GEOL. CROAT. 50/2 1289 -2981 6 Figs. 1 ZAGREB 1997 Hydrogeological Evaluation of Water Retaining Properties of the Kosinj Reservoir (Lika, Croatia) Ante PAVICIC Key words: Karst hydrogeology, Karstification , Water retaining, Hydrogeological zoning, Kosinj reservoir, Lika, Croatia. Abstract Constructi on of the Kos inj reservoir was planned in a karst ter­ rain on the lower par! of the river Lika. Preceding slUd ies of this area did not compl etely de fi ne the waler constraining clements of the ter­ rain, and therefore the more reliable Kruscica reservoir was buill. Fur­ ther studies were undertaken in order 10 evaluate in detail the terrain o f the wlIicipalcd location of the dam impoundment at Kosinj . Due to the lithological homogeneity of the karst terrain, the s tudi es focused on tectonics and geomo rphology, wilh emphas is on karstification. Terrain zoning was made on the basis of hydrogeo logical favourabi ­ lily for water rctaining in the reservoir. The Vcl cbit hydrogeolog ical barrier innuences the general water run off in the hinterl and , as well as the depth and the direction of adva ncement of recent karsti ficati on processes . In the hinterland of the Velebit barrier the karstifi cat ion is shallow, increases in depth to­ wards the sea, and is characterized by the zoned distribution of spri­ ngs, estavelles and ponors. Th is concept played a major role in laler hydrogeological stud ies o f the proposed reservoir. 1. INTRODUCTION The completion o r the Krusciea reservoir with a vol ~ umc of 134 x 10(' m3 on the Lika river did not sat isfy the needs for reservoir space, because the achieved volume is only 15% of the averagc annual recharge of the Lika rive r (Qmcan= 27.9 111 3 S· I ) . Together the headwaters of the Gacka and Lika rive rs are utilized by the hydroelec~ trie plant HE Senj but, due to the shortage of wate r stor~ age space, a po rti on o f the high wate rs is d ischarged in to ponors, which in turn causes floodin g of the polje. In order to utili ze larger volumes of water and to el imi ~ nate flooding, studies which would enable the construc­ tion of the Kosinj reservo ir with a vo lume of 433 mil­ lion 111 3 were co ntinued in 1972. Th e upper course of the Bako vac tributary lies in an es tave lle zone and, the refo re, this va lley will be separated by a dam from the lake. Institute o f Geology, Sachsova 2, P.O. Box 268, HR· ] 0000 Zagreb, Croatia. The waters from the Bakovac tributary are to be di s~ charged int o the Lika rive r through a tunnel down ~ stream from the lake and utilized in the HE Senj. During exploratory drilling , the rim o f the future reservoir, at the side of the Lipovo polje, and the bound~ ary area by the Gacka rive r were investi gated. The majority of the drilled holes contain equipment whi ch has produced water level data for over 30 years. Recen­ tly drilled observation boreholes were a lso used for monitoring groundwater level variations. Groundwater flow tracing tests were performed (from the easte rn side of the lake and in the Bakovac valley) which enabled the expansion of flow data obtained in previous studies (rrom areas surrounding Pazariste and Studcnci). De tai ­ led geophysical studies were performed in areas where injection barriers were planned and the narrow locati ons where damming was to be perfol111ed. To perform these rather co mplex studies, a large volume of varied data, from both published papers and proress ional studies was required. The fund amental publi shed papers were the result of studies done during the construction of the Kruscica artificial lake. BAHUN (1962) dcscribed in deta il the lithological composition o f the rocks, with emphasis on the l elar beds. The same author (BAHUN, 1973), re lated the karst processcs and fluvial e ros ion in the L ika ri ver area and gives an acco unt o f karstificat ion phases. On the bas is o r data from the region surrounding the mtificial Kruscica lake, PAVLlN (1970) delineated a ponor zone. an es tavelle zone and a spring zone. PAVICH: (1984) also included a large part of th is area when dealing with the Velebit groundwater divide. BAH UN & FRITZ (1975) stud ied the hydrogeological features of the lelar beds, while FR ITZ & PAVICH': (197 5) and PAVICIC & FRITZ (1976) outlined in deta il C re taceous beds and the l e lar beds, dete rmined num erous fault s in the area anci described the hydrogeological re lationsh ips in the lower course of the Lika river. PRELOGOVIC (1989) descri ­ bed the Neotectonic movements in the area of northern Velebit and a part of Lika and provided new insight to the structu ral and tectonic re lat ionships. The da ta 0 11 the Th is paper was presented at the scientific meeting: dedicated to the 80th a nniversary of the life of Professor Mi lan Herak, held on March 5th, 1997 in Zagreb origin of structures and faults and Neotectonics enabled bette r interpretation of the hydrogeological relation­ ships and groundwaLer flow. PA VICI<': (1995) in hi s PhD Lhesis described the hydrogeological condiLions for the construction of water reservoirs in the karst of the VclebiL hinLeriand , where he also sLudied the area around Kosinj . and thi s was used as the basis for the present paper. 