HOLOCENE HIGH FLOODS ON THE PLANINA POLJE, CLASSICAL DINARIC KARST, SLOVENIA VISOKE POPLAVE V HOLOCENU NA PLANINSKEM POLJU, klasični DINARSKI KRAS, SLOVENIJA Uroš STEPIŠNIK1, Mateja FERK2, Petra GOSTINČAR3 & Luka ČERNUTA4 Abstract UDC 911.2:556.166(497.471Planinsko polje) 556.166:551.435.8(497.471Planinsko polje) Uroš Stepišnik, Mateja Ferk, Petra Gostinčar & Luka Černuta: Holocene high floods on the Planina Polje, Classical Dinaric Karst, Slovenia The Planina Polje is located in the northwestern part of Notranjsko Podolje, Slovenia. Annual floods cover the flattened floor of the polje at elevation 445 m a.s.l. and reach the depth of approximately 8 meters. Loamy sediments which were found on surface and subsurface features from the inflow part of Planina Polje up to the elevations of about 495 m a.s.l., indirectly show that floods in the past must have been much more extensive than the recent ones. Radiocarbon dating of flowstone layer from side passage Tiha Jama in Planina Cave revealed that the last such extreme floods appeared around 5,706 ± 49 BP. The time frame of the flood roughly corresponds with the Altith-ermal (8,000-5,000 BP). More humid mid-Holocene climate might be the main cause for the high floods on Planina Polje. Keywords: geomorphology, floods, radiocarbon dating (14C), Dinaric Karst, Planina Polje, Planina Cave. Izvleček UDK 911.2:556.166(497.471Planinsko polje) 556.166:551.435.8(497.471Planinsko polje) Uroš Stepišnik, Mateja Ferk, Petra Gostinčar & Luka Černuta: Visoke poplave v holocenu na Planinskem polju, klasični Dinarski kras, Slovenija Planinsko polje je kraško polje, ki leži v severozahodnem delu Notranjskega podolja v Sloveniji. Vsakoletne poplavne vode poplavijo celotno uravnano dno polja na nadmorski višini 445 m in dosežejo globino približno 8 metrov. Ilovnati sedi-menti na površju in v jamah na pritočni strani Planinskega polja, najdeni do okoli 495 m n. v., posredno kažejo, da so bile poplave v preteklosti veliko bolj obsežne kot današnje. Radio-karbonsko datiranje plasti sige iz stranskega rova Tiha Jama v Planinski jami je pokazalo, da so bile zadnje ekstremne poplave v času okoli 5.706 ± 49 BP. Časovni okvir poplav približno ustreza obdobju altitermala (8.000-5.000 BP). Vlažno podnebje srednjega holocena bi lahko bil tudi glavni razlog za visoke poplave na Planinskem polju. Ključne besede: geomorfologija, poplave, radiokarbonsko datiranje (14C), Dinarski kras, Planinsko polje, Planinska jama. INTRODUCTION Planina Polje (Planinsko polje) is one of the karst poljes ki Mts.). Due to its morphological and hydrological char- on Classical Karst in Slovenia which is a part of the Di-naric Karst. It is situated in the northwestern part of Notranjsko Podolje and represents the most important confluence of karst waters in the basin of the Ljubljanica River (Cerkniško Polje, the Pivka Basin, and the Javorni- acteristics it is defined as the most typical karst polje in Slovenia (Gams 2004). During floods the water covers the whole floor of polje, forming a lake of approximate 8 meters in depth (Gospodarič & Habič 1976). 1 Faculty of Arts, Department of Geography, Aškerčeva 2, SI-1000 Ljubljana, Slovenia, e-mail: uros.stepisnik@gmail.com 2 Anton Melik Geographical Institute ZRC SAZU, Gosposka ulica 13, SI-1000 Ljubljana, Slovenia, e-mail: mateja.ferk@zrc-sazu.si 3 Karst Research Institute ZRC SAZU, Titov trg 2, SI-6230 Postojna, e-mail: petra.gostincar@zrc-sazu.si 4 Brdo 70, SI-5230 Bovec, Slovenia, e-mail: crni.lukc@gmail.com Received/Prejeto: 17.5.2010 ACTA CARSOLOGICA 41/1, 5-13, POSTOJNA 2012 Detailed investigations of the inflow part of Planina Polje and the caves in the hinterland revealed that the early Holocene floods reached much higher elevations than recent floods do. The aim of this study was to re- construct the extent, sedimentation mechanisms and time incidence of extreme Holocene floods on Planina Polje. The study is a contribution to the understanding of morphology and hydrology of the polje. METHODOLOGY In order to acquire data about the occurrence and extent of floods at higher elevations, an extensive research was carried out. The investigation based on detailed geomor-phological mapping and morphometric analyses in the hinterland of the inflow part of Planina Polje. Furthermore, detailed speleological analyses