The Extremes ofthe Extremes: Exlraordinan' Floods (Proceedings ol'a symposium held at Reykjavik. Iceland. July 2000). IAHS Publ. no. 271. 2002. 133 Extraordinary summer flood in a karst area: case study in Croatia BORIS BERAKOVIC Faculty of Civil Engineering, University of Zagreb, Kaciceva 26. 10000 Zagreb, Croatia e-mail: boris.berakovic@public.srcc.hr KSENIJA CESAREC Meteorological and Hydrological Service, Gric 3, 10000 Zagreb, Croatia e-mail: kseniiac@vahoo.com Abstract In July 1999 a sudden flood in the Ogulin area in Croatia surprised even the water management experts. The town of Ogulin is situated in a karstic area. Waters from this area are effluents of the Sava River through a number of sinks. In this paper a review of earlier flooding is given; the work to prevent flooding is described including a hydroelectric power plant which reduces the water flow to the sinks. An analysis of the 1999 extreme flood is given. Also damage caused by the flood and measures to improve protection are described. Key words flood; rainfall; karst; sink; hydropower plant; reservoir; torrent; case study GORNJA DOBRA RIVER The Gornja Dobra River is situated in the middle karst area of Croatia. It has a basin area of 380 km". One part of the water flowing in that part of the karst comprises waters from surrounding areas, the other part is water from poljes or fields (karstic phenomenon) situated at higher levels farther away. These waters disappear through sinks and appear again miles away at a lower level. The Gornja Dobra River receives mainly waters from the orographic basin. High waters appear as a consequence of rainfall or snowmelt. The river's spring is near Skrad (530 m a.m.s.l.) and it sinks in the centre of the town Ogulin (320 m a.m.s.l.) into Djula Sink (Figs 1 and 2). This sink is situated on a plateau where Ogulin is the largest town. Besides the Gornja Dobra River on this plateau there is also the Zagorska Mreznica River, which collects waters from its direct orographic basin and waters from several higher poljes through underground flows. Whereas the Gornja Dobra River, upstream from Ogulin, has several more sinks, the Zagorska Mreznica River also has a number of sinks east of Ogulin that are separated from the Gornja Dobra River. Waters from the Gornja Dobra River emerge at a lower level as the Donja (Lower) Dobra River, while the Zagorska Mreznica River appears at a lower level as the Primisljanska Mreznica. At these lower levels they do not sink again and are tributaries of the Sava River (Black Sea basin). The Ogulin Plateau is made up of Jurassic limestone and dolomite and Cretaceous limestone with the surface broken up through past tectonic activities. In the lower areas the limestone and dolomite are partially covered by alluvial deposits. This geological make up facilitated the development of the very specific karstic phenomena
134 Boris Berakovic & Ksenija Cesarec Golik MVICA '/ m i,? 5 V**-' -O S " I 1 '/'! lës^-yjtë**> Y* u â ^ * s v ' Moravice ; i/ Jumi asv V, \ \ - / V1 -. (f, VuSkovici ' C I..., """ N. / if. Komleriici, 1 11 " > ^i^-ueskovaurago 4 keskova Draga ^ {/ \ \SenJSko VSi V, 'stubics- ^AaflS.t- (. -;'%'als3ina"\ 2aloaina~ / 4s?s: :{!KP - - -, \./ V. Jablan \ -* Iff,Sri.- ~_ Tuk \ St. Lazfo. t / I «T*.'".I i X 1/ S! Catchment boundary «C ' v ->j'i V i.,, /' ".AV - ' ^HambanMo. l.\ f H ' Brezova tel / \,!».'' Catchment area JjjVjî \ i, ' sk ~ CroiTurie - ' ''"(vlrkopalj V _ J W ' : ' VMrkopa&SKI 1 K - Bresto\actt \_. 41 \\ v - ' U ^ Diula Smk Turkovici, - JK X > i ~ V ^ " ogunnski y ^ ^ ^ "y' ^ J. i p'ua-kanàf ^ ^!*N^Gi)LIN ui\ l' Reservoir ^ ~ / Wl N C5S*'T u \ ^W^'nikovnik s ^ ««S, Sabljak-Selo] i\. ;' f //' > Ravnicô»ï 2\ \ ' Ù Zagorje 0 2 4 6 km -tii,., \r-.... l /Zenon»' Fig. 1 Drainage area and hydrometric stations of the Gornja Dobra River. characteristic of this area. There are many sinks in the north part of the plateau. The Djula Sink is the largest of all. It has been explored many times. So far some 15 km of the underground flow through caves has been explored (Bozicevic, 2000; Cepelak, 1985). Through the process of karstification in the past the water level has sunk, and today we are speaking about the fourth active generation of sinks. But the earlier higher levels sinks still become active during high waters. It is interesting that even during drought periods underground flows were registered when there was almost no surface flow. The median flow rate of the Gornja Dobra River at the Turkovici measuring station (6.2 km upstream from Ogulin) is about 10 m J s" 1. Yearly high waters in the measurement period (1963-1998) are between 100 m J s" 1 (224 cm level, December 1967) and 210 m 1 s" 1 (310 cm level, December 1966). The extraordinary flood of 1999 is estimated to have had a flow rate of 270 m 3 s"' (358 cm level, July 1999).