2. HYDROGEOLOGICAL RELATIONSHIPS IN THE KOSINJ RESERVOIR AREA 2.1. HYDROGEOLOGICAL PROPERTIES OF ROCKS On the hydrogeological map presenLed in Fig. 2 Lhe main lithostratigraphic members outlined are grouped by Lhe ir hydrogeological properties. In the studied recharge area Triassic. Jurassic, Cretaceous and Lower Palaeogene carbonate rocks dominate. The rocks are divided into five groups according to the ir permeability. The limestones in alternation with dolomites and limestone breccia (T; , J, K ,) are very permeable rocks. Partly impenmeable are the Jelar beds (Pg2,,), while the dolom ites (TJ) are part ly permeable. The shales and sandstone (T " T ;) and tuff and tuffite veOiogm \..roallCiI ' VI/. Fig. 1 Kosinj reservoir layout. Legend: I ) Kosinj rcsclvoir: 2) Kosinj dam and HPP: 3) Sedlo dam: 4) Bakovac dam: 5) Skl ­ ope dam and HPP; 6) Krusc ica rese rvoir; 7) Bakovac - Lika tunnel; 8) Bakovac - Lika e1la­ nnel; 9) Se Ji ste compcnsat ion basin. are impermeable. The Quaternary deposits are mainly represented by colluvial beds, deposited in the poljes, and their relics can be found on elevated surfaces of the terrain. These clay-silty deposits ean be very thick and impermeable in some areas, allowing surface water flow in the polje where the groundwater level is below the water bed (e.g. Lipovo polje). 2.2. TECTONIC BASIS OF HYDROGEOLOGICAL CONDITIONS Almost all of the area studied belongs to the Dinar­ icum megastructural unit (HERAK, 1986, 1991). The geodynamics of the studied area is characterized by structures resulting from foldin g and faulting which occurred after deposition of the Eocene flysch deposits. These movements defined the existing structures, which are outlined by regional fault s which are often accom ­ panied by the Jelar beds (BAHUN, 1974, 1984; HER­ AK & BAH UN, 1980). Thesc tectonic structures were formed during the Upper Oligocene and Miocene, prin­ cipally by tangential movements (BAHUN, 1974; PRELOGOVIC, 1989). The final phase of these move­ ment s occurred in the Upper Pliocene and the Quater­ nary when transcurrent faults were active and were par­ ticularly intensive in the Vclebit region. The Neotecto­ nic movements have influenced recent karstificati on Pavicic: I-Iydrogeological Evaluation of Water Retaining Propenies of the Kosinj Reservoir (lib, Croatia) 29 1 ~ I -- 2 3 • 4 0 5 <> • 8 7 ,(? 8 ~ 9 D'O ~11 S" Em3313 I ii " 0"",=",,,,_=,,-,,5'0 J Fig. 2 General hydrogeo logical map. Legend: 1) geo logical boundary; 2) fault; 3) assumed direclion of g roundwater discharge; 4) permancnt karSI spring; 5) intermittent karst spring; 6) intermittent coaslal sp ring; 7) swallow hole (ponor); 8) submarine spring (vrulja); 9) defined underground connecti on betwccn a ponor and a spring; 10) permeable zone; II ) Velebit hydrogeological balTier; 12) partly impcrmeable rocks; 13) partly pcrmeable rocks; 14) impcnncable rocks. and recent water now patterns. Although the prevailing geological structures in the studied area are governed by deep tangential and reverse tectonics, the hydrogeo­ logical conditions can be interpreted main ly from sur­ face outlines of the structures. Alternatively, the water dynamics of the studied area is confined to relati vely sha llow parts of the terrain, well above the conlact between megas lructu ra l unit s, so the deep langent ial lectonics has no influence on water emergence and flow. On the basis of the geology of the studied area and the basic tectonics involved, distinctive areas. structural units and fault s that inlluencc the hydrogeology of the terrajn arc presented in Fig. 3. The Velebi t mountain range form s (he south-west­ ern limb of an extensive regional fo ld (BAHUN, 1974) thrusted durin g the first phase, and later disintegrated by radial NeoleelOnics. Tn the core of the anticline, alo­ ng the fault on the edge of Licko poljc, to the west from the water accumu lations Kruscica and Kosinj , outcrops of Pal aeozoi c and Lowe r Triassic, and to a lesser de­ gree Middle Triassic impemleable clast ic rocks occur. The posi tion of these impenneable rocks in the Velebit massif, the uplift of which began after the Miocene and co ncluded in the Upper Pliocene and the Quaternary (PRELOGOYI C, 1989), influenced the hydrogeological conditions in the hinterland , especially as reflected in the direction of kars tification and the groundwater level in the area of recharge surrounding the water reservoirs. The occurrence of Neogene marls in a deep sinkhole (borehole K-4-2, Fig. 4) confirms the existence of vcry young fau lt tectoni cs . The thi ckening marl and mor­ phology of the lerrain indicate a throw along the fault plane of over 100 Ill. The fau lts, IOgether with the struclural and lithologi­ ca l features of the rocks, are the most important e le­ ments that influe nce the karstification process and groundwater flo w in karsl terrai ns, as first stressed by HERAK (1957). More recent studies (FR ITZ, 1972, 199 1, 1992; FRITZ & PAYICIC, 1986; BAI-IUN & FRITZ, 1987) suggest that the principal influence is that of recent karstifi cation , which is assoc