of the caves from the inflow part of the polje were conducted. Sediment and flowstone were sampled from selected surface karst features and caves. The mechanism of sedimentation and the origin of loamy sediment were established in the laboratory with grain-size and petrographic analyses. Petrographic analysis of sandy fractions was performed in order to determine the origin of the loamy sediment. Extent and volume of high floods were established with GIS using Golden Software Surfer 9. Digital elevation model (DEM) with cells 12.5 m was used. The approximate age of the sedimentation was established with Radiocarbon dating of flowstone from the Tiha Jama which is a side passage of the Planina Cave. The dating analysis was performed with radioactive isotope 14C in the Laboratory for Measurements of Low-level Radioactivity at the Ruder Boškovic Institute, Zagreb, Croatia. Subsurface structure of loamy sediment fills in selected surface karst features were established with the use of Electrical resistivity imaging (ERI) and the interpretation of output data. The SuperSting R1/IP earth resistivity meter (Advanced Geosciences, Inc.) was used for data collection. The survey was conducted with the dipole-dipole array with 5 m spacing between two electrode pairs. The data were processed to generate two-dimensional resistivity models using EarthImager 2D resistivity inversion software. REGIONAL SETTING Planina Polje is situated on the Classical Karst between Postojna and Vrhnika. The Unica River, which flows across Planina Polje, is a part of the Ljubljanica River catchment. The catchment area of the Unica River is about 800 km2 (Žibrik et al. 1976). The river drains allogenic catchment which includes a series of karst poljes in the Notranjsko Podolje and its surroundings. It also receives some autogenic recharge from the higher areas of Snežnik and Javorniki (Žibrik et al. 1976). Planina Polje is elongated in northwest-southeast direction and covers an area of over 10 km2. The rim of the polje is closed and in the lowest part it is about 55 m above the flattened floor which is on elevation between 445 and 450 m. The floor is almost uniformly flat with some small-scale irregularities represented by various surface karst features (Ravnik 1976) which are up to two meters high. On the southern rim of the polje there are two pocket valleys where the permanent springs of the Unica River and the Malenščica River emerge. The waters from the two springs join together into the Unica River. Nearby there is Škratovka, a group of intermittent karst springs. In the northern and northeastern part of the polje the water sinks in numerous ponors and also two small blind valleys are formed (Fig. 1). Planina Polje lies within lowered karst surface of Idrija strike-slip fault zone. Its geometry seems is strongly influenced by geological structures (Vrabec 1994). Planina Polje is situated mostly in fractured Upper Triassic and Lower Jurassic dolomite (Fig. 1). The northern, northeastern and inflow part in the south are stratigraphically positioned in Lower and Middle Cretaceous limestones (Buser et al. 1970). Tectonically deformed dolomite underlying sedimentary floor provides a hydrological barrier to the karst aquifer. The polje is covered with clayey and sandy deposits which are commonly up to 5 m thick, but near the eastern and northern rims, where the bedrock consists of limestone, depth of the sediment cover is Fig. 1: Geological and hydro-logical characteristics of Planina Polje. 1. Elevation, 2. Peak, 3. Settlement, 4. Cave, 5. Source, 6. Surface flow, 7. Visible and covered fault, 8. Visible thrust, 9. Fault zone, 10. Milonite, 11. Quaternary, 12. Cretaceous limestone, 13. Jurassic limestone and dolomite, 14. jurassic limestone, 15. Jurassic limestone and dolomite, 16. jurassic dolomite, 17. triassic dolomite, 18. Rim of the polje. up to 25 m. Grain-size of the deposits ranges from clay to sand (Ravnik 1976). As floods appear regularly the Planina Polje can be classified as an overflow type of polje (Gams 1994). Permanent tributaries of the polje are the Unica River that flows out of the Planina Cave (Reg. No. 748) and the Malenščica River that has its spring in the Malni Pocket Valley. At higher discharges the rivers Škratovka in the south and Hotenjka in the northwest also flow