The possible negative consequences of underground dam and ... dam in coastal areas...2042 T. Roje-Bonacci and O. Bonacci: Underground dam in coastal karst catchment area is the starting

Nat Hazards Earth Syst Sci 13 2041ndash2052 2013wwwnat-hazards-earth-syst-scinet1320412013doi105194nhess-13-2041-2013copy Author(s) 2013 CC Attribution 30 License

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The possible negative consequences of underground dam andreservoir construction and operation in coastal karst areasan example of the hydro-electric power plant (HEPP) Ombla nearDubrovnik (Croatia)

T Roje-Bonacci and O Bonacci

Faculty of Civil Engineering Architecture and Geodesy Split University 21000 Split Matice hrvatske 15 Croatia

Correspondence toO Bonacci (obonaccigradsthr)

Received 5 April 2013 ndash Published in Nat Hazards Earth Syst Sci Discuss 19 April 2013Revised 8 July 2013 ndash Accepted 9 July 2013 ndash Published 15 August 2013

Abstract The Ombla Spring represents a typical abundantcoastal karst spring located in the vicinity of the town ofDubrovnik (Croatia) Its outlet is at an altitude of 25 mabove sea level (m asl) and the water from it immediatelyflows into the Adriatic Sea The minimum and maximummeasured discharges are 396 m3 sminus1 and 117 m3 sminus1 re-spectively The Trebišnjica River traverses through its catch-ment The mean annual discharge after the canalization ofover 60 km of its watercourse with spray concrete (in thetime span 1981ndash2011) is 2405 m3 sminus1 Before massive civilengineering work which took place during 1968ndash1980 themean annual discharge was 2835 m3 sminus1 There is a projectfor construction of the hydro-electric power plant (HEPP)Ombla which will exclusively use groundwater from theOmbla Spring karst aquifer The underground dam will beconstructed about 200 m behind the existing karst spring out-flow in the karst massif by injecting a grout curtain Thetop of the grout curtain is planned to be at an altitude of130 m asl This karst system is complex sensitive vulner-able and ecologically extremely valuable The grout cur-tain as well as the HEPP Ombla development could leadto extremely dangerous technical and environmental conse-quences In this paper some probable negative consequencesof the HEPP Ombla construction and development are ex-plained The HEPP Ombla could result in many large andhard-to-predict negative consequences which are specific forthis particular HEPP for example (1) severe spring dischargechange (2) unpredictable regional groundwater redistribu-tion (3) threatening of endemic fauna (4) induced seis-micity (5) induced sinkholes (6) occurrence of landslides

(7) conflict regarding internationally shared karst aquifers(8) intensification of karst flash floods (9) sea water intru-sion in coastal karst aquifer etc

1 Introduction

Natural as well as anthropogenic works in karst areas causestrong sudden hardly predictable and mostly dangerous pro-cesses with immeasurable consequences for the environmentand social systems (eg Beck 1984 1989 2005 Bonacciand Jelin 1988 Drew and Houmltzl 1999 Breznik 1998 Mi-lanovic 2000 2002 Beck and Herring 2001 Bonacci 2004Waltham et al 2005 Parise and Gunn 2007 Bonacci et al2009 etc)

Karst is defined as a terrain generally underlain by lime-stone or dolomite in which the topography is chiefly formedby the dissolution of rock and which is characterized bysinkholes sinking streams closed depressions subterraneandrainage and caves A wide range of closed surface depres-sions a well-developed underground drainage system andstrong interaction between the circulation of surface waterand groundwater typify karst

Diversity is considered the main feature of karstic sys-tems which are known to change very fast and in a hardlypredictable manner over time and in space so that an in-vestigation of each specific system is required In karst ter-rains groundwater and surface water constitute a single dy-namic system The hydrocomplexity of karst regions is enor-mous The determination of catchment boundaries and the

Published by Copernicus Publications on behalf of the European Geosciences Union

2042 T Roje-Bonacci and O Bonacci Underground dam in coastal karst

catchment area is the starting point in all hydrologic anal-yses and one of the essential pieces of information whichserve as the basis for all hydrologic calculations In karstterrains this is always a very complex task (Gunn 2007)Groundwater exchanges with adjacent aquifers through un-derground piracy routes or via inflows from surface streamsConsequently only in exceptional cases do surface (topo-graphic or orographic) and subsurface (hydrologic or hydro-geologic) catchment boundaries coincide The problem is ad-ditionally complicated by the time-variant hydrologic bound-aries which are dependent on fluctuations of groundwaterlevels (GWLs) (Bonacci 1987 2002 Kresic and Stevanovic2010)

Any kind of engineering action in a karst terrain presentsnumerous challenges to the engineer including the unpre-dictable occurrence of cavities and well-developed hydraulicconduits which could lead to sinkhole development The sizeand shape of a cavity in limestone depend on the interactionbetween the characteristics of geological structures such asinternal fractures or discontinuities and water activity

Human intervention especially the construction of damsand reservoirs realized by creating large grout curtains(Bonacci et al 2009) as well as interbasin water transfersthrough long tunnels and pipelines can introduce instanta-neous and distinct changes in catchment areas and bound-aries and in turn on the hydrological hydrogeological andecological regimes (limitation of energy flow negative im-pacts on food cycle blocking of convections between habi-tats and species pollution of groundwater and undergroundenvironment etc) (Bonacci 2004)

In recent times there have been many high disturbances of-ten caused by human activities in karsts North et al (2009)gave interregional comparison of karst disturbance in west-central Florida and southeast Italy while van Beynen etal (2012) discussed a sustainability index for karst environ-ment Both papers deal with indices for evaluating impactsand sustainability in karsts and are therefore closely relatedto the issue dealt with here

The project for construction of the hydro-electric powerplant (HEPP) Ombla will exclusively use groundwater fromthe Ombla Spring karst aquifer The underground dam willbe constructed about 200 m behind the existing karst springoutflow in the karst massif by injection of a grout curtain

The main goal of this paper is to encourage further de-tailed interdisciplinary measurements and analyses in orderto understand the possible negative consequences caused bythe construction and development of the HEPP Ombla Theintention of the paper is to identify potential problems and tosolve them before they occur in complex and poorly inves-tigated karst environments The HEPP Ombla system is in-ternationally shared between Croatia and Bosnia and Herze-govina Due to this reason one of the goals of the paper is topoint out the specific characteristics of the karst aquifer andits water management which can represent a real trigger forpossible conflicts between the two neighbouring states

22

Figure 1 Fig 1Location map indicating the study area the site of the OmblaSpring with its supposed catchment area the Zaton Spring the Tre-bišnjica River the boundary limit between Croatia and Bosnia andHerzegovina as well as the position of existing dams reservoirs andHEPPs

The HEPP Ombla example apart from being an interest-ing case study can be valuable for many other karst areasworldwide where engineering works and construction aretaking place especially those based on grout curtains Thispaper focuses on the critical issues and priorities needed toaddress karst region risks and governance for disaster pre-vention