Extraordinary summer flood in a karst area: case study in Croatia 135 Fig. 2 Flooded area in Ogulin. THE GOJAK HYDROPOWER PLANT In 1959 the Gojak hydropower plant (Elektroprojekt, 1953) with a head of 135 m was completed to catch the waters of both the Gornja Dobra and the Zagorska Mreznica on the Ogulin Plateau. The power station is situated directly downstream of the Donja Dobra River spring, and the water from it flows into the Donja Dobra River. The design capacity is 50 m 3 s" 1. On the Zagorska Mreznica River the Sabljaci Reservoir was built (total volume 4.1 10 6 m J ; useful volume 3.7 x 10 6 m') and on the Gornja Dobra River the Bukovnik Reservoir (total volume 0.245 x 10 6 m J ; useful volume 0.238 x 10 6 m 3 ). Both reservoirs are connected by a tunnel (length 2 km) and are used together (maximum levels of the Sabljaci Reservoir and Bukovnik Reservoir are respectively 320.20 m a.m.s.l. and 320.15 m a.m.s.l.). The power plant operates on the natural flow when there is enough water, and when the natural flow is low the reservoirs are temporarily used. Because the rivers downstream from the reservoirs take only water that overflows from the Sabljaci and Bukovnik reservoirs, high waters as well as flooding intensity are partially reduced. FLOODS ON THE OGULIN PLATEAU On the Ogulin Plateau floods are not rare. Both rivers on the plateau flood the lower areas, and sometimes also some settlements. The floods are the result of more water incoming to the plateau than the sinks can swallow. After the construction of the hydropower reservoirs the intensity of the flooding was reduced. The Gornja Dobra River floods the region of Ogulin. The Zagorska Mreznica River flows besides the
136 Boris Berakovic & Ksenija Cesarec villages of Otok, Ostarije and Skradnik. Because of the different ways both rivers receive water there are also differences of their respective high waters. The Gomja Dobra River has a pronounced torrential character: high waters appear suddenly, levels are high, do not last long and water flows away fast (Fig. 3(a)-(d)). In the Zagorska 600,- - Ï -, horyears 1995 and 1996 tie-data available ' 1 f u 0 1 2 3 4 5 6 7 1960 1970 1980 1990 2000 Distance from Djula Sink (km) Years (e) Ti me (hours- si ar! at six o'cl ock on 04.11.1998) Fig. 3 River Gomja Dobra flood waves: (a) 1999 water levels; (b) 1999 discharges; (c) 1987 and 1999; (d) 1988; (e) longitudinal profile maximum water level on 29 July 1999; (f) hydrometric station Turkovici maximum water level (yearly and July). (f)
Extraordinary summer flood in a karst area: case study in Croatia 137 Mreznica River high waters appear slowly, are of lower intensity and last longer. The lowest part of the Ogulin Plateau is at 320 m a.m.s.l. It is flooded more frequently. Figure 3(f) shows the highest yearly levels and highest levels in July at the Turkovici measuring station. It can be seen that the highest yearly levels are between 250 and 300 cm (flows between 130 and 200 m 3 s" 1 ). Flooding in Ogulin occurs when the level is above 280 cm (flow higher than 170 m 3 s" 1 ). There are no data for some parts of 1995 and 1996 when the measuring station was out of operation. From Fig. 3(f) it can be seen that in 35 years in Ogulin there have been 10 floods of different intensity (four floods in both October and December, and one flood in May and June). The Djula Sink (Fig. 4), the main effluent for waters from the Ogulin Plateau, was explored in the nineteenth century (Bozicevic, 2000) (Fig. 4(a)). Primarily the morphology of the cave effluent system was explored. Much less attention was given to the sink's capacity. From explorations in 1987 the maximum capacity in the sink area was estimated to be 90 m J s"' at level of 