High ows from Gornja Dobra basin

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


Introduction
The Dobra River can be divided into three characteristic areas of ow.The rst, known as the Gornja (Upper) or Ogulinska Dobra, is 51.2 km long, stretches from the spring on the north-eastern slopes of the Mlada Gora through Skrad, Kupjak and Ravna Gora settlements to the town of Ogulin where the river sinks into ulin ponor.From the ulin ponor, the underground stream continues through the natural system of karstic canals and caverns, about 16,400 m in length, and on the surface, it ows to the village of Gojak and thus forms the second part of the Dobra River.The ulin ponor represents a natural drainage system for the high waters of the Upper Dobra and its tributaries.The height difference between the highest and lowest point is 83 m ( epelak, 1985).The third part of this watercourse, which ows into the river Kupa, takes on the characteristics of the lowland watercourse called Donja Dobra.Downstream of the hydroelectric power plant Gojak, the hydroelectric power plant Leš e was built on Donja Dobra.
The Dobra River is among the subterranean rivers of Europe and is an important tributary of the Kupa River.Since the Dobra basin is a part of the Dinaric karst, it is characterized by a complex geological structure and shows great variation in hydrological and hydrogeological properties.Consequently, it can be said that karstic aquifers represent heterogeneous systems which are difcult to de ne due to the constant interaction between surface and groundwater.
The main objective of this paper is to determine the dependence of mean maximum annual speci c discharge q M on the size of the in uential basin, to de ne the frequency and ow-duration curves of mean daily ows and to analyse the most recent ood event that occurred in Ogulin in September 2017.The analysis was carried out on the basis of data collected over 31 years from the period (1981 -2011), based on the data of the mean and maximum annual ows from hydrological stations in Gornja Dobra and Vitunj ica.

Research area and previous studies of research area
Most of the current research, except for the Gojak hydroelectric power plant, is based on nding appropriate solutions for the protection of Ogulin, which is endangered by the emergence of high waters.Based on this, a brief overview of the ood in 1999 in Ogulin is presented, the problem of the limited capacity of the ulin ponor, which is directly related to this issue and the inuence of the Gojak hydroelectric power plant on the whole river system is discussed.The importance of solving the problem of ooding the city Ogulin was conrmed by the latest ood in September 2017.The Dobra River is a karstic river that runs partly as a surface and partly as an underground river.On its way, it passes through karstic conduits of different dimensions (caves and other underground karstic forms), which can be either free water face or under pressure (Prelovšek et al., 2008).Thus, on certain watercourse lines, the Dobra River, depending on the hydrogeological situation, simultaneously loses part of the water in the karst subterranean, but is also fed with water from the karst aquifers.Mean daily ow rates data used in this analysis are taken from the Meteorological and Hydrological Service of the Republic of Croatia (DHMZ) and collected from hydrological stations Moravice, Luke and Turkovi i on Gornja (Ogulinska) Dobra and Brestovac stations on Vitunj ica, an important tributary of Gornja Dobra.Vitunj ica ows in Gornja Dobra after 4 km of ow near the Turkovi i village.Figure 1 shows the characteristic parts of the river Dobra, the river catchment, the mentioned hydrological stations, ulin ponor and the hydroelectric power plant Gojak.
In the hydrogeological sense, the Dobra River basin is a transitional area connecting the Dinaric and Pannonian areas, where three distinct units can also be distinguished.Gornja Dobra begins as a surface runoff with clastics of Paleozoic age in the area of Skrad, and it retains such characteristics to Vrbovsko.Signi cant karst springs are formed in in the area of Vrbovsko where permeable and impermeable deposits intersect, (IGH, 2009).The watercourse continues on the terrain that is made of limestone and dolomite of Jurassic age.While owing through the dolomites, no loss of water occurs, but upon contact with limestone, there are sinkholes, as is the case with ulin ponor.At the intersection of the Triassic clastites and carbonate deposits, the Zagorska Mrežnica River, which, as well as the Gornja Dobra, sinks upon contact between dolomites and limestones.The last part of the Dobra watercourse is Donja Since the town of Ogulin is located in the central part of the river Dobra, there is a great in uence of ulin ponor (see Figure 2), which represents the natural drainage system of high waters.It is noted that ow through the underground system, after the entrance of water into the sinkhole, primarily depends on the underlying condition that is dif cult to de ne.The capacity of ulin ponor during the ood in 1999 was evaluated: Q p = 72.0m 3 /s (Brki , 2001).Due to this limited capacity, a very large water wave cannot completely drain from this area.The Gojak hydroelectric power plant with the Sabljaki and Bukovnik accumulation system plays an important role in the defence against oods.
For the purpose of ood protection, Croatian Waters (the legal entity for water management in Croatia) are planning to build a retention in Ogulin on the upstream part of the basin.The retention will result in a reduction of peak ow of high waters and the retention of oating objects, which by entering the ulin ponor would reduce the ow of the sinkhole.This will ensure the appropriate degree of protection for the city of Ogulin.
The most downstream hydrological pro le of Turkovi i on Gornja Dobra is located upstream from Ogulin and the small reservoir lake Bukovnik.Data collected at the Turkovi i pro le is the most important hydrological data for this work.An interesting fact is the size of the basin of the Gornja Dobra, in uential to the Turkovi i pro le, which was earlier A T ' = 296 km 2 , and according to recent research A T = 357 km 2 (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 Pavli , 2013).Furthermore, it is important to say that the Turkovi i pro le has a discharge into inundation area of high waters from the river bed (see Figure 3).Figure 3 shows the water stage of maximum level recorded in July 29 th 1999 and the water stage of the most recent ood event in September 2017, which was 327.01 m a.s.l.
The stage-discharge curve for the Turkovi i pro le from 2004, which is in use today (see Figure 4), is considered the most reliable due to additional river ow measurements in the high water area after the great ood in Ogulin in 1999.
It was found that the longitudinal slopes of the watercourse are such that there is no damming action because there are several smaller cascades along the river.Thus, the damming will not act on the stage-discharge curve and can be uniquely determined.The fourth or nal prole of this analysis is Brestovac on Vitunj ica, an important tributary of Gornja Dobra, whose in uence is covered by the Turkovi i hydrological station.

