APPROACH TO THE PROBLEM OF THE UNDERGROUND WATER LEAKAGE FROM THE STORAGES IN KARST REGIONS. KARST STORAGES BUSK0 BLATO, PERUCA

AND KRUSCICA Mladen BORELLI

et

Boris PAVLIN

SUMMARY

Three reservoirs in Karstic regions of Yugoslavia, one of them (PeruCa) already constructed, the second (KrusCica) in construction and the third (BuSko Biato) in design stage, with very complete field data, have been utilised by the authors to conduct a quantitative study on water losses in karstic reservo~rs, which is of paramount importance for the economic design of watertight curtain and other similar measures concerning preventing of water losses.

Hydrogeologic and technical conditions were investigated, at first, by which the bedrock of the reservoir may be defined as a porous medium of a definite type; field investigations were analysed subsequently, especially the data on permeability and on water table in order to obtain the necessary parameters to define the bedrock as a ..

porous medium. The first stage of field investigations, which is at the same time the starting point

of the schematisation of the bedrock, is the determination of the zones where the permeability is so high that it can be considered (in point of view of storage reservoir) as zones of practically infinite permeability.

If these zones can be located, they can later on be isolated by means of dams or by watertight curtains, permitting the quantitative definition of the permeability of the remaining zones of bedrock. The zones of undetermined permeability will define the boundary conditions only.

The statistical analysis of permeability is based essentially on the hypothesis of correlation between the general conditions of fissured rock and the existence of indi- vidual large caverns end channels (a relation which normally exists, by there may be exeptions, too). This analysis is very useful for the determination of the zones of undetermined high permeability and of the relative variance of permeability with depth, or for derivation of a relation between the permeability of particular regions. For fissured rock, however, and especially in carstic regions, the statistical analysis can lead to very rough information, only, on the average value of bedrock permeability as a whole, or of different parts of bedrock. This data are, however, necessary for the calculation of water losses from storage.

Water table observations will give far more reliable informations on this matter. Great attention was attached consequently to the analysis of piezometric data, and their use.

At the end the effect of impermeable soils was considered as it usually covers the limostone bedrock in the carstic valleys of Yugoslavia.

RESUME

Contribuiion à ì‘étudedes eaux souterraines en uue de la conitruction de réseruoirs dans les régions karstiques (d’après les cas des réseruoirs de BuSko Biato de Perufa et Peruda)

Trois réservoirs situés dans des régions karstiques, de Yougoslavie, dont le premier (PeruCa) est construit, le second (KruZfcica) est en construction et le troisième (BuHko Blato) est à l’état de projet, et des observations trks complètes effectuées sur le terrain ont été utilisées par les auteurs pour procéder à une étude quantitative des pertes d’eau dans les réservoirs construits en région karstique. Cette étude est d’une extrême importance pour l’établissement de murs d’étanchéité et l’adoption d’autres mesures du m ê m e genre, visant à arrêter ces pertes.

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Les auteurs ont commencé par étudier les conditions hydrogéologiques et techniques qui permettent de définir la roche en place du réservoir comme un milieu poreux d’un type déterminé; ils ont ensuite analysé les observations faites sur le terrain, en particulier les données relatives ii la perméabilité et au niveau de la nappe, afin d’obtenir les paramètres nécessaires pour définir la roche en piace c o m m e un milieu poreux.

L a première étape des études sur le terrain, qui constitue en m ê m e temps le point de départ de la schématisation de la roche en place, consiste à déterminer les zones OU la perméabilité est si forte qu’elles peuvent être considérées (du point de vue de la réserve d’eau) comme des zones de perméabilité pratiquement illimitées.

Si ces zones peuvent être localisées, elles pourront ultérieurement être isolées au moyen de barrages ou de murs d’étanchéité, ce qui permettra de déterminer quanti- tativement la perméabilité de la zone restante de la roche en place. Les zones de perméabilité non déterminée ne définiront que les conditions marginales.

L’analyse statistique de la perméabilité repose essentiellement sur l’hypothèse de la corrélation entre les caractères généraux des roches fissurées et l’existence de vastes cavernes et chenaux (relation qui existe normalement, mais qui n’est pas sans exceptions). Cette analyse est très utile pour la détermination des zones de forte perméabilité non déterminée, pour la détermination des variations de la perméabilité avec la profondeur et pour la détermination d’un rapport entre les différentes perméa- bilités de régions particulières. Mais dans le cas de roches fissurées, notamment en région karstique, il se peut que l’analyse statistique ne fournisse que des données très approximatives sur la valeur moyenne de la perméabilité de l’ensemble ou de diverses parties de la roche en place. Ces données sont cependant nécessaires pour le calcul des pertes en eau d’un réservoir.

Les observations du niveau de la nappe donneront à ce sujet des informations beaucoup plus sûres. Par conséquent, on a apporté une grande attention à l’analyse des données piezométriques et à leur utilisation.

Les auteurs étudient enfin le rôle des sols imperméables, qui recouvrent d’ordinaire le fond rocheux calcaire des vallées karstiques de Yougoslavie.

1. INTRODUCTION

Abundant research works accompanying the designing of karst storage basins render useful data about the characteristics of karst rocks underlying these reservoirs.

Particular attention should be paid to the observation of the karst storage basins already completed, especially to these which were designed in accordance to the numerous research works. This article deals with the results yielded by the research works and observations of three Yugoslav karst reservoirs, the one of which-PeruCa- has been exploited for five years. The data presented here will probably contribute to the general knowledge of the karst terrains, though the aim of the article is restricted to the problem of leakage from karst reservoirs. This should be mentioned since the data and information necessary of the estimation and calculation of the water leakage from reservoirs are specific, differing from the other technical problems in karst, particularly from the probiem of water capture of the influence of backing to the inflow of underground water into karst storage basins. The differences among these problems will be pointed out in the further text.

The main characteristic of the fissure porosity in general and particularly of karst rocks in the existence of preferential channel ways. In natural circumstances these ways offen in general almost illimited transport possibilities, as to the formation of reservoirs at least. The realisation of a karst reservoir implicates the necessity to cut off main water drains in order to impede the water leakage. Consequently, the estimation ofwater leakage is reduced to the determination of the water loss through the remaining fissured rock mass.

There are two ways of calculating the leakage of water as well as the analytic treatment of flows in general: a) To observe only the preferential ways applying the equations valid for tubes and

canals. This way of calculating can be used if the size of preferential ways, faults, tubes and canals are known:

33

b) To observe the entire rock mass performing the calculation in accordance with the scheme of continuum and schematizing the bedrock of reservoir as a porous medium through which the water flows at a discharge velocity V. In this case the rock mass is defined, from the standpoint of water flow, by some of the parameters determining the permeability. Such is, for example, Darcy’s coefficient of filtration.