2 Short description of the natural characteristics ofOmbla Spring and its catchment

Figure 1 represents a map of the study area the site of theOmbla Spring with its supposed catchment area the ZatonSpring the Trebišnjica River the boundary between Croatiaand Bosnia and Herzegovina as well as the position of exist-ing dams reservoirs and HEPPs The photograph presentedin Fig 2 shows the Ombla Spring from an areal view

The catchment area and boundaries shown in Fig 1 havebeen determined by using only geological data without ade-quate hydrogeological data The catchment area is estimatedto be about 600 km2 (Milanovic 1996 Sever 2003) Ac-cording to the authorrsquos investigations which were based onwater-budget analyses the catchment area is much larger andranges between 900 km2 and 1000 km2 (Žugaj and Bonacci1994)

Nat Hazards Earth Syst Sci 13 2041ndash2052 2013 wwwnat-hazards-earth-syst-scinet1320412013

T Roje-Bonacci and O Bonacci Underground dam in coastal karst 2043

23

Figure 2Fig 2Areal photograph of the Ombla Spring and surrounding area(taken by Antun Maškaric)

The geological setting is mostly composed of highlypermeable Jurassic limestone poorly permeable Triassicdolomite and Eocene flysch which represent an imperme-able hydrogeological barrier (Buljan et al 2000 Sever2003 Paviša 1998 2003) The Quaternary sediments coverthe bottom of the numerous poljes in the karst They rep-resent low-gradient features in the landscape As a conse-quence of intense tectonic activity these poljes have beenformed as terraces from an altitude of 1000 m asl to thesea level From the hydrological and hydrogeological pointof view the poljes function as part of a wider system Theyrepresent more-or-less interconnected subsystems within theprocess of surface and groundwater flow through the OmblaSpring karst catchment The basic structural and stratigraphicunits are elongated in a NWndashSE direction which follows theextension of the Dinarides

About 90 of the whole Ombla Spring catchment is cov-ered by limestone Because of its high secondary and tertiaryporosity the water infiltration takes place very rapidly TheTrebišnjica River traverses the Ombla Spring catchment (seeFig 1) and loses surface water through numerous sinks lo-cated along the bottom of this section which is longer than60 km thus recharging the Ombla Spring In order to reduce

losses more than 60 km of the Trebišnjica River watercoursebottom have been covered with spray concrete The maxi-mum discharge capacity of this canal is about 50 m3 sminus1 Theaverage annual discharge of the Trebišnjica River in its nat-ural state was about 68 m3 sminus1 It should be noted that theriverrsquos natural regime has strongly changed due to the BileceReservoir operations and the HEPP Trebinje development

The climate of the Ombla Spring outlet measured at theDubrovnik climatological station is typically MediterraneanIn other parts of the catchment the climate changes as a func-tion of the distance from the Adriatic Sea coast moving fromthe Mediterranean to the continental and mountainous areas

The average annual rainfall exceeds 1500 mm result-ing from intensive convective precipitation of short dura-tion through the entire year but particularly in October andNovember About 70 of the total annual rainfall appearsduring the cold period of the year from October to AprilDuring thunderstorms intense precipitations of more than10 mm hminus1 occurs on average twice a year (Bonacci 19952004 Bonacci and Roje-Bonacci 2000)

The average annual air temperature measured inDubrovnik (1961ndash2012) is 165C During the last twodecades the annual average air temperature in Dubrovnikwas 07C higher than in the previous subperiod (1961ndash1990) This statistically significant trend of an increasein air temperature is found throughout the whole broaderregion The average annual air temperature over the wholecatchment is assessed to be about 13C

The average annual water temperature measured at theoutlet of the Ombla Spring (1962ndash2010) is 130C In thecase of water temperature no trend of increase or decreasehas been found

The minimum measured discharge of the Ombla Spring is396 m3 sminus1 Maximum measured discharges never exceededa value of 117 m3 sminus1 despite the fact that the spring catch-ment area is about 1000 km2 and precipitations are very in-tensive and abundant The mean annual discharge after canal-ization of the Trebišnjica River watercourse with spray con-crete is 2405 m3 sminus1 Before this massive civil engineeringwork which took place during 1968ndash1980 the mean annualdischarge was 2835 m3 sminus1 The Ombla Spring is a typicalkarst spring with a limited outflow capacity (Bonacci 2001)Despite frequent and intense precipitation falling over itslarge catchment the maximum possible discharge from theOmbla Spring is lower that 120 m3 sminus1 The possible rea-sons for this limitation on its maximum flow rate can bethe following (1) limited size of the karst conduit (2) pres-sure flow (3) inter-catchment overflow (4) overflow fromthe main spring-flow system to intermittent springs withinthe same catchment and (5) water storage in the zone abovethe karst aquifer (Williams 1983 Bonacci 2001)

The GWL changes extremely rapidly (up to 100 m in 24 h)and the total difference between the maximum and the mini-mum GWL in one piezometer can reach up to 200 m (Paviša1998 2003) The maximum measured GWL rising intensity

wwwnat-hazards-earth-syst-scinet1320412013 Nat Hazards Earth Syst Sci 13 2041ndash2052 2013


was 292 m per hour The maximum measured GWL fallingintensity does not exceed a value of 2 m per hour (Bonacci1995)

The regime of suspended sediment which outflows fromthe Ombla Spring outlet was measured from 2003 to2010 The annual quantity of sediment varies between88times 103 t yrminus1 and 474times 103 t yrminus1 Most of the sediment(more than 90 of whole annual amount) is delivered dur-ing the intensive short rainfall in a few hours (Denic-Jukicet al 2012)

3 Underground dam and the HEPP Ombla

In karsts laminar and turbulent flow exist at the same timeand management and development of karst undergrounddams is very specific and different than in other more ho-mogeneous media In the first part of this section some in-formation about underground dams in karst regions will begiven In the second part the main characteristic of the HEPPOmbla underground dam will be explained

An underground dam is a facility that stores groundwa-ter in the pores of strata Recently the concept of the un-derground dam has spread and many have been constructed(Ishida et al 2011) Only few of the recently built under-ground dams have been realized by using grout curtains Thegrout curtain height typically ranging from 10 to 40 m

The most underground karst underground dams havebeen constructed in China Carbonate rocks cover about450 000 km2 of the Chinese territory mostly located in thesouthern tropical and subtropical region All undergroundreservoirs are constructed by clogging large karst conduitsand underground cave rivers Their storage capacity is rathersmall therefore water is practically only stored in the con-duits and caves (Lin Hua 1989)

During the 1990s the Japan Green Resources Agency con-structed subsurface dams in a limestone aquifer on Miyako-jima Island located in Japanrsquos subtropical climate zone Theproject was completed in 2001 The dams are composed ofa cut-off wall with a maximum height of 50 m by which thegroundwater flow is dammed and the intrusion of sea water isprevented (Ishida et al 2003 2011) The cut-off walls werebuilt using the in situ churning method For irrigation pur-poses the groundwater is drown up by 147 tubes and storedin farm ponds (water tanks) until it is supplied through thepipeline systems that serve the entire island In the Ryukyulimestone of Miyakojima Island only small caves severalcm in diameter exist

Gilly and Mangin (1994) described field investigationsmade in order to create an underground reservoir in Franceby closing up the outlet of a small karst spring This projecthas not been realized so far

