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P011-1

The Chain of Hydropower Plants on the Lower Part of the in : Hybrid Hydrodynamic Modelling of High Waters Crossing Hydropower Plants and Inundations

Matija Bogdan MARINČEK1, Leon GOSAR2 , Gašper RAK2, Tanja PREŠEREN2

1 Consulting, designing, agency and research Matija Bogdan Marinček s.p., Teharska cesta 13, SI-3000 , Slovenia, e-mail: [email protected] 2 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Chair of Fluid Mechanics with Laboratory, Jamova ul. 2, Ljubljana, Slovenia, e-mails: [email protected]; gasper.@fgg.uni-lj.si; [email protected]

Keywords Sava River, hydropower plants, high water conditions, flood prone areas, retention areas, physical modelling, mathematical modelling, DHI MIKE FLOOD.

Abstract Slovenia is constructing hydropower plants on the lower part of the Sava River. The chain of six hydropower plants contains the hydro systems of Vrhovo, Boštanj, Blanca, Krško, Breţice and Mokrice. The first three plants have already been built, the construction of the forth plant is underway, and the fifth plant, Breţice, is at the start of the design phase. While the upper hydropower plants are located in the relatively narrow part of the Sava River valley, the locations of the other plants are in the middle of alluvial valleys, where there are wide flood prone areas along the Sava. The high water of the river could flood these areas for several hundred meters wide and cause significant flood damage to the beneficiaries of the space. The foreseen hydropower plants could influence the run-off water regime, particularly the flood discharges which are crucial boundary conditions on the state border to Croatia. Therefore, careful investigation of the water regime and operational influences on the regime is needed. Two parallel approaches are used, and thus the processes of coupling the results of the investigation on the physical model and the results of the simulations using the mathematical model DHI MIKE FLOOD are discussed.

INTRODUCTION

Due to many morphological, hydrological, hydraulic and anthropogenic features, the designing of the exploitation of the hydropower potentials of the Sava River in the area between Krško and the state border to Croatia is very complex. To implement the designs properly, exhaustive and accurate hydrological and hydraulic data on the current status and the status after the implementation of the planned facilities as well as the impact of the planned construction on the runoff regime of the Sava River in the area concerned must be provided before and during designing. For a suitable draining of part of the flood discharges, the entire retention area on both banks of the Sava River outside the levees restricting the direct area of the Breţice and Mokrice hydropower plants must necessarily be activated simultaneously with the area of the Sava River bed itself. In addition to the protection of the Krško Nuclear Power Plant, Čateţ ob Savi and Čateţke toplice, the activation of the retention areas will also satisfy the demands of the neighbour, the Republic of Croatia, not to increase the flood waves peak by excluding the retention areas as an active discharge profile for the draining of high water of low probability.

P011-2 In addition to the data obtained through on-site measurements, the designing of construction in the water and riparian areas crucially depends on the results of appropriate simulations realised by means of physical and mathematical modelling of water flow in different spatial, hydrological and operational circumstances. The complexity and mutual connectedness of the issues concerning the runoff of the Sava River through the area in question during high water as well as the sensitivity of the area itself (the Krško Nuclear Power Plant, large flood prone areas, state border, etc.) require high accuracy of input data for designing, which can only be provided through properly implemented hybrid modelling. This is the reason for using physical models with high accuracy of simulated phenomena and high level of similarity of the modelled phenomena with the events occurring in the nature (generally and locally), with a simultaneous use of mathematical models, enabling the coverage of a wide area with a relatively large number of different flow variants. The results of the physical models will also provide all the necessary data that cannot be obtained through on-site measurements and which will be used as the basis to set up the mathematical models of the areas concerned. Parallel modelling also enables mutual control and verification of the physical and mathematical models.