According to our opinion the research works may render more precise information only on the biggest canals which should be cut off in any case in order to perform a karst reservoir, so that the characteristics of these canals are not particularly interesting in that sense. Under these conditions the only possible method of the leakage estimate is: the calculation after the continuum scheme. Since there are many difficulties appearing in karst when applying the continuum scheme of the rock mass, it is necessary to explain such a scheme. It is a particular question whether and under which conditions the permeability of the rock mass, necessary for the estimate of leakage according to the continuum scheme, can be obtained by means of research works.

2. POSSIBILITIES OF SCHEMATIZING THE RESERVOIR BEDROCK AS CONTINUUM

More water can pass through a karst channel of larger dimensions than through thousands, or even million cubic meters of the surrounding fissured rock mass. Naturally, under such conditions it is not logical to use the continuum scheme including the inactive rock masses undetermined in size which practically do not take part in flow. Taking into account the new rock mass when determining the discharge velocity provokes the apparent reduction of this velocity. In such a way these velocities become entirely arbitrarily. This is the reason why the problem of observing the reservoir bedrock as a continuum scheme does not seem convenient or, at least, it requires a special treatment. W e would like to remind of this fact that in a basically homogenous porous granular mass there are determined unhomogeneities requiring the medium coeîñcient of permeability to be determined on the basis of more tests and to cover, by investigating the permeability, a certain not too small size of terrain (Labye, 1960). By rock mass, even if regularly and uniformly fissured, the investigated size of terrain may be larger. The number of specimens must also be greater according to the statistic rules because the standard deviation here is far more greater than by granular mass. Karst terrains are substantially an extreme example of the mentioned phenomenon.

Generally, they can be treated in the same way as the granular material, but in a considerably larger scale, ofen in the regional framework. In consequence, the difference between granular masses and karst is just a question of scale.

The possibility of schematizing limestone as a permeable medium is facilitated by such an interpretation. This point of view also explains why rock mass with karst phenomenon near a water capture should not be considered as a continuum. In the first place we are interested in the preferential channel ways, i.e. the localities of the rock mass, essential for this phenomenon, and which are different from the mass as a whole. Furthermore, the regime of ground water flow is distinct from the flow in natural condition only in the close vicinity of water capture in a very limited volume of rock mass. Both aforesaid moments prove that the application of discharge velocities is not justified in the case of capturing the underground water in karst.

The method of schematizing the bedrock of reservoir, at least of the big one, as a continuum for the determination of leakage offers the following advantages: - for the reservoirs in karst it is necessary to cut off the principal preferential

passages i.e. those parts of rocks which represent basic singularities, different from the remaining rock mass;

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- in the case of big karst reservoir the water flows through millions of cubic meters of rock, after the main preferential passages were cut off. Than w e may consider that the flow becomes in a certain way uniform. The closing of main waterways can be achieved by filling the caves by a grout

curtain or by the cutoff embankment separating those parts of reservoir area where the karst phenomena are especially developed. The degree of cutting off the preferential passages depends on hydrogeological conditions, sufficiency of research works and economic moments. If the zones of the exceptionally high permeability are distributed in height and space in such a way as to justify their separation and cutoff, the remaining rock mass may be considered as a continuum and the water losses may be estimated accordingly. Whether a mass may be considered as a continuum or not depends not only on the hydrogeological conditions but also on technical-economic moments. The exact calculation after the scheme is conditioned by the same moments.

It is to be emphasized that the reservoir bedrock as a continuum does not necessarily mean a hypothesis of homogeneity or isotropy of rock, so far as the permeability is concerned. If the research works are sufficient the differences in permea- bility of separate sections, even anisotropy of the rock mass, can be taken into account.

The law of flow has not been dealt with in this article since it is not essential in this stage of the discussion on the possibility of schematization of the reservoir bedrock. Once the schematization has been done (or the researche works utilized for the deter- mination of the corresponding permeability coefficients) a certain regime of flow (laminar, turbulent, transitional) should be accepted in order to enable the estimate of leakage.

This complex problem has been left aside in the present article. According to our opinion the problem of leakage through bedrock, after cutting off the preferential waterways, the laminar regime (*)-in accordance with Darcy’s filtration coefficient - c a n be accepted. It exaggerates the leakage (although not in an exceptionally large scale).

The karst valleys are often covered by relatively less permeable material but which is never entirely continuous or impermeable. The role of the cover as a water tightening bed is treated in laboratory (Babac 1965) and in situ. It has been proved that this cover should not be considered as a basic element for water tightening. This fact increases the interest in the possibilities to calculate the leakage through the reservoir bedrock.

3. SOME REMARKS ABOUT WATER PERMEABILITY TESTS AND PIEZOMETRIC DATA

In this article two kinds of investigation works: water permeability tests and the analysis of the ground water levels will only be considered. They are partly described hereinafter and partly in the following chapters (BuSko Blato, Perda, KruSEica). Before we start a detailed analysis it should be mentioned that the importance of research works is greater in cases when the geological results offer wide possibilities of interpretation, leaving the flow conditions insufficiently defined. The authors of the article will be glad if this study contributes to the knowledge of the water permeability survey and the analysis of the ground water table. The Statistics and other data introduced in the article are only the auxiliary means for solving these problems. In the final interpretation of the results it is necessary to bear in mind all the moments determining the permeabiìity of the fissured rock mass.

The permeability of rocks is determined by water permeability tests (WPT) and is expressed by means of Lugeons. One of the problems to be faced is the relation between the WPT expressed in Lugeon units and the permeability coefficient.

(*) Physical models for any type of flow can be utilized, as shown by Ollös, 1961.

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For the laminar flow of the underground water w e can accept(*)

1 Lugeon = 0,07-0,15 m/day

This relation can be easily established by taking common diameters of drillholes of two extreme cases:

a) Entirely homogenous environment; b) Premise of pervious strata situated between two impervious strata.

Obviously 1 Lugeon is a rather small permeability unit; even 100 Lugeon is not disastrous for water leakage if the permeability would really correspond to this value and if it would not increase due to scouring of fissures. The WPT technique cannot often achieve 100 Lugeon. It is obvious that the technical instruments offer rather limited possibilities in determination of WPT. There are numerous cases when the water escaped without pressure, i.e. the WPT value was undefined(**), which is the result of the insufficient capacity of the WPT equipment. This is the first important problem to be faced when analysing the results of water permeability tests; under certain conditions it is simply impossible to give any interpretation of the WF'T data. Naturally, whenever a horizon with the undefined high permeability occurs there cannot be determined the average values of WPT of the observed drillhole.