Results and discussion
A statistical analysis of maximum annual ows and volumes of isolated water waves is shown in Figures 5 The maximum ow from the 1999 ood, which was 254 m 3 /s, had a little bit more than 100-year return period, according to Žugaj and Pavli (2013).The volume of that water wave was 21.3 • 10 6 m 3 , which was about a 20-year return period.Figure 7 shows the double water wave that occurred in September 2017 together with the water wave that occurred in July 1999, as a comparison.
The rst water wave in 2017 had a maximum ow Q M1 = 196 m 3 /s and volume V 1 = 26.89• 10 6 m 3 .The maximum ow is a 10-year return period, while its volume is more than a 100-year return period.The second water wave in 2017 had a maximum ow Q M2 = 136 m 3 /s and volume V 2 = 17.57• 10 6 m 3 .The maximum ow of the second water wave is less than a 2-year return period and its volume is little less than a 5-year return period.
In the rst water wave, the return periods did not coincide with ow and volume, while in the second water wave they approximately coincided.Regardless of the fact that the maximum ow and volume of the second water wave was of a smaller return period, it represented a danger because the rst water wave saturated the ground with water, which made it possible to create a major ood.Double water waves on the Gornja Dobra basin can cause major ooding, and in particular, attention should be paid to them since they have appeared 11 times since 1963.
It is clear that high waters, in the form of single or double water waves occur regularly on Gornja Dobra, which is a threat to the town of Ogulin.For the purpose of protecting the city of Ogulin from oods, the plan is to build a dam in Turkovi i.This dam should atten the water wave that comes to Ogulin, but the question is whether the dam and the associated retention will be able to capture the entire volume of high water waves, since the area for predicted accumulation is limited by the railway line along the Gornja Dobra, and the shponds on the left bank of Vitunj ica tributary (IGH, 2013).This suggests that an alternative solution to this problem should be considered, such as the retention of water upstream.
The dependence of mean maximum annual speci c discharge q M on the size of the catchment area A is derived using data from three hydrological stations: Moravice and Turkovi i at Gornja Dobra and Brestovac at Vitunj ica.Data from the pro le Luke at Gornja Dobra was not used because value at that station was unrealistically high with respect to derived dependency, as shown in Figure 8.The value of mean maximum annual ow at pro le Turkovi i is Q M = 124 m 3 /s, which is quite a high value.
The hydrological pro le Luke on the Gornja Dobra is unstable in the hydrological sense, and the stage-discharge curve is extrapolated in a wide range of high waters.When de ning the stage-discharge curve in the Turkovi i pro le, river ow measurements of the 5 and 7-year return periods were used, and the pro le in the hydrological sense was considerably more stable than the Luke pro le.
Figure 9 shows frequency and ow-duration curves de ned on the basis of daily mean ows in the Turkovi i pro le.The frequency curve shows an asymmetric shape with a high peak where ows ranging from 2.50 m 3 /s to 7.50 m 3 /s occur more than 35% of the time during a year.From the ow-duration curve it can be concluded that the mean ows of Gornja Dobra at the Turkovi i pro le (Q = 10.5 m 3 /s) and higher, last for 30% of the total time during a year, while the ows lower than the mean ows last for 70% of the total time during a year.Therefore, the main feature of the ow-duration curve has quite a favourable distribution for energy use.
Frequency and ow-duration curves for values of ow greater than 75.0 m 3 /s show very low frequencies and short durations of such ow rates.

Conclusion
The Dobra River represents a karst phenomenon characterized by a great changeability of hydrological and hydrogeological features of the catchment area.By part of its ow, it ows as a surface, and partly as a subterranean stream, which makes it into the most famous waterways of that type.

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

Figure 5 :Figure 6 :and 6 .
Figure 5: Exceedance probability of maximum annual ows according to Pearson 3 distribution at Turkovi i hydrological pro le

Figure 7 :
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.

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