iated with the youngesl Neotectonic fault s. During map e labora­ tion, the data from studies of the northern Ve lebit and parts ofLika (PRELOGOVIC, 1989) was used. As a consequence of regional tangential stress, com­ press ion faults occur within smaller or larger areas. These are usually manifesled as hinge, transcurrent and reverse faults. Tension fault s, that occur as fi ssures par­ allel with the principal stress direction, havc the least pronounced strike displacement, and they are con fin ed to smaller areas, usually within tectonic blocks. They are important for local groundwater flow conditions. The transcurrenl fault s are of primary importance in groundwater flow , since the rocks in which they occu r are compressed, and thus can act as a barrier 10 ground­ water flow perpendicular to the fault plane. The gro­ undwater flow is directed parallel to the fault plane, and 292 " '" 9 • 10 natural re tention can occur on the upstream side of the fault. This group of fa ults is represented by the Vra­ tnik - Senjsko Bilo - Peru sic fault on the eastern margin of the Senjsko Bilo - Krasno structural unit , and the Lika fault , west of the studied water reservoir locations. These faults are characterized by groundwater flow par­ allel to the fault plane, while groundwater flow perpen­ dicular to the fault plane is of a lower degree, parlieu- m Q.S.1. A 20' 600 0 0 00 0 o 0 ~ 00 o 0 0 0 550 o 00 0 0 000 0°0 00 0 00 0 0 0°°0 0 5 DO ooPg 23oo o 0 ~ do 000 000 00 00° "0 B ~ /,. 100 100 300 400m , , Fig.4 Cross-section through the borehole K-4-2, showing the posi­ tion of the Neogene marls. 6 <> EIill IMnMij 2 IA,C~ 3 Geologia Croatica 5U/2 Fig. 3 Map showing the tectonic structure and hydrogeological function of the terrain. Legend: § 4 ~ 5 J) Velebit hydrogeological bar­ rier; 2) Apatisan relative barrier: 3) hydrogeological zones: A - spring zone, B - estavelle zone, C - ponor zone; 4) nonnal fau ll : 5) reverse fault; 6) anticline axis; 7) permanent karst spring; 8) intermittent karst spring: 9) estavelle; 10) permanent coastal spri ng; 11) intermittent coastal spring; 12) submarine spring; 13 swallow hole (ponor). • 7 " 8 11 vO 12 0 13 lady in recently active fault zones. The Neotectonic active faults in the area of northern Velebit are vertical to very steep wi1h a WNW-ESE strike. 2.3 HYDROGEOLOGICAL FUNCTIONS OF THE TERRAIN The hydrogeological functions of the studied terrain were determined on the basis of the hydrogeological features of rocks, tectonics, and the spatial distribution and relative positions of gcological bodies and their morphology. In the studied area the following hydroge­ ological functions of different parts of the terrain were determined: futl barriers, partial barriers and permeable areas. 2.3.1 Hydrogeological barriers Barriers can be classified according to the degree that they obstruct water flow as full or partial (relative) balTiers. In the stud ied area the principal hydrogeological feature is the Velebit full barrier which extends from the Bakovae valley to 1hc Stikadsko polje (Fig. 3). The str ike of this barrier is confined within the Velebit structural unit. The existence of this barrier is deter­ mined by impermeable Palaeozoic and Lower Triassic deposits the e levat ion of which is much higher than the Pavicit: I lydrogeological Evaluation of Water Retaining Prope rties of the Kosinj Reservoir (Uk;!, Croatia) 293 m (0 S I) 1000 ., 500 ) m (0 s I) 1000., YOUNGER WURM 1580 OLDER HOLOCENE <>-+--<> Jama (ha((>1 River L iko PRESENT m (0 s I) 1000 ZONE OF SINKHOLES KrCE'vine Zl vulJo ZONE OF I NTE RMIT~ 5PRING5 (""""j" Z 0 N E OF SPR'NGS ern a Vr(>fo KOSINJSKI Koslnjsko polle : Lokv/co A- 6 BMOVAC groundwater level in the Licko polje. The impemleable Middle Triassic pyrocl astic rocks also supplement the hyd rogeo logica l barrier in the area to west from Pazariste. This part of the Velcbit structure is an impcr­ meab le barrier wh ich cannot be penetrated either by surface or g round water from the hinterland. It forces the waters to flow ei ther in an eastward and south-east­ ward d irect ion or northward and north-eastward. The function of the Velebit barrier as a full barrier is not diminished by the occurrence o r permeable Middle Triass ic deposits, because the loss of water is confined with in the penneable rocks, and the occurrence of deep karst forms, such as jamas (karst shans), is only local. The Upper Tri ass ic clas tic rocks and dolomites pre­ scnt in arcas where no tectonic di sruption occurred , aet as hangi ng barriers with in a full barrier. Anot her partial barrier is the Apat isan anticline sit u· ated wi thi n the Senjsko Bilo structural unit , wherc the Li assic dcposits in the core of the antic line rcduce the fl oor and alter the direction of groundwater flow , which result s in relatively hi gh groundwat er level s in the Lipovo polje and the occurrence of intermittent springs and nooding. River L/ko Fig. 5 Deve lopm ent of the Bako· vac valley and wat er dminage. Legend: I) pe rmanent s urface discharge into the Likil river; 2) intermittent surface diseha· rgc into lhe Uka river; 3) gro­ undwater discharge; 4) grou­ ndwater level: 5) zonal wa te r di vide: 6) remains of a terrace; 7) observ •• L----=:j Fig. 6 Hydrogeological zon ing of permeable areas. Legend: I) grou nd water above the Lika river stage: 2) Jehu deposits impenneablc as a unit; 3) rclative ly uplifled tcctonic block; 4) terrain under a highcr influence of clay deposi ts; 5) low groundwatcr under Lika river: 6) faul l; 7) major karst spri ng. pennanent; 8) major karst spring. inte rmittent; 9) permanen t karst spring. di scharge less than I l/s: 10) estaveJle; II) swallow hole (ponor); 12) observation exploratory borehole; 13) wate r table contour line (mainly low water); 14) assumed boundary of Ihe arca without vertica l leakage; 15) boundary of the area where groundwater is above 540 m a.s.l. during the rainy season; 16) water divide between the Lib and Gacka rivcrs; 17) local water divide; 18) groundw,llcr flow dircction; 19) detennincd underground hyd raulic conne· clion (belween a ponor and a spri ng); 20) Kosinj reservoir. and low flow. Along these river valleys permanent and intermiltent springs ex ist, but es tavelles and ponors are absen t. The watcr div idc in this region is shown in Fig. 6. The water divide between the Lika and Gacka rivers is confirmed by the exis tence of numerous springs alo­ ng the eastern bank of the Lika river and by the ground­ water levels in the boreholes. The permeable zone, that is drained towards the sea, incorpo rates the area of es tavcllcs from the Bakovac valley, through the Lipovo polje to the Gaeka river (Ar­ ea B in Fig. 3). The main features of this area are the exis tence of ponors and es tavelles, and rarely some intermittent springs of low discharge rate. The Gacka and Lika rivers in this part of the terrain have their lev­ els above groundwater levels for almost all of the year and arc considered as "hanging" surface-water streams. Lower groundwater levels in this area are a conse­ quence of cessati on of the function of the Velebit and Apatisan barriers, and progradation of recent karstifica­ tion from the coast towards the hinterland. The estavelle zonc in the downstream direction passes into the ponor area. During all hydrological con­ dit ions, the groundwater level in the ponor area is con­ siderably lower than the stream that is sinking. More detailed zoning of thc karst terrain, and parti­ cularly the permeable areas a t the edge of the Velebi t barrier was performed duri ng inves tigations for the Kosinj water reservoir project. 2.4. KARSTIFICATION Since the Kosinj reservo ir recharge area is in a rela­ tively litho log ically homogeneous karst te rrain , an important element for the evaluation of reservoir water constrain ing capacity and reservoir feasi bility, arc the processes of tectonism and karsti fication. The geolog i­ cal conditions in the studied terrain indicate, that before the Neotectonic uplift which preceded the forming of the current landscape, several emersion events occurred in the Dinarides. The first favo urable conditions for karstification occurred in the Dinaride area after f'ly sch deposition, when, during the Pyrenean orogeny, large masses of carbonate rocks were exposed to exogenous processes . During this orogenic event , the clas tic-car­ bonate l elar beds were form ed in the Lika region. The mol asse characteristics of these sediments indicate that the flattening and erasing of the elevated landscape was contemporaneo us with their [ormation (BA HUN , 1990). Pav icic: Ilydrogcological baluation of Water Ret;lin ing Properties of the Kosinj Reservoir (Lika, Croalia) 295 The Ncotcctonic movements which commenced in the Miocene and were intensified toward the end of Pli ocene and the beginning of Pleistocene played a major ro le in the reshaping of the land scape and the developmelll of karstirication. Velebit was then uplifted together with other mountain ranges, whi le depressions developed as isolated karst plateaux and poljes. In somc deprcss ions, deposition of Plio-Quaternary sediments occu rred, the remnants of which are round in sevcral rccent depress ions and karst poljes. The Neotectoni c movement s changed not only the shape of the land­ scape but also influenced the change of groundwatcr drainagc th rough previously form cd channels and cavi­ ties. The links bctween the subsurface channels wcre cut , and so mc water-bearing cavities were scparated from the new ly form ed ground water dynamics. The principal drainage d ircction from the Velebit hinterland towards the Adriat ic basin was preserved, but the local erosion base levels changed. Together wi th the upli ft of Velcbit the impenn eablc Palaeozoic and Lower Triassic deposi ts were also raised closer 10 the surfacc, resulting in their action as a hydrogeological barrier to the south of the Bakovac fa ult from the time of uplift un til the prcsen t. To the north of the Bakovac fault , the imper­ mea bl e rocks lie deeper beneath thc surface, enabling deeper karstification and groundwater drainage from the Velebit karst hinterland towards the Adriatic sea. The dept h of karstification in the Lika region , (man­ ifested in the ex istance and depth of spclacolog ical phe­ nomena), the groundwater level conditions, the occur­ rence or ponors, estavelles, intermittent and permancn t springs and their zoned d ist ribtHion , are the conse­ qucnccs of thc positive role of the Velebit hydrogeolo­ gica l barrier. The karstification process generally pro­ gresses from the sca towards the hinterland. In the Ilear hinte rl and of the Velebit barrier, the current eros ion base levels are thc Li ka river and the secti on of Gacka river th at is upstream from the zone of permanent springs. The depth of rcccnt karstification is approxi­ mately the same as the depth of the rivcr beds, towards which grou ndwater drainage occurs. Only small local variations ca n occur as a conscqucncc of the most reccnt movements of tcctonic blocks. Karstifi cation deeper than the level of the river bed in the area under thc influence of the Velebil barrier is linkcd to an earl i­ e r phase of karstification. Thi s was determined in the area of the KrLlscica rcservoi r, whcre at a depth of 250 111 be low the level of the river bed, caverns fill ed with quartz sand were cncountered during exploration dril ­ ling. T hese finding s indicate that these deep cave rns were once a part of the ground water dynamics. The subsurface cav iti es and channels, developed in earlier phases of karstificati on, locally can be " inherited" into the complex underground systcm s. However, for th e interpretation of current hydrogeological conditions, the landscape and karstification arc a conscquence of exo­ dynamic processes that were active in the Pl eistocene and Holoce ne, and where the eros ion base level s are ri vers (Lika and Gacka) or the sca. To evaluate water retention potential of a rcservoir in karst terrains, it is of prime im portance that the study of landscape develop­ ment and karstification is pe rform ed (FRITZ, 1992). Recent studies performed in the Dinaric karst terrains indicate that the present landscape is very young. The majOlity of river valleys and lake depressions were cre­ ated during the Lower Pleistocene and Holocene. The analysis of landscape development and karstifi ­ cation processes is performed through the study of river va tteys. The analysis involves not only the relationship between the valleys and thc clcvated surrou ndings in a karst environment, but also utili zes the dat a on hydro­ logical characteristics, the occurrence of springs and ponors and the water-leve l flu ctuations in the terrain through which the river nows. The most abundant data, that can be used for analysis of landscape developmcnt and karstification, is available for the a reas of the Licko-Gracac plateau, the Lika river and the Bakovac valley, while correlation is poss ible with the sou thern part of Velebit in the area surrou nding the cave area Cerovacke pecine. In the Lipovo polje , the Lika ri vcr bed partly cuts through Quaternary sediment s. Arter thi s incision , ncw terraces were not form ed. The left ridge of the valley , upstream from thc hydroe lec tric plant Sklope, consists of Plio-Quaternary marl s, over­ lain by Pleistocene colluvial clays, which were deposi ­ ted both in the Kosinjsko and Lipovo polje. Along the river banks, intermittent and pcrmanent springs occur, with no noticeable water loss , and the g roundwater lev­ els are higher in the banks than in the river bed. The stream in the Bakovac valley, (the nort hern tributary of Lika river), cuts through Triass ic and Juras­ sic depos its in its upper course, and in its mi dd le and lower course through the l elar beds. In the va lley, depo­ sition of Pleistocene and Holocene sed iments of various genetic types took place. Tn the hi gher part of the val­ ley, rem nants of loose moraine materi al and so lid bed­ ded fluvi oglacial cong lomerates can be found. On thc va ll ey slopes and the valley fl oor collu vial clays were deposi ted. On the basis of relatively small patches of Quater­ nary deposits preserved in the higher pan of thc Bako­ vac valley and Velebit, it can be concluded that valley incision occurred in two phases . The incis ion of a rela­ ti vely shallow valley occurred before the deposition of rIuviogiac ial conglomerales which are preserved in the area of Krcevine at 720-780 m a.s.!. In the highest pan of the valley on the slopes of Velebit , moraine material is preserved. The downstream part of the valley was at that period cither deeper, or was lowcred along a YOUJ1 - ger diagona l fault in the area downstream from the Zivulje spring. In this part of the va lley, flu viog lacial conglomerates arc situated at an elevation of 580 m a.s.!. During deposition of colluvi al clays in the broader area of the Lika plateau, thc Bakovac vall ey was co n­ siderably shallower than today. The association of Bakovac valley deve lopment with the evolution of the Lika plateau and the canyon inc is ion of the Lika river, is the consequence of the 296 function of the Velebit hydrogeological barrier (Fig. 3). The Bakovac valley is situated at the edge of influence of the Velebit full barrie r, i.e. on the boundary towards northern Velebit and Senjsko Bilo. In this region , the karstification is much deeper and the waters