Since the horizons of the undefined high permeability also occur the data of WPT have to be classified into two parts: into determined and undetermined high perme- ability. The latter is marked P = 03. If an exceptionally high permeability with P = 03 is really assumed to occur in certain zones they should be obligatorily be cut by a grout curtain. As w e shall see later in the case of PeruCa, such a point of view should be corrected, but w e accept it for the present. W h e n schematizing the storage bedrock as a continuum for the laminar flow, the zones of unlimited permeability can be consid- ered as equipiezometric lines. In another words, the losses of piezometric levels by flowing of water through these zones can be neglected. This standpoint obviously speaks in favour of security, since at any rate, even in the zones of undefined high permeability, the corresponding resistance appears.

Let us suppose that the zones of undefined high permeability were limited in space and height and that they could be cut off by dikes or grout curtains. N o w a question emerges whether the permeability coefficients of the remaining rock mass could be determined from the WPT data. The essential difficulty for this transition does not seem to be either the flow regime or the scheme utilized for the calculation of permeability (from the WPT data), but the very physical aspect of the phenomenon. Larger faults and karst channels are comparatively scarce, i.e. there is little probability of their registration.

According to the aforesaid w e may finally conclude that the WPT data cannot convey directly the permeability value of the reservoir bedrock, which will be illustrated by the example of BuBko Blato, where the difference between the permeability value obtained by the analyses of WPT and the logical average value will be clearly manifested.

Although the water permeability tests fail in this case, they are still very precious. The WPT data could be very useful to determine the relations in permeability among different parts of a storage basin, as well as for the relative decrease of permeability corresponding to depth.

For this purpose it is convenient to use the method of cumulative frequency

(*) See: I1 Giornale del Genio Civile II. 1962. (**) Certain cases of the undefined high permeability occurred due to the water

escape around packer.

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represented in galtons scale: logarithmus values of the WPT versus frequency, which is used in all the further examples (BuSko Blato, PeruCa, KruSEica).

The fact that the permeability decrease, almost as a rule with the depth enables the introduction of the conception of “effective depth” by which the analytical treatment of losses is considerably alleviated. Let us consider a certain level having the permeability K = KO. Let us assume arbitrarily for this level that 2 = 20 = O. If the permeability decreases with a depth more rapidly, than l/Z, what is almost a rule, then the “effective depth” .Zopf may be defined as the depth of a layer through which at the same gradient the same discharge would be filtered in the horizontal direction through the layer of homogenous permeability KO, as through a given porous medium. If w e assume that the exponential law of permeability decrease according to depth

K = KO e(-Z) (2) then the effective depth .Zerr

1 Zeff = - ß (3)

B is a characteristic exponent of permeability decrease. The value of this exponent is calculated in the analysed examples.

The piezometric levels integrate in certain sense the conditions of a larger part of the rock mass, at least in comparison with the WPT, and that is why they are precious. The piezometric observations are not free of the local properties of the rock mass, so that these data should be studied simultaneously with the WPT data. The level of the underground water registered in the piezometer corresponds to the level of the ground water around the piezometer provided that the area containing ground water is sufficiently pervious.

If the active part of piezometer is not separated, the piezometer gives a kind of the average piezometric level of all strata within the influence of ‘he piezometric drillhole. If the dissimilar piezometric levels prevail in different strata the water flows from one stratum to another. The piezometers could be filled through inclined faults. As the upper parts of rocks, where the piezometer is located, has the considerably higher permeability than lower parts, the piezometer is taster filled than discharged. Therefore the piezometer generally shows higher levels than the real ones. This is particularly characteristic of the minimum water levels.

The way of use of piezometric data, as applied in the storage basins described in the article, will be explained by the example of BuSko Blato. Here w e would like to stress the following two moments: - It is often considered that minimum underground water levels correspond to the

elevation of impervious stratum. This can be true only under exceptional conditions, i.e. if a stratum of very high permeability (collector) overlies the inclined impervious stratum. In such a case the water level diagram shows in its declination position a clear change into the horizontal direction. In the minimum level of the ground water is the level to which the water table drops before the new precipitation water comes (the end of the dry season).

- Under the adequate conditions (if the approximate boundary of the rock mass through which the water filtrates can be determined) and other favourable possi- bilities, the average values of the rock permeability couid be obtained from piezometric data. It is a very important fact which w e tried in vain to get by means of WPT. In the following text w e shall dwell on the description of the three Yugoslav karst

storage basins (BuGko Blato, PeruCa, KruSCica), starting with BuSko Blato for which the project design is being made. The abundant investigation works have been performed in ail these reservoirs.

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4. T H E BUSK, ßLATO STORAGE BASIN

The BuSko Blato Storage Basin (figs. 1, 2) is situated at the south east part of Livanjsko Polje over the surface of 50 km2. The storage basin is separated from Livanjsko Polje by a natural dike called Kraljicin Nasip. The other flanks of BuSko Blato are rocky. To the west, Busk0 Blato, the bottom of which is between 700-710 meter elevation ad., borders upon the slopers of the Dinara mountain (fig. i) which separates if from Sinjsko Polje (300 meters elevation). Southward there is ArZano valley which at the distance of 3-5 km descends to the elevation 600 m, forming Ariansko Polje and ViniEko Polje. T o the east two mountains, Grahovica and TuSnica, separate BuSko Blato from the higher situated Duvanjsko Polje (850 m elevation).

In its natural condition BuSko Blato is a periodical lake which draws water springs in its eastern part. The most important water discharge comes from the RiEina River spring. The RiEina is the only surface flow in the polje having Qmed = 12 cu.m/sec. During summer there is practically no inflow into the polje.

The drainage channels of BuSko Blato are the karst sinkholes on the west and south-west part of the polje. The largest one is Stara Mlinica, into which the RiEina River sinks, then Sinjski Ponor and the sinkhole Proidrikoza. A programme of closing the sinkholes and other cutoff measures has been made, including the strengthening of the dike KraljiEin Nasip (or alternatively the erection of a new one) in order lo level the water utilizing then the waterfall of 400 meters to Sinjsko Polje and downstream to the Split hydroelectric power station. In such a way a reservoir could be formed with the useful capacity of water retain equal to 8OO,ûûO,OOO cu.m approximately at the water elevation of 716.4 meters.

The geological explorations of BuSko Blato have shown that in its bottom there is a cover of diluvial age, mostly 10 to 20 m thick, which wedges out toward the border. In most part the cover is impermeable, being composed of clay, but it has many weak points since the lenses of sand and even gravel appear on a considerable surface, especially on its northern and eastern edge. The western border of the cover is damaged by a series of mostly small sinkholes, distanced even up to 500 m (the example of the hole near the limestone elevation MatkovaEa). The northern edge of the polje, as well as the part of the bedrock on which the cover is situated in this part of the polje, is composed of the impermeable tertiary marl deposits. A certain portion of the bedrock is composed of the Promina conglomerates and breccias. However, the biggest part of the bedrock is formed of the limestone layers from the Upper and Middle Cretaceous, in which the dolomite appear in the form of lenses and thin layers.

