GEOL. CROAT. 50/2 1289 -2981 6 Figs. 1 ZAGREB 1997

Hydrogeological Evaluation of Water Retaining Properties of the Reservoir (, )

Ante PAVICIC

Key words: Karst hydrogeology, Karstification, Water The waters from the Bakovac tributary are to be di s~ retaining, Hydrogeological zoning, Kosinj reservoir, charged int o the Lika rive r through a tunnel d o w n ~ Li ka, Croatia. stream from the lake and utilized in the HE Senj. During exploratory drilling, the rim of the future reservoir, at the side of the Lipovo polje, and th e bound ~ Abstract ary area by the river were investigated. The 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 majority of the drilled holes contain equipment which did not completely de fi ne the waler constraining clements of the ter­ has produced water level data for over 30 years. Recen­ rain, and therefore the more reliable Kruscica reservoir was buill. Fur­ tly drilled observation boreholes were a lso used for ther studies were undertaken in order 10 evaluate in detail the terrain of the wlIicipalcd location of the dam impoundment at Kosinj. Due to monitoring groundwater level variations. Groundwater the lithological homogeneity of the karst terrain, the studi es focused flow tracing tests were performed (from the eastern side on tectonics and geomorphology, wilh emphas is on karstifi cation. of th e lake and in the Bakovac valley) which enabled Terrain zoning was made on the basis of hydrogeological fa vourabi­ the expansion of flow data obtained in previous studies lily for water rctaining in the reservoir. The Vcl cbit hydrogeolog ical barrier innuences the general water (rrom areas surrounding Pazariste and Studcnci). Detai ­ run off in the hinterl and , as well as the depth and the direction of led geophysical studies were performed in areas where advancement of recent kars ti ficati on processes. In the hinterland of injection barriers were planned and the narrow locati ons the barrier the karstifi cat ion is shallow, increases in depth to­ where damming was to be perfol111 ed. wards the sea, and is characterized by the zoned distribution of spri­ ngs, estavelles and ponors. This concept played a major role in laler To perform these rather co mplex studies, a large hydrogeological stud ies of the proposed reservoir. volume of varied data, from both published papers and proressional studies was required. The fundamental publi shed papers were the result of studies done during the construction of the Kruscica artificial lake. BAHUN (1962) dcscribed in detail th e lithological composition 1. INTRODUCTION of the rocks, with emphasis on th e l elar beds. The same author (BAHUN, 1973), related the karst processcs and

The completi on o r the Krusciea reservoir with a vo l ~ fluvial erosion in the Lika ri ver area and gives an um c of 134 x 10(' m3 on the Lika river did not satisfy the acco unt of karstification phases. On the basis o r data needs for reservoir space, because th e ac hi eved volume from th e region surrounding the mtificial Kruscica lake, is only 15 % of th e averagc annual recharge of the Lika PAVLlN (1970) delineated a ponor zone. an es tavelle 3 I river (Qmcan= 27.9 111 S· ) . Together th e headwaters of zone and a spring zone. PAVICH: (1984) also included th e Gacka and Lika rivers are utilized by the hydroelec ~ a large part of this area when dealing with the Velebit trie plant HE Senj but, due to the shortage of water s tor~ groundwater divide. BAH UN & FRITZ (1975) studied age space, a po rti on of the hi gh waters is discharged the hydrogeological features of th e lelar beds, while in to ponors, which in turn causes flooding of the polje. FRITZ & PAVICH': (1975) and PAVICIC & FRITZ In order to utili ze larger volumes of water and to e l imi ~ (1976) outlined in detail Cretaceous beds and the l elar nate flooding, studies which woul d enable the construc­ beds, determined numerous faults in the area anci tion of the Kosinj reservoir with a vo lume of 433 mil­ described th e hydrogeological relationsh ips in th e lower 3 lion 111 were continued in 1972. The upper course of course of the Lika river. PRELOGOVIC (1989) descri ­ th e Bako vac tributary lies in an estavell e zone and, bed the Neotectonic movement s in the area of northern therefo re, this va ll ey will be separated by a dam from Velebit and a part of Lika and provided new insight to the lake. the structural and tectonic relat ionships. The data 0 11 the

This paper was presented at the scientific meeting: dedicated to the 80th a nniversary of the life of Professor Milan Herak, Institute of Geology, Sachsova 2, P.O. Box 268, HR · ] 0000 Zagreb, held on March 5th, 1997 in Zagreb Croatia. veOiogm \..roallCiI ' VI/.

