FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ
The Devon Karst Research Society & Speleološko Društvo "Zelena Brda"
Istočna Hercegovina, Bosna i Hercegovina.
Источна Херцеговина, Босна и Херцеговина,
FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ
Fatnica Polje, Eastern Herzegovina, Bosnia & Herzegovina.
Webpage last updated 30 January 2006.
Section 8.0.    THE "OBOD" ESTAVELLE AND THE FATNIČKO POLJE TRUNK-AQUIFER (CONDUIT) CAVE SYSTEM  -
-  ESTAVELA "OBOD"  -  ЕСТАВЕЛА "ОБОД" :
["Obod" = brim or edge.]
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Fatničko Polje Section 8. Webpage Contents :
Section 8.1.  Location. (opposite);
Section 8.2.  Description. (opposite);
Sub-Section 8.2.1.  Surface Area; (opposite);
Sub-Section 8.2.2.  Hydro-engineering Infrastructure;
Sub- Section 8.2.3.  Speleography;
Section 8.3.  Cave exploration in Estavela "Obod";
Section 8.4.  Geological Setting;
Section 8.5.  Hydrological Characteristics;
Sub-Section 8.5.4.  The Experimental Closing of Estavela "Obod";
Sub-Section 8.5.5.  Observations on Changes in Functionality of Estavela "Obod";
Sub-Section 8.5.6.  Physico-Chemical Characteristics of the "Obod" Water;
Section 8.6.  Sub-Project Reports for Authorized Users.

Click on the underlined Links above to move down to the Sections more quickly.
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Photo FP8.2.  (Digital Sony FD71.)  (Photo :  Bogdan Prelević, 08 September 2001.)
There is a standing figure on the lower right hand side of the view by the ground foliage.
 



HISTORICAL :   Photo FP8.3. (below) (Digitally reproduced from Lithograph.)
"DIE OBODQUELLE AM RANDE DES POLJES VON FATNICA (HERZEGOWINA)."
Translation :-  "The Obod Spring at the Edge of the Polje of Fatnica (Herzegovina)."


This amazing feature was formally described in great detail by Grund (1910)
in his "Das Karstphänomen" in which he also published a black & white
photograph of the estavelle, which we have reproduced here.
The necessity of including a recognizable scale in this view does not seem to
have occurred to the photographer!


 

8.1.    LOCATION :
[GPS Identifiers :..43° 01' 06'' N : 018° 20' 54'' E.]
As a karst geomorphological, hydrological and speleological feature, this estavelle is one of the most dramatic, spectacular and imposing karst and features in Eastern Herzegovina, both when it is overflowing with water or absorbing water and when it is completely dry.
It is situated below the main road running along the northern perimeter of the polje, just west of Orahovica Village and below an obvious Stećci site situated beside the main road near Kalac Village. The gigantic mouth of the Estavelle is easily accessible at polje-floor level in the Upper Polje. The lip of the Entrance Shaft at the top of the Mill Dam at polje-floor level is situated at an elevation of 476.17 m. asl.  It is such an enormous karst feature that it poses a practical problem when it comes to photographing it!!  Ideally, the photography should be done from the air high above the ground. Until we can arrange for this to be done, we have no alternative but to try our best at recording it from the ground. We have replaced some of our earlier images of this remarkable feature with more recent ones, in the hope that we can capture its enormity.

8.2.    DESCRIPTION :
..........8.2.1.  SURFACE AREA DESCRIPTION :
Its wider opening at the surface has the maximum dimensions of 30 x 60 metres, (98 ft x 197 ft.). From the polje-floor level up to the top of the feature, just below road level above, the vast opening forms a deeply-cut curving cliff rising upwards to a height of 100 metres from the edge of the entrance shaft.. This inward curving cliff is a bisected vertical shaft, eroded out from its south side by the sand-laden water issuing from the estavelle and pouring out onto the polje floor, which itself has gradually been lowered by the same fluvial processes. From the edge of the entrance shaft (top of the Mill Dam) at polje-floor level down to the foot of the entrance shaft, it is a maximum vertical distance of 50 metres (164 feet) to bedrock. However, there was an accumulation of about 6 metres of sand and pebbles at the bottom of this shaft on our visit during September 2001. In cross-section, the Estavela "Obod" entrance shaft takes the shape of a deformed funnel.
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Photo FP8.1.  (Photo :  Tibor Dianovszki, 17 July 2004.)
A general view of Estavela "Obod" from the polje floor opposite, indicating the Estavelle Dam, the two Water Mills. The view also shows the vertical cliff behind the Dam which is an upward extension of the far side of the entrance shaft.
The dry streambed of the Obod River is represented by the earthy ground approaching the camera position.
 