The dolomites are more intensely represented in the background of the eastern group of periodical springs (MukiHnica, Agino Vrelo, Kuielj, and others) where they appear intermittantly with limestone in two anticlines with a syncline between them passing by the village Korito (“The Korito Syncline”). The eastern part of the syncline is covered by Tertiary deposits, mostly by Promina breccias, so that it cannot be clarified if the syncline closes toward the catchment area of Imotsko Polje.

To the south of BuSko Blato, on the line from Ariano to Sinjsko Polje, we can find outcrops of Mesozoic, Cretaceous-Jurassic limestones and dolomites and besides this line, the outcrops of Werfen slate appear very near to Sinjsko Polje. This complex of deposits represent a barrier impermeable to water.

The terrain of BuSko Blato is strongly folded with a series of microfaults. One of the micro faults is good expressed. Situated at the south-eastern part of the polje, this fault passes through the area of the drillholes No. P-19 B and P-3 B. Especially in this area a deep karstification is possible.

The underground flows which connect BuSko Blato with the other karst poljes were stated by means of colour tests. Thus, it was shown that the RiEina water comes by underground ways from Duvanjsko Polje but on the way a certain quantity of water

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is deflected, which is in so much greater, expressed in percentage, as the RiEina discharge decreases. The water which does not appear in BuSko Blato flows, as it seems as we shall see further, only in the surface part of the bedrock, immediately below the alluvial cover.

The colour tests in the sinkholes Proidri Koza, Sinjski Ponor and Stara Mlinica have shown that the water from these sinkholes flows subterraneously to a group of karst springs Grab and Ruda in Sinjsko Polje, about 20 km far from BuSko Blato. The water from BuSko Blato travels in this direction, which is iiot the shortest, just because of the mentioned barrier composed of the dolomite from Lower Cretaceous and Jurassic limestones and partly of Werfen slates.

Fig. 2 - Situation Map of BuSko Blato Showing Performed. Observation Boreholes- Piezometers.

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From the standpoint of the design, the principal condition for the formation of the Buiko Blato Storage Basin was to find the answer to the question whether it was possible or not to reduce the runoff by economically safe measures. Bisides, it was important to establish the eventual necessity of separating by dike the south-east part of the polje from the rest of the reservoir.

Jn answer to these questions it was necessary to perform numerous investigation works: structural core drilling, water permeability tests as well as the year-long observations of ground water levels. The obtained analyses will be presented hereinafter.

The analysis of the water permeability(*) was done according to the principles emphasized in the chapter 2. First, the zones of the undefined high permeability, with P = m, were found out. The frequency of the piezometric stage, with P := m, in subordination to elevations, is represented in figure 3.

700

650

600

5 5 0

a BOREHOLES IN FIELD BOREHOLES IN THE SOUTHWEST SIDE

BOREHOLES IN THE WEST SIOE

Fig. 3 - BuBko Blato. The Frequency of Stages with Undefined High Permeability in Relation to the Elevation Point.

It can be seen that the zone of the undefined high permeability is limited to the relatively narrow belt of the reservoir bedrock above the elevation 640 meters.

In the statistic elaboration of the cumulative frequency of maximum WPT, the frequency “P” is expressed after Hazen p = (m-O,5)/N where ‘‘in’’ is the ordinal number of phenomena classified according to the size, and “ N’’-the total number of phenomena. The results are represented in Galton’scale: logarithms-frequency. Since the real aim of the data analysis was to determine the degree of water permea- bility decrease in relation to the depth, the data concerning the stages 650-600 meters and 600-550 meters were separately observed. The diagram in figure 4, clearly shows a

(*) The data of the permeability and the ground water levels have been given by Prof. Dr. Nouveiller and Mr. NikoliC, C. E. from the Geoistraliivanja enterprise who were studying the BuSko Blato Reservoir. Results of investigation works directed by Electroprojekt, Zagreb, also have been used.

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decrease of WPT in relation to depth. The factor of the permeability decrease p, detìned by the equations 3, is:

1 50

p = - 111 1.3 = 0,0052 The effective depth would be:

1 2 =- = 192m ß

5

4

3

VI c z O s oi a 4

1 O0 9

.2 7 Q o 6

.? 8 * 5 F c

t 3 9

2

10 9

7 6 5 I

3

a

2

1 0,l 0.2 0.5 I 2 5 10 20 30 60 50 60 70 80 90 95 98 99 99599A 99.9

Cumulative frequency x

Fig. 4 - BuHko Blato. Cumulative Frequency of the Maximum WPT: - for the Stage 650-600 meters a d . ; - for the Stage 600-550 meters a.s.1.

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The average value of the maximum WPT for the stage 600-550 meters, would amount to 9,2 Lugeons units which corresponds to Darcy’s coefficient of permeability of:

0.1 -0.2 m day This permeability is very low and in case it really exist, the artificial reduce of losses

from the reservoir would be meaningless. In fact, as further piezometric data analyses will show, the real permeability is much higher even in the zone below the elevation 600 meters a.s.1.

Another goal of the WPT data analysis was the establishment of the relation between the water permeability Pi of the southwest edge of the polje and the permeability of the remaining part of the western edge. The results obtained are represented in figure 5, from which we could derive

The south-west sector, although more permeable, is not characterized by extremely high permeability which could justify its separation by a dike from the rest of the reservoir. This conclusion will be checked up by the analysis of the ground water levels.

The analysis of underground water lecels. The year-long observations on a series of piezometers enabled to establish some laws concerning the underground levels. The main characteristics of the piezometric data are represented in table 1, shortly to be commented. It should be mentioned that the piezometric data in case of BuSko Blato have been used for a qualitatively higher stage of elaboration, i.e. for the determining the approx. value of the mean permeability coefficient of the western and southwestern edge.

The analysis of the piezometric data proved the existence of a kind of stabilized level of the ground water close to the maximum water level, characteristic of over- flowing. All observed piezometers do not have this stabilized level (the cases when the stabilized levels are not clearly defined are marked in the table by an asterisk). The stabilized underground level indicates the existence of a very karsty and permeable zone permitting the outfiow of water with low charge. The piezometers with registered stabilized maximum water levels are situated along the west and south-west edge of the polje. In principle, there is no wonder that the stabilized levels appear because sinkholes act as overflows. More interesting is the fact that the stabilized levels occur even below the elevation of the polje which points at the existence of a very permeable surface zone of the reservoir bedrock.

The stabiliied maximum levels never drop below the elevation 660 meters. This, or somewhat lower elevation, could be considered as the presupposition for any more complex cutoff of the BuSko Blato Reservoir bedrock.

It is necessary to emphasize that this conclusion is completely conformed to the results obtained by WPT test according to which the zones of undefined high permeability were stated above the elevation 640-650 meters.