Fig. 1 Kosinj re servoir layout. Legend : I ) Kosinj rcsclvoir: 2) K osinj dam and HPP: 3) Sedl o dam: 4) Bakovac dam: 5) Skl ­ ope dam and HPP; 6) Kru sc ica reservoir; 7) Bakovac - Lika tunnel; 8) Bakovac - Lika e1la­ nnel; 9) Se Ji ste compcnsat ion basin. origin of structures and faults and Neotectoni cs enabled are impermeable. The Quatern ary deposits are mainly bette r interpretation of th e hydrogeological relation­ represented by coll uvial beds, deposited in the poljes, ships and groundwaLer flow. PA VICI<': (1995) in hi s and their relics can be found on elevated surfaces of th e PhD Lhesis described the hydrogeological condiLions for terrain. These clay-silty deposit s ean be very thick and the construction of water reservoirs in the karst of the impermeab le in some areas, allowing surface water VclebiL hinLeriand, where he al so sLudied the area flow in the polje where the groundwater level is below around Kosinj . and thi s was used as the basis for the the water bed (e.g. Lipovo polje). present paper. 2.2. TECTONIC BASIS OF HYDROGEOLOGICAL CONDITIONS 2. HYDROGEOLOGICAL RELATIONSHIPS IN THE KOSINJ RESERVOIR AREA Almost all of the area studied belongs to the Dinar­ icum megastructural unit (HERAK, 1986, 1991). The 2.1. HYDROGEOLOGICAL PROPERTIES OF geodynamics of the studied area is characteri zed by ROCKS structures resulting from folding and faulting which occurred after deposition of the Eocene flysch deposit s. On the hydrogeological map presenLed in Fig. 2 Lhe These movements defined the existing structures, which main lithostratigraphic members outlined are grouped are outlined by regional fault s which are often accom ­ by Lheir hydrogeological properties. In the studied panied by the Jelar beds (BAHUN, 1974, 1984; HER­ recharge area Triassic. Jurassic, Cretaceous and Lower AK & BAH UN, 1980). Thesc tectonic structures were Palaeogene carbonate rocks dominate. formed during the Upper Oligocene and Miocene, prin­ The rocks are divided into fi ve groups according to cipally by tangential movements (BAHUN, 1974; their permeability. The limestones in alternation with PRELOGOVIC, 1989). The final phase of these move­ dolomites and limestone breccia (T;, J, K,) are very ments occurred in the Upper Pliocene and the Quater­ perm eable rocks. Partly impenmeable are the Jelar beds nary when transcurrent faults were active and were par­

(Pg2,,), while th e dolom it es (TJ) are partly permeable. ticularly intensive in the Vclebit region. The Neotecto­ The shales and sandstone (T " T;) and tuff and tuffite ni c movement s have influenced recent karstificati on Pa vicic: I-Iydrogeological Evaluation of Water Retaining Propenies of the Kosinj Reservoir (lib, Croatia) 291

~ I -- 2 3 • 4 0 5 <> • 8 7 ,(? 8 ~ 9 D'O ~11 S" Em3313 I ii"

0"",=",,,,_=,,-,,5'0

J

Fig. 2 General hydrogeological map. Legend: 1) geological boundary; 2) fault; 3) assumed direclion of groundwater 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 th e prevailing The occurrence of Neogene marls in a deep sin khole geological structures in th e studied area are governed (borehole K-4-2, Fig. 4) confirms the existence of vcry by deep tangenti al and reverse tectonics, the hydrogeo­ young fau lt tectonics. The thi ckening marl and mor­ logical conditions can be interpreted main ly from sur­ phology of the lerrain indicate a throw along th e fault fa ce outlines of the structures. Alternatively, the water plane of over 100 Ill. dynamics of the stu died area is confined to relati vely The fau lts, IOgether with the stru clu ral and lithologi­ shallow parts of the terrain, well above the conlact cal features of the rocks, are th e most important ele­ between megaslructu ral unit s, so the deep langential ments that influence the karstification process and lectonics has no influence on water emergence and groundwater flo w in karsl terrains, as first stressed by flow. HERAK (1957). More recent studies (FRITZ, 1972, On the basis of the geology of the studied area and 199 1, 1992; FRITZ & PAYICIC, 19 86; BAI-IUN & the basic tectoni cs involved, distinctive areas. structural FRITZ, 1987) suggest that the principal influence is units and fault s that inlluencc the hydrogeology of the that of recent karstifi cation, which is associated with terrajn arc present ed in Fig. 3. th e youngesl Neotectonic fault s. During map elabora­ The Velebit mountain range form s (he south-west­ tion, the data from studies of the northern Ve lebit and ern limb of an extensive regional fo ld (BAHUN, 1974) parts ofLika (PRELOGOVIC, 1989) was used. thrusted during th e first phase, and later disintegrated As a consequence of regional tangential stress, com­ by radial NeoleelOnics. Tn the core of the anticline, alo­ press ion faults occur within smaller or larger areas. ng the fault on th e edge of Licko poljc, to the west from These are usually manifesled as hinge, transcurrent and th e water accumulations Kruscica and Kosinj , outcrops reverse faults. Tension fault s, that occur as fi ssures par­ of Pal aeozoic and Lower Triassic, and to a lesser de­ all el with the principal stress direction, havc the least gree Middle Triassic impemleable clast ic rocks occur. pronounced strike displacement , and they are confin ed The posi ti on of these impenneable rocks in the Velebit to smaller areas, usuall y within tectonic blocks. They massif, the uplift of which began after the Miocene and are important for local groundwater flow conditions. concluded in th e Upper Pliocene and the Quaternary The transcurrenl fault s are of primary import ance in (PRELOGOYIC, 1989), influenced th e hydrogeological groundwater flow, since the rocks in which th ey occu r conditions in th e hinterland, especially as reflected in are compressed, and thus can act as a barrier 10 ground­ th e direction of karstification and the groundwater level water flow perpendicular to the fault plane. The gro ­ in the area of recharge surrounding the water reservoirs. undwater flow is directed parallel to the fault plane, and 292 Geologia Croatica 5U/2 EIill IMnMij 2

