High Flows from Gornja Dobra Basin

The Mining-Geology-Petroleum Engineering Bulletin High ƪ ows from Gornja Dobra basin UDC: 556.5 DOI: 10.17794/rgn.2018.3.5

Professional paper

Krešimir Pavli©1; Daniel Jakobovi©2 1 University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Pierottijeva 6, p.p. 390, 10000 Zagreb 2 HIS d.o.o. Donja Višnjica 61D, 42255 Donja Višnjica

Abstract This paper presents basic hydrological indicators based on data from the four existing hydrological stations in the Gorn- ja Dobra basin. The frequency and ƪ ow-duration curves of mean daily ƪ ow in the Turkovi©i proƤ le are also deƤ ned and presented. The dependence of the mean maximum annual speciƤ c ƪ ows on the size of the inƪ uential basin to the considered hydrological proƤ les was considered. A review of the most recent high water event in 2017 was given.

Keywords Gornja Dobra, frequency and ƪ ow-duration curve, maximum speciƤ c ƪ ow, stage-discharge curve, big water wave.

1. Introduction The main objective of this paper is to determine the dependence of mean maximum annual speci¿ c dis- The Dobra River can be divided into three character- charge qժ M on the size of the inÀ uential basin, to de¿ ne istic areas of À ow. The ¿ rst, known as the Gornja (Up- the frequency and À ow-duration curves of mean daily per) or Ogulinska Dobra, is 51.2 km long, stretches from À ows and to analyse the most recent À ood event that oc- the spring on the north-eastern slopes of the Mlada Gora curred in Ogulin in September 2017. The analysis was through Skrad, Kupjak and Ravna Gora settlements to carried out on the basis of data collected over 31 years the town of Ogulin where the river sinks into Ĉulin from the period (1981 - 2011), based on the data of the ponor. From the Ĉulin ponor, the underground stream mean and maximum annual À ows from hydrological sta- continues through the natural system of karstic canals tions in Gornja Dobra and Vitunjþica. and caverns, about 16,400 m in length, and on the surface, it À ows to the village of Gojak and thus forms the 2. Research area and previous studies second part of the Dobra River. The Ĉulin ponor repre- of research area sents a natural drainage system for the high waters of the Most of the current research, except for the Gojak hy- Upper Dobra and its tributaries. The height difference droelectric power plant, is based on ¿ nding appropriate between the highest and lowest point is 83 m (ýepelak, solutions for the protection of Ogulin, which is endan- 1985). The third part of this watercourse, which À ows gered by the emergence of high waters. Based on this, a into the river Kupa, takes on the characteristics of the brief overview of the À ood in 1999 in Ogulin is present- lowland watercourse called Donja Dobra. Downstream ed, the problem of the limited capacity of the Ĉulin of the hydroelectric power plant Gojak, the hydroelec- ponor, which is directly related to this issue and the in- tric power plant Lešüe was built on Donja Dobra. À uence of the Gojak hydroelectric power plant on the The Dobra River is among the subterranean rivers of whole river system is discussed. The importance of solv- Europe and is an important tributary of the Kupa River. ing the problem of À ooding the city Ogulin was con- Since the Dobra basin is a part of the Dinaric karst, it is ¿ rmed by the latest À ood in September 2017. characterized by a complex geological structure and The Dobra River is a karstic river that runs partly as a shows great variation in hydrological and hydrogeologi- surface and partly as an underground river. On its way, it cal properties. Consequently, it can be said that karstic passes through karstic conduits of different dimensions aquifers represent heterogeneous systems which are dif- (caves and other underground karstic forms), which can ¿ cult to de¿ ne due to the constant interaction between be either free water face or under pressure (Prelovšek et surface and groundwater. al., 2008). Thus, on certain watercourse lines, the Dobra River, depending on the hydrogeological situation, si- Corresponding author: Krešimir Pavliü multaneously loses part of the water in the karst subter- e-mail address: [email protected] ranean, but is also fed with water from the karst aquifers.

