X_m = 1.710;
• Standard deviation—σ = 0.1273;
• Asymmetry coefficient—C_s = 0.4837;
• Coefficient—K = 2.674;
• Gross rainfall—P_gr(p_1%) = 112.266 mm.

$${\mathrm{X}}_{\mathrm{m}}={\displaystyle \frac{\uparrow \sum {\mathrm{X}}_{\mathrm{i}}}{\mathrm{n}}}$$

(6)

Theorem 6.

X_m is the mean of series of members. ↑∑X_i is the Sum of members in the sequence: n is the Number of members in the sequence [38].

$$\mathsf{\sigma }=\sqrt{{\displaystyle \frac{\sum {\left({\mathrm{X}}_{\mathrm{i}}{-\mathrm{X}}_{\mathrm{m}}\right)}^2}{\mathrm{n}-1}}}$$

(7)

Theorem 7.

σ is the standard deviation [38].

$${\mathrm{C}}_{\mathrm{s}}={\displaystyle \frac{\mathrm{n}·{\left({\mathrm{X}}_{\mathrm{i}}{-\mathrm{X}}_{\mathrm{m}}\right)}^3}{\left(\mathrm{n}-1\right)\left(\mathrm{n}-2\right){\mathsf{\sigma }}^3}}$$

(8)

Theorem 8.

C_s is the asymmetry coefficient [38].

Coefficient K is a function of the value C_s and a certain probability p (%)obtained from the table [38].

P_gr = 10^Xm+K·σ

(9)

Theorem 9.

P_gr is the gross rainfall [38].

• Average humidity—CN_avg(PVT2) = 78.69;
• Above-average humidity—CN_avg(PVT3) = 90.19;
• Infiltration potential—d = 27.62.

CN_avg(PVT2) = CN · Fn

(10)

Theorem 10.

CN_avg(PVT2) is the average humidity. CN is the Runoff curve that depends on the hydrological class of the soil, the manner of use, and the hydrological characteristics obtained from the table; F_n is the size of surfaces of specific purposes (km²) [38].

CN_avg(PVT3) = CN · Fn

(11)

Theorem 11.

CN_avg(PVT3) is the above-average humidity [38].

$$\mathrm{d}=25.4·\left({\displaystyle \frac{1000}{{\mathrm{C}\mathrm{N}}_{\mathrm{a}\mathrm{v}\mathrm{g}}}}-10\right)$$

(12)

Theorem 12.

d is the infiltration potential [38].

$${\mathrm{T}}_{\mathrm{p}}={{\mathrm{t}}_{\mathrm{p}}}_{ }+{\displaystyle \frac{{\mathrm{T}}_{\mathrm{k}}}{2}}$$

(13)

Theorem 13.

T_p is the Rise time of the hydrograph (h). T_k is the Duration of effective rain (h) [38].

T_r = k·T_p

(14)

Theorem 14.

T_r is the Decline time of the hydrograph [38].

T_b = T_p + T_r

(15)

Theorem 15.

T_b is the Time base of the synthetic unit hydrograph [38].

$${\mathrm{q}}_{\max }={\displaystyle \frac{0.56·\mathrm{F}·1.0}{{\mathrm{T}}_{\mathrm{b}}}}$$

(16)

Theorem 16.

q_max is the Peak (maximum) ordinate of synthetic unit triangular hydrograph (m³·s⁻¹·mm⁻¹) [38].

$${\mathrm{H}}_{(\mathrm{T},\mathrm{P})}={\displaystyle \frac{\mathrm{a}·{\mathrm{T}}_{\mathrm{k}}}{1440}}·{\left({\displaystyle \frac{1440·\mathrm{A}+1}{\mathrm{A}·\mathrm{T}+1}}\right)}^{\mathrm{B}}·{\mathrm{P}}_{\mathrm{gr}}$$

(17)

Theorem 17.

H_(T,P) is the relevant rain of occurrence probability duration (mm). a is the constant approximate to the value 1; A is the constant value 0.3; B is the coefficient obtained using isoline maps for the Republic of Serbia (0.815) [38].

$${\mathrm{P}}_{\mathrm{ef}}={\displaystyle \frac{{({\mathrm{H}}_{\mathrm{T},\mathrm{P}}-0.2·\mathrm{d})}^{2}2}{{\mathrm{H}}_{\mathrm{T},\mathrm{P}}+0.8·\mathrm{d}}}$$

(18)

Theorem 18.