drain towa­ rds the sea. The progression of deep karstification deve­ lops from the sea towards the mainland in the Lika region through the Bakovac valley, and on through to the lower cou rse of the Lika ri vcr, and then into the Lipovo poljc and the periphery of Kosinjsko polje. The progression of the karstification process is manifes ted in chan ges of stream flow patcrns, the occurrence of intermittent sprin gs, the presence of estavclles and panors, and an increase of depth to groundwate r. The advance of thc karstification process from thc sea to the hinterland has affected the Bakovac valley in its higher and middle parts. As a resu lt, part of the wa­ ter drains beneath the valley floor through its northern side directly into the sea . During heavy rainfall, when water flows in the stream bed through the middle part of the Bakovac valley, the groundwater level there is below the valley floor. From the activity of this ponor, it can be concluded that the groundwater level there is always below the stream bed, which was confirmed with measurements performed in the observation bore­ holc A-8. The spclaeological phenomena situated along the middle and lower course of the Lika river, and the regi­ on between the Lika and Gacka rivers occur at shalow depths (BOZICEV IC, 1965, 1969) which corresponds to the relatively high erosional base level encounte red therc. This is a resu lt of the function of the Velebit full barrier. The development of the karstification process can also be viewed in terms of sufficient sinking of the paleoplateau on the southeastern part of the Velebit bar­ rier, in the cave area Cerovacke peeine along the south­ ern cdgc of the Graeacko polje (MALEZ, 1965). 3. GENERAL CONDITIONS FOR THE CONSTRUCTION OF THE KOSINJ RESERVOIR The construction of the Kosinj reservoir is planned for part of the lower course of the Lika river. The dam is to be built some 800 m upst ream from the bridge (Kosinjski most), in fact downstream from the existing reservoi r Kruscica. In order to dete rmine the hydrogeological condi­ tions that would allow the feasibility of reservoir con­ struction, the area downstream from the reservoir "Kruscica" in the valley of Lika river was also studied, as we ll as the broader area of reservoir recharge towards the Bakovac valley, Lipovo polje and the boun­ dary zone towards the Gacka river. Detailed hydrogeo­ logical mapping was performed on a scale of I :5,000 of the local area and the margins of the reservoir. A better understanding of tectonic conditions of both the imme­ diate and broader regions of reservoir recharge was enhanced by Neotectonic studies of the same area (PRELOGOVIC, 1989). Geologia Cro;llica 50/2 Extensive exploration drilling was performed, acco­ mpani ed by permeability measurements, geophysical tests, and water tracing tests. The grou ndwater level monitoring was conducted for many years, in over 50 observation boreholes in the recharge area of the future reservoir. In the groundwater-level studies, data were used [rom observation and other exploraLion boreholes made during the elaboration of the Kruscica reservoir. The Kosinj reservoir area mainly consists of pennc­ able Cretaceous carbonate rocks and the Jelar beds, so it is a hydrogeologically homogeneous terrain . There­ fore, the hydrogeological conditions and water retaining characteristics are explained by zoning of the broader region surrounding the reservoir, according to the hyd­ rogeological functions of the terrain (Fig. 3). The Velebit hydrogeological barrier prevented dee­ per karstification of the transition region, which is refle­ cted by high g roundwater levels and the drainage of water towards the Lika river. Outside the area of barrier influence, ponors and estavclles exist, and the streams are in tennittent or "hang" above the deeper groundwa­ ter levels. Under these conditions, in order to perform the studies, to evaluate the permeability of the broader area, and to plan the necessary intervening procedures, detailed hydrogeological zoning of the terrain recharg­ ing the reservoir (Fig. 6) was performed. The following [our zones with unique hydrogeological features were outlined: The area with permeable surface conditions and groundwater above the water level of the Lika river (1) covers a large part of the Kosinj reservoir bOllom su rface. All the waters in this zone flow towards the Lika river. Along the stream bed of the Lika river and its tributaries Bakovac and Mlakvenski potok, several permanent and intcrmittent springs exist. Tn this part of the terrain, only one ponor exists which drains the waters from the Mlakvenski potok. The tracing tests, performed in the borehole LG-5 on the northern bank of the reservoir, indirectly proved that the water of Mlak­ venski potok drains through Ponomc into the Lika river. In the terrain within this zone, all the waters either drain towards the Lika or Gaeka rivers. The impermeable Jelar beds (2) arc situated in the southwestern part of the terrain on which the Kruscica reservoir lies. The lelar beds in this part of the terrain contain breccia with clayey marl cement and lenses of marl. The water