All until including the design of the Krško hydropower plant, hydraulic calculations of the river flow on the lower Sava needed to provide the boundary conditions for designing and physical modelling were possible by using simple 1D mathematical models. Since the flow nature is prevailingly one-dimensional (also during high water peaks), the calculations provided sufficiently accurate results. Downstream of Krško, during flood discharges the Sava spills over vast flood prone areas, on which distinctly 2D flows appear. The latter require the use of more complex mathematical models. For the concept stage, combinations of the proper 2D and quasi-2D mathematical models have already been used, while higher project stages will necessitate the use of combined physical and mathematical hybrid models.

The physical models primarily provide the data that not even the latest mathematical models can satisfactorily describe (especially turbulence and various 3D phenomena connected with it), and the mathematical models complement the physical ones where a large number of variants is needed, e.g. to determine the necessary level of overflow to the flood prone areas or to design the dominant floodways along the retention areas. The verification of the results and solutions obtained by means of the mathematical model is provided by simulating the variants selected on the physical model. The results of mathematical modelling can also be used to address the difficulties arising from model similarity problems.

Such method of a combined use of both model families is optimal and also ensures sufficient result accuracy (quantitative and not only qualitative accuracy) necessary to construct individual facilities. The appropriateness of this approach has also been confirmed by practice, as physical modelling of at least zero status is still envisaged for the construction of specific facilities. In no case, mathematical modelling can fully replace the physical one. Such a hybrid model (on-site measurements - mathematical model - physical model) will be able to provide the designers with the data necessary to ensure the required levels of safety and technical merit of the design as well as the manner of conveying high water.

The main strength of calibrated mathematical models will also be shown after the conclusion of the examinations for the Breţice and Mokrice hydropower plants on physical models, as in the future it will be possible to use them to complement the hydraulic calculations both for the impact of developments on the modelled areas and for the impact of upstream developments, like the planned construction of a chain of hydropower plants on the middle Sava. P011-3

The paper presents the hybrid hydraulic research of the hydropower plants on the lower Sava and the initial results of the models which will join the physical and mathematical modelling of the lower part of the Sava River and its flood prone areas. For mathematical modelling, the software package DHI MIKE FLOOD was chosen because of the large presence in the analyses of different water levels and since it enables the investigation of 2D water flows on flood prone areas.

SAVA RIVER

The Sava River is a tributary to the River, into which it flows at Belgrade. It is the central Slovenian river, rising under the Julian Alps, which are part of the central European Alps mountain range. The headwaters of the Sava River consist of two basic streams in the upper part, i.e. and . The significant tributaries are the , and Sotla . After 173 km, it crosses the border to the Republic of Croatia.

The Sava River has all the characteristics of a flashy stream, like: - relatively large amount of precipitation in the river basin - up to 4,100 mm annually; - steep and short-term flood waves of 1 to 3 days; - the discharge relations between low and high waters are up to 1:100 - with extremes also up to 1:250; - steep bottom slope - only downstream of the town of Krško, the drop of the bottom is lower than 1‰; - large erodability and large amounts of sediments.

Along the flow, the water regime of the Sava River changes with the alterations in the climate and ground configuration. The snow-rain regime prevails in the upper reach, transiting to the rain-snow regime in the middle and lower reaches. Annual maximums appear in the spring and autumn months, while minimums occur in the summer and winter months.

The typical passing of precipitation fronts, especially in the autumn, results in a high level of concurrence of the high waters of the Sava River and its tributaries. Although the annual flows of the Sava are low - below the of the Sava with its tributary Savinja only around 220 m3/s - the large slope of the riverbed provides the opportunity for the exploitation of its hydropower potentials.

In terms of hydropower, the flow of the Sava River is divided to the upper, middle and lower Sava. The upper Sava is fully exploited in terms of its energy, while three out of six envisaged hydropower plants have been constructed on the lower Sava.