45-52 Pavli©, K. and Jakobovi©, D. 46

Figure 1: The situation where the river Dobra is marked with 0ulin ponor, HPP Gojak, accumulation of Bukovnik and Sabljaki, the positions of all hydrological stations and river catchment

Mean daily À ow rates data used in this analysis are (Gojaþka) Dobra, whose À ow mostly depends on the op- taken from the Meteorological and Hydrological Service eration of the hydroelectric power plant Gojak (Opala of the Republic of Croatia (DHMZ) and collected from and Ožaniü, 2010). hydrological stations Moravice, Luke and Turkoviüi on Some recent hydrological surveys covered by the Do- Gornja (Ogulinska) Dobra and Brestovac stations on bra River basin were carried out by Žugaj et al. (1998- Vitunjþica, an important tributary of Gornja Dobra. 2007), R. Biondiü (2005), D. Biondiü (2005), Pavliü Vitunjþica À ows in Gornja Dobra after 4 km of À ow near (2016) and Pavliü et al. (2017), but geological research the Turkoviüi village. Figure 1 shows the characteristic of the wider area of the karst of Croatia has been con- parts of the river Dobra, the river catchment, the men- ducted by Sudar et al. (2016) and Veliü et al. (2017). tioned hydrological stations, Ĉulin ponor and the hydro- Since the town of Ogulin is located in the central part electric power plant Gojak. of the river Dobra, there is a great inÀ uence of Ĉulin In the hydrogeological sense, the Dobra River basin is ponor (see Figure 2), which represents the natural drain- a transitional area connecting the Dinaric and Pannonian age system of high waters. It is noted that À ow through areas, where three distinct units can also be distin- the underground system, after the entrance of water into guished. Gornja Dobra begins as a surface runoff with the sinkhole, primarily depends on the underlying condi- clastics of Paleozoic age in the area of Skrad, and it tion that is dif¿ cult to de¿ ne. The capacity of Ĉulin

retains such characteristics to Vrbovsko. Signi¿ cant ponor during the À ood in 1999 was evaluated: Qÿp = 72.0 karst springs are formed in in the area of Vrbovsko m3/s (Brkiü, 2001). Due to this limited capacity, a very where permeable and impermeable deposits intersect, large water wave cannot completely drain from this area. (IGH, 2009). The watercourse continues on the terrain The Gojak hydroelectric power plant with the Sabljaki that is made of limestone and dolomite of Jurassic age. and Bukovnik accumulation system plays an important While À owing through the dolomites, no loss of water role in the defence against À oods. occurs, but upon contact with limestone, there are sink- For the purpose of À ood protection, Croatian Waters holes, as is the case with Ĉulin ponor. At the intersection (the legal entity for water management in Croatia) are of the Triassic clastites and carbonate deposits, the planning to build a retention in Ogulin on the upstream Zagorska Mrežnica River, which, as well as the Gornja part of the basin. The retention will result in a reduction Dobra, sinks upon contact between dolomites and lime- of peak À ow of high waters and the retention of À oating stones. The last part of the Dobra watercourse is Donja objects, which by entering the Ĉulin ponor would reduce

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 47 High ƪ ows from Gornja Dobra basin

the À ow of the sinkhole. This will ensure the appropriate degree of protection for the city of Ogulin. The most downstream hydrological pro¿ le of Turko- viüi on Gornja Dobra is located upstream from Ogulin and the small reservoir lake Bukovnik. Data collected at the Turkoviüi pro¿ le is the most important hydrological data for this work. An interesting fact is the size of the basin of the Gornja Dobra, inÀ uential to the Turkoviüi 2 pro¿ le, which was earlier AT ‘= 296 km , and according to 2 recent research AT = 357 km (Žugaj et al., 1998-2007), which compared to the earlier value is 20.6% larger. Since it is a karst area and it is dif¿ cult to determine the surface area of the river basin, these sizes should be considered approximate (Žugaj and Pavliü, 2013). Furthermore, it is important to say that the Turkoviüi pro¿ le has a discharge Figure 2: 0ulin ponor into inundation area of high waters from the river bed (see

Figure 3: Turkovi©i hydrological proƤ le (modiƤ ed after VPB (2001))

Figure 4: Stage-discharge curves from 2002 and 2004 at Turkovi©i hydrological proƤ le (VPB, 2005)

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 Pavli©, K. and Jakobovi©, D. 48