retaining characteristics of the Krus­ cica reservoir is attributed to the favourable hydrogeo­ logical features of th~se deposits. The zone of the uplifted tectonic block and the terrain under the larger influence of the now eroded Neogene clay deposits (3) - on the map showing hyd­ rogeological provinces, this area is situated in the mid­ dle part, and the eastern margin of the Kosinj reservoir. This is also the water divide between the Lika and Gac~ ka rivers. The existence of this water divide is the con­ sequence of the uplifted tectonic block Mlakvena greda - Poijan, where reccnt karstification is fairly l'aviCic: Il ydrogeologic:II EV3lu:lIion of W:uer Rel3ining Propcn ics oflhe Kosinj Reservoir (Lib. emalia) 297 shallow. The occurre nce of Neogene marl deposits in fossi l depress ions, found in boreholes K-4-2 and K-9, has a certain impermeable role in this terrain. The boreholes revealed fi ss ures and cavities con­ taining compressed clay, a feature also indicated by the geophys ica l exploration performed a long the route of the planned grout curtain. Spelaeological phenomena in thi s zone occur at relat ively shallow depth and are above the Kosinj reservoir backwater leveL The groundwater level measurement s performed in the dri ll hole over a longcr period showcd that the gro­ undwater leve ls alo ng the watcr divide towards the Gaeka ri ver do not fall bellow 520 m a.s.l. (Fig. 6). The precise position o r the water divide was de tennincd by traci ng or water drainagc from the dr ill holes. The area in which the low waters are below the Lika river stream flow, and the erosion base level is the Adriatic sea (4) occupies the te rrain north and northwcst from the Kosinj reservoir. Downstream from the Kosinj dam is the Lipovo polj e and it s recharge zone. In the upstream part of the Lipovo polje on its eastern and western slopes a st ring o r intermittent spri­ ngs and cs tave tles exisI. Ponors arc located in the downstream part or the polje and, through them, water from the Lika rivcr drains to the sec. The estavelle zone during the d ry season "passes" from the upstream part o r the Lipovo polje into the area of the Kosinj reservoir. The groundwater level decline below the level or the Lika ri ver stream bed was observed o n the wes tern bank be tween thc Lipovo polje and Kosinj sko polj e, where the 480 m water- table contour enters the area of the reservo ir. Nca r the borehol e BK-4, the boundary between the sp ring zone and the estavelle zone is close to the reservoir. The groundwater leve l dec line in this reservoir bank waS also observed in the borehole BK-2. In the Bakovac va lley this boundary is upstream from the position of the dam. Hydrogeological zoning (Fig. 6) is the basis for Ihe understanding of hydrogeo logical co nditi ons and the eval uation or the water retaining properties of the Kos­ inj reservoir; it a lso allowed the separation of imperme­ ab le areas from the permeable ones, the definition of locations for the construction of dams and the identifi­ cation of banks which have to be made impermeable by grouting. FUrlher studies (drilling exploration, geophys ical exploration, detailed study of Neogene marls in depres­ sions on the East reservoir bank) should more precisely determ ine the conditions that will enab le the constru­ ct ion of the reservoi r, i.e. produce the da ta necessary for the construct ion of the grout curtain. 4. CONCLUSION The necessa ry studies performed for the constru ­ ction of a suffic iently impermeable reservoir in a karst terrain, for a big hydroelectric plant , arc presented throug h the results obtained in the case of the Kosinj reservoir. The reservoir is situated in te rrain composed or penneable limestone and limestone breccia (C reta­ ceous and l e lar deposit s). The karst terrain in the hin­ te rland of the Velebit barrier is divided int o a spring lone, an estavelle zone and a ponor zone. Because the location of the Kosinj reservoi r is almos t com ple te ly situated within the spring zone, a basis for further st ud­ ies exists, although this location was prev iou sly reje­ cted . The homogeneous karst terrain, largely composed of mass ive lelar deposits with relatively undefined re la­ tionships with the Cre taceolls deposits , requ ired a spe­ cific approach, so the studies were focu sed on the struc­ tural and tectonic setting, the landscape development and the karstification processes. In order to evaluate the permeability of the broader area and to plan the necessary intervening procedures, detailed hydrogeological zoning in the area or reservoir influcnce was performed and several zones with similar hydrogeological features were out lined. From the explorati on boreholes it was poss ib le to obtain the data about groundwater levels, the character and depth of karstification. By groundwater traci ng, the flow directions and the positions of groundwater divides were defined. The Bakovac valley development is associated with the incision of the Lika river into the Lika plateau as a continuou s process of the recent karstifi cat ion. Thi s karstifica tion incorporates the spelaeological phenome­ na