CHAIN OF HYDROPOWER PLANTS ON THE LOWER PART OF RIVER SAVA

In mid twentieth century, the construction of large power plants on the Sava River intensified with the construction of the Moste HPP (1952) and the Medvode HPP (1953). Initially, a construction of an entire chain of hydropower plants all to the border to the Republic of Croatia was planned, but it was postponed by 30 years due to various reasons, until the construction of the Mavčiče HPP (1986) and the Vrhovo HPP (1994). Because of the delays in the construction, the hydropower potentials of the Sava basin are the least exploited among the main Slovenian rivers. Only poor 12% of the available hydropower potential is exploited, which only accounts for 7.4% of the total hydropower produced in Slovenia. P011-4

The construction of the chain on the lower Sava has already been in preparation since mid 1980s. The idea about the construction of hydropower plants on manmade deviation channels, which would preserve the existing river and riparian ecosystems, was abandoned (experience with the River). The tendency to type the facilities and equipment of hydropower plants led to the decision in favour of the same energy head of 8 m. This provided the basis for the location of 6 hydropower plants: Vrhovo, Boštanj, Blanca, Krško, Breţice and Mokrice directly in the bed of the Sava itself.

Figure 1: The locations of hydropower plants on the lower Sava. The Vrhovo, Boštanj and Blanca hydropower plants have already been built, the Krško HPP is being designed, and the Brežice and Mokrice plants are in the concept stage.

Before the beginning of the construction of the hydropower plants on the lower Sava, in 1991 a hydrological and hydraulic study of the so-called zero status of the Sava River was also made. The mathematical model MIKE 11 was used for hydraulic analysis. The calculation area comprised the Sava bed from Litija to the border to the Republic of Croatia (middle and lower Sava). It also included sections of the tributaries Savinja, Krka and Sotla, as well as retention and flood prone areas along the Sava downstream of Krško. On the then PCs, still supported by the MS DOS operating system, the calculation time was 1 day, if the simulation was successful, of course. The model was calibrated to the data on the Sava high water in 1990, which according to assessments was of probability of hundred years. The highest water levels were indeed determined on the area along the flow. These data were important for the calibration of the model. The findings of the study were also one of the bases for the preparation of the concession contract for the exploitation of water on the lower Sava for hydropower purposes. Later, many hydrological and hydraulic investigations have been conducted, representing the appropriate basis for further procedures in the construction and optimisation of the operation of the whole chain of hydropower plants, e.g. Rajar and Četina, 1994; Brilly et al., 2003; Četina et al., 2003; Zakrajšek and Četina, 2004; Mlačnik et al., 2004; Četina and Krzyk, 2005; Zakrajšek and Četina, 2005; Četina and Krzyk, 2007; Steinman et al., 2007; Širca et al., 2007, Ţagar et al., 2008.

RIVER AND FLOOD PLAIN MODELLING OF BREŢICE AND MOKRICE HPPs – FIRST RESULTS

P011-5 The hydraulic model of the Breţice HPP, i.e. Krško-breţiško polje, and the Mokrice HPP consists of the physical and mathematical models which both cover the same spatial area (Figure 2). The two models complement each other and together enable the simulation of all relevant hydrological situations and different variants of construction measures and their combinations. The two models differ from each other by the type of water flow investigated: the physical model precisely simulates the stationary conditions and, by means of these data, provides the mathematical model with the appropriate data for calibration, on the basis of which the mathematical model is able to simulate the dynamic phenomena determining the manner of high water conveyance in flood prone areas. The calibration of the mathematical model to the results of the physical model will increase the accuracy of the mathematical model, which will, also after the project is concluded and the physical model is removed, remain an appropriate tool for examining any subsequent developments in the modelled area, for modelling the impact of upstream developments influencing the flow of the Sava River (e.g. the chain of hydropower plants on the middle Sava). Therefore, DHI MIKE FLOOD has been selected as the most suitable mathematical model. The concept of this model is such that it is widely accessible and suitable for subsequent use by any qualified user, and can be included in a more complex model of the whole chain of hydropower plants on the Sava River.

Figure 2: The physical hydraulic model of a section of the middle Sava downstream of Krško (July 2008).

