45 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 con- sidered 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 sur- face, 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.