The HEPP Ombla underground dam will be constructedabout 200 m behind the existing spring outflow in the karstmassif (Fig 3) by injecting a grout curtain The projected

24

Figure 3

Fig 3 Location map indicating the position of the projected groutcurtain five piezometers (P6 P8 P9 P14 and P18) the data ofwhich are used in this article and the adjacent karst springs (Za-ton Ombla Slavljan and Zavrelje)

total area of the curtain is about 300 000 m2 The total lengthof the curtain will be about 1470 m while its maximum depthwill be 410 m Figure 4 shows a cross-section of the projectedgrout curtain (underground dam) (modified after Ravnik andRajver 1998 Sever 2003) The top of the grout curtain isprojected to be at an altitude of 130 m asl It is presumedthat the grout curtain will be founded on an impervious flyschbarrier The grout curtain is planned to be constructed fromthree grouting galleries set up at different elevations (Fig 4)

The installed discharge of the HEPP Ombla is 60 m3 sminus1

(Sever 2003) Four generating units will be located in thepower house Two of them will have Frances turbines with arated discharge of 24 m3 sminus1 and 30 MVA synchronous gen-erators The other two generating units will have a rated dis-charge of 6 m3 sminus1 and 8 MVA synchronous generators Ithas been assessed that the total HEPP Ombla rated powerwill be 68 MV with a mean annual output of about 225 GWhThe bottom outlet has a multipurpose role Besides function-ing as the bottom outlet it will be used as an emergencyspillway and turbine discharge regulator as well as for wa-ter evacuation during and after the construction of the HEPPOmbla Its maximum discharge capacity will be 113 m3 sminus1

4 Possible negative consequences of the HEPP Ombladevelopment

Milanovic (2002) has cited the following negative conse-quences of human activities and engineering construction inkarst regions (1) severe spring discharge change (2) ground-water quality deterioration (3) threatening of endemic fauna(4) waste disposal failures (5) induced seismicity and (6) in-duced sinkholes Bonacci (2004) has added the follow-ing hazardous consequences (1) occurrence of landslides(2) intensification of floods and droughts (3) regional water



25

Figure 4

Fig 4Cross-section through the projected grout curtain (underground dam) (modified after Ravnik and Rajver 1998 Sever 2003)

redistribution (4) conflict regarding internationally sharedkarst aquifers (5) water losses from reservoirs (Bonacci andRubinic 2009 Bonacci and Roje-Bonacci 2012) and (6) in-stability of dams built in karsts (Dreybrodt et al 2002) Aspreviously stressed in karst terrains the relationship betweensurface water and groundwater is very closely connected andinterrelated

The construction and development of the HEPP Omblacan result in many large and hard-to-predict negative conse-quences Some of them are specific for this particular HEPP

It is not possible to determine the volume and extent of theunderground reservoir as well as its functioning The cru-cial question until now unsolved is to what extent it will bepossible to manage the water from a completely undergroundkarst reservoir in which more than 95 of the groundwater isstored in small karst fissures and joints where diffuse laminarflow prevails (Boumlgli 1980 Milanovic 1981 Bonacci 1987Ford and Williams 2007) The natural behaviour of the karstaquifer through the Ombla Spring will be drastically changedby operation of the HEPP Ombla but is not easy to predictin what manner

41 Changes in water regime

The Ombla Spring mean annual discharges will be dimin-ished because of increases in the GWL over the whole karstaquifer It will cause more water overflow from the OmblaSpring catchment to other surrounding permanent and tem-porary karst spring catchments Due to increasing of theGWL caused by grout curtain construction some temporarykarst springs in the local Ombla Spring catchment area couldbecome permanent In some cases these intermittent springsmight remain intermittent but their mean discharges couldbe higher For the same reason minimum spring dischargesmay be magnified The maximum discharges may be magni-fied

26

Figure 5 Fig 5 Relationship between hourly Ombla Spring discharges andthe GWL measured in piezometer P8 (modified according Bonacci1995)

Figure 5 represents the relationship between the hourlyvalues of the GWL in piezometer P8 (Fig 3) GWL P8 inm asl and the respective discharges of the Ombla SpringQOMBLA in m3 sminus1 (Bonacci 1995) It is of crucial impor-tance to notice that the GWL stagnates in the period when thehydrograph increases from 932 m3 sminus1 to 549 m3 sminus1 (over11 h) The intensive rainfall which caused this hydrographstarted 2 h earlier This phenomenon can be explained by thefact that during 11 h large karst conduits were filled andflow which was under pressure was formed only in the largekarst conduits and caves where turbulent flow is present Atthis time the small karst fissures in which diffuse laminarflow prevails were not filled by water (Bonacci 1995) If thisassumption is correct it is possible to determine the volumeof the large caverns to be about 15times 106 m3 The relation-ship between GWL P8 andQOMBLA differs during the risingand descending parts of the hydrograph The formation ofthis loop can be explained by the different influence of tur-bulent and laminar flow



27

Figure 6

Fig 6 Time data series of hourly GWL measured by piezometersP8 and P14 during high water caused by intensive precipitation

Because of the much higher GWL caused by the groutcurtain increase in the number and duration of karst flashfloods in the area of Mokošica (Fig 3) a densely popu-lated suburb of Dubrovnik can be expected High water dueto emptying of the underground reservoir will cause higherflood levels a likely cause of destruction At the same timeit is realistic to expect the occurrence of landslides alongthe Mokošica flysch slopes The calculation of the Mokošicaarea slope stability had been performed with GWL at an al-titude of 100 m asl The expectation of landslide occurrenceis based on comparison with the Vajont landslide (Paronuzziand Bolla 2012) due to similarity of their geological set-tings The Mokošica low permeable slope is built by Eoceneflysch layers which spread over the limestone and dolomiteThe flysch layers can be influenced by high uplift caused byfast GWL rising The siltstone layers in the flysch are verysoft rocks and sensitive to water content In combinationwith low effective stresses and changes of its water contenttheir shear resistance can be reduced

It is very difficult to work out a reliable hydrological-hydrogeological-hydraulic model of the functioning of theunderground karst reservoir In many cases such as in the op-eration of the HEPP Ombla this task is not possible becausethe exact boundaries of the catchment are not known It is ob-vious that they change over time being dependent on the fastand unknown changes of the GWL in this large dynamic andinadequately investigated complex karst aquifer The prob-lem is that only a few active caves and conduits in the vicin-ity of the spring outflow are partly known No other largeunderground karst space has been investigated well enough

One good example of the fast and unexpected changes ofthe GWL in natural conditions in the Ombla Spring karstaquifer can be seen in the graph provided in Fig 6 where twotime data series of hourly GWL measured by piezometers P8and P14 (Fig 3) during high water are represented At thesame time this representation can be used to explain the in-fluences of the GWL changes on catchment boundary move-ment and the overflowing of groundwater from the Ombla

28

Figure 7

Fig 7 Relationship between hourly discharges of the Ombla andZaton springs (modified according Bonacci 1995)