In table 1 we have given the absolute maximum levels (column 2.2.) which for certain piezometers only slightly differ from the stabilized maximum ground water levels. In the column 6 of table 1 there is a fact of the maximum velocity of the level diagram rise, VmaY Provided that only the vertical flow existed V,,, would be Zma/rz, where I,,, maximum intensity of infiltration is equal to “vi”, and where “r]” is the infiltration coefficient, “i” the intensity of precipitation, and “n” the coefficent of effective porosity. According to the rainfall measurement data for the given conditions, we could take i = 0.1 m/day and since the rise of level occurs in the rainy season the approximate value of r] could be 7 = 0.8. Regarding the data from the table we could consider ‘‘n” of approx. value of 0.01 i.e. 1 /,. Such a little permeability coefficient is completely

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conformed to the some data which can be found in literature. Thus for instance Plotnikov (1957) considers that for the karst terrains “n” is 0.05-0.01 and for less karstified terrains it is 0.01-0.005. Since the permeability decreases with the depth for the upper stages “n” should he: n > 0.01 and in lower stages it is the same or even less. The next important item of the piezometric observations is the data of the minimum

P:

3 Cumulative frequency 7.

Fig. 5 - BuSko Blato. Cumulative Frequency of the MaximumJWPT: A. South-Western Border Zone; B. Western Border Zone.

water levels. This piezometric level shows the general drop in the west and south-west directions (same as the maximum and medium levels). However, the decrease in these directions is not monotonous. Obviously the terrain where the piezometers are situated is not homogenous and the collecting zone can be below the mesured ground water minimum levels in most piezometers. The absolute maximum levels in each section

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TABLE 1 Characteristic piezometric datu of Buiko Blato

1 2 3 4 5 6

2.1. I 2.2. Maximum water level

4.1. I 4.2. Lowering of water :vel durini the last month of recession period

1.1. I 1.2. Observation well Mini-

m u m water level

,mpiitude of wate level fluctuations

daximum rate of water level rise Maximum

in observed period

Eleva- ;ion of :round urface m

706,7 707,7 703,4 71 I ,9 740,O 767,9 762,3 782,8 743>4 854,O

Stabi- lized 2.1.-3.) :2.2.-3.] No.

E - 1 E - 4 E - 5 E - 6 E - 7 E - 8 E -13 E - II E - 29 E - 14

m/month m

26,O 50.0 80,O 68,O 67,O 82,O 39,O 86,O 102,o 58,O

m

706,O 708,O 704,O 704,O 712,O 730,O 724,O 736,O 740,O 808,O

m

680,O 656,O 624,O 636,O 645,O 648,O 685,O 650,O 638,O 750,O

m

25,O 48,O 78,O 64,O 55,O 60,O 19,O 54,O 72,O 10,o

m

705,O 704,O 702,O 700,O 700,O 708,O 704,O 704,O 710,O 760,O

E - 19 E -26 E -28 E - 10 E-I2 E -27 E -17 E -22 E -24 E - 20

757,9 755,O 848,O 768,6 714,2 858,O 738,2 763,8 751,3 753,6

734,O 705,O

726,O 700,O

704,O 750,O 740,O 710,O

-

-

740,O 728,O 748,O 730,O 704,O 740,O 714,O 760,O 746,O 740,O

684,O 620.0 686,O 71 5,O 654,O 695,O 630,O 660,O 660,O 604,O

50,O 85,O 2

1 l,o 46,O

74,O 90,O 80,O 106,O

-

337 10,o

11,o

12,5 18,O 15,O 10,5

I - -

56,O 108,O 62,O 15,O 50,O 45,O 84,O 100,o 86,O 136,O

128,O 70,O 80,O 82,O 50,O 57,O 37,O 42,O 76,O 72,O

56,O 72,O

92,O 73,O

84,O

- -

- - 40,O

19,O 20,o 10,o 935 15,O 11,o 5,7 3,3 17,O 7,3

746,O 690,O 700,O 698,O 696,O 665,O 660,O 668,O 696,O 680,O

674,O 678,O 700,O 700,O 700,O 680,O 702,O 700.0 682,O 660,O

680,O 682,O 680,O 696,O 690,O 670,O

626,O 630,O 622,O 618,O 652,O 623,O 628,O 628,O 624,O 624,O

632,O 628,O 631,O 614,O 610,O 610,O 700,O 700,O 618,O 664,O

120,o 60,O 78,O 80,O 44,O 42,O 32,O 40,O 72,O 56,O

42,O 50,O 69,O 86,O 90,O 70,O 2,o 070 64,O -

E -21 E -23 P -13 P -14 P -15 P - 3 P - 4 P - 5 P -16 P - 17 P -18 P -19 B -la B -2a B - 3a E -25 K- 3 K- 4 P - 8 P - 10 P - 1 1 P -22 P -12 P -20 P -21 P - 7

778,3 769,8

722,9 730,3 723,2 71 7,2 717,7 735,O 715,5

736,7 722,8 700,O 700,O 699,9 729,3 70?,5 700,9 717,9 704,4

716,4 695,6 707,2 697,4 803,5 707,9

- 754,O 700,O 702,O 700,O 702,O 674,O 665,O 770,O 700,O 696,O

688,O 690,O

702,O 683,O

- -

- - 702,O 704,O

692,O 684,O 700,O 728,O 702,O 708,O

624,O 636,O 660,O

666,O 630,O

-

-

56,C 46,O 20,o

24,O 40.0

-

-

68,O

40,O

36,O 78,O

48,O -

- Cases where the stabiliseted maximum level is not well defined.

are of special interest because the other higher levels could be considered as a sort of the suspended water table. The existence of some irregularities in the piezometric level do not compromise the afore stated regularity of the level drop related to the depth, but it shows that this regularity should be comprised as a general tendency admitting some local exceptions. As far as the leakage from the BuSko Blato Reservoir is concerned, it, is interesting to consider the fact of relatively low water levels in the piezometers of the Korito Syncline which indicates the possibility of the eventual leakage in this direction. As emphasized in item 3, the minimum levels do not correspond to the situation

of impervious strata. The minimum level position is a function of a series of parameters. Since the phenomena of the level drop from the maximum to the minimum are very complicated due to the fact that the water level passes through zones of different permeability and undergoes the influence of precipitation, we shall analyze only the phenomena near the minimum water levels where it is much simpler. For this purpose we have taken from the level diagram the value of the ground water level drop in course of one month, marked by AH (table 1, column 6). As it can be seen A H varies between 2-14 m. Hence we can obtain the mean permeability of the rock mass by means of the equations of the unsteady filtration and presuming the corresponding scheme of flow. W e have accepted the scheme of the runoff of the mass without infiltration or inflow figure 6.

Fig. 6 - Scheme for a Water Level Drop Estimate: - the Area of Undefined High Permeability; - the Area of Defined Permeability; - the Free Surface Line.