to the polje. Hydrological characteristics of Planina Polje are strongly influenced by its geological characteristics. Karst springs and ponors on the polje appear mostly on Fig. 2: Floods on Planina Polje (Photo: I. Marentič). Cretaceous limestone areas (the Malenščica River, the Unica River) or on the contact between Cretaceous limestone and Triassic dolomite (Škratovka) (Gospodarič & Habič 1976). The recorded inflow on the polje ranges between 1.4 m3/s and 130 m3/s and regularly exceeds the maximum outflow capacity of the ponors. This is why Planina Polje represents a storage basin for high flood waters. Floods occur when the discharge of the Unica River at the Hasberg gauging station exceeds 60 m3/s (Ravnik 1976). During floods, lasting on average 41 days per year (Gams 2004), the water level increases up to 8 m and reaches 450.2 m a.s.l. (Kovačič & Ravbar 2011). The volume of water at that elevation calculated with DEM reaches approximately 59 Mm3, with the lake surface area of 9.1 km2. However, the highest recorded floods reached the elevation of around 458 m a.s.l. (Gams 1979). The volume of the lake at this elevation was 136 Mm3, with the lake surface area of 10.6 km2. According to the notes of w. Putick, the locals were reporting floods reaching as high as 464 m (Gams 1979), but it seems rather unlikely. The volume of the lake at this elevation would be about 202 Mm3 with an area of 11.2 km2. TRACES OF HIGH FLOODS ON PLANINA POLJE Detailed investigations of Planina Polje and the cave systems of its inflow part (Fig. 3) have revealed numerous traces of floods which do not correspond with the level of known recent floods (Gospodarič 1976; Stepišnik 2010). These traces were identified in the surface and cave morphology and sediments. Patches of allochtoneous loamy deposits can be found on some parts of the surface and in the caves. Fig. 3: Inflow part of Planina Polje. 1. Settlement, 2. Cave, 3. Karst spring, 4. Sampling point (1 -Planinska jama, 2 - Lisičina v Nartu, 3-6 - Malni, 7 - Mrzli Dol), 5. Surface flow, 6. ERI profile location, 7. Loamy deposits, 8. Rim of the polje. MALNI POCKET VALLEY The Malni Pocket Valley is situated on the southern part of the polje. The pocket valley is 1.1 km long and, on average, about 250 m wide. The rim of the pocket valley is 55 m high in the northern outflow side and 120 m high in the southern part. The floor in the southern part of the pocket valley lies at the elevation of about 500 m a.s.l. and it gradually lowers towards Planina Polje in the north. The springs of the Malenščica River, a tributary of the Unica River on Planina Polje, are situated in the middle part of the pocket valley. The discharge of the Malenščica River varies between 1.1 to 9.9 m3/s (Kolbezen & Pristov 1998). The upper part of Malni Pocket Valley is filled with loamy sediment up to 490 m a.s.l. The sediment covers 0.05 km2 and is deposited over limestone rubble which is believed to be a result of intensive mechanical weathering during ice ages (Gams 1966). Grain-size analysis of the sediment revealed that the loam consists mainly of clay loam particles (Fig. 7). Petrographic analysis of sandy fractions of the sediment revealed flysch-derived particles of chert, limonite, quartz sandstone and mica. The results of ERI of the cross-section in the upper part of the Malni Pocket Valley has shown that its bedrock slopes, with resistivity values higher than 1,000 ohm-m (Fig. 4; red and orange colour), are covered with scree or weathered bedrock with resistivity values between 150 and 1,000 ohm-m (Fig. 4; yellow and green colour). The floor is entirely filled with less resistant loamy sediment with resistivity values up to 150 ohm-m (Fig. 4; blue colour) (Stepišnik & Mihevc 2008). Thickness of the loamy material exceeds 22 m. COLLAPSE DOLINE MRZLI DOL Mrzli Dol is a collapse doline situated 200 m west of Malni Pocket Valley and about 500 m north of the passages of the Planina Cave. The longer diameter of Mrzli Dol is 224 m, the shorter diameter is 218 m and the average depth is 46 m. Approximate volume of the collapse do- line is 0.8 Mm3. The slopes are covered with scree and a thin layer of soil. The floor is flattened with loamy sediment at 494 m a.s.l. where the average diameter is about 15 m (Stepišnik 2010). The fine-grained sediment fill