situated along the Kruscica dam and in the region between the Lika and Gaeka ri vers. T he perfo rmed studies confirmed the ex istence of karst ification bene­ ath the Lika riverbed (the river is incised int o a previ­ o usly karstified ter rain). Neve rtheless, thi s docs no t dimini sh the hydrogeological importance of th e Lika river as a recent erosional base level, because the o lder subsurrace systems and groundwate r flows were inter­ rupted by Neotec tonics, or because the dra inage chan­ nels arc clogged with c lay mate rial. The results of the studies confirmed that the recent karstifica tion , with which the present water dynami cs is assoc iated, is relat ively shallow in the Kosinj reservoir area. Therefore, it is poss ible to achieve an impermea­ ble reservoir with the aid of addit ional co nstruct ion work. 5. REFERENCES BAH UN, s. (1962): Vapnenei Promina-nas laga 1I pod­ rucju Kruscice u Lic! (Limestones in thc Prom ina Deposi ts of the te rritory of Krusc ica in Lika.).­ Geo /. vjesnik, 15/1 , 101 -106. BAH UN. S. (1973): Odnos krskog proeesa i Ollvijalnc erozije u poclrucju Like (Relationship between kars­ tifi cation and fluvi atil e erosi on in the reg ion of Lika, cenlral Croatia).- Krs JlIgOS/. , 8/5, 91 - 100, Zagreb. 298 BAHUN, S. ( 1974): Tcktogencza Vclebita i postanak Jelar· naslaga (The tectogenesis of Mt. Velebit and the formation of l e lar deposils) .- Oeol. vjesnik, 27, 35·51, BAHUN, S. (1984): Tectonic and hydrogeological sig· nificance of the areas composed of the lelar forma­ tion (Tektonsko i hidrogeo!osko znacenje podrucja izgradenih od Jelar· fonnacija). · Krs Jugosl. , 11/1, 1· 11, Zagreb. BAHUN, S. (1990): Stupnjevi razvoja zaravni u dinar· skorn krsu (Stages of formation of the plains in the Dinaric karst).· Krs Jugosl. 12/6, 147·158, Zagreb. BAHUN, S. & FRITZ, F. (1975): Hidrogeoloske speci· ficnosti Jelar-naslaga (Hydrogeolog ical properties of Jelar deposits, Lika, Croatia.).· Geol. vjesnik, 28, 345· 355. BAI-TUN, S. & FRITZ, F. (1987): Postanak izvora u d in arskom orogenskom akumuliranom krsu (The origin of springs in Dinaric orogenic acc umulated karsl.) . Krs Jugosl. 12/2,27·37, Zagreb. BOZICEVIC, S. (1965): Poljakova pecina.· Geol. vjes· nik, 18/1, 141·157. BOZICEV IC, S. (1969): Horvatova pecina uz branu Sklope (The Horvat cave at the Slope dam). · Geol. vjesnik, 22, 501·510. FRITZ, F. (1972): Razvitak gornjeg toka rijeke Zrman· jc (Morpholog ica l evolution of the upper Zrmanja course).· Krs Jugosl., 8, 1· 16, Zagreb, FRITZ, F. (1991): Utjeeaj reeentnog okrsavanja na zah· vacanje voda (The influence of the recent karst ifica­ tion on water cxtraction).- Geol. vjesnik, 44, 281- 288. FRITZ, F. (1992): Effect of recent sea level change on the development o f Karst phcnomena. - Proc. Intern. symp. "Geomorphology and sea" and Meeting Geo­ morpho comm. Carpatho-Balcan countries, Mali Losinj, 85-92, Zagreb. FRITZ, F. & PAVICIC, A. ( 1975): Tektonski odnosi u podrucju razvoja krednih i Je lar nas laga kod Kosi ­ nja u Lici (Tectonical pattern o[ Cretaceous and Je l­ ar depos its at Kosinj , Lika, Croatia).- Geo!. vjesnik, 28, 35-42. FRITZ, F. & PAVICIC, A. (1987): 0 problemu zastit· nih zona u krsu - primjer problematike zastite izvora Geolog ia Croatica 50/2 Jadro.- Zast ita izvorista voda za vodoopskrbu, 255- 262, Split. HERAK, M. (1957): Geoloska osnova nekih hidrolo· skih pojava u dinarskom krsu (Gcologischc grundla­ gen e iniger hydrogeologischen Erschinungen in Dinarisehen Karst).· II. kongr. geo!. Jugos !. , 523· 539, Sarajevo. HERAK, M. (1986): A new concept of geotectonics of the Dinarides.· Acta geoL, 16/1, 1-42, Zagreb. HERAK, M. ( 1991): Dinaridi. Mobilistieki osvrr na genezu i strukturu (Dinarides. Mobilistic view of the genesis and strukrure).· Acta geoL, 2 1/2, 35· 117, Zagreb. HERAK, M. & BAH UN, S. (1980): The role of the eal· carous breccias (Je lar-formalion) in the tectonic interpretation of the hi gh Karst zonc of thc Dinar­ ides. · GeoL vjesnik, 31, 49·59. MALEZ, M. (1965): Cerovaeke peci ne (D ie Hohlen von Cerovac) .- Spe leol. drustvo Hrvat ske, 1, 1-44, Zagreb. PA VICIC, A. (1984): Geoloska osnoya Veicbi tske raz· vodnice (Geological foundation of the Vele bit waler div ide, Croalia).- 8 . Jugos!. s imp. hidrogeol. in z . geoL, 1,537·547, Budya. PAVICIC, A. ( 1995): Hidrogeoloski uvjeti za ostyare· nje akumulacije u krsu zaleda Velebita (Hydrogeo· logical conditions [or the construction of reservoirs in the Velebit Ml. hinterland, Croatia).· Unpub· lished PhD Thesis, University of Zagreb, 113 p. PA VICIC, A. & FRITZ, F. ( 1976): l-lidrogeologija sir· eg podrueja donjeg toka Like (Pazariste, Krs, Otoeae) (Hydrogeology of the lower Lika river region (Pazariste - Krs - Otocac». - 4. Jugos!. simp. hidrogeoL inz. geoL, 1,291·304, Skopje. PAVLlN, B. (1970): Kruseica storage basin in the cav· ernous Karst area.- Comision Intcrnational e des Grandes Barrages . Dixi eme Congres des Grands Barrages, 209·224, MontreaL PRELOGOVIC, E. (1989): Neotektonski pokreti u pod· rucju sjevernog Velebita i dijela Likc.- Geol. vjes­ nik,42, 133· 147 . Manuscript received April 14, 1997. Revised manuscript acccpted November 10, 1997.