Spring to other neighbouring karst springs shown in Fig 3Two piezometers which are not very far from each other reactvery differently to the same precipitation The GWL in P14 isgenerally higher than the GWL in P8 This means that duringthis period groundwater from P14 flows to the Ombla Springaquifer In the case when the GWL in P8 is higher than theGWL in P14 the overflow of groundwater from the OmblaSpring to the Zaton Spring (Figs 1 and 3) appears This over-flow started when the GWL in both piezometers reached analtitude of 74 m asl and finished when both were at an alti-tude of 71 m asl

Figure 7 shows the relationship of the simultaneous hourlydischarge of the Ombla and Zaton springs It can be seen thatthe discharge of the Zaton Spring suddenly increased whenthat of the Ombla Spring exceeded values from 60 m3 sminus1 to70 m3 sminus1 The Zaton Spring has a maximum capacity be-tween 5 m3 sminus1 and 6 m3 sminus1 of the water discharging fromits own catchment Discharges above these values result fromthe overflow of water from the Ombla Spring catchment It isnecessary to notice the different behaviour in the relationshipanalysed in Fig 7 during the rising and descending section ofthe GWL hydrograph Bonacci (2004) showed that the over-flow from the Ombla Spring catchment to the Zaton Springcatchment started when the GWLs at P9 P6 and P18 reachedaltitudes of 75 m asl 135 m asl and 165 m asl respec-tively Piezometer P9 is only 14 km away from the pro-jected grout curtain This means that when the GWL in theunderground reservoir will be higher than about 70 m asloverflow from the Ombla Spring catchment to the ZatonSpring catchment (and maybe in some other neighbouringkarst springs) will occur

42 Influence of the grout curtain

In this subsection only the possible negative technical as-pects of the grout curtain will be discussed Grouting inkarst terrains represents the injection of appropriate materi-als under pressure into rock through drilled holes to change



29

Figure 8

Fig 8Cross-section through the system of explored karst caves and large conduits in the vicinity of the Ombla Spring indicating the locationof some key parts of the projected HEPP Ombla grout curtains and concrete blocks (modified after Ravnik and Rajver 1998 Sever 2003)

the physical characteristics of the karst formation Its con-sequences should be the sealing of all kinds of karst voids inorder to make the injected part of the karst massif less perme-able and stronger From the engineering point of view grout-ing essentially consists of the waterproofing of a rock masswhere there is a network of karst conduits cracks joints fis-sures and excessive fragmentation (Nonveiller 1989)

Injection pressure should not exceed the overburden stressbecause lifting on the horizontal planes could occur In prac-tice the injection pressures used often exceed the maximumrecommended values

Figure 8 shows a cross-section through the system of ex-plored karst caves and large conduits in the vicinity of theOmbla Spring indicating the location of some key parts ofthe projected HEPP Ombla the grout curtains and the con-crete blocks (modified after Ravnik and Rajver 1998 Sever2003) Attention should be drawn to the fact that three knownlarge active karst conduits will be clogged by concrete pack-ers The designer is aware that it is not possible to plugsuch large karst cavities by injection The main problem isthat there might be additional active or large fossil cavi-ties Milanovic (2002) pointed to the problem that high non-homogeneity in the permeability of karsts makes it a mostdifficult target for grouting Particular problems which canarise during grouting are zones of concentrated undergroundflow

In the case of the grout curtain constructed at the ETHaleReservoir (Cetina River in Dinaric karst of Croatia) Bonacciand Roje-Bonacci (2012) showed how quick and serious itsimpact was on the groundwater regime in karsts

It is not possible to prevent water leakage from grout cur-tains especially over long periods This is especially impor-tant in the case of the HEPP Ombla grout curtain with its

maximum height of 410 m When the grout curtain is finishedthe upgradient of the curtain is increased to levels which havenever been previously realized This increased pressure even-tually results in ldquoclay plugsrdquo being expelled from some ofthe cavities adjacent to where the grout penetrates openingup new cavities which might gradually enlarge as effectiveunderflow conduits The general conclusion is that the largehydraulic gradient imposed by the reservoir accelerates leak-age These high hydraulic gradients accelerate by orders ofmagnitudes through the dissolutional widening of fracturesand bedding planes (Dreybrodt et al 2002 Romanov et alm2003 Turkmen 2003 Karimi et al 2007 Rogers 2007Unal et al 2007 Kaufmann and Romanov 2008 Bonacciand Rubinic 2009 Hiller et al 2011 Bonacci and Roje-Bonacci 2012 Zubac and Boškovic 2012)

Under the pressure of the underground reservoir head in-ternal erosion of the cavity infilling material creates pipingWater rinses filling of the karst features and widens open-ings which results in an increasing volume of lost wateran increase in its velocity and higher erosive potential Ul-timately this can result in the formation of collapse sink-holes In Fig 4 two large karst conduits are shown Ravnikand Rajver (1998) discovered them by measuring ground-water temperatures in 30 deep piezometers drilled from aninvestigation gallery (gallery 1 in Fig 4) These two karstconduits are active Nobody knows how many fossil and in-active karst conduits there are in this space Some of thesecould be reactivated due to the influence of high hydrostaticpressure which will be formed by the grout curtain Regard-ing the functioning of the HEPP grout curtain Ravnik and Ra-jver (1998) have stressed that ldquoThe positions and dimensionsof all the basic structural elements of this power plant havenow after 20 yr of interrupted investigations been outlined



However one very important problem remained unsolvedconcerning the degree of impenetrability of the planned un-derground dam and especially of its lower part which is lo-cated inside the rock massif below sea levelrdquo

As the Ombla Spring is a coastal karst spring there is afurther likely problem About 25 000 yr ago the Adriatic Sealevel was 964 m lower than at present (Šegota 1968) thusrepresenting the karst base level Due to this reason thereare many submarine springs along the Croatian Adriatic Seacoast In the case of the currently active Ombla Spring thepossibility of inactive and blocked fossil conduits which areexpected to open under the influence of the new and muchhigher (than in its natural state) hydrostatic pressure causedby the grout curtain (underground dam) construction cannotbe excluded

Regarding the operation of the HEPP Ombla grout curtainit is of special importance to point to the case of the groutcurtain of the Gorica Reservoir on the Trebišnjica River (seeFig 1) which is constructed in the same catchment and ge-ological setting as the HEPP Ombla at a distance of about20 km The loss of water from the Gorica Reservoir has a ten-dency toward constant growth which creates a huge problemfor water management in Trebišnjicarsquos HEPP system Zubacand Boškovic (2012) propose building a new grout curtainwhich should restore the existing degraded one which wasbuilt in 1981 only 33 yr ago This case indicates a probablefate of the HEPP Ombla grout curtain which is at least fivetimes higher

The construction of the HEPP Ombla underground damwill definitely destroy the natural equilibrium between saltsea water and fresh water The grout curtain will preventfresh groundwater flow from the Ombla Spring aquifer tothe downstream coastal area Because of this sea water willpenetrate the local area downstream of the grout curtain Theconsequences of this process should be investigated