 
 
 
 
 
 
 
 

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In Photos FP8.2. (left-side column, top) and FP8.4. (immediate-left; Photo by Tibor Dianovszki, 16 July 2004.), we have offered two modern views of this feature indicating the same two extremely large boulders in the middle-left of the view, still in virtually the same position 94 years later after Grund's photo in FP8.3. (left-side column), though recorded from a slightly different viewpoint.
Photo FP8.5. (above)  (Photo : Tibor Dianovszki, 17 July 2004.)
This view was taken from a high position on one side of the inside of the entrance shaft.
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Photo FP8.9. (Digital Sony FD71.)  (Photo :  B. Prelević, 08 September 2001.)
A view of the "Obod" Estavelle Dam.
The Figure in the foreground, to the left below the Dam, provides a scale.

Photo FP8.10. (Digital Sony FD71.)   (Photo :  B. Prelević, 08 September 2001.)
A view of the central stanchion that used to support a pedestrian wooden-bridge access between
the two Water Mills when the Dam was overflowing. The pillar is set on the limestone bedrock of
the Obod streambed. The standing figure gives a scale.
..........8.2.2.    ESTAVELA "OBOD" DAM, WATER MILLS AND BRIDGE INFRASTRUCTURE - Description of the individual elements of the Hydro-engineering Infrastructure :


 
 
 

In front of Estavela "Obod" at polje-floor level, a Dam has been formerly constructed across its mouth on the limestone bedrock forming its natural overflow lip. At each end of the Dam are the remains of a Water Mill. The Dam impounds the water to feed the Water Mills.
To complete this ancient industrial infrastructure are the remains of a bridge, which formerly enabled foot-access between the two mills when the Dam was overflowing; any alternative route being totally impractical. On top of the central stone stanchion of the footbridge are the remains of iron anchor-points to which the wooden bridge was fixed. This prevented it being washed away during periods of high flood.
 
 
 
 
 

Photo FP8.6. (left), (Photo :  B. Lewarne, 08 September 2001., Digital Sony FD71.)
The view on first approaching the "Obod" Estavelle Dam, two Water Mills (one on either side)
and remaining standing central stone pillar of the Bridge Foundations (centre).
 
 
 
 

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Photo FP8.7. (above left)  (Photo :  A.J..Satterley, 17 July 2004, Digital Sony FD75.)
A view of the Babić Mill beside the Estavelle Dam. In the right-foreground, is central stone stanchion that used to support a wooden bridge.
Photo FP8.8. (above right) (Photo :  L. Satterley, 17 July 2004, Digital Sony FD75.)
A view of the Babić Mill beside the Estavelle Dam. In the right-foreground, is central stone stanchion that used to support a wooden bridge.
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..........8.2.2.    ESTAVELA "OBOD" DAM, WATER MILLS AND BRIDGE INFRASTRUCTURE - Ownership of the Hydro-engineering Infrastructure :
We have confirmed the true ownership of the two mills, which we believe were constructed no later than during the Austro-Hungarian Occupation - and maybe even earlier.
The Babić Mill, situated to the left of the dam in Photo FP8.6., Photo FP8.7. and Photo FP8.8. above, is named after one of the two principal owners. A senior member of the local Babić family died in 2001 and is buried in the cemetery contiguous with the old Stećci Site above Estavela "Obod". Other members of this family are presently scattered around the Bileća and Trebinje areas.
Although the Kundačina Mill on the right side of the dam reflects the family name of another owner, (probably the Mill operator), the other principal owner is the Bajramović family. This family have now "emigrated".
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We are aware that the HET Company in Trebinje claims ownership of Estavela "Obod" and its superficial infrastructure. However, we have received good local legal advice that this natural feature cannot be owned by this Company and is certainly the property of the State. We bear in mind the similarly false claims of this Company to the ownership of all the Ponor Mills along the riverbanks of the regulated Trebišnjica River in Popovo Polje. The Estavelle Dam was formerly a shared asset in the joint ownership of the owners of the two Mills.
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..........8.2.2.    ESTAVELA "OBOD" DAM, WATER MILLS AND BRIDGE INFRASTRUCTURE - Condition and Functionality of the Hydro-engineering Infrastructure :
The condition of each of these components has sadly deteriorated.
-..As of 08 September 2001.
..........-..The Estavelle Dam still functioned perfectly but was in dire need of repair due to the collapse of the lower part of its outer lining at its east end. The collapsed debris lay at the foot of the Dam.
..........-..The Babić Mill is in a better condition than the Kundačina Mill and is missing its roof tiles but the wooden roof-frame is still in position. The Mill is in a generally very bad condition, but is recoverable.
..........-..The Kundačina Mill is missing most of the superstructure although the foundations survive. Mill Stones are present in both Mills together with an interesting design of water supply channels within each mill structure.
..........-..The Wooden Bridge is long gone but the foundations at each end are in position, as is the central stone stanchion, as indicated in Photo FP8.10. above.