Fig. 3 Map showing the tectonic structure and hydrogeological function of the terrain. Legend: J) Velebit hydrogeological bar­ rier; 2) Apatisan relative barrier: 3) hydrogeological zones: A - spring zone, B - estavelle zone, IA,C~ 3 C - ponor zone; 4) nonnal fau ll : " 5) reverse fault; 6) anticline § 4 5 axis; 7) permanent karst spring; ~ 8) intermittent karst spring: 9) estavelle; 10) permanent coastal 6 7 8 • " spri ng; 11) intermittent coastal spring; 12) submarine spring; • 10 11 vO 12 0 13 '" 9 <> 13 swallow hole (ponor). natural retention can occur on the upstream side of the lady in recently active fault zones. The Neotectonic fault. This group of fa ults is represented by the Vra­ active faults in the area of northern Velebit are vertical tnik - Senjsko Bilo - Peru sic fault on the eastern margin to very steep wi1h a WNW-ESE strike. of the Senjsko Bilo - Krasno structural unit, and the Lika fault, west of the studied water reservoir locations. 2.3 HYDROGEOLOGICAL FUNCTIONS OF These faults are characteri zed by groundwater flow par­ THE TERRAIN allel to the fault plane, while groundwater flow perpen­ dicular to the fault plane is of a lower degree, parlieu- The hydrogeological functions of the studied terrain were determined on the basis of the hydrogeological features of rocks, tectonics, and the spatial distribution A and relative positions of gcological bodies and their B morphology. In the studied area the following hydroge­ m 20' Q.S.1. ological functions of different parts of the terrain were 600 0 determined: futl barriers, partial barriers and permeable 0

00 0 areas. o 0 ~ 00 o 0 0 0 2.3.1 Hydrogeological barriers 550 o 00 0 0 000 0°0 00 0 00 Barriers can be classified according to the degree 0 0 0 0°°0 that they obstruct water flow as full or partial (relative) ooPg o 5 DO 23o o 0 ~ do balTiers. 000 000 ~ In the studied area the principal hydrogeological 00 00° feature is the Velebit full barrier which extends from "0 /,. the Bakovae valley to 1hc Stikadsko polje (Fig. 3). The strike of this barrier is confined within the Velebit 100 100 300, 400m, structural unit. The existence of this barrier is deter­ mined by impermeable Palaeozoic and Lower Triassic Fig.4 Cross-section through the borehole K-4-2, showing the posi­ tion of the Neogene marls. deposits the elevation 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) YOUNGER WURM 1000.,

1580

500 )

m (0 s I) OLDER HOLOCENE 1000., <>-+--<> Jama (ha((>1 River L iko

PRESENT

m (0 s I) ZONE OF I NTE R M IT~ ZONE OF SINKHOLES OF SPR'NGS 1000 5PRING5 (""""j" Z 0 N E KrCE'vine ZlvulJo KOSINJSKI Koslnjsko polle ern a Vr(>fo BMOVAC : Lokv/co A- 6 River L/ko Fig. 5 Development of the Bako· vac valley and wat er dminage. Legend: I) permanent s urface discharge into the Likil river; 2) intermittent surface diseha· rgc into lhe Uka river; 3) gro­ un dwater discharge; 4) grou­ ndwater level: 5) zonal water di vide: 6) remains of a terrace; 7) observ