43 Some other possible negative consequences andproblems

The questions are as follows (1) what will be the spreadof the underground reservoir under different GWL condi-tions (2) What will be the maximum GWL altitudes innew conditions Problems can be expected when intensiveand abundant precipitation falls on the catchment when theunderground reservoir will be full In this case the GWLcan quickly rise more than 100 m It will cause not onlythe overflow of groundwater into neighbouring permanentand temporary karst spring catchments but also the over-flow of groundwater downstream and upstream of the under-ground dam where all the installation of the HEPP Omblaare planned The rising of the GWL is a very rapid pro-cess which will not be possible to control efficiently by theHEPP Ombla operation (controlled releasing of water fromthe reservoir) as is possible in the case of a classic HEPP withsurface reservoirs

30

Figure 9 Fig 9Photograph of GWL overflow through three fossil karst con-duits on 22 October 2010

The photograph in Fig 9 shows three torrential streamswhich formed on 22 November 2010 as a consequence ofheavy intense rainfall They outflowed from three fossilcaves where temporary karst springs were produced by arapid rise in the GWL These torrential streams as well asmany other smaller GWL breakthroughs from the karst mas-sif in the broader area around the Ombla Spring flooded thesuburb of Dubrovnik Mokošica causing serious damages

At the same time it is realistic to expect the occurrenceof landslides along the Mokošica steep and unstable flyschslopes part of which are urbanized

A higher-than-the-natural-state GWL caused by the groutcurtain will diminish the vadose zone and increase thephreatic zone The regime of suspended sediment will be def-initely altered but it is hard to assess to what extent Most ofthe sediment will be separated through sedimentation in thekarst matrix upstream of the grout curtain The suspendedsediment (mostly clay ndash terra rossa) will close the karst frac-tures fissures and joints of the limestone and dolomite in theformer vadose zone Hydraulic conductivity in most of thekarst matrix will decrease capacity of the underground reser-voir diminish and groundwater circulation change

It is still a challenge to develop underground construc-tions in limestone areas where the possibility of sinkholesand subsidence events always exists (Parise and Gunn 2007)Identifying and predicting karst cavity networks which couldcause sinkholes and subsidence on the ground surface wouldhelp engineers overcome this challenge (Waltham et al2005)

Dynamic interplay between sea water and flow discharg-ing from the Ombla Spring in the Rijeka Dubrovacka has notbeen investigated sufficiently so far It is known that duringlow discharges (until about 10 m3 sminus1) fresh water flows oversea water without mixing For higher discharges fresh springwater displaces sea water along different long sections of the



Rijeka Dubrovacka (see Fig 3) For greater discharges thesesections are longer but to what extent has not been yet in-vestigated It should be stressed that in the natural state dur-ing high water severe floods appear regularly once to twiceyearly The operation of the HEPP Ombla may aggravate thisdangerous situation

One of the greatest problems for conducting reliable anal-yses or modelling hydrological-hydrogeological processes inthe analysed catchment is that the exact catchment boundaryand area are not defined The heterogeneity and anisotropyof the study area are extreme and not well understood Forthis reason modelling the groundwater behaviour representsa great challenge Modelling water circulation in the OmblaSpring catchment requires a mixture of surface water con-cepts and groundwater concepts Karst hydrology and hy-drogeology need all kinds of models and modelling as wellas the new scientific approaches methods and technologiesAt the same time one should be profoundly aware that theyare only a useful tool but not a panacea For complex karstsystems no single model can be all-embracing so differ-ent models may be needed for different purposes includ-ing explanation prediction and control (Bonacci and Roje-Bonacci 2012) This is especially true when prediction ofthe behaviour of a karst system due to HEPP development isattempted

Jovic (2003) made a numerical model of flows in the un-derground storage reservoir of the HEPP Ombla The great-est and until now unsolved obstacles for flow modelling thevadose and phreatic zones of the large Ombla Spring under-ground karst massif are the geological setting and hydrogeo-logical characteristics as well as the dimensions and positionsof the main karst conduits The local area around the OmblaSpring has until now been only partly investigated

Bonacci (1995 2000b) has shown how fast unpredictablyand over what a large range the values of hydraulic conduc-tivity K change in time and space upstream of the OmblaSpring underground aquifer This means that conditions forthe reliable modelling of the functioning of this system arenot fulfilled and the real question is whether it is possible atall to fulfil such conditions in this large deep and complexkarst area This is especially true in the case where mod-elling should take into account the anthropogenic influenceof HEPP development on the regime of groundwater circu-lation The previously explained phenomenon of the limitedmaximum discharge capacity of the Ombla Spring should beincluded in a rainfall-runoff model Probably one of the rea-sons for the present poor quality of hydrological modellingin karsts is that this phenomenon has not been taken into con-sideration

It is becoming increasingly critical for scientists and deci-sion makers to become aware of the ways in which their workis influenced by legal and policy aspects This is especiallytrue for the case of the HEPP Ombla which uses water froma transboundary karst aquifer divided by Croatia and Bosniaand Herzegovina The large majority of this aquifer as well

as the Ombla Spring catchment is in Bosnia and Herzegov-ina The operation of the HEPP Ombla could cause troublewithin the territory of Bosnia and Herzegovina which couldprovoke international disputes It can be expected that anyproblem in the part of the Ombla Spring catchment whichbelongs to Bosnia and Herzegovina will be explained as aconsequence of the HEPP Ombla operation

44 Induced seismicity

Large-scale fluid injections induce both seismic and aseis-mic motion (Cornet 2012) The injection of fluid intoa rock mass results in the variation of effective stressesthat sometimes generate induced seismicity An increase inpore pressure may induce some microseismic activity Roje-Bonacci (1995) analysed the interdependence between thechange in rock mass stress conditions caused by changes inpiezometric pressures due to the impoundment of large reser-voirs and induced seismicity observed in such structuresThere are many studies of earthquakes triggered by artificialwater reservoirs (eg Talwani and Acree 1984 Stojic andLalic 1994 Gupta 2002 Fan Xiao 2012)

The oscillations of the GWL in the Ombla Spring aquiferare sudden and on a large scale The changes they causewithin the rock mass are so strong that they can lead to aredistribution of stress states and as such can cause sinkholecollapses (Roje-Bonacci 1997)

Occurrences of a series of minor earthquakes and roars inthe coastal karst Adriatic Sea area are quite frequent thosewhich occurred in the area of Dubrovnik were particularly in-tense (Cvijanovic 1971 Garašic and Cvijanovic 1991 Sto-ji c and Lalic 1994) The seasonal character leads to the as-sessment that they are caused by abundant precipitation andcombined to presence of the large karst caves and conduits inthe area Garašic and Cvijanovic (1986) maintain that earth-quakes and speleological phenomena are related to the mainfault zones and to the tectonic processes which have takenplace

Milanovic (2002) believes that the abrupt rise of the GWLin the analysed Dinaric karst area compresses air trapped inkarst conduits caves and siphons A trapped ldquoair pillowrdquo maythus escape creating strong explosions felt by the inhabitantsand recorded by the nearby seismological stations

As concerns the HEPP Ombla it is of crucial importanceto take into consideration that induced seismicity has beentriggered during the operation of large dams in the neigh-bouring Dinaric karst area of Grancarevo and PivaMratinje(Milanovic 2002)

Undoubtedly the massive injections which should be donefor the HEPP Ombla grout curtain construction will inducemany small seismic motions followed by unpleasant acousticeffects