P_ef is the effective rain (mm) [38].

$${\mathrm{Q}}_{\mathrm{m}\mathrm{a}\mathrm{x}\left(1\mathrm{\%}\right)}={\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}·{\mathrm{P}}_{\mathrm{e}\mathrm{f}}$$

(19)

Theorem 19.

Q_max(1%) is the maximum flow (m³·s⁻¹) [38].

When calculating the capacity of the natural bed of the Jablanica river to accept certain flows, Chezy’s and Manning’s formula was used. On the right bank, there are no residential buildings on the profile itself, while there are residential and other buildings in the immediate vicinity of the riverbed.

• Chezy’s and Manning’s coefficient—0.035;
• Area of the flow profile—A = 26.51 m²;
• Surface-area-to-volume ratio—R = 14.60 m;
• Stream slope on the calculated profile—I = 0.7%;
• Flow for the natural bed—Q = 94.318 m³·s⁻¹.

$$\mathrm{Q}={\displaystyle \frac{1}{\mathrm{n}}}·\mathrm{A}·{\mathrm{R}}^{{\displaystyle \frac{2}{3}}}·{\mathrm{I}}^{{\displaystyle \frac{1}{2}}}$$

(20)

Theorem 20.

Q is the Chezy’s and Manning’s formula (m³·s⁻¹) [38].

4. Results

One of the main reasons for the planning and project design of the Stubo–Rovni reservoir is the formation of the regional system for water supply to the population and the economy, water protection, and protection against high waters. This capital water-management facility should provide 1140 L/s for the needs of the population and the economy.

4.1. Impact of Reservoir Construction on Forest Resources

The protection and preservation of forest resources is the main prerequisite for protecting the basin from erosion, sediment production, torrential flows and the filling of the reservoir with sediment. Given this, one hundred percent forest cover of the basin is the most favorable for achieving an optimal condition. The structure of forests, observed through the prism of surface coverage, is just as important as the forest cover of the basin in the function of complete protection.

Some of the measures to prevent soil erosion, sediment production, water protection, and surface runoff prevention were defined during the planning and construction of the Stubo–Rovni reservoir. The prohibition of frequent felling and the application of selective stand structure methods as one of the ways of forest stewardship are some of the positive measures based on scientific knowledge and experience in practice so far. Also, an important segment is defining the purpose of existing and future forests, which should be defined as anti-erosion and, as such, should have different treatments in the manner of management and stewardship. In the function of increasing anti-erosion forests, an increase of surfaces of the land of the VI–VIII capability class by around 1371 ha was defined, which would increase the forest cover to around 40%, which can be considered satisfactory compared to the existing situation. In addition to banning frequent felling and increasing forest coverage, the following measures are applied:

• The introduction of more intensive stewardship to ensure as complete stability and durability as possible (in terms of permanence) of these forests through constant and permanent cultivation interventions in all stages of age and development;
• Protection and nurturing of stands of high growth form, primarily of beech (Fagus) forests;
• Melioration of thickets and copses;
• Protection and cultivation of forest crops, primarily the white pine (Pinus sylvestris), black pine (Pinus nigra) and spruce (Picea);
• For protection, it is necessary to establish a forest order at the time of felling and thinning and to encourage individual trees of deciduous species that have been found in these localities as a result of thinning;
• To give up the further massive introduction of conifers into this basin to minimize the risk of biological instability;
• As a rule, conifers should only be introduced as pioneer species in completely degraded habitats;
• Melioration of coppice forests;
• Development of special bases for forest stewardship;
• Development of stewardship programs for private forests, etc.