Since individual modelling techniques are limited, it is indispensable to use all appropriate research and designing methods in order to provide the answers to all technical questions concerning the input design data for the designing the hydropower plants and the related water management, infrastructural and environmental arrangements. Consequently, the goals and results of the mathematical modelling of water flow through the sections concerned must be tied directly to the other methods with which they complement each other. In certain cases, the results of mathematical modelling serve as support to physical P011-6 modelling, while in other cases the results of mathematical modelling must upgrade the results of physical modelling. The main goals of mathematical modelling are the following:

- determination of the conveyance of the retention area separately for the left bank and right bank areas for design variants, - determination of the main retention ways of the Sava River outside its bed and valuation of the relation of the discharges along individual ways across the retention areas, taking into account the variants of technical measures possible, - determination of the delineation of the discharge across the retention areas and in the bed before and after the developments, - determination of the most appropriate locations and the capacity of flood relief facilities needed, - determination of the need for the technology (type) of flood relief facilities (fixed weir or regulation facilities), - determination of the amounts, macro-location and the paths along which the flow returns from the retention areas to the main bed, - determination of the extent of any earthmoving work to correct the terrain which is to enable an uninterrupted and well synchronised return of the flow from the retention areas into the main bed, - determination of the need to examine anti-erosion measures on the inland side of the flood and power production levees and the retention areas in the area concerned, - determination of the extent of any protective measures needed for the Ljubljana - Zagreb motorway in the relevant section, - determination of other parameters for the calibration and improvement of the physical models of the Sava flow along its bed and along the retention areas.

Down to the confluence with the Savinja River, the Sava River has a steep and diversified bottom. It flows along a narrow valley which it hollowed out of carbonate rock and has no major tributaries. Down to the confluence with the Krka River, it only managed to create a few minor alluvial plains. Below the confluence with the Krka, it flows on its own alluvia. Its flow moderates, the bed is shallower. Thus, high water primarily floods and endangers the left bank all to the outfall of the Sotla (Figure 3 - the green line presents the left bank of the riverbed, and the brown line presents the right one). In the mathematical model, these hydraulic features had to be illustrated by means of several branches and loops presenting the floodways outside the main bed. The calculations in the model confirmed the on-site observation revealing that in 1990 part of the flood water did not return to the bed of the Sava River. The water flew on across the border river Sotla and further across the border to Croatia, parallel to the main Sava flow. The parallel flows also required two output boundary conditions.

Figure 4 shows large concurrence of the wave peaks of the Sava and its tributary Savinja. The two peaks arrived almost simultaneously to the confluence. This is characteristic of autumn precipitation fronts, spreading from the west towards the east. A somewhat delayed arrival of the front to the Savinja basin is compensated by a somewhat shorter and faster flow of the Savinja River.

P011-7

Figure 3: The longitudinal stream profile of the Sava River and its maximal water level.

Figure 4: The hydrographs of the 1990 flood wave in certain cross-sections calculated by the mathematical model MIKE 11.

Consequently, although having a smaller catchment area, the Savinja contributed almost half of the Sava peak discharge. The flood waves of the Sava and Savinja rivers are steep, which is characteristic of the watercourses with a flashy discharge regime. The flood waves of the tributaries Krka and Sotla are more flat and appear later and delay the lowering of the Sava flood wave. Figure 4 also shows the transformation of the Sava wave along the flow and the impact of the tributaries. P011-8 Until the Croatian border, the Sava wave peak below the confluence with the Savinja decreases by 10% due to the influence of the bed volume and the retention/flood prone areas.

CONCLUSIONS

Since individual modelling techniques are limited, it is indispensable to use all appropriate research and designing methods in order to provide the answers to all technical questions concerning the input design data for the designing of the Breţice and Mokrice hydropower plants and the related water management, infrastructural and environmental arrangements. Consequently, the goals and results of the mathematical modelling of water flow through the sections concerned must be tied directly to the other methods with which they complement each other. In certain cases, the results of mathematical modelling serve as support to physical modelling, while in other cases the results of mathematical modelling must upgrade the results of physical modelling.

The main goals of the whole project are the synthesis and interpretation of the results of combined physical and mathematical (hybrid) models, constituting a relatively new approach in Slovenia, which will consequently require somewhat increased communication with the expert public. The objective will primarily be to show the possible reasonable expectations concerning the results of modelling and, by presenting the technical contents, to encourage other spatial planners to include the boundary conditions analysed (and then included in the background documents for administrative acts) into their planning and programme documents in the impact area of the designed developments.