5 Influence on the ecosystem

The local area of the Ombla Spring is a nature park withprotected flora and fauna since this particular locality wasrecognized as an especially valuable and vulnerable coastalkarst environment

In regard to the ecological issues in karst terrains Bonacciet al (2009) stated ldquoThe karst environment has very dif-ferent characteristics than all other environments The bio-logical importance and particularities of karst are enormousThe evolution of entire karst landscapes is thought to be bio-logically controlled through the interrelationships of vegeta-tive cover erosion and dissolution rates Subterranean karstecosystems are sensitive to environmental changes that oc-cur on the surface The importance of maintaining biologi-cal diversity goes far beyond mere protection of endangeredspecies and beautiful landscaperdquo This was also stated in thepreviously mentioned work by North et al (2009) and was atthe origin of the ldquobiologyrdquo category in the karst disturbanceindex (KDI) evaluation

Injection will definitely lead to many negative conse-quences for the underground ecosystem The constructionof the grout curtains will rashly strongly and dangerouslydisrupt the existing vulnerable ecological equilibrium of thekarst underground system The natural food chain and thecontact between species and their habitats will be lost in thisvery large but unknown underground space

The main ingredients of a grout curtain (Portland cementclay bentonite sand asphalt and some chemicals) may betoxic neurotoxic or carcinogenic irritant to the skin or cor-rosive Their use is dangerous for human beings and the envi-ronment Such toxic components can pollute the karst aquiferand cause long-lasting hazardous consequences on karst un-derground species (Bonacci et al 2009)

The penetration of salt sea water into the local area be-tween the sea coast and the grout curtain will destroy thevulnerable ecological equilibrium

In the valuable and vulnerable environment of the OmblaSpring the hot spot is represented by Vilina Cave (see its po-sition in Fig 8) In this cave seven sorts of bat have theirhabitats The guano which they produce plays a crucial rolefor sustaining the life of all the other terrestrial as well asaquatic species in the Vilina CavendashOmbla Spring ecosys-tem The construction of this extremely high grout curtaincan have a strong and negative influence on the natural foodchain in the Vilina CavendashOmbla Spring ecosystem whereeight cave species five terrestrial and three aquatic (endemicand endangered) are found (Sket 1997 Schuumltt 2000 Be-dek et al 2006) The vadose zone will be decreased and allterrestrial species which have habitats in this zone will bedestroyed

Despite the fact that the grout curtain will be constructedbelow the bottom of the Vilina Cave the cave will be tem-porarily flooded After heavy rainfall a rapid and uncon-trolled rise in GWL will occur It is realistic to expect that

the Vilina Cave will function as a temporary spring for thefast water evacuation from the karst aquifer to the surface Inthis case all habitats and species from the Vilina Cave will bewashed away and destroyed

6 Concluding remarks

Problems discussed in this paper are extremely complex Inaccordance with existing literature and authorsrsquo experienceall (or at least most) of the risks previously discussed be-long to the so-called ldquoshallow uncertainties hazardsrdquo whicharise when the probabilities of outcomes are reasonably wellknown Deep uncertainties arise when the probabilities ofoutcomes are poorly known unknown or unknowable (Steinand Geller 2012 Stein 2013 Stein and Stein 2013)

This is the first underground dam and reservoir of such sizeand height to be constructed in this deep weakly investigatedand extremely heterogeneous coastal area of Dinaric karstThe available data and information are not ample enough tomake definite and reliable conclusions and offer technical so-lutions On the basis of the analyses developed in this paperit is very probable that the negative consequences of the con-struction and development of the HEPP Ombla will be veryserious

Generally speaking each grout curtain acts individuallyTo understand its role and its influence on the environmentit is absolutely necessary to have long-lasting continuousmonitoring (before and after construction) at least for GWLgroundwater temperature and the chemical composition ofthe water itself (Bonacci and Roje-Bonacci 2012)

The present-day ecological and environmental aspects ofgrouting ie its influence on this wonderful valuable andvulnerable karst underground environment has not drawnmuch attention if any This problem could be very seriousand due to this reason grouting in karsts should be treatedwith much great caution (Bonacci et al 2009)

To obtain harmonious reliable and sustainable develop-ment in this very complex valuable and vulnerable ecolog-ical and social system it is necessary to take the complexinteractive technical social economic environmental andcultural aspects of water resources management into accountin decision making

In the analysed case issues of the transboundary karst wa-ter resources management represent an especially difficultand dubious task Water policy is becoming progressivelymore relevant across different sectors (including energy landuse natural resource management health and the environ-ment) and effective implementation is crucial for the sus-tainable development of all of these aspects

The possibilities for overcoming karst system complex-ity can be found in close co-operation between hydrologyand hydrogeology as well as between different branches ofthe geosciences and in implementation of detailed monitor-ing The need for a better understanding of the deep and



long-lasting mutual relationship between human activitiesand natural processes in karst terrains is of crucial impor-tance In order to achieve their real sustainable developmentit is necessary to fill in the gaps which remain in the under-standing of hydrological and ecological behaviour

Edited by M PariseReviewed by M Petitta and T Bechtel

References

Beck B F Sinkholes Their geology engineering and environmen-tal impact Proceedings of the first multidisciplinary conferenceon sinkholes Orlando 1984

Beck B F Engineering and environmental impacts of sinkholesand karst Proceedings of the third multidisciplinary conferenceon sinkholes Orlando 1989

Beck B F Sinkholes and the engineering and environmental im-pacts of karst Proceedings of the tenth multidisciplinary confer-ence on sinkholes San Antonio 2005

Beck B F and Herring J G Geotechnical and environmen-tal applications of karst geology and hydrology Proceedings ofthe eighth multidisciplinary conference on sinkholes Louisville2001

Bedek J Gottstein-Matocec S Jalžic B Ozimec R and Šta-mol V Katalog tipskih špiljskih lokaliteta faune Hrvatske (Cat-alogue of cave type localities of Croatian fauna) Natura Croat-ica 15 1ndash154 2006

Boumlgli A Karst hydrology and physical speleology Springer Ver-lag Berlin 1980

Bonacci O Karst hydrology with special reference to Dinarickarst Springer Verlag Berlin 1987

Bonacci O Ground water behaviour in karst example of theOmbla Spring (Croatia) J Hydrol 165 113ndash134 1995

Bonacci O Interpretation of groundwater level monitoring resultsin karts aquifers examples from the Dinaric karst Hydrol Pro-cess 14 2423ndash2438 2000a

Bonacci O Heterogeneity of hydrologic and hydrogeologic pa-rameters in karst example from Dinaric karst Proceedings ofthe 6th international symposium and field seminar ldquoPresent stateand future trends of karst studiesldquo Marmaris 393ndash399 2000b

Bonacci O Analysis of the maximum discharge of karst springsHydrogeol J 9 328ndash338 2001

Bonacci O Development of catchment area in karst as a resultof natural and anthropological factors in Evolution of karstfrom prekarst to cessation edited by Gabrovšek F Inštitut zaRaziskovanje Krasa Postojna 359ndash365 2002