4.2. Impact of Reservoir Construction on Agricultural Land

The impact of reservoir construction on agricultural land is reflected in the defining of management measures. One of the basic measures in the function of protecting agricultural land from erosion is defining the method of soil cultivation on terrains with certain degrees of slope. Increasing the slope usually decreases the depth of the humus horizon and increases erodibility, i.e., lowers the soil resistance to erosion processes. The result of this phenomenon is the defining of measures (in line with the planning regulations [30] and scientific references [44]) that would enable the afforestation of all agricultural areas above a 30% slope, not counting the areas up to 45% of the slope on which a grass cover has been formed, which has a protective role in the function of soil stability and, as such, is of great economic importance for the local population (livestock breeding, development of tourism, etc.). The measures of afforestation and preservation of grassland on agricultural land on slopes greater than 30% enable the improvement of the structure and quality of soil, i.e., an increase in land capability. The result of the implementation of these measures represents the economic justification, the profitability of production on the one hand, and the prevention of sediment as a consequence of soil erosion, i.e., the filling of the reservoir and water pollution, on the other. The terrains suitable for agricultural production occupy around 30% of leveled and moderately inclined lands, on which the application of anti-erosion protection measures (measures of protection against soil erosion, the formation of sediments, torrential flows, and the arrival of sediments in reservoirs) and measures of the regime for the use of chemical agents and other industrial substances are mandatory (measures to protect water from pollution).

4.3. Impact of Reservoir Construction on Soil Erosion and Flash Floods

By applying the EPM in the basin area of the Jablanica river from the profile of the Stubo–Rovni reservoir, erosion processes are detected in almost all forms—Figure 5, soil erosion map. According to the general development of erosion processes, the basin area of the Jablanica river can be classified as a weakly eroded area under weak erosion, with an average erosion coefficient Z_m = 0.332, with values ranging from 0.007 to 2.39. Excessive erosion processes occur on 3.64 km², i.e., 3.31% of the total basin area. Strong erosion occurs in an area of 7.05 km², i.e., 6.42% of the researched area. The erosion of average intensity affects 23.67 km² of land, i.e., 21.55% of the total area, while 15.29 km², i.e., 13.92% of the total researched area, is affected by weak erosion. The largest area is affected by very weak erosion: 60.19 km² or 54.80% of the researched area.

The total production of erosion material (W_yr) and specific production of erosion material (W_yrsp) were calculated based on the raster, where each pixel “carries” a certain value of production of erosion material (m³∙km⁻²∙yr⁻¹). The total production of erosion material in the Jablanica river basin is W_yr = 79,693.74 m³∙yr⁻¹, while the specific production is W_yrsp = 725.61 m³∙km⁻²∙yr⁻¹. The range of specific production is W_yrsp = 1.89–11,921.9 m³∙km⁻²∙yr⁻¹, with all five categories of destructiveness present (Figure 6). The surface representation of specific erosion production W_yrsp in the area of the Jablanica river basin is shown in

Table 2. Specific erosion production in the Jablanica river basin from the Stubo–Rovni profile.

Erosion Category	Severity of Erosion Processes in the Basin	Quantity (m3∙km−2∙yr−1)	km2	%
I	Excessive erosion	>3000	1.25	3.74
II	Heavy erosion	1500–3000	0.85	13.05
III	Medium erosion	1000–1500	0.55	11.69
IV	Slight erosion	500–1000	0.30	4.91
V	Very slight erosion	0–500	0.1	66.61
∑			109.83	100.00

.