-..As of early 2004., the already poor condition of the lower part of the outer lining at the east end has further deteriorated due to the abnormal pressure of water issuing from the estavelle during the extremely wet winter of 2003 / 2004. The result is that a hole has been punched right through the structure of the Dam. This situation has affected the natural functionality of the estavelle, as further detailed below.
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Photo FP8.11.  (Photo :  B. Lewarne, 08 September 2001., Digital Sony FD71.)
The abandoned high-level apse.

Photo FP8.13.  (Photo :  B. Prelević, 08 September 2001.Digital Sony FD71.)
The blocked cave passage near the base of the Entrance Shaft of "Obod" Estavelle.
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..........8.2.3.  UNDERGROUND DESCRIPTION OR SPELEOGRAPHY :
Within the confines of the surface shaft, there are several lateral side passages at various levels.
..........-..The highest one that we have observed is far above the polje-floor level and appears to take the form of an abandoned, high-level apse, pictured in Photo FP8.11. below.
..........-..There also appears to be large cave entrances on either side of the shaft, not far above polje-floor level, and directly associated with the major fault that passes across the estavelle mouth.
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Click on this Link to observe the relative positions of these high level caves above the flooded entrance shaft.
Video Sequence MPEG8.2.   *..Obod Surface Shaft high-level caves.
(60 seconds duration; 1503 frames.)
 
 
 
 

Photo FP8.12. (left).  (Digital Sony FD92.)  (Photo :  B. Lewarne, 13 July 2004.)
One of the high-level cave entrances in the entrance shaft of Estavela "Obod", zoomed into from a position on the Dam.
 
 
 

..........-.There is a further cave entrance on the east side of the shaft near its base. At its entrance, it is 2.5 m in height. (see Photo FP8.13. left.) The passage is blocked with loose sediment and rock.
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A full assessment of the cave passages leading off from the exterior top part of the Estavela "Obod" Surface Shaft will be undertaken in due course and their inter-relationship plotted.
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At the base of the dramatic 50 m. (164 feet) deep entrance shaft, the karstified Upper Cretaceous Limestone bedrock is exposed, revealing erosional and corrosional karrenforms on an impressive scale. (see Photos FP 8.14., 8.15. and 8.16. below.)
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Speleomorphology at the Base of the Estavela "Obod" Surface Shaft.

Photo FP8.14.  (Photo :  B. Lewarne, 08 September 2001.Digital Sony FD71.)
A view of the deeply cut karrenforms on the limestone bedrock in
the entrance to the Obod Cave at the base of the shaft.
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Photo FP8.15. (above left).  (Photo :  B. Lewarne, 08 September 2001., Digital Sony FD71.)
The first of two images detailing the view looking down at the base of the Obod Surface Shaft, indicating the karrenforms. The next image (Photo FP8.16) indicates the same area looking out from the interior.
Photo FP8.16. (above right).  (Photo :  B. Lewarne, 08 September 2001., Digital Sony FD71.)
Detail of the area at the base of Estavela "Obod" Entrance Shaft, looking out from the position of the concrete plug. The view indicates the karrenforms and the sediment accumulation.
There would appear to be a another blocked cave passage at the foot of the cave wall on the right-hand side of the view. The standing figure giving a scale to the image is Bogdan Prelević.
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At the time of our last visit on 08 September 2003, there were about 6 metres of sand and pebble deposits at the bottom of the entrance shaft.
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The main way on takes the form of a short section of neo-horizontal cave passage, whose floor is washed limestone gravel. This accumulation of sediments and debris obviously conceals the full size of the passage at the base of the Estavela "Obod" Surface Shaft. Ahead lies the infamous concrete plug which has an access hole at its top end. At the time of our last inspection, there was a climb of 2.0 metres up to the access hole from the gravel beach floor of the cave.
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The access hole leads directly to a short length of high passage, which itself leads to the top of the 1st Underground Vertical Pitch. We have estimated that this 1st Vertical is only 18 m. (59 feet) deep to the cave floor below, (now permanently submerged).