1

" 1 -520- 13 -480- 14 -_5'0 __ 15 --"-"- 16 a 20 ~ 17 o 1000 2000"1 ~ 18 ... _ ..J ...... >•• 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) rclatively uplifled tcctonic block; 4) terrain under a highcr influence of clay deposits; 5) low groundwatcr under Lika river: 6) faul l; 7) major karst spring. pennanent; 8) major karst spring. inte rmittent; 9) permanent 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 vertical 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 More detailed zoning of thc karst terrain, and parti­ intermiltent springs exist, but estave ll es and ponors are cularly the permeable areas at the edge of the Velebit absent. The watcr dividc in this region is shown in Fig. barrier was performed during investi gations for th e 6. The water divide between the Lika and Gacka rivers Kosinj water reservoir project. is confirmed by th e existence of numerous springs alo­ ng the eastern bank of the Lika river and by the ground­ 2.4. KARSTIFICATION water levels in th e boreholes. Sin ce the Kosinj reservoir recharge area is in a rela­ The permeable zone, that is drained towards the sea, tively lithologically homogeneous karst te rrain , an incorporates th e area of estavcllcs from th e Bakovac important element for the evaluation of reservoir water valley, through the Lipovo polje to the Gaeka river (Ar­ constraining capacity and reservoir feasi bility, arc the ea B in Fig. 3). The main features of this area are the processes of tectonism and karsti fication. The geologi­ existence of ponors and estavell es, and rarely some cal conditions in the studied terrain indicate, that before intermittent springs of low discharge rate. The Gacka the Neotectonic uplift whi ch preceded the forming of and Lika rivers in this part of the terrain have th eir lev­ the current land scape, several emersion events occurred els above groundwater levels for almost all of the year in th e Dinarides. The first favo urable conditions for and arc considered as "hanging" surface-water streams. karstification occurred in th e Dinaride area after f'ly sch Lower groundwater levels in this area are a conse­ deposition, when, during the Pyrenean orogeny, large quence of cessati on of the function of the Velebit and masses of carbonate rocks were exposed to exogenous Apatisan barriers, and progradation of recent karstifica­ processes. During this orogeni c event , th e clasti c-car­ tion from the coast towards the hinterland. bonate l elar beds were formed in the Lika region. The The estavell e zonc in the downstream direction mol asse characteristics of these sediments indicate that passes into th e ponor area. During all hydrological con­ the flattening and erasing of the elevated landscape was dit ions, the groundwater level in the ponor area is con­ cont emporaneous with their [ormation (BA HUN , siderably lower than the stream that is sinking. 1990). Pav icic: Ilydrogcological baluation of Water Ret;lin ing Properties of the Kosinj Reservoir (Lika, Croalia) 295