Bonacci O Hazards caused by natural and anthropogenic changesof catchment area in karst Nat Hazards Earth Syst Sci 4 655ndash661 doi105194nhess-4-655-2004 2004

Bonacci O and Jelin J Identification of a karst hydrological sys-tem in the Dinaric karst (Yugoslavia) Hydrol Sci J 33 483ndash497 1988

Bonacci O and Roje-Bonacci T Interpretation of groundwaterlevel monitoring results in karst aquifer examples from the Di-naric karst Hydrol Process 14 2423ndash2438 2000

Bonacci O and Roje-Bonacci T Impact of grout curtains on karstgroundwater behaviour an example from the Dinaric karst Hy-

drol Process 26 2765ndash2772 2012Bonacci O and Rubinic J Water losses from a reservoir built in

karst the example of the Boljuncica reservoir (Istria Croatia)Environ Geol 58 339ndash345 2009

Bonacci O Gottstein S and Roje-Bonacci T Negative impactsof grouting on the underground karst environment Ecohydrol2 492ndash502 2009

Bonacci O Pipan T and Culver D C A framework for karstecohydrology Environ Geol 56 891ndash900 2009

Breznik M Storage reservoirs and deep wells in karst regionsBalkema Rotterdam 1998

Buljan R Prelogovic E and Paviša T Izvorište Ombla (Thespring area of Ombla) Proceedings of 2nd Croatian geologicalcongress 555ndash560 2000

Cornet F H The relationship between seismic and aseismic mo-tions induced by forced fluid injections Hydrogeol J 20 1463ndash1466 2012

Cvijanovic D Seizmicnost dubrovackog podrucja (Dubrovnik re-gion seismicity) Acta Seismologica Iugoslavia 1 31ndash56 1971

Denic-Jukic V Juras T Plenkovic M Kadic A and Jukic DTurbidity dynamics of the karst spring Ombla (Croatia) Geo-phys Res Abstr EGU2012-12428 EGU General Assembly2012 Vienna Austria 2012

Drew D and Houmltzl H Karst hydrogeology and human activitiesndash impacts consequences and implications Balkema Rotterdam1999

Dreybrodt W Romanov D and Gabrovšek F Karstification bel-low dam sites A model increasing leakage in reservoirs EnvironGeol 42 518ndash524 2002

Fan Xiao Did the Zipingpu Dam trigger Chinarsquos 2008 earthquakeThe scientific case Probe International 2012

Ford D and Williams P Karst hydrogeology and geomorphologyJohn Wiley Chichester 2007

Garašic M and Cvijanovic D Speleological phenomena and seis-mic activity in Dinaric karst area in Yugoslavia Proceedingsof 9th international speleological congress Barcelona I 94ndash971986

Garašic M and Cvijanovic D Seasonal occurrence of series ofminor earthquakes and roars in the area of Dubrovnik Ston andMljet Naš Krš 30 55ndash62 1991

Gilli E and Mangan C Le prototype de Coaraze (Alpes-Maritime) exemple de creacuteation drsquoune reacuteserve drsquoeau souterrainepar obturation drsquoune source karstique (The Coaraze prototype anexample of water underground storage by karstic spring closing(Alpes-Maritime France) Karstologia 24 37ndash40 1994

Gunn J Contributory area definition for groundwater source pro-tection and hazard mitigation in carbonate aquifers in Naturaland anthropogenic hazards in karst areas recognition analysisand mitigation edited by Parise M and Gunn J GeologicalSociety of London Special Publication 279 97ndash109 2007

Gupta H K A review of recent studies of triggered earthquakes byartificial water reservoirs with special emphasis on earthquakesin Koyna India Earth-Sci Rev 58 279ndash310 2002

Hiller T Kaufmann G and Romanov D Karstification beneathdam-sites From conceptual models to realistic scenario J Hy-drol 398 201ndash211 2011

Ishida S Kotoku M Abe E Faza M A Tsuchihara T andImaizumi M Construction of subsurface dams and their impacton the environment RMZ Material Geoenviron 50 149ndash152



2003Ishida S Tsuchihara T Yoshimoto S and Imaizumi M Sus-

tainable use of groundwater with underground dams Japan AgrRes Quarterly 45 51ndash61 2011

Jovic V Numerical model of flows in the underground storagereservoir of the Ombla HE plant ndash natural state and the stateplanned by the project HYDRO 2003 conference proceedings1 483ndash498 2003

Karimi H Keshavarz T Mohammadi Z and Raeisi E Poten-tial leakage at the Khersan 3 Dam Site Iran a hydrogeologicalapproach B Eng Geol Environ 66 269ndash278 2007

Kaufmann G and Romanov D Leakage of dam sites in karst ter-rains Geophys Res Abstr 10 EGU2008-A-02229 EGU Gen-eral Assembly 2008 Vienna Austria 2008

Kresic N and Stevanovic Z Groundwater hydrology of springsElsevier Amsterdam 2010

Lin Hua S Subsurface reservoirs and karst geomorphology Pro-ceedings of the third multidisciplinary conference on sinkholesBalkema Rotterdam 369ndash376 1989

Milanovic P Karst hydrogeology Water Resources PublicationLittleton 1981

Milanovic P Ombla Spring Croatia Environ Geol 27 105ndash1071996

Milanovic P Geological engineering in karst Zebra Pbl CoBeograd 2000

Milanovic P The environmental impacts of human activities andengineering constructions in karst regions Episodes 25 13ndash212002

Nonveiller E Grouting theory and practice Elsevier Amsterdam1989

North L A van Beynen P E and Parise M Interregional com-parison of karst disturbance West-central Florida and southeastItaly J Environ Manage 90 1770ndash1781 2009

Parise M and Gunn J (Eds) Natural and anthropogenic hazardsin karst areas Recognition Analysis and Mitigation Geol SocLondon Special Publications 279 2007

Paronuzzi P and Bolla A The prehistoric Vajont rockslide Anupdated geological mode Geomorph 169ndash170 165ndash191 2012

Paviša T Podzemna akumulacija u krškom terenu (Undergroundreservoir in karst terrain) Hrvatska Vodoprivreda VII 14ndash251998

Paviša T HPP Ombla in Croatia ndash proposed use of energy fromgroundwater in karst aquifer HYDRO 2003 conference proceed-ings 1 463ndash470 2003

Ravnik D and Rajver D The use of inverse geotherms for deter-mining underground water flow at the Ombla karst spring nearDubrovnik Croatia J Appl Geophys 3 177ndash190 1998

Rogers D Hales Bar Dam and the pitfalls of constructing dams onkarst foundation Missouri University of Science amp Technologyavailable athttpwebmstedu~rogerssdadam(last access 18April 2013) 2007

Roje-Bonacci T Piezometarski pritisci efektivna naprezanja in-ducirani potresi (Piezometric pressures effective stresses in-duced seismicity) Proceeding of 1st Croatian water conferenceDubrovnik 107ndash113 1995

Roje-Bonacci T Influence of the fluctuation of groundwater lev-els upon the formation of sinkholes in Engineering geology andthe environment edited by Marinos P G Koukis G C Tsi-ambaos G C and Stournaras G C Balkema Rotterdam 997ndash1002 1997