Figure 3). Figure 3 shows the water stage of maximum there are several smaller cascades along the river. Thus, level recorded in July 29th 1999 and the water stage of the the damming will not act on the stage-discharge curve most recent À ood event in September 2017, which was and can be uniquely determined. The fourth or ¿ nal pro- 327.01 m a.s.l. ¿ le of this analysis is Brestovac on Vitunjþica, an impor- The stage-discharge curve for the Turkoviüi pro¿ le tant tributary of Gornja Dobra, whose inÀ uence is cov- from 2004, which is in use today (see Figure 4), is con- ered by the Turkoviüi hydrological station. sidered the most reliable due to additional river À ow measurements in the high water area after the great À ood 3. Results and discussion in Ogulin in 1999. It was found that the longitudinal slopes of the water- A statistical analysis of maximum annual À ows and course are such that there is no damming action because volumes of isolated water waves is shown in Figures 5

Figure 5: Exceedance probability of maximum annual ƪ ows according to Pearson 3 distribution at Turkovi©i hydrological proƤ le

Figure 6: Exceedance probability of maximum volumes of water waves according to Pearson 3 distribution at Turkovi©i hydrological proƤ le

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 49 High ƪ ows from Gornja Dobra basin

Figure 7: Double water wave recorded at hydrological station Turkovi©i that occurred in September 2017 together with water wave from July 1999 as a comparison. Plotted values of discharge are hourly values. and 6. The selected relevant distribution for maximum whether the dam and the associated retention will be À ows and volumes of large water waves is Pearson 3. able to capture the entire volume of high water waves, The maximum À ow from the 1999 À ood, which was since the area for predicted accumulation is limited by 254 m3/s, had a little bit more than 100-year return peri- the railway line along the Gornja Dobra, and the ¿ sh- od, according to Žugaj and Pavliü (2013). The volume of ponds on the left bank of Vitunjþica tributary (IGH, that water wave was 21.3 · 106 m3, which was about a 2013). This suggests that an alternative solution to this 20-year return period. Figure 7 shows the double water problem should be considered, such as the retention of wave that occurred in September 2017 together with the water upstream. water wave that occurred in July 1999, as a comparison. The dependence of mean maximum annual speci¿ c

The ¿ rst water wave in 2017 had a maximum À ow discharge qժ M on the size of the catchment area A is de- 3 6 3 rived using data from three hydrological stations: QM1 = 196 m /s and volume V1 = 26.89 · 10 m . The maximum À ow is a 10-year return period, while its vol- Moravice and Turkoviüi at Gornja Dobra and Brestovac ume is more than a 100-year return period. The second at Vitunjþica. Data from the pro¿ le Luke at Gornja Do- 3 bra was not used because value at that station was unre- water wave in 2017 had a maximum À ow QM2 = 136 m /s 6 3 alistically high with respect to derived dependency, as and volume V2 = 17.57 · 10 m . The maximum À ow of the second water wave is less than a 2-year return period shown in Figure 8. The value of mean maximum annual 3 and its volume is little less than a 5-year return period. À ow at pro¿ le Turkoviüi is Qժ M = 124 m /s, which is quite In the ¿ rst water wave, the return periods did not co- a high value. incide with À ow and volume, while in the second water The hydrological pro¿ le Luke on the Gornja Dobra is wave they approximately coincided. Regardless of the unstable in the hydrological sense, and the stage-dis- fact that the maximum À ow and volume of the second charge curve is extrapolated in a wide range of high wa- water wave was of a smaller return period, it represented ters. When de¿ ning the stage-discharge curve in the a danger because the ¿ rst water wave saturated the Turkoviüi pro¿ le, river À ow measurements of the 5 and ground with water, which made it possible to create a 7-year return periods were used, and the pro¿ le in the major À ood. Double water waves on the Gornja Dobra hydrological sense was considerably more stable than basin can cause major À ooding, and in particular, atten- the Luke pro¿ le. tion should be paid to them since they have appeared 11 Figure 9 shows frequency and À ow-duration curves times since 1963. de¿ ned on the basis of daily mean À ows in the Turkoviüi It is clear that high waters, in the form of single or pro¿ le. The frequency curve shows an asymmetric shape double water waves occur regularly on Gornja Dobra, with a high peak where À ows ranging from 2.50 m3/s to which is a threat to the town of Ogulin. For the purpose 7.50 m3/s occur more than 35% of the time during a year. of protecting the city of Ogulin from À oods, the plan is From the À ow-duration curve it can be concluded that to build a dam in Turkoviüi. This dam should À atten the the mean À ows of Gornja Dobra at the Turkoviüi pro¿ le water wave that comes to Ogulin, but the question is (Q = 10.5 m3/s) and higher, last for 30% of the total time

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 Pavli©, K. and Jakobovi©, D. 50

Figure 8: The dependence of mean maximum annual speciƤ c discharge qժ M on the size of the catchment area A on Gornja Dobra. Data from Luke hydrological station was not used for deriving the relationship.