It is supposed that in the initial moment the level of outflow is horizontal and that the rock mass empties in consequence of a sudden drop in level of the surface water. In this case the drop of the ground water level is the function of parameter

HO the initial depth, t time, K coefficient of filtration, n porosity.

by the integral After Polubarinova-Kocina (1 952) the interdependence HIHO can be represented

H HO -=JFo

Where

47

The value of the above integral of the random processes can be found in mathematical and technical handbooks.

For further calculation we have taken the coefficient of prosity n = 0.01. Regarding the distance x (the boundary of the porous massive) it is certain that x

is smaller than the distance between the sinkhole edge of BuSko Blato and the eastern edge of Sinjsko Polje (20 km approx.) because the existence of big springs on the flank of Sinjsko Polje which shows that the fissure porosity of the rock mass must at a certain moment turn into higher porosity. O n the other hand, this boundary cannot be in the vicinity of the polje. This can be concluded from the regularity of decrease of most piezometric levels, from the fact that the permeability in the zone of reservoir decreases very rapidly with the depth, from that the sinkholls through which the lake empties from time to time, have outstanding hozirontal outlets etc. It has been consid- ered that the reasonable border x should he between 5 and 10 km. Having the supposed values x and n the further calculation do not pose any difficulty. From the equation 6 we have

where A H is the ground water level drop in an accepted time interval, in our case one month, given in the column 6 table 1, and vi, and q~ are the values of parameter q, given in the equation 5, in the moments ti and f2, its interval being one month.

For the calculation we accepted tl = 60 and tz = 90 days. The calculation was done in such a way that the distance x was established for a series of values AH, for several depths Ho, as well as for three values of the permeability coefficient, K =0.01, K = 0.1, K = 1 m/day. The results are represented in a diagram (fig. 7). As indicated in the diagram, only the permeability of about K T 1 m/day conveys reasonable values of the distance x. Accordingly, the values of permeability coefficient of a porous mass in the area i.e. below the minimum levels, should be approximately the same. It can be seen, as well, that the depth of a water bearing layer should be at least some 200 meters. Just to remind that when determining the average maximum WPT in the zone of minimum water levels we obtained K = 0.1 m/day. The real values, as seen in the diagram, are considerably higher. Although the above calculations are not precise, because of many introduced presumptions, we consider that the result (obtained by the analysis of the piezometric data) is much closer to reality than the WPT data. Hence the conclusion which does not speak in favour of WPT data because it is often impossible to find from these data the approximate values of the average permeability of the rock mass.

In order to establish the difference between the permeability Pi of the south-west sector and the permeability PZ of the remaining part of the western edge of the polje, we have first calculated the mean values AH. For the south-west edge A H = 6 m and for the rest of the western flank it amounts LI to N = 3.3 m/month.

By using this fact îor the supposed Ho = 300 m, the permeability relation KlIK2 = 1.30 was obtained. This value, which corresponds to the one obtained by the WPT analysis, shows that on the south-west part a greater leakage could be expected, but still of the same approximate values as on the south-west edge. It is up to a detailed calculation and economic estimate to decide whether this sector should be separated or not.

The presented calculation was done by one of the authors but the results of the calculation as well as some problems in connection with the grout curtain have not been given regarding the character of the article.

The limestone bedrock of the BuSko Blato Storage Basin is reaching to a practically limitless depth. This bedrock mass suffered from orogenetic movements owing to which a certain extent of fissuration occurred. The rate of fissuration however is

48

decreasing with the depth. The ground water percolation is also decreasing with the depth, owing to the fact that in greater depths hydraulic gradients are smaller and the conditions are therefore less convenient for the development of a natural karst drainage under the corrosive influence of the percolation waters there, than in the zone with higher hydraulic gradients. Here in the zone of more moderate hydraulic gradients with very slow percolation effects much better conditions are existing for sedimentation of terra rossa, i.e. for the filling of fissures through which the ground water made its way.

Data obtained by investigations in BuSko Blato are in accordance with this explanation.

Fig. 7 - Correlation between H (Water Level Drop During the‘last Month of the Recession Period), between the Permeability Coefficient K”, between the Calculative Distance to the Edge Zone of the Porous Rock Mass X, and the Staring Depth HO of the Water table calculated by Means of equation 8.

. . . . . HO = 700 meters. .-.-.-. HO = 500 meters. ----_ HO = 300 meters.

HO = 200 meters.

The relative inclination between Bubko Blato and the karst springs in Sinjsko Polje (the latter are in fact just discharge outlets of the natural under-ground drainage flows by which waters are flowing off the BuSko Blato karst Field), is comparatively small and amounts to 4 per cent only. The high permeability zone in BuSko Blato is reachinginto the bedrock-according to data available now-some 50 to 60 meters only.

Water percolation in vertical direction-with a high gradient consequently-is occurring during low waters in the zone of this high permeability, starting from the surface and downwards to the ground water table.

49

The more the ground water table rises, the more the height of the zone of such a percolation decreases and gives place to a drainage with relatively low gradients, which the area formed according to conditions of the flow with the continuous groundwater table through fissure systems and karst natural drains of this permeable superficial

STABILISED MAXIMUM WATER LEVEL

Fig. 8 - BuSko Blato. Correlation between the Maximum Stabilized Love1 and the Terrian Elevation.

karst zone of bedrock and of its further karst channels towards Sinjsko Polje. This direction of the ground water drainage is predominant in this area and therefore the main karst drainage channels are developed here in this direction, both the sinkhole channels and the spring channels.

Although the zone under observation has been strongly karstified and the sinkholes are of a great capacity indeed, it is out of question that the underground flow-ways have not developed to Such an extent, as to make possible an unhindered water Row. The following facts point to this: u) During high waters BuSko Blato is inundated; b) A certain correlation exists between the terrain surface level and the maximum

stabilized water levels (the latter are shown on fig. 8);

50

c) In the subsoil of the Dinara Mountains, in the area of the karst sinkhole drains from BuSko Blato, during the raining season comparatively high ground waters are formed, and they put a limit to the draining capacity of the BuSko Blato sinkholes. During low waters there is no such draining limitation, and then there are more convenient conditions for the ground water flow-off from BuSko Blato towards Sinjsko Polje.

5. THE PERUCA WATER STORAGE BASIN

The Peruta Water Storage Basin (fig. 9) is extending 20 kilometers in ienght and is situated in the karst area of the Upper Cetina Valley. With a waterhead of 60 meters this reservoir accumulates 540 million cubic meters, this being some 30 per cent of the average annual Cetina River water discharge at this particular profile. This water storage has been formed to accumulate water for the Split Hydro-Electric Power Station in the lower course of the Cetina River (fig. i).