forming the flat floor of the collapse doline was examined. Grain-size analysis of the sediment revealed that the loam consists of silty clay particles (Fig. 7). Petrographic analysis of the loam revealed flysch-derived minerals of mica, quartz, sand and chert. Fig. 4: ERIprofile of Malni Pocket Valley. Fig. 5: ERI profile of the collapse doline Mrzli Dol floor. Results of ERI have shown that the floor of the collapse doline is filled with less resistant (up to 150 ohm-m) loamy sediment (Fig. 5; blue colour), overlying scree footslope of the collapse doline with resistivity values higher than 150 ohm-m (Fig. 5; yellow and green colour) (Stepišnik & Mihevc 2008). Thickness of the loamy sediment is about 8 m. THE PLANINA CAVE The Planina Cave is situateds in the southwestern part of Planina Polje, with its entrance under a 50 m high wall at the end of a 1,200 m long pocket valley. The entrance lies on the elevation of 453 m a.s.l. and the inner parts of the cave are at a slightly higher elevation. The Planina Cave is a horizontal epiphreatic cave that discharges the main spring of the Unica River. The total discharge varies between 1.1 and 100 m3/s, whereas the mean discharge is 21 m3/s (Frantar 2008). The cave is 6,656 m long with hydrologically active passages. Only some small side passages at higher elevations are inactive. These inactive passages are about 15 m wide and 15 m high (Cave Register of JZS 2011). The Planina Cave is the longest and the largest cave of the inflow part of the Planina Polje. The Planina Cave is filled with various sediments, mostly limestone gravel, chert gravel and laminar clay. The mineral composition of the deposits indicates that the waters depositing the clastic load in the cave must have flown from the Pivka Basin where Eocene flysch rocks prevail. In addition, the cave is partly filled with limestone gravel, which according to Gospodarič (1976) originated from the collapse doline Planinska Koliševka. Gospodarič (1976) described sediments from the Planina Cave in the following stratigraphic succession: the oldest are coloured chert gravels and older laminated loam (Middle Quaternary), white chert gravel (Riss), flowstone (Riss-Würm), younger laminated loam (Lower würm), flowstone (Middle würm), flood loam (Upper würm), flowstone (Postglacial) and the youngest flowstone (Ho-locene). Nevertheless, loam deposits which, according to Gospodarič, are of flood origin, are situated at the elevation around 495 m a.s.l. above all other stages of sedimentation in the cave. Furthermore, it is also situated above the limestone rubble from Planinska Koliševka which was dated to the upper würm (Gospodarič 1976). Paleomagnetic dating in the Planina Cave (Zupan Hajna et al. 2008) showed that all sediments sampled in the side passage Rudolfov Rov (at cca. 460-475 m a.s.l.), have normal polarization. The sediments were thus considered to be younger than 0.73 Ma and in addition, suggesting that the results are in good accordance with Min-del age (0.35-0.59 Ma) proposed by Gospodarič (1981) (Zupan Hajna et al. 2008). Patches of loamy sediment overlie other sediments in the entire cave at the elevations between 470 and 495 m a.s.l. (Gospodarič 1976). In the hydrologically active channels, the loamy sediment is mostly absent, while it is well-preserved along the whole length of some inactive side passages. In side passages Katernov Rov, Paradiž and Tiha Jama the flood loam reaches the elevation of 495 m a.s.l. The highest loam deposits were also mentioned by Gospodarič (1976) who concluded that these sediments could be a result of higher floods in the underground. However, Gospodarič did not study the floods in detail and did not associate underground floods to flooding of the polje. Tiha Jama is a small side passage of the Planina Cave. The deposits of loamy sediment are covering the passage floor up to the elevation of 495 m a.s.l. The sediment consists of thin stratigraphic layers with a thickness Fig. 6: Ground plan of Planina Cave. 1. Cave entrances of surrounding caves, 2. Sampling point. 100 A 90 / \ 60 / 10 \ 20 41 lUnl^htK X HAlrri PMhAt VtUftr * MnllDDl It Pbrlru Cwe (Titu tirna} T V Mmi 1 * Kövflflrt rvd Mli™ w / 60 / T \ 30 \ lO 50 / 50 ^ • \ 60 ■to / X X 30 / It, " \ 70 / ♦ 20 / ♦ ■A- y ■ \ 80 10 / \ 90 I \ \ irw 100 50 SO 70 60 50