Photo FP8.17.. (Photo :  B. Lewarne, 13 July, 2004., Digital Sony FD92.)
A view at the bottom of the entrance shaft looking in at the concrete plug. Most of the concrete plug
remains in situ though buried on the outside below a pebble beach.
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Photo FP8.18. (above).  (Photo :  Dianovszki Tibor, 13 July, 2004., Digital Z1)
A view standing near the top of the 1st Underground Vertical, looking back out towards the concrete plug. The right-hand wall is covered with sinter and rises up into the blackness of this impressive cave passage.
Photo FP8.19. (left).  (Photo :  B. Lewarne, 13 July, 2004., Digital Sony FD92.)
A view standing at the top inner edge of the 1st Underground Vertical, looking down at a descending group.

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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ The roof of the very large underground chamber on the inside of the concrete plug rises up into darkness. The wall ahead on the left side is covered with a large mature stalagmite flow, varying in colour from brown to whitish-cream. On the right side, the ceiling is not so high and is very water-worn with solution pockets and large karren forms.
Below, can be seen the top half of the first underground vertical pitch, which is a steeply-sloping stalagmitic free-climb leading down to an overhanging-to-vertical lower half, containing a large solutional pocket part way down which forms a convenient ledge. The descent of this leads one immediately to land into the Obod River below. 
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We have estimated that this 1st Vertical is only 18 m. (59 feet) deep to the cave floor below.
Underground, in the short section of known cave system, Estavela "Obod" is again impressive in its dimensions. The immediately accessible passage at the bottom of the 1st underground vertical, is terminated at both its upstream and downstream ends by syphon lakes. These have been dived during the 1960's in the period of investigation before the plugging experiment described below. We currently have no detailed information about the passages beyond the syphon pools, although as of September 2003, the Society has begun to sponsor the investigation of the permanently phreatic sections of the estavelle. Further details about this can be seen in the Section 18. of the Fatničko Polje Webpages.
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The accessible passage at the time of the 1960's investigations totalled about 76 m. (250 feet) in length, including the airspace above the syphon lakes. Along this length of passage, there was a difference in elevation between the surface of each of the two siphon lakes of 13.22 metres. (43 feet.) The southerly Downstream Siphon Lake surface was at an elevation of 416.795 m. and the northerly Upstream Siphon Lake surface was at an elevation of 403.575 m.
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This length-dimension of the accessible cave passage will of course change dramatically as water levels fall or rise above or below those prevailing at the time of the survey, which was undertaken during the dry summer period in 1964. During the period of estavelle activity in either its spring or ponor mode, there would be no vadose air-space anywhere in this short section of known cave passage. Details of the cave survey that was prepared, are given below. We do not have access to the indicated cross- sections. Some structural geological information is provided in the cave survey below, which has lost its original scale of 1 : 500.
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Click on this Link to view a JPEG image of the   *..1964 Obod Cave Map.
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The problem that has now become apparent to us in recent years is that the section of impressive vadose cave passage surveyed in the early 1960's and as presented in the Obod Cave Map Link, seems no longer to be above water at low-water levels!  This annoying scenario is yet another effect of the creation of the HET Company's Bileća Lake and its drowning of the Outlet Springs at the principal Local Base Level of Erosion.
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This comprises the maximum extent of the accessible vadose passages at low water conditions.
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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ ..........8.3.   CAVE EXPLORATION IN ESTAVELA "OBOD" :
This magnificent estavelle offers enormous potential for speleological exploration. The many entrances in the top half of the entrance shaft will be investigated as part of separate sub-Projects. The greatest potential for new or previously unexplored passages are within the Fatničko Polje Trunk-Aquifer (or Master Conduit) at the bottom of the 1st Underground Vertical Pitch. Downstream exploration of this trunk-aquifer from the downstream siphon, will inevitably lead to a connection with Estavela "Baba Jama". (Further downstream exploration of the trunk aquifer in this direction beyond Estavela "Baba Jama" will be dealt with in Section 9. of these webpages).
However. it is with the exploration of the Upstream Siphon that the greatest potential exists. The section of the Fatničko Polje Trunk-Aquifer upstream of Estavela "Obod" could eventually lead to the ponors of Cerničko Polje, a minimum straight-line distance of 16km.
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There is now a new Section 18. for the Fatničko Polje Webpages which are dedicated to the CAVE DIVING PROGRAMME in this remarkable area and includes cave diving investigations in Estavela "Obod". The information about this which was formerly located here, has now been transferred to Section 18. Further information about the interior of the Estavela "Obod" Cave System in the Upstream Siphon is now available to us, as a result of the Hungarian Cave Diving operations in September 2003., July 2004. and August 2005.
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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ .
8.4.    GEOLOGICAL SETTING :
Estavela "Obod" is located in Upper Cretaceous Limestone near its contact with Eocene flysch sediments. This contact takes the form of a reverse fault, whose trend is continued along the north-eastern edge of the polje. At the mouth of the Estavelle are two major fractures, perpendicular to the direction of the contact direction.
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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ ..........8.5.    HYDROLOGICAL CHARACTERISTICS OF ESTAVELA "OBOD" :
..........8.5.1.  FUNCTIONALITY :
By definition, an estavelle performs the functions of both a karst spring and those of a karst ponor. The hydrologic functionality of the Estavela "Obod" is principally that of a gigantic karst spring.
During its periods of hydro-activity, its function as a ponor is considerably shorter in duration than when it is functioning as a spring (vrelo). From the point of view of contributing to the drainage of Fatničko Polje, its ponor function is almost irrelevant. This is due to its position at the edge of the Upper Polje, where it is always left abandoned by the falling flood-water levels of the periodic lake, which discharges into the ponors at the lower polje.
However, when it does perform as a ponor, during the initial stages of receding water-levels from a high-level flooding event, it receives and swallows at a rate of Qmax  = 2 m3 sec-1  of the periodic lake.
When functioning as a karst spring, it is classified as a "vrelo" spring-type and could be classified as a Vauclusian-type (ascending), B-Type (Allogenic) karst spring. The maximum yield that it has been possible to actually measure has been 35.8 m3 sec-1. The maximum estimated yield has been calculated as being Qmax= 60 m3 sec-1.