The geological and technical problems confronting the formation of this water storage and the technical solution chosen for the task (which was subsequently accomplished) have been described by one of the authors at two International Congresses on Large Dams in 1955 and in 1961, (see the attached List of References) and therefore in this report just a summarized description will be given. Supplementing data concerning leakage, which have been recorded over a period of five years since the utilization of this reservoir, and the interpretation of WPT statistical data by the method explained above, both are now providing new information, and this should help to throw further light on the problem of water storages in karst areas generally.

The bed of the PeruCa water storage is situated in an area of cretaceous limestones along syncline (shown on fig. 9). This limestone section is in its flanks and in its bottom impounded by concordantly lying lower cretaceous dolomites. At the upstream and at the downstream and it is impounded by tectonic outcrops of werfen slates. This means that water cannot be lost either to another catchment area or to the sea. The only other possibility would be that the water would bypass the dam (this dam is constructed on the same limestone bedrock), but it must appear again in any case before the aforementioned geological barrier. This makes it possible that all the waters of the Cetina River, including therefore the leakage from the Peruca water storage also. are completely utilized in the Split Hydro-Electric Power Station further downstream, and just for the requirements of this Station the Perda reservoir has been principally formed.

During investigation works two possibilities of water leakage from the water storage have been considered. The first possibility of water leakage could occur by flowing-off through the net of karst underground channels, of which the upstream ones feed the strong springs which are coming under hydrostatic pressure of the 35 meter high water of the storage, and the rest of them are feeding the other strong springs located downstream of the dam site. The second possibility is water percolation through the fissured limestone area at the dam site profile.

By means of watercoloration tests the spot was determined at which the ground water flows of these karst springs are branching. Placed piezometric boreholes have shown that ground water levels do not fall there below the foreseen maximum waterhead of this water storage. Consequently through this net of karst channels water cannot be lost from the Peruta artificial lake.

The position and size of the grout curtain for the cout-o8 of the dam area has been determined by investigations. Some eighty drillholes in a total length of more than 14,000 meters have been made, core samples were taken and water permeability tests performed. Later these drillholes were used to take records about the height and about variations of the ground water table.

51

500 7

400

300 -

200-

1 O0

52

1

53

Cumulative frequency of the maximum WPT of PeruCa is shown in the attached diagrams (fig. 10, fig. 11 and fig. 12), separately for the central part of the reservoir bedrock at the dam profile, and separately for the left and for the right flank. Data are grouped into stages of 50 meter each. As we can see, the percentage of the boreholes with an undetermined intense permeability: P = rm is relatively considerable. P = cci

5 4

3

u)

g 2 & 4

4 I00 .b .Y : 8

e 6

.- 2 7 $ 5 $ 4

t 3 2

10

8 7 6 5

c

3

2

1 , 1 , , , , , , , , , , I 1 , , . , , , , , , , , , , . , I , , , I , , , , , , , , , I , , , , , I , I

Cumdative frequency y. 0.1 0.2 O S 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99599.899.9

Fig. 10 - PeruCa. Cumulative Frequency of the Maximum WPT. Observation Boreholes in the Central Area.

appears for the boreholes at the left flank and in the central part including the stage 250-200 meters of depth, and on the right one even deeper (stage 200-150 meters). At the left flank there exists a distinctive collector zone at the elevation 250-200, which is the most permeable of all the stages of 50 meters at the dam site. The perme- ability at the deeper stages decreases with the depth. The decrease exponent (equation

54

no. 3) is 0.0158 which gives an actual depth of the permeable stratum of 62 meters. At the central part the decrease of permeability is in relation to the depth very distinc- tively below the stage 250-200, and therefore the effective depth in relation to the mean height of the stage 250-200, i.e. for the elevation 225: Zef = 45 meters. At the right flank the decrease of permeability with the depth could also be observed (here also

i) Cumulative frequency *i.

Fig. 11 - Peruta. Cumulative Frequency of the Boreholes in the Left Side Flank.

Maximum WPT. Observation

below the stage 250-200), but here it is less distinctive due to the different course of the cumulative frequency curve of the individual stages of 50 meters. Concerning this analysis of the WPT data we would like to point out an important feature in connection with the left flank at the stage 300-250, for which there are practically only excep- tionally high and very low permeabilities, i.e. for which there is no proper correlation

55

between the high and the low permeability. From which one may conclude that in karst even in a comparatively flat mass without fissures, considerable canals may be found. This is directly allowed in a number of cases by boring the tunnels in karst.

According to the principle mentioned earlier, the grout curtain should cut through all the sectors of the undefined high permeability, and this would mean that the grout

Fig. 12 - Peruta. Boreholes in the

Cumulative Frequency Right Side Flank.

+ Cumulative frequency Y.

of the Maximum WPT. Observation

curtain should be laid across the whole bedrock beyond the elevation 200 (and at the right flank even below this elevation). Such a criterion however would hardly be acceptable here for economic reasons. Hydrologic conditions make it possible to disregard this serious criterion owing to the following reasons: In the given conditions water flows which formed the channel network in the karst could appear-owing to

56

the direction of the flow-in general only perpendicularly to the valley, because in this direction only a stronger piezometric runoff gradient was occurring. However, from the aspect of the water leakage around the d a m the ground water channels parallel with the valley are much more important.

I 1

I - l 2 3 D I S C H A R G E mysec. -

Fig. 13 - Spring Discharge along the Cetina River downstream of the PeruCa D a m for the Following Profiles: Profile B. is immediately downstream of the dam; Profile Z. is 100 meters downstream of the dam; Profile ZZ. is 200 meters downstream .of the dam; Profile ZII. is 750 meters downstream of the dam; Profile IV. is 1500 meters downstream of the dam.

5

4000-

900-

800 -

700-

600-

500 -

Velebit Mountain

4

58

Fig. 14 - KrubCica Water storage: 1. Drillhole; 2. Ground Water Level:

3. Sinkhole; 4. Underground karst Water W a y tested by Colouring.

u) Maximum, b) Minimum;

59

This possibility to avoid to cut off all these drains in the zone of undefined high permeability on the left flank has been confirmed by colour tests and by piezometric data as well, which have-as mentioned earlier-pointed to the fact that the ground water drains branch at an elevation which is higher than the maximum waterhead in the reservoir, and also to the fact that along this flank there is in existence even a natural barrier of ground water, and therefore the grout curtain was executed to lean against this water barrier.

The curtain was formed 1800 meters in length and from 150 to 250 meters deep, as shown on plate 9.

Since the PeruCa reservoir was put into operation it was found that all the waters lost from the water storage are returning into the Cetina River bed downstream of the dam by means of permanent springs which are to be found predominantly on the left bank of the river. Systematic ground water level measurements have been started during the period of researce works, but have been continued ever since, and new measurements were undertaken after the reservoir formation in order to collect data concerning the quantity of water leakage. The main difficulty for determination of the actual water leakage quantity was encountered in connection with the proper assess- ment of the original water discharge of these springs to the exclusion of water quantities due to water leakage in the reservoir. Owing to the fact that these water springs are dispersed along the Cetina river bed, their discharge could not be measured but after the dam was constructed and the reservoir formed. But then the influence of water leakage during the high waterhead in the reservoir appeared in the water springs.