Romanov D Gabrovšek F and Dreybrodt W Dam sites in solu-ble rocks a model of increasing leakage by dissolutional widen-ing of fractures beneath a dam Eng Geol 70 17ndash35 2003

Schuumltt H Die Hohenmollusken der Ombla-Quelle (The subter-ranean molluscs of the Ombla Spring) Natura Croatica 9 203ndash215 2000

Šegota T Morska razina u halocenu i mladem wuumlrmu (Sea level inHalocen and Wuumlrm) Geografski Glasnik 30 15ndash39 1968

Sever Z The Ombla HPP multipurpose project HYDRO 2003conference proceedings 1 455ndash462 2003

Shapiro S A Rothert E Rath V and Rindschwentner J Char-acterization of fluid transport properties of reservoirs using in-duced seismicity Geophysics 67 212ndash220 2002

Sket B Distribution ofProteus(Amphibia Urodela Proteidae)and its possible explanation J Biogeogr 24 263ndash280 1997

Stein S Shallow versus deep uncertainties in natural hazard as-sessment Eos Trans AGU 94 133ndash134 2013

Stein S and Geller R J Communicating uncertainties in naturalhazard forecasts Eos Trans AGU 93 361ndash362 2012

Stein S and Stein J L How good do natural hazard assessmentsneeds to be GSA Today 23 60ndash61 2013

Stojic P and Lalic R Potresi izazvani akumuliranom vodom(Earthquakes induced by reservoirs) Gradevinski Fakultet Split1994

Talwani P and Acree S Pore pressure diffusion and the mecha-nism of reservoir-induced seismicity Pure Appl Geophys 122947ndash965 1984

Turkmen S Treatment of the seepage problem at the Kalecik Dam(Turkey) Eng Geol 68 159ndash169 2003

Unal B Eren M and Yalcin M G Investigating of leakage atAtaturk dam and hydro electric power plant by means of hydro-metric measurements Eng Geol 93 45ndash63 2007

van Beynen P Brinkmann R and van Beynen K Karst a sustain-ability index for karst environments J Cave Karst Studies 74221ndash234 2012

Waltham T Bell F and Culshaw M Sinkholes and subsidencendash Karst and cavernous rocks in engineering and constructionsSpringer Verlag amp Praxis Publishing Heidelberg amp Chichester2005

Williams P V The role of subcutaneous zone in karst hydrologyJ Hydrol 61 45ndash67 1983

Zubac Ž and Boškovic Ž Problem vododrživosti akumulacijeldquoGoricardquo-Trebinje (Issue of losing waters from Goricarsquos reser-voir) Vodoprivreda 44 273ndash276 2012

Žugaj R and Bonacci O HE Ombla hidrološka obrada (HEPPOmbla hydrological analyses) Elektroprojekt Consulting Engi-neers Zagreb 1994 (unpublished)



catchment area is the starting point in all hydrologic anal-yses and one of the essential pieces of information whichserve as the basis for all hydrologic calculations In karstterrains this is always a very complex task (Gunn 2007)Groundwater exchanges with adjacent aquifers through un-derground piracy routes or via inflows from surface streamsConsequently only in exceptional cases do surface (topo-graphic or orographic) and subsurface (hydrologic or hydro-geologic) catchment boundaries coincide The problem is ad-ditionally complicated by the time-variant hydrologic bound-aries which are dependent on fluctuations of groundwaterlevels (GWLs) (Bonacci 1987 2002 Kresic and Stevanovic2010)

Any kind of engineering action in a karst terrain presentsnumerous challenges to the engineer including the unpre-dictable occurrence of cavities and well-developed hydraulicconduits which could lead to sinkhole development The sizeand shape of a cavity in limestone depend on the interactionbetween the characteristics of geological structures such asinternal fractures or discontinuities and water activity

Human intervention especially the construction of damsand reservoirs realized by creating large grout curtains(Bonacci et al 2009) as well as interbasin water transfersthrough long tunnels and pipelines can introduce instanta-neous and distinct changes in catchment areas and bound-aries and in turn on the hydrological hydrogeological andecological regimes (limitation of energy flow negative im-pacts on food cycle blocking of convections between habi-tats and species pollution of groundwater and undergroundenvironment etc) (Bonacci 2004)

In recent times there have been many high disturbances of-ten caused by human activities in karsts North et al (2009)gave interregional comparison of karst disturbance in west-central Florida and southeast Italy while van Beynen etal (2012) discussed a sustainability index for karst environ-ment Both papers deal with indices for evaluating impactsand sustainability in karsts and are therefore closely relatedto the issue dealt with here

The project for construction of the hydro-electric powerplant (HEPP) Ombla will exclusively use groundwater fromthe Ombla Spring karst aquifer The underground dam willbe constructed about 200 m behind the existing karst springoutflow in the karst massif by injection of a grout curtain

The main goal of this paper is to encourage further de-tailed interdisciplinary measurements and analyses in orderto understand the possible negative consequences caused bythe construction and development of the HEPP Ombla Theintention of the paper is to identify potential problems and tosolve them before they occur in complex and poorly inves-tigated karst environments The HEPP Ombla system is in-ternationally shared between Croatia and Bosnia and Herze-govina Due to this reason one of the goals of the paper is topoint out the specific characteristics of the karst aquifer andits water management which can represent a real trigger forpossible conflicts between the two neighbouring states

22

Figure 1 Fig 1Location map indicating the study area the site of the OmblaSpring with its supposed catchment area the Zaton Spring the Tre-bišnjica River the boundary limit between Croatia and Bosnia andHerzegovina as well as the position of existing dams reservoirs andHEPPs

The HEPP Ombla example apart from being an interest-ing case study can be valuable for many other karst areasworldwide where engineering works and construction aretaking place especially those based on grout curtains Thispaper focuses on the critical issues and priorities needed toaddress karst region risks and governance for disaster pre-vention

2 Short description of the natural characteristics ofOmbla Spring and its catchment

Figure 1 represents a map of the study area the site of theOmbla Spring with its supposed catchment area the ZatonSpring the Trebišnjica River the boundary between Croatiaand Bosnia and Herzegovina as well as the position of exist-ing dams reservoirs and HEPPs The photograph presentedin Fig 2 shows the Ombla Spring from an areal view

The catchment area and boundaries shown in Fig 1 havebeen determined by using only geological data without ade-quate hydrogeological data The catchment area is estimatedto be about 600 km2 (Milanovic 1996 Sever 2003) Ac-cording to the authorrsquos investigations which were based onwater-budget analyses the catchment area is much larger andranges between 900 km2 and 1000 km2 (Žugaj and Bonacci1994)



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Figure 3









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Figure 4







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Figure 6





28

Figure 7








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Figure 8

















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Figure 3









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Figure 6





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Figure 7








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Figure 8

















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Figure 3









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Figure 4







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Figure 6





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Figure 7








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Figure 4







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Figure 6





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Figure 7








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Figure 8

















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27

Figure 6





28

Figure 7








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Figure 8

















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Figure 8

















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Documents

The possible negative consequences of underground dam and ... dam in coastal areas...2042 T. Roje-Bonacci and O. Bonacci: Underground dam in coastal karst catchment area is the starting