The periodic karst stream that issues forth from this Estavelle is known as the Obod River (Oboda rijeka).

The apparent discrepancy in the values between what it can yield as a spring and what it can swallow as a ponor, is due to the prevailing, relative piezometric levels below the polje floor during the flooded and the subsequent drainage periods, respectively.

Photo FP8.20. (above).  (Photo :  Bogdan Prelević, 11 November 2001., Zenith SLR film camera.)
The "Obod" Estavelle functioning as a vrelo-type karst spring, recorded in early November 2001.
The entire surface shaft is now full of rising karst water and, if it were not for the Dam, would be fully overflowing its natural bedrock lip and discharging into the polje floor. The respective water-levels between the estavelle and the polje floor indicate that it is in a "spring mode" of functionality and not a "ponor mode".

Photo FP8.21. (left).  (Photo :  Bogdan Prelević, 11 November 2001., Zenit SLR film camera.)
The "Obod" Estavelle functioning as a Vrelo-type karst spring, recorded in early November 2001 after a prolonged drought in Eastern Herzegovina. Only now is the poor waterproof condition of the Dam evident. The bottom right-hand corner of the Dam structure is in the worse condition, and could be washed out completely under a severe flood conditions.
 
 

..........8.5.2.  ESTABLISHED UNDERGROUND FLOWLINE DATA :
.............a) Date of Test  =  summer 1961.
.............Method  =  Na-fluorescein.
.............Water Velocity (sink to rising)  =
.............Connection  =  with Trebišnjica Izvorište (Trebišnjica Spring Group) near Bileća.
.............It has also been proven by Na-fluorescein dye testing that incoming waters feeding the Estavela "Obod" supply originate from :-
....................Cerničko Polje - Ključki River Ponor,  (18 November 1961.)
....................Gatačko Polje - Srđevići Ponor,  (19 October 1964.)
....................Gatačko Polje - Turčinovac Ponor,   (22 November 1961.)