The original discharge values of these springs along the Cetina River bed are being assessed by hydrologic interpretation of the correspondent of the discharge data of the Mali Rumin Spring. This was made possible owing to the fact that this spring received its water from the same karst hydrologic system as all the other considered springs, and on the other hand it was possible to bring forth ample proof that the leakage waters from the PeruLa reservoir do not mix with the original water of the Mali Rumin Spring.

In figure 13 results of water discharge measurements of the aforementioned springs along the Cetina River downstream of the PeruCa D a m have been shown and placed into correlation to the elevation of the water level in the reservoir in the moment of measurement. The first curve with the indication mark “B” indicates the springs discharge immediately along the dam, and each of the curves marked I, II, III and IV indicate the total spring discharge up to 100, 200, 750 and 1500 meters downstream of the dam. Measurement points ZZZ and ZV indicate that all the water leakage from the reservoir returns into the Cetina River bed not further than at the profile III, i.e. within the sector of up to 750 meters downstream of the dam. Further downstream of the profile ZII no water leakage return could be evidenced. As aforesaid for determining suitable unbacked discharge the spring Mali Rumin

was used. Its discharge is indicated as R in figure 13. If its discharge is really represen- tative the reservoir losses should be given by a bunch of lines each of which would correspond to its constant discharge. According to a small number of date it was not possible to draw the bunch of curves. In figure 13 only two curves are drawn from which the full one corresponds to the most logical correlation at less discharges of the Mali Rumin spring, and the dobted one corresponds to the discharge Q = O of the Mali Rumin spring only at low discharges. The latter may be considered as the correlative dependence which gives the upper limits of losses. They amount to Qmax = 1,7 m3/sec and an average annuel discharge of 1,20 cubic meter per second, what represented about 2,O per cent of the annual water discharge of the Cetine River at the PeruCa Dams.

Water discharge data which in figure 13 fall to the right of the curve for the profiles ZII and IV have a much higher natural original discharge than the data according to which this curve has been formed what can be seen from the indicated

60

water discharges for the Mali Rumin Spring. It is therefore quite natural that these are data deviating so much from the curve mentioned.

A n analysis of the produced data confirm the correctness of the conception which has been chosen within the design, that under certain conditions it is both possible and necessary to recede from the mentioned principle that all the zones with undefined high results of water leakage tests should be cut through. Such decisions must be made very carefully and they must rely on very through hydrogeological studies.

6. THE KRU~CICA WATER STORAGE

The KruStica Reservoir (fig. 14) is being built on the Lika River whose waters are utilized together with the waters of the Gacka River (having a waterhead of 432 meters) in the Senj Hydro-Electric Power Station.

Cumulative frequency %

Fig. 15 - KruSEica Water Storage. Cumulative Frequency of the Maximum WPT. 61

The reservoir will take in 240 million cubic meters of water at a waterhead of 80 meters, and this amount to some 22 per cent of the average annual water discharge of the Lika River.

The river bed of the KruSfica Storage Basin is Located in a rocky ground consisting of eocene-Oligocene breccia and limestone. These strata are lying atratigraphically discordant over a substratum consisting of cretaceous and jurassic limestones with dolomites. The triassic series, following the next ones and consisting of thick strata of dolomites and slates, form along the Velebit Mountain ridge a high impervious barrier, which is impounding the Lika River Valley towards the sea (fig. 14:, and therefore in the area of the KruSfica water storage area there are no sinkholes. This barrier in the Velehit Mountain declines lower than the Lika River level as far as some10 kilometers downstream of the KruEica Dam, and there we encounter sinkholes, whose karst ground channels convey the water to the sea. The KruSEica dam site on the Lika River is far enough upstream from this sinkhole area.

In order to obtain necessary information on bedrock of this water storage and on prevajling hydrogeological conditions, some fifty drillholes have been made with coring and water permeability tests, in total length of 9000 meters. Later on all the drillholes have been used for piezometers. The largest part of these drillholes has been made in the area ofthe downstream zone of this reservoir and in its flanks far into the background (fig. 14). Data obtained by these drillholes and by colour tests of the under- ground flows in the water storage flanks have shown that the Lika River Valley in the area of the KruSGca Reservoir and of the immediate downstream zone is in fact the lowest draining recepient of this area. W e come to the conclusion therefore that water leakage losses from the water storage KruSEica will return into the Lika River and will be utilized in the Senj Hydro-Electric Power Station.

Due to aforementioned statements no demand has been imposed to the effect that the water storage should he completely watertight, but only that the water leakage losses should be reduced to a reasonable rate, similarly to the case of the PeruCa Reservoir.

The interpretation of the maximum WPT statistica1 data, as it can he seen for three stages of 50 meters each in figure 15, is showing that here the zones of undefined high permeability appear in the upper 100 meter region. Here the permeability decreas with the depth more obviously than in the other two water storages. The exponent of permeability decrease with the depth is here 0.032, and this gives an actual depth of 31 meters.

In the case of the KruSEica Water Storage the size of the grout curtain has been determined by applying the same principle as in the case of the Peruta Water Storage, owing to the mutually corresponding facts, but most of all owing to the fact that any waters by-passing the dam will return into the water flow upstream of the Senj Hydro-Electric Power Station, for the sake of which the water storage is being formed. The grout curtain, as adopted in the design, will he 700 meters long and 100 meters deep.

LIST OF REFERENCES D. BABAC, Sur l’influence de la couche superficielle peu perméable sur pertes d’eau

des retenues (Etude en cours). Y. LABYE, Étude statistique du coefficient Kz de filtration verticale dans une zone

pédologiquement homogène; Vfe Journées de l’Hydraulique, Nancy, 1960. G. OLLÖS, Possibilities of model investigations into water movement occurring in

fissured rocks, Ninth Coni>enfion of IAHR, Dubrovnik, 1961. B. PAVLIN, L. MLADINEO, V. STUBIEAN and E. NONVEILLER, Bassin d’accumulation

de PeruCa dans le Karst Dinarique, (Q 18, R22), Ve Congrès des grunds barrages, Paris 1955.

B. PAVLIN, Réalisation du bassin d’accumulation de PeruCa, (Q25, R85), Vffe Congrès des grands barrages, Rome 1961.

N. J. PLOTNIKOV, M.V. SIROVATKO, and D. J. SHEGOLEV, Podzemnie vodi rudnih mestoroidenia, Moskva 1957.

P. XA, Polubariiiova Kochina. Teorija dviienia gruntovih vod, Moskva 1952.

62