Figure 9: Frequency and ƪ ow-duration curves of mean daily ƪ ows of Gornja Dobra in Turkovi©i proƤ le (1981 - 2011) (Jakobovi©, 2014) during a year, while the À ows lower than the mean À ows 4. Conclusion last for 70% of the total time during a year. Therefore, The Dobra River represents a karst phenomenon the main feature of the À ow-duration curve has quite a characterized by a great changeability of hydrological favourable distribution for energy use. and hydrogeological features of the catchment area. By Frequency and À ow-duration curves for values of part of its À ow, it À ows as a surface, and partly as a sub- À ow greater than 75.0 m3/s show very low frequencies terranean stream, which makes it into the most famous and short durations of such À ow rates. waterways of that type.

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 51 High ƪ ows from Gornja Dobra basin

The appearance of high waters on the Gornja Dobra Institut IGH d.d. (2013): Retencija Ogulin s pripadajuüim basin is especially dangerous in the case of double water graÿevinama (Ogulin retention with related buildings). waves. From 1963 to the present day, 11 double high IGH Projekt, Zavod za geotehniku, Zagreb. (in Croatian) water waves appeared. Therefore, the solution for the Jakoboviü, D. (2014): Karakteristiþni protoci sa sliva Gornje protection of the city of Ogulin from À oods should take Dobre (Characteristic À ows from Gornja Dobra basin). particular account of the occurrence of such water waves University of Zagreb, Faculty of Mining, Geology and and consider the possibility of retention of water up- Peroleum Engineering, Bachelor’s Thesis. (in Croatian) stream from Turkoviüi. Opala, I. and Ožaniü, N. (2010): Hidrološka analiza sliva ri- jeke Dobre (Hydrological Analysis of Dobra River Catch- The speci¿ c mean maximum discharges qࡄ on the M ment Area), Zbornik radova, knjiga VIII, Graÿevinski Moravice pro¿ le show lower values than those assumed, fakultet Sveuþilišta u Rijeci, Rijeka, 11-33. (in Croatian) which is explained by water losses in the underground. Prelovšek, M., Turk, J. and Gabrovšek, F. (2008): Hydrody- The hydrometric pro¿ le of Luke, in which the À ow val- namic aspect of caves. International Journal of Speleology, ues of high waters are estimated on the basis of an unre- 37 (1), 11-26. liable stage-discharge curve, shows large deviations and Pavliü, K. (2016): Regionalna Hidrološka analiza krškog data from that station were not used for de¿ ning the de- porijeþja Kupe (Regional Hydrological Analysis of the pendence of mean maximum annual speci¿ c discharge Kupa Karstic Basin). University of Zagreb, Faculty of qժ M on the size of catchment area A. The speci¿ c mean Mining, Geology and Peroleum Engineering, PhD Thesis. maximum discharges qMࡄ on the Turkoviüi pro¿ le show (in Croatian) assumed values, so the stage-discharge curve that is now Pavliü, K., Kovaþ, Z. and Jurlina, T. (2017): Trend analysis of used in the high waters is adequate. mean and high À ows in response to climate warming-Evi- Gornja Dobra shows a favourable À ow distribution in dence from karstic catchments in Croatia. Geo¿ zika, 34 the Turkoviüi pro¿ le. Mean À ow and À ow rates higher (1), 157-174. than mean last 30% of the total time while À ows lower Sudar, V., Aljinoviü, D., Smirþiü, D. and Barudžija, U. (2016): than mean last 70% of the time, which is a good distribu- Composition and provenance of Gröden sandstone from tion for energy generation use. the Velebit Mts. The Mining-Geology-Petroleum Engi- neering