..........See separate entries under these Polja (when available) for further details.

..........8.5.3.  UNUSUALLY HIGH MAGNITUDE OF HYDRAULIC CHARACTERISTICS :
It has been long noted by local inhabitants that in the 15 to 30 hours preceding the Estavela "Obod" functionality as a karst spring, strong periodic seismic disturbances or ground tremors can be felt in the area, together with muffled sounds resembling explosions.
To confirm this singular observation, a single-component seismograph was installed in a concrete bunker on the high cliffs above the entrance to the estavelle. The results of one year's seismic observations have scientifically confirmed the experiences of the local inhabitants.
When considering the morphology of the base of the surface shaft of the estavelle, together with that of its cave system, the seismic tremors are the result of tremendous volumes of water passing through the karst channels under enormous pressures and in doing so, causing cavitation on a grand scale! This phenomenon is to be expected under the circumstances.

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The Experimental Closing of Estavela "Obod".


"Pogled na Betonski Čep. HE Trebišnjica-II Etapa. Zatvaranje Estavele 'Obod'.
Nacrt Betonskog Čepa."


Translation :
 "View of Concrete Plug. HE Trebišnjica-Stage II. Closing of the Estavela 'Obod'. 
Sketch of the Concrete Plug."
(Document extract taken from the HET Archive, Trebinje.)

The above sketch shows a part of Estavela "Obod" that we can no longer see.
The heavy dark line inside the main drawing indicates the original size of the
bedrock orifice leading into the underground at the base of the entrance shaft.
The straight lines passing through this heavy dark line out to the broken line at the
perimeter of the drawing indicate the position and extent of penetration of steel
reinforcing bolts into the limestone bedrock around the cave orifice, which was
completely plugged with steel-reinforced concrete.
This drawing indicates that the vertical range of the natural cave passage at this
point was about 11m. (36 feet) in maximum height and 3.5 m. (11.5 feet) in
maximum width. The clearly-indicated access hole at the top of the drawing,
together with a few metres of concrete plug below it, are all that can be seen today.
Until this concrete abomination is completely removed, the full natural functionality
of Estavela "Obod" will never fully return.
 

..........8.5.4.  THE 1964 EXPERIMENTAL HYDRO-TECHNICAL CLOSURE OF ESTAVELA "OBOD" BY THE HET Co., TREBINJE. /  ZATVARANJE ESTEVELE "OBOD" :
..........Introduction :
To reduce both the period of inundation of Fatničko Polje and the unwanted loss of water to the Bregava River Catchment to the west, an experiment to seal the "Obod" Estavelle was proposed in 1964. It was also thought that this may have the effect of creating a longer-lasting inundation in Cerničko Polje. After speleological investigations had been concluded within the Estavela "Obod" Cave System by certain Speleological Groups, the sealing of the estavelle at the narrowest point at the base of the entrance shaft was undertaken and the results were observed.
As mentioned above, a maximum of 15% of the water arriving at Fatničko Polje would, under natural conditions, then proceed to discharge to the Bregava Springs, via certain ponors in the polje-floor. So, the intention was to "force" that part of the water which would normally outflow into Fatničko Polje, to remain as an underground flow and discharge wholly towards the Trebišnjica Catchment, to the benefit of both the Hydro-electric Plant at Grančarevo (Trebinje) and further on at the Ombla Hydro-electric Plant at Dubrovnik.
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..........Pre-experiment Precautions :
In order to observe the effects that the plugging of the Estavela "Obod" would have on the Trebišnjica Spring Group, the experiment would have to be undertaken before the creation of the Bileća Lake, which would inundate the springs at the principal local base level of erosion.
Being mindful that the vertical head of water feeding into Fatničko Polje from the main supply at Cerničko Polje is ca. 400 metres, any concrete plug being set into the base of Estavela "Obod" would have to be constructed to withstand a minimum pressure of 40 atm.
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Also being mindful that a catastrophe may occur as a result of impounding water right up to the level of Cerničko Polje and of causing problems around the Estavela "Obod" area arising from the great pressure of water trapped in the underground system, a "safety valve" would have to be constructed within the plug-infrastructure to permit a rapid restoration of the natural conditions.
The "safety valve" was chosen to be an explosive charge, which had to be waterproofed against ingress from the cave side of the plug as well as the exterior side of the plug which would be inundated under natural conditions when the polje flooded. The explosive device had to be guaranteed to remain waterproof for a minimum of two years.
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To enable the pressure to be measured remotely, a manometer was also constructed within the fabric of the concrete plug. The outside of this device was led up and out of the mouth of Estavela "Obod" to a safe and permanently dry place in the polje above.
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..........Constructional Details :
Accordingly, a massive concrete plug was designed by the engineers of the Energoinvest Co. of Sarajevo. The on-site construction was undertaken by the Jugometal Enterprise of Ljubinje under the technical supervision of the Energoinvest Engineers. The emplacement of the explosive device was undertaken by the enterprise Pirotehnika from Sarajevo. The pipes which carried the detonator wires up to the polje floor were even protected against lightening strikes.
Thus the concrete plug was cast in situ. Its dimensions were 11 m. high and 2.7 - 3.5 m. in width. At the top, a hole of dimensions 1.53 m. x 1.40 m. was left in the structure. This was eventually covered with a steel-reinforced concrete slab containing 10 kg. explosives. The plug was finally designed to be able to withstand a pressure of  40 Bar. The pressure behind the plug was measured using a pressure guage connected via an in-built pipe, which was extended up and out to a place in the polje above, beyond the level of inundation. The experiment was planned to be on-going for one "hydrological year".
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..........Observations :
The dramatic events that took place soon after the installation of the plug had been completed, are as follows :-