The Ncotcctonic movements which commenced in retention potential of a rcservoir in karst terrains, it is of th e Miocene and were intensified toward the end of prime im portance that the study of landscape develop­ Pliocene and the beginning of Pleistocene played a ment and karstification is pe rformed (FRITZ, 1992). major role in th e reshaping of th e landscape and the Recent studies performed in th e Dinaric karst terrains developmelll of karstiri cation. Velebit was th en uplifted indicate that th e present landscape is very young. The together with other mountain ranges, whi le depressions majOlity of river vall eys and lake depressions were cre­ developed as isolated karst plateaux and po lj es. In somc ated during the Lower Pleistocene and Holocene. deprcssions, deposition of Plio-Quaternary sediments The analysis of landscape development and karstifi ­ occurred, th e remnants of which are round in sevcral cation processes is performed through th e study of river rccent depressions and karst poljes. The Neotectoni c vatt eys. The analysis involves not onl y the relationship movements changed not only the shape of th e land­ between the valleys and th c clcvated surrou ndings in a scape but also influenced the change of groundwatcr karst environment, but also utili zes th e data on hydro­ drainagc th ro ugh previously form cd channels and cavi­ lo gical characteristics, the occurrence of springs and ti es. The links bctween the subsurface chann els wcre ponors and the water-level flu ctuations in the terrain cut , and so mc water-bearing cavities were scparated through which the river nows. The most abundant data, from the newly formed groundwater dynamics. The th at can be used for analysis of landscape developmcnt principal drainage dircc tion from the Velebit hinterland and karstification, is available for the a reas of the towards the Adriat ic basin was preserved, but the local Li cko-Gracac plateau, the Lika ri ver and the Bakovac erosion base levels changed. Together wi th the upli ft of valley, while correlation is possible with th e sou thern Velcbit the impenn eablc Palaeozoic and Lower Triassic part of Velebit in the area surrou nding the cave area deposits were also raised closer 10 the surfacc, resulting Cerovacke pecine. In th e Lipovo polje, the Lika ri vcr in th eir action as a hydrogeological barrier to th e south bed partly cuts through Quaternary sediment s. Arter of the Bakovac fa ult from the time of uplift until the thi s in cision, ncw terraces were not form ed. The left prcsent. To the north of the Bakovac fault , th e imper­ ridge of the valley, upstream from thc hydroelectric meabl e rocks li e deeper beneath thc surfa ce, enabling plant Sklope, consists of Plio-Quaternary marls, over­ deeper karstification and groundwater drainage from lain by Pleistocene colluvial clays, which were deposi­ th e Velebit karst hinterl and towards the . ted both in the Kosinjsko and Lipovo polje. Along the The dept h of karstification in the Lika region , (man­ river banks, intermittent and pcrm anent springs occur, ifested in the ex istance and depth of spclacolog ical phe­ with no noticeable water loss , and th e groundwater lev­ nomena), the groundwater level conditions, the occur­ els are higher in th e banks than in the river bed. rence or ponors, estavell es, intermittent and perm ancn t The stream in the Bakovac vall ey, (the northern springs and their zoned d ist ribtHion, are the conse­ tributary of Lika river), cuts through Triassic and Juras­ qucnccs of th c positive role of th e Velebit hydrogeolo­ sic deposits in its upper course, and in its mi dd le and gical barrier. The karstifi cation process generally pro­ lower course through the l elar beds. In the vall ey, depo­ gresses from the sca towards th e hinterland. In the Ilear sition of Pleistocene and Holocene sedim ent s of various hinte rl and of the Velebit barrier, the current erosion geneti c types took place. Tn the hi gher part of the val­ base levels are th c Li ka river and the secti on of Gacka ley, remnants of loose moraine materi al and solid bed­ river th at is upstream from the zone of permanent ded fluvi oglacial conglomerates can be found. On thc springs. The depth of rcccnt karstification is approxi­ va ll ey slopes and th e vall ey fl oor collu vial clays were mately th e same as the depth of the rivcr beds, toward s deposi ted. which grou ndwater drainage occurs. Only small local On the basis of relatively small patches of Quater­ variations can occur as a conscqucncc of the most nary deposits preserved in the hi gher pan of th c Bako­ reccnt movements of tcctonic blocks. Karstifi cation vac va lley and Velebit, it can be concluded th at valley deeper th an th e level of the river bed in th e area under incision occurred in two phases. The in cision of a rela­ thc influence of the Velebil barrier is linkcd to an earli­ ti ve ly shallow valley occurred before the deposition of e r phase of karstification. This was determined in the rIu viogiacial conglomerales which are preserved in the area of the KrLl scica rcservoi r, whcre at a depth of 250 area of Krcevine at 720-780 m a.s.!. In the highest pan 111 below the level of the river bed, cave rn s fill ed with of the valley on the slopes of Velebit , mora ine material quartz sand were cncountered during expl oration dril ­ is preserved. The downstream part of the va ll ey was at ling. T hese findings indicate that th ese deep cave rn s that period cither deeper, or was lowcred along a YOUJ1 - were once a part of the groundwater dynamics. The ger diagonal fault in the area downstream from th e subsurface caviti es and channels, developed in earlier Zivulje spring. In this part of th e va ll ey, flu vioglacial phases of karstificati on, locally can be " inherited" into conglomerates arc situated at an elevation of 580 m th e complex underground systcms. However, for th e a. s.!. During deposition of colluvi al clays in th e broader int erpretati on of current hydrogeological conditions, the area of the Lika plateau, th c Bakovac valley was con­ landscape and