These objectives are extremely important, especially because new water (flood) corridors are envisaged in directions where no significant flood risk has appeared until now. Although the design will envisage the compensation measures for design discharges, many entities will also have to be informed about any circumstances on exceptional occurrences (averages etc.).

The main advantage of calibrated mathematical models will be shown after the conclusion of the research, when it will be possible to use them for spatial zoning (hazard mapping) and for examining other spatial developments in terms of their exposure to risk of water-related processes.

REFERENCES

Marinček M., Kovačič I., Burja D., Globevnik L., Rebolj D. (1993) Hidrološko- hidravlične raziskave vodnega reţima Save v Sloveniji do sotočja s Sotlo, Vodnogospodarski inštitut, Ljubljana.

Rajar, R., Četina, M. (1994) Two-dimensional modeling of flow in the river Sava. Modeling of flood propagation over initially dry areas : proceedings of the Specialty conference. New York: American Society of Civil Engineers, cop. pp 309-323.

Brilly, M., Mikoš, M., Četina, M., Šraj, M., Vidmar, A., Zakrajšek, M., Zidarič, M. (2003) Inventarizacija ključnih stanj prostora, okolja in infrastrukture na vplivnem območju Spodnje Save z ločevanjem ukrepov za izboljšanje sedanjega stanja in ukrepov zaradi izgradnje hidroelektrarn. UL FGG, Katedra za splošno hidrotehniko, Ljubljana.

P011-9 Četina, M., Širca, A., Zakrajšek, M., Ţagar, D., Brecelj, M. (2003) Optimizacija obratovanja hidroelektrarn na reki Savi. UL FGG, Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

Zakrajšek, M., Četina, M. (2004) Optimizacija zajezitve in energetske proizvodnje. Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

Mlačnik, J., Vošnjak, S., Ciuha, D. (2004) Razvoj podslapij na spodnjesavski verigi hidroelektrarn. Goljevščkov spominski dan, Ljubljana.

Četina, M., Krzyk, M. (2005) Moţne rešitve za izboljšavo zaščite NEK pred poplavami, Analiza obstoječega stanja - hidravlika. Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

Zakrajšek, M., Četina, M. (2005) Optimizacija zajezitve in energetske proizvodnje : določitev ukrepov ob prehodu poplavnih valov, račun obratovalnih valov in opredelitev načina obratovanja v verigi - HE Krško. Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

Četina, M., Krzyk, M. (2007) 2D modeliranje poplavnih valov na območju HE Mokrice. Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem; IBE, Ljubljana.

Steinman, F., Rajar, R., Četina, M., Rak, G., Krzyk, M., Zakrajšek, M., Prešeren, T., Gosar, L. (2007) Recenzija elaborata o pretočnosti pri visokih vodah Save v povezavi s HE Krško : Končno poročilo. Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

Četina, M., ZAKRAJŠEK, Majda, SIRNIK, Nataša. Izdelava predinvesticijske tehnično prostorske dokumentacije za HE na srednji Savi. Sklop 1, Hidravlični model gladin v sedanjem in bodočem (zajezenem) stanju za odsek srednje Save med HE Medvode in HE Vrhovo ter v fazi izgradnje : končno poročilo. Ljubljana: Univerza v Ljubljani, FGG - Katedra za mehaniko tekočin z laboratorijem, 2007.

Širca, A., Rodič, B., Zadnik, B., Josipovič, Z., Kvaternik, K., Četina, M., Zakrajšek, M. (2007) HE na spodnji Savi, Porušitveni valovi. IBE, Ljubljana.

Ţagar, D., Četina, M., Rajar, R. (2008) Študija določitve pogojev plovbe na motorni pogon in moţnega vpliva tokov in valov zaradi plovbe na breţine akumulacije HE Boštanj. Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Katedra za mehaniko tekočin z laboratorijem, Ljubljana.

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