11 October 1964 :   Extremely heavy rainfall is experienced in the Estavela "Obod" catchment area, with average rates of precipitation measured in excess of 100 mm. (3.9 ins.). Some rain guages measured 230 mm. (9.0 ins.) of precipitation. Estavela "Baba Jama"  began to issue water at 1830hrs. and at Estavela "Pribabići"  at 1800hrs.
11-12 October :   Above and around Estavela "Obod", new springs appeared. The highest ones appeared at levels (overnight) of 90 - 100 m. (295 - 328 feet) above the level of Estavela "Obod", with some springs appearing in the houses 80 m. above the level of Estavela "Obod". The total yield of water from these new springs was calculated from both actual and estimated values. It was found to be of the order of 11 m3 sec-1. Five very strong earth tremors and a large number of smaller ones were felt in the area.
12 October 1964  At 0700hrs., the pressure guage indicated 7.0 bar. At 1400hrs., the Estavela "Obod" Dam started to overflow. By 1800hrs, the pressure had risen to 9.25 Bar. Very loud noises were heard down the entrance of Estavela "Obod" and more strong ground tremors were felt in its vicinity. The road above the Estavela "Obod" now starts to slide away and there are landslips near the well above "Obod". The highest pressure recorded was 10.6 Bar (12 Atm.) and the water in the underground was dammed up 120 metres above the Estavela "Obod" Dam overflow. At 1400hrs., the new springs appearing at the houses above Estavela "Obod" are yielding 100 litres sec-1.  Later in the day, new springs were appearing above Estavela "Baba Jama"  and to the west in Fatnica Village.
14 October 1964 :   All traffic was prevented from using the road above Estavela "Obod".
The result of hydrological analysis (Petrović 1965.) indicated that about 30 m3 sec-1 of water was prevented from discharging into Fatnčiko Polje during this period.
A first attempt at mining the plug failed. A comparatively stabilised situation was now observed over the next few days, with no appreciable increase in pressure and no new springs appearing. A period of equilibrium had been achieved whereby Estavela "Baba Jama" and Estavela "Pribabići" were coping with the new artificial conditions being imposed by the concrete plug.
18 October 1964 Owing to the increasingly critical situation, destruction of the concrete plug by the detonation of its in-built explosive device was successfully undertaken at 1100hrs., using an increased voltage of 220 volts. The water level over the Dam at the exit of Estavela "Obod" rose from 14 cm. to 130 cm. within 5 minutes, which by calculation equated to a flow of 55  m3 sec-1. This level slowly dropped over the course of the next few days until at 0700hrs. on the 21st October, it was 110 cm. The highest of the new springs above Estavela "Obod" stopped flowing at 1200hrs. By 1345hrs, all the new springs above the road level stopped flowing.
The mining of the concrete plug destroyed the pressure guage pipe making it impossible to track the rate of pressure decrease in the Estavela "Obod" system. All new springs above Estavela "Obod" stopped flowing during the next 3-6 hours, further indicating the rapid rate of pressure decrease in the underground. The road surface above Estavela "Obod" settled by 50 cm. (20 ins.) along a length of 30 m. (100 ft.). Nine houses on the slope above Estavela "Obod" at a distance of 250 - 300 m. were damaged so badly that the households had to be evacuated.
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Conclusions :
Heavy rains affected the experiment to the extent that instead of lasting for one hydrological year, it was ended after only one week of heavy rain !!  The increased underground pressure and the activation of the aeration zone caused many problems on the surface of the terrain. As a result, it was clear that the expected effects of the sealing of the estavelle were not achieved under the prevailing hydrogeological conditions in Fatničko Polje. ie :-
..........-  the geological conditions prevent the formation of an underground accumulation of water, and
..........-  the transitory "base level of erosion" that exists, (and which causes the bifurcation of the watershed), is only relevant at high water levels.
The yield of Estavela "Baba Jama", the second largest karst vrelo in Fatničko Polje, belonging to the source zone of the same catchment as Estavela "Obod", increased due to the underground pressure rise, by 5m3..sec-1. Under natural conditions, this estavelle starts functioning as a spring at least half a day after Estavela "Obod".
The yield of Estavela "Pribabići" during the experimental period had attained an estimated 2 - 3 m3 .sec -1.