karstification arc a conscquence of exo­ siderably shallower than today. dynamic processes that were active in the Pl eistocene The association of Bakovac valley development and Holocene, and where the erosion base level s are with the evolution of the Lika plateau and th e canyon ri vers (Lika and Gacka) or the sca. To evalu ate water incision of the Lika river, is th e consequence of th e 296 Geologia Cro;llica 50/2 function of the Velebit hydrogeological barrier (Fig. 3). Extensive exploration drilling was performed, acco­ The Bakovac valley is situated at the edge of influence mpanied by permeability measurements, geophysical of the Velebit full barrier, i.e. on the boundary towards tests, and water tracing tests. The groundwater level northern Velebit and Senjsko Bilo. In this region, the monitoring was conducted for many years, in over 50 karstification is much deeper and the waters drain towa­ observation boreholes in the recharge area of the future rd s the sea. The progression of deep karstification deve­ reservoir. In th e groundwater-level studies, data were lops from the sea towards the mainland in the Lika used [rom observation and other exploraLion boreholes region through the Bakovac valley, and on through to made during the elaboration of the Kruscica reservoir. the lower course of th e Lika ri vcr, and then into the The Kosinj reservoir area mainly consists of pennc­ Lipovo poljc and the periphery of Kosinjsko polje. The able Cretaceous carbonate rocks and the Jelar beds, so progression of the karstification process is manifested it is a hydrogeologically homogeneous terrain. There­ in changes of stream flow patcrns, the occurrence of fore, the hydrogeological conditions and water retaining intermittent springs, the presence of estavclles and characteristics are explained by zoning of the broader panors, and an increase of depth to groundwater. region surrounding the reservoir, according to the hyd­ The advance of thc karstification process from thc rogeological functions of the terrain (Fig. 3). sea to th e hinterland has affected the Bakovac valley in The Velebit hydrogeological barrier prevented dee­ its higher and middle parts. As a result, part of the wa­ per karstification of the transition region, which is refle­ ter drains beneath the valley floor through its northern cted by high g roundwater levels and the drainage of side directly into the sea. During heavy rainfall, when water towards the Lika river. Outside the area of barrier water flows in th e stream bed through the middle part influence, ponors and estavclles exist, and the streams of the Bakovac valley, the groundwater level there is are intennittent or "hang" above the deeper groundwa­ below the valley floor. From the activity of this ponor, ter levels. Under th ese conditions, in order to perform it can be concluded that th e groundwater level there is the studies, to evaluate the permeability of the broader always below the stream bed, which was confirmed area, and to plan the necessary intervening procedures, with measurements performed in the observation bore­ detailed hydrogeological zoning of the terrain recharg­ holc A-8. ing the reservoir (Fig. 6) was performed. The following The spclaeological phenomena situated along the [our zones with unique hydrogeological features were middle and lower course of the Lika river, and the regi­ outlined: on between the Lika and Gacka rivers occur at shalow The area with permeable surface conditions and depths (BOZICEVIC, 1965, 1969) which corresponds groundwater above the water level of the Lika river to the relatively high erosional base level encountered (1) covers a large part of the Kosinj reservoir bOllom therc. This is a result of the function of the Velebit full su rface. All the waters in this zone flow towards th e barrier. The development of the karstification process Lika river. Along the stream bed of the Lika river and can also be viewed in terms of sufficient sinking of the its tributaries Bakovac and Mlakvenski potok, several paleoplateau on the southeastern part of the Velebit bar­ permanent and intcrmittent springs exist. Tn this part of rier, in the cave area Cerovacke peeine along the south­ the terrain, only one ponor exists which drains the ern cdgc of the Graeacko polje (MALEZ, 1965). waters from the Mlakvenski potok. The tracing tests, performed in the borehole LG-5 on the northern bank of 3. GENERAL CONDITIONS FOR THE the reservoir, indirectly proved that the water of Mlak­ CONSTRUCTION OF THE KOSINJ RESERVOIR venski potok drains through Ponomc into the Lika river. In the terrain within this zone, all the waters either drain The construction of the Kosinj reservoir is planned towards the Lika or Gaeka rivers. for part of the lower course of the Lika river. The dam The impermeable Jelar beds (2) arc situated in the is to be built some 800 m upst ream from the bridge southwestern part of the terrain on which the Kruscica (Kosinjski most), in fact downstream from the existing reservoi r Kruscica. reservoir lies. The lelar beds in this part of the terrain In order to determine the hydrogeological condi­ contain breccia with clayey marl cement and lenses of tions that would allow the feasibility of reservoir con­ marl. The water retaining characteristics of the Krus­ struction, the area downstream from the reservoir cica reservoir is attributed to the favourable hydrogeo­ "Kruscica" in the valley of Lika river was also studied, logical features of th ~se deposits. as we ll as the broader area of reservoir recharge The zone of the uplifted tectonic block and the towards the Bakovac valley, Lipovo polje and the boun­ terrain under the larger influence of the now eroded dary zone towards the Gacka river. Detailed hydrogeo­ Neogene clay deposits (3) - on the map showing hyd­ logical mapping was performed on a scale of I :5,000 of rogeological provinces, this area is situated in the mid­ the local area and the margins of the reservoir. A better dle part, and the eastern margin of the Kosinj reservoir. understanding of tectonic conditions of both the imme­ This is also