The construction of the Stubo–Rovni reservoir, i.e., the use of the basin area, has a positive impact on soil erosion and the occurrence of torrential flows. These impacts are reflected through planning and project management of the basin area and the reservoir. The direct results of the management are related to an increase in the forest cover of the basin (no organized afforestation was carried out, but only sporadic by the “Boranja” forest estate, but population migration and measures within the sanitary protection of the basin and the reservoir had a positive effect on the natural renewal of forest resources). Anti-erosion Protection Project from 1986 envisaged afforestation, melioration and grass seeding of certain areas. By 2013, 70% of the planned activities had been carried out, i.e., some 260 ha had been afforested, and some 724 ha covered with grass [59]. In the said period (1986–2013), forest cover increased by 25% (55% of the total area of the basin) [59], while the percentage is somewhat bigger today (no updated cadastar data). The barren land share decreased from 1.2% in 1986 to 0.1% in 2013. The measures defined in the planning and project documentation resulted in the prevention of erosion processes, sediment production, the occurrence of torrential flows, and the sediment reaching the reservoir. Some of the measures to prevent erosion processes include the anti-erosion treatment of agricultural land, prevention of felling of existing forests in areas where there are risks of soil erosion, afforestation and grassing of bare areas, restoration and afforestation of areas with neglected and poor quality forests, possibilities of irrigation in dry cycles, monitoring of the management of land and forest resources, etc. In line with the above and the sublimation of results, it can be concluded that the erosion coefficient has permanently decreased from 1986 onwards: Z_m1986 = 0.40, Z_m2013 = 0.37 [59], and Z_m2024 = 0.33. Specific sediment production reduced by around 11% in 2013 (from 916.49 m³ ∙km⁻² ∙yr⁻¹ to 815.34 m³∙km⁻²∙yr⁻¹) [59], and/or some 20% in 2024 (from 916.49 m³∙km⁻²∙yr⁻¹ to 725.61 m³∙km⁻²∙yr⁻¹). The occurrence of torrential flows and floods is fully regulated on the Jablanica river downstream of the dam because a controlled flow regime was established in the downstream part of the stream. On the tributaries and the main stream upstream of the reservoir, the establishment of anti-erosion management in the basin area had a significant impact on the occurrence of torrential floods and flows.

Given all the above, it can be concluded that with the construction of the Stubo–Rovni reservoir, anti-erosion works and defined measures must be carried out to prevent the above phenomena, i.e., to prevent sediment from reaching the reservoir and to prevent water pollution. The construction of the Stubo–Rovni reservoir provides integral protection of the basin against erosion processes, torrential flows, and floods.

4.4. Calculation of Flow Rate with Occurrence Probability of 50%, 20%, 10%, 2%, 5%, 1%, 0.5% for the Jablanica River (before the Reservoir Construction) and the Calculation of the Capacity of the Jablanica Riverbed

What follows are the results of calculations made for the Jablanica river with occurrence probabilities listed in

Table 3. Calculation for the occurrence probability of 50%, 20%, 10%, 2%, 5%, 1%, 0.5% Q = 5, 10, 20, 50, 100, and 200 years.

Tk (h)	Tb (h)	qmax (m3·s−1·mm−1)	HTP (mm)	Pef (mm)	Q (m3·s−1)	Rec. Int. in Year
6	23.512	3.702	37.749	17.352	64.244	2
6	23.512	3.702	48.989	26.577	98.397	5
5	22.162	3.928	54.811	31.586	124.069	10
5	22.162	3.928	65.023	40.636	159.614	20
5	22.162	3.928	73.060	47.933	188.278	50
5	22.162	3.928	81.464	55.686	218.731	100
4	20.812	4.183	86.172	60.074	251.274	200

before the reservoir construction. The obtained results indicate that the capacity of Jablanica’s natural bed on the profile around 3 km upstream from the source of the Kolubara is sufficient to accept a flow of around 94 m³/s (Figure 7), which represents a flow with the probability of occurrence between 50% and 20%. In the event that there is no Stubo–Rovni reservoir, which controls the flow of the Jablanica river downstream of the dam, this profile would flood very often, while the flows of 100-year-old water would cause great material damage and jeopardize the safety of the population. The flow rate of 298 m³/s causes flooding of the regional road and increases traffic and population safety risks (Figure 7).

The completion of construction works and the start of operation of the reservoir reduced flood risks downstream. The water reservoir management plan and the program ensure controlled water release and, consequently, the control of the flow rate downstream from the reservoir. Controlled inflows imply that the amount of water released downstream must not be lower than the biological minimum of 150 L/s. Other flow rates achieved in the planned reservoir water release must not pose flood risks downstream from the dam and depend on various factors such as precipitation, water consumption, maintenance of the facility, and others. Before the construction of the reservoir, the average flow rate was approximately 1.15 m³/s.

5. Discussion and Conclusions

The protection and use of water resources, as well as water protection is one of the basic prerequisites for economic, social, and spatial development. The Stubo–Rovni reservoir, as one of the most important water facilities, provides security for the supply of water to the population and the economic sector in the Valjevo region and the entire Kolubara region.