Bulletin, 31 (1), 53-67. Veliü, J., Veliü, I., Kljajo, D., Protrka, K., Škrabiü, H. and Špo- 6. References ljar, Z. (2017): An geological overview of glacial accumulation and erosional occurrences at the Velebit and the Biondiü, D. (2005): Velike vode hrvatskih vodotoka crnomor- Biokovo Mts., Croatia. The Mining-Geology-Petroleum skog sliva (High waters of Croatian watercourses of the Engineering Bulletin, 32 (4), 77-96. Black Sea basin). University of Zagreb, Faculty of Civil VPB (2001): Zaštita grada Ogulina od velikih voda: Knjiga 1 Engineering, PhD Thesis. (in Croatian) - Hidrološke obrade (Protecting the City of Ogulin from Biondiü, R. (2005): Zaštita voda gornjega dijela sliva Kupe the High Waters: Book 1 - Hydrological Processing). Za- (Protection of the water from the upper part of the Kupa greb. (in Croatian) basin). University of Zagreb, Faculty of Mining, Geology VPB (2005): Struþna podloga za izradu studije o utjecaju na and Peroleum Engineering, PhD Thesis. (in Croatian) okoliš retencije Ogulin: hidrološko-hidrauliþki proraþun Brkiü, B. (2001): Zaštita grada Ogulina od velikih voda, idej- (Expertise for the Study on Environmental Impact Assess- no rješenje (Protection of city Ogulin from high waters, a ment of Ogulin: Hydrological and Hydraulic Processing). conceptual solution). Zagreb: Vodoprivredno-projektni Zagreb. (in Croatian) biro (VPB) d.d., Hidrološke obrade, knjiga 1. (in Croatian) Žugaj, R., Andreiü, Ž., Pavliü, K. and Fuštar, L. (2011): Kriv- ýepelak, M. (1985): Špiljski sustav Ĉula-Medvedica (Ĉula- ulje trajanja protoka (Flow duration curves). Graÿevinar, Medvedica cave system). Speleolog, 2-24. (in Croatian) 63 (12), 1061-1068. (in Croatian) HIDROINŽENJERING d.o.o. (2015): Retencija Ogulin s Žugaj, R. and Pavliü, K. (2013): Peer Review of Preliminary pripadajuüim graÿevinama - Elaborat zaštite okoliša (Ogu- Project “Ogulin Retention in Turkoviüi” in Terms of the lin retention with related buildings - Environmental Elabo- Acceptability of Adopted Hydrological Data and Analyses ration), Zagreb. (in Croatian) with Implementation of Additional Hydrological Analyses Institut IGH d.d. (2009): Plan navodnjavanja Karlovaþke of Gornja Dobra High Waters. (in Croatian) županije (Irrigation Plan of Karlovac County). IGH Pro- Žugaj, R., Jurak, V., Plantiü, K., Galoviü, I., Košüal, S. (1998- jekt, Zavod za hidrotehniþko projektiranje, Zagreb. (in 2007): Regionalna analiza slivova Kupe i pritoka u Kršu, Croatian) Znanstveni projekt MZOŠ, RGNF, Zagreb. (in Croatian)

The Mining-Geology-Petroleum Engineering Bulletin and the authors ©, 2018, pp. 45-52, DOI: 10.17794/rgn.2018.3.5 Pavli©, K. and Jakobovi©, D. 52

SAŽETAK Velike vode na slijevu Gornje Dobre U radu su na osnovi podataka s «etiriju postoje©ih hidroloških stanica na slijevu Gornje Dobre razmatrani osnovni hidro- loški pokazatelji. DeƤ nirane su i graƤ «ki prikazane krivulja u«estalosti i krivulja trajanja srednjih dnevnih protoka u proƤ lu Turkovi©i. Razmatrana je zakonitost veze maksimalnih srednjih godišnjih speciƤ «nih dotoka u ovisnosti o veli«ini slijevova te o razmatranim hidrološkim proƤ lima. Razmatrani su veliki vodni valovi iz 1999. i 2017. godine.

Klju«ne rije«i: Gornja Dobra, krivulja u«estalosti i trajanja, maksimalni speciƤ «ni dotoci, proto«na krivulja, veliki vodni val

Authors contributions: Krešimir Pavlic (Dsc.) carried out a statistical analysis and analysed the most recent major water wave. Daniel Jako- bovi© gave an overview of the current research and research area and analysed the historical data.