the water divide between the Lika and Gac~ diate and broader regions of reservoir recharge was ka rivers. The existence of this water divide is the con­ enhanced by Neotectonic studies of the same area sequence of the uplifted tectonic block Mlakvena (PRELOGOVIC, 1989). greda - Poijan, where reccnt karstification is fairly l'aviCic: Il ydrogeologic:II EV3lu:lIion of W:uer Rel3ining Propcnics oflhe Kosinj Reservoir (Li b. emalia) 297 shallow. The occurrence of Neogene marl deposits in reservoir. The reservoir is situated in terrain composed fossi l depressions, found in boreholes K-4-2 and K-9, or penneable limestone and limestone breccia (Creta­ has a certain impermeable role in this terrain. ceous and l elar deposit s). The karst terrain in the hin­ The boreholes revealed fi ssures and cavities con­ terland of the Velebit barrier is divided into a spring taining compressed clay, a feature also indicated by the lone, an estavelle zone and a ponor zone. Because the geophysical exploration performed along the route of location of the Kosinj reservoir is almos t complete ly the planned grout curtain. Spelaeological phenomena in situated within the spring zone, a basis for further st ud­ thi s zone occur at relat ively shallow depth and are ies exists, although this location was previously reje­ above the Kosinj reservoir backwater leveL cted. The groundwater level measurement s performed in The homogeneous karst terrain, largely composed the dri ll hole over a longcr period showcd that the gro­ of massive lelar deposits with relatively undefined rela­ undwater leve ls along the watcr divide towards the tionships with the Cretaceoll s deposits, required a spe­ Gaeka ri ver do not fall bellow 520 m a. s.l. (Fig. 6). The cific approach, so the studies were focused on th e struc­ precise position o r th e water divide was detennincd by tural and tectonic setting, the landscape development traci ng or water drainagc from the drill holes. and the karstification processes. The area in which the low waters are below the In order to evaluate the permeability of the broader Lika river stream flow, and the erosion base level is area and to plan th e necessary intervening procedures, the Adriatic sea (4) occupies the terrain north and detailed hydrogeological zoning in the area or reservoir northwcst fr om th e Kosinj reservoir. Downstream from influcnce was performed and several zones with similar the Kosinj dam is the Lipovo polje and it s recharge hydrogeological features were outlined. zone. In the upstream part of the Lipovo polje on its From the exploration boreholes it was possible to eastern and western slopes a st rin g o r intermittent spri­ obtain the data about groundwater levels, the character ngs and cstavetl es exisI. Ponors arc located in the and depth of karstification. By groundwater traci ng, the downstream part or th e polje and, through them, water flow directions and th e positions of groundwater from the Lika rivcr drains to the sec. The estavelle zone divides were defined. during the d ry season "passes" from the upstream part The Bakovac valley development is associated with o r the Lipovo polje into the area of th e Kosinj reservoir. th e incision of the Lika river into the Lika plateau as a The groundwater level decline below the level or the continuous process of th e recent karstification. This Lika ri ver stream bed was observed o n the western karstification incorporates the spelaeological phenome­ bank between thc Lipovo polje and Kosinjsko polje, na situated along the Kruscica dam and in the region where the 480 m water-table contour enters the area of between the Lika and Gaeka ri vers. T he performed th e reservoir. Ncar the borehole BK-4, the boundary studies confirmed the ex istence of karstification bene­ between the sp rin g zone and the estavell e zone is close ath the Lika ri verbed (the river is incised int o a previ­ to the reservoir. The groundwater level decline in this o usly karstified terrain). Neve rtheless, this docs no t reservoir bank waS also observed in the borehole BK-2. diminish the hydrogeological importance of th e Lika In the Bakovac vall ey this boundary is upstream from ri ver as a recent erosional base level, because the o lder th e position of the dam. subsurrace systems and groundwater flows were inter­ Hydrogeological zoning (Fig. 6) is the basis for Ihe rupted by Neotectoni cs, or because the drainage chan­ understanding of hydrogeological conditions and the nels arc clogged with clay materi al. evaluation or the water retaining properti es of the Kos­ The results of th e studies confirmed that th e recent inj reservoir; it also allowed the separation of imperme­ karstification , with which the present water dynamics is able areas from th e permeable ones, th e definition of associated, is relatively shallow in the Kosinj reservoir locations for the construction of dams and the identifi­ area. Therefore, it is possible to achieve an impermea­ cation of banks which have to be made impermeable by ble reservoir with the aid of additional construct ion grouting. work. FUrlher studies (drilling exploration, geophysical exploration, detail ed study of Neogene marls in depres­ sions on the East reservoir bank) should more precisely 5. REFERENCES determ in e th e conditions that will enable the constru­ ction of th e reservoi r, i.e. produce th e data necessary BAH UN, s. (1962): Vapnenei Promina-naslaga 1I pod­ for the construct ion of the grout curtain. rucju Kruscice u Lic! (Limestones in thc Prom ina Deposi ts of the territory of Kruscica in Lika.).­ Geo/. vjesnik, 15/1 , 101 -106. 4. CONCLUSION BAH UN. S. (1973): Odnos krskog proeesa i Ollvijalnc The necessary studies performed for the constru­ erozije u poclrucju Like (Relationship between kars­ ction of a sufficiently impermeable reservoir in a karst tification and fluvi atil e erosi on in the region of terrain, for a big hydroelectric plant, arc presented Lika, cenlral Croatia).- Krs JlIgOS/. , 8/5, 91 - 100, throug h th e results obtained in th e case of th e Kosinj Zagreb. 298 Geolog ia Croatica 50/2