The planning and construction of the Stubo–Rovni regional water-management system resulted in setting special objectives for the use and arrangement of forest land. They are reflected in the maintenance of the optimal degree of forest cover and the specific structural form of stands in the basin area while controlling the introduction of others and for the purpose of maintaining and preserving indigenous tree species. Given the above facts, it can be stated that the stewardship and management of forest resources, as one of the basic measures during the planning and construction of the Stubo–Rovni reservoir, must be directed towards the reduction of the total unvegetated area of forests in order to ensure the integral protection of the basin and a reduction of structural breaks in tall pure beech stands. The implementation of measures results in the stewardship treatment aimed at increasing forest areas and forest restoration, which is reflected in the afforestation of forest land of VI, VII, and partly of the VIII capability class. An increase in forested areas by about 25% compared to the period before construction and a reduction in barren land areas from approximately 1.2% to about 0.1% are some of the results of the implementation of the above measures.

In terms of the impact of the Stubo–Rovni water reservoir on agricultural resources, it is obvious that the construction of the reservoir requires occupying certain areas of agricultural land and the implementation of significant restrictions in the processes of agricultural production. There is no doubt, however, that the cultivation and use of agricultural land are the economic basis of the existence of rural settlements. The primary goal of implementing measures in agricultural production (from the aspect of soil erosion) is to prevent the negative impact of agricultural production on the state of water in the basin area of the Stubo–Rovni reservoir, i.e., to prevent erosion processes and sediment production. The results of the applied Stubo–Rovni reservoir protection measures, defined by planning and project design, include the reduction in the share of intensive crops (fields and orchards) from 27% to 18.3%, i.e., an increase in the share of forest and grassland ecosystems from 69.6% to 76.6%, along with more rational use of reduced agricultural areas.

By summing up the research results, it can be concluded that the construction of the Stubo–Rovni reservoir on the Jablanica river contributed to a series of positive effects, both from the aspect of protection against soil erosion and in terms of the environment in general, predominantly referring to the reduced sediment production by approximately 20% and the erosion coefficient by 10% compared to the period prior to the construction of the reservoir in 1986. The said anti-erosion measures are afforestation and renewal of forest resources, changes in the methods of cultivating agricultural land, construction of landfill barriers, regulation of water flows, etc. The accumulation of sediment before the construction of the reservoir on the profile of the conflux with the Obnica river was up to 8000 m³ for one flood cycle, while now it is almost insignificant. In terms of protecting the basin from soil erosion, flash floods, and high waters, the Stubo–Rovni reservoir has fully achieved the integral protection of the basin and its greater area, securing its sustainable development.

Certain restrictions to the use of land were minimized compared to the benefits brought about by the construction of the dam and the reservoir formation. They considerably reduce the risk of a flood wave, of the disruption in the supply of water to the population and the economy, and contribute to the development of tourism. The hydrological study that was carried out for the Jablanica river for the case without a reservoir found that the flow for the probability of occurrence of 1% on the chosen profile significantly exceeds the capacity of the transverse profile, i.e., the ability to receive and carry the amount of 100-year water. The results of the mentioned hydrological study and flow modeling indicate that a flow rate of approximately 298 m³/s causes flooding of the regional road, while the centennial water flow rate comes very close to the said road, significantly increasing the risk to traffic and people safety. Additionally, the flood zone includes certain residential and other buildings, as well as inhabitants, who would be at risk even with much smaller flow rates than those of the centennial waters. After the construction of the reservoir, controlled regulation of the water level in the Jablanica river, i.e., the flow rate downstream of the embankment dam, is carried out.

To sum up the above facts, despite the great challenges in the planning, design, and construction phases, the Stubo–Rovni reservoir has fully justified and strengthened the positions defined by the strategic development documents and planning solutions presented in this paper, which are related to the protection of the basin from erosion processes, torrential floods, particularly the regulation of the water balance in the territory of the city of Valjevo and the broader area. This case study, i.e., the experiences and lessons learned, has significantly contributed to the development of other water-management systems in Serbia, especially the Selova Water Reservoir near Kuršumlija, South Serbia, and Svračkovo Water Reservoir, near Arilje, Western Serbia, both under construction. These reservoirs would considerably reduce the risk of high waters and meet the water supply needs of the population and the regional economy.