320.46 m a.m.s.l. (Power Management Institute, 1987) (Fig. 3(c)). (a) A drawing ofthe Djula Sink from 1890; and (b) a photo of Djula Sink. THE EXTREME FLOOD OF 1999 The flood of July 1999 surprised the inhabitants of Ogulin, the management of the hydropower plant and water management experts firstly because of the time of its occurrence (summer) and by its intensity which surpassed all registered records and those in the memory of the inhabitants. The flood was a consequence of abundant rain in the whole of the Gornja Dobra basin. From 20:00 h on 28 July to 11:00 h on 29 July 156 mm of rainfall were recorded at the Brod Moravice measuring station in the spring area ofthe Gornja Dobra River, 130 mm at the Ravna Gora station and 78 mm at the
138 Boris Berakovic & Ksenija Cesarec station in Ogulin. One-hundred-year 1-day rainfall is estimated to be 93 mm at the Ravna Gora rainfall measuring station, which is substantially less than the July 1999 rainfall. The recorded rainfall in Ogulin corresponds to a return period of 50 years. The maximum levels on the Gornja Dobra River were recorded at the upstream stations during the night on 29 July 1999, and in the Ogulin area after 13:00 h on the same day, which indicates a very fast arrival of the flood wave and reflects the distinctive torrential character of Gornja Dobra River. The high water lasted less than two days. The flood wave is shown on Fig. 3(a), which has levels for all the measuring stations on the Gomja Dobra River and its two main tributaries (the Ribnjak and Vitunjcica). In the same figure the total rainfall recorded at the measuring station in Ogulin is shown. On Fig. 3(b) the estimated flow on the same measuring stations are shown. The longitudinal profile of the maximum levels in the area from the Djula Sink up to the Turkovici measuring station (6.2 km) is shown on Fig. 3(e). For comparison the flood waves in 1987 (May) are shown in Fig. 3(c) when the capacity of the sink in Ogulin as well as levels in the sink area were measured. It can be noticed that water from Ogulin flows fast away after the water wave has passed. As an illustration of another high water event the levels of the flood of November 1998 are shown in Fig. 3(d). All these floods resulted in water levels in the Ogulin area of up to 320.5 m a.m.s.l. which is 2 m lower than the extraordinary 1999 flood (Fig. 2). At the time of flood the hydropower plant was being revamped and as there was scaffolding in the tunnel it could not be used to redirect part of the water. It is a usual practice to do maintenance work only in July as there is usually no rain in that time. Figure 5 illustrates the flood in the town. Fig. 5 Flood in Ogulin on 29 July 1999.
Extraordinary summer flood in a karst area: case study in Croatia 139 FLOOD PROTECTION MEASURES The flood protection measures applied so far, including sink maintenance and trials of increasing its capacity and the use of the hydropower plant were shown to be inadequate under such exceptional circumstances. One of the most useful suggestions is the construction of a retention area. This would cause short-term flooding of meadows and fields upstream from Ogulin, but will cause less damage than the flooding of the town. REFERENCES Bozicevic, S. (2000) Poplave u Oguiinu (Floods in Ogulin) (in Croatian). Hrvatska Vodoprivreda 90, 10-12. Cepelak, M. (1985) Spiljski sustav Djula-Medvedica (The Djula-Medvedica cave system) (in Croatian). Speleolog. 125, 2-24. Elektroprojekt ( 1953) HPP Gojak Design Documentation (in Croatian). Elektroprojekt, Zagreb. Power Management Institute (1987) Mjerenja kapaciteta Djulinog ponora (Measurement of Djula Sink capacity) (in Croatian). Report no. 0105. Power Management Institute, Zagreb.