BAHUN, S. ( 1974): Tcktogencza Vclebita i postanak .- Zastita izvorista voda za vodoopskrbu, 255- Jelar· naslaga (The tectogenesis of Mt. Velebit and 262, Split. the formation of l elar deposils).- Oeol. vjesnik, 27, HERAK, M. (1957): Geoloska osnova nekih hidrolo· 35·51, skih pojava u dinarskom krsu (Gcologischc grundla­ BAHUN, S. (1984): Tectonic and hydrogeological sig· gen e iniger hydrogeologischen Erschinungen in nificance of the areas composed of the lelar forma­ Dinarisehen Karst).· II. kongr. geo!. Jugos!. , 523· tion (Tektonsko i hidrogeo!osko znacenj e podrucja 539, Sarajevo. izgrad enih od Jelar· fonnacija). · Krs Jugosl., 11/1, 1· HERAK, M. (1986): A new concept of geotectoni cs of 11, Zagreb. the Dinarides.· Acta geoL, 16/1, 1-42, Zagreb. BAHUN, S. (1990): Stupnjevi razvoja zaravni u dinar· HERAK, M. ( 1991): Dinaridi. Mobilistieki osvrr na skorn krsu (Stages of formation of the plains in the genezu i strukturu (Dinarides. Mobilistic view of the Dinaric karst).· Krs Jugosl. 12/6, 147·158, Zagreb. genesis and strukrure).· Acta geoL, 2 1/2, 35· 117, BAHUN, S. & FRITZ, F. (1975): Hidrogeoloske speci· Zagreb. ficnosti Jelar-naslaga (Hydrogeological properties HERAK, M. & BAH UN, S. (1980): The rol e of th e eal· of Jelar deposits, Lika, Croatia.).· Geol. vjesnik, 28, carous breccias (Jelar-formalion) in the tectonic 345· 355. interpretation of th e hi gh Karst zonc of thc Dinar­ BAI-TUN, S. & FRITZ, F. (1987): Postanak izvora u ides.· GeoL vjesnik, 31, 49·59. d in arskom orogenskom akumuliranom krsu (The MALEZ, M. (1965): Cerovaeke peci ne (Die Hohlen origin of springs in Dinaric orogenic acc umulated von Cerovac).- Spe leol. drustvo Hrvat ske, 1, 1-44, karsl.) . Krs Jugosl. 12/2,27·37, Zagreb. Zagreb. BOZICEVIC, S. (1965): Poljakova pecina.· Geol. vjes· PA VICIC, A. (1984): Geoloska osnoya Veicbitske raz· nik, 18/1, 141·157. vodnice (Geological foundation of the Vele bit waler BOZICEVIC, S. (1969): Horvatova pecina uz branu divide, Croalia).- 8 . Jugos!. simp. hidrogeol. in z. Sklope (The Horvat cave at the Slope dam).· Geol. geoL, 1,537·547, Budya. vjesnik, 22, 501·510. PAVICIC, A. ( 1995): Hidrogeoloski uvjeti za ostyare· FRITZ, F. (1972): Razvitak gornjeg toka rijeke Zrman· nj e akumulacije u krsu zaleda Velebita (Hydrogeo· jc (Morphological evolution of the upper logical conditions [or the construction of reservoirs course).· Krs Ju gosl., 8, 1· 16, Zagreb, in the Velebit Ml. hinterland, Croatia).· Unpub· lished PhD Thesis, Uni versity of Zagreb, 11 3 p. FRITZ, F. (1991): Utjeeaj reeentnog okrsavanja na zah· vacanj e voda (The influence of the recent karst ifica­ PA VICIC, A. & FRITZ, F. ( 1976): l-lidrogeologija sir· tion on wat er cxtraction).- Geol. vjesnik, 44, 281- eg podrueja donjeg toka Like (Pazariste, Krs, 288. Otoeae) (Hydrogeology of the lower Lika river region (Pazariste - Krs - Otocac».- 4. Ju gos!. simp. FRITZ, F. (1992): Effect of recent sea level change on hidrogeoL inz. geoL, 1,291·304, Skopje. th e development of Karst phcnomena. - Proc. Intern. symp. "Geomorphology and sea" and Meeting Geo­ PAVLlN, B. (1970): Kruseica storage basin in the cav· morpho comm. Carpatho-Balcan countries, Mali ernous Karst area.- Comision Intcrnationale des Losinj, 85-92, Zagreb. Grandes Barrages. Dixieme Congres des Grands Barrages, 209·224, MontreaL FRITZ, F. & PAVICIC, A. ( 1975): Tektonski odnosi u podrucju razvoja krednih i Je lar naslaga kod Kosi­ PRELOGOVIC, E. (1989): Neotektonski pokreti u pod· nja u Lici (Tectonical pattern o[ Cretaceous and Je l­ rucju sjevernog Velebita i dijela Likc.- Geol. vjes­ ar deposits at Kosinj , Lika, Croatia).- Geo!. vjesnik, nik,42, 133·147. 28, 35-42. FRITZ, F. & PAVICIC, A. (1987): 0 problemu zastit· nih zona u krsu - primjer problematike zastite izvora

Manuscript received April 14, 1997. Revised manuscript acccpted November 10, 1997.