The conclusions that were drawn were that the sealing of Estavela "Obod" for the single reason of preventing the discharge of its water into Fatničko Polje and thereby forcing its flow solely towards the Trebišnjica River Catchment, is linked with the co-creation of insurmountable side-effects elsewhere on the karst terrain, the sheer scale of which had been entirely unpredictable. The expected gains in hydro-energy generation do not justify such risks.

Click on this Link to view an image of the   *.."Obod" Surface Shaft Concrete Plug Location.

Click on this Link to view an image of the   *.."Obod" Surface Shaft Concrete Plug Construction Details.

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..........A Final Disconcerting Note :
It should also be stated that the partial removal of the concrete during the emergency mining operation at the termination of the "Obod" Estavelle Closure Experiment, has not fully restored the levels of functionality of the Estavelle. In its original condition, there were two large orifices at the base of the entrance shaft, through which water flowed in or out. It should be clearly understood that 90% of the concrete plug remains in situ where it continues to plug the lower of the two original orifices and restricts flow through the upper orifice. Hence, one of the effects is to lessen the power of the emerging water which now drops most of its heavier debris once it has been carried to the outside of the concrete plug area. The base of the "Obod" Estavelle Surface Shaft has silted up. We refer you to the Links above which indicate the constructional details of the "Obod" Estavelle Concrete Plug.
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Photo FP8.26.  (Digital Sony FD92.)  (Photo :  B. Lewarne, 13 July, 2004.)
A contextual view of the change to the base of the Estavelle Entrance Shaft from the inside with
the new accumulation of debris washed down from higher up the slope.
..........8.5.5.   OBSERVATIONS ON RECENT SIGNIFICANT CHANGES IN THE NATURAL FUNCTIONALITY OF ESTAVELA "OBOD" :
13 July 2004 :
As is apparent from the details shown in Photo FP8.21. above, the structural integrity of the Estavelle Dam has been a questionable matter for several years. The issue has now become more interesting because during the extremely wet winter months of November 2003 to April 2004 during the "spring" mode of the Estavelle, the Dam structure has deteriorated even further with the appearance of a large hole and the washing away of much of the base of the masonry structure around the hole. The Estavelle has now partly recovered the original, natural elevation of the lip of its entrance shaft, with the resultant effect of prolonging the period of functionality when the Estavelle is in the "ponor mode" and more significantly, of enhancing the potential energy of the incoming water, which is no longer fully restricted by the porous nature of the Dam structure. Photos FP8.22. and FP8.23. below indicate the recent damage to the Estavelle Dam.

Photo FP8.22. (above left).  (Photo :  L. Satterley, 17 July, 2004., Digital Sony FD75.)
A view of the damage to the Estavelle Dam from the outside. The Figure is pointing to the hole.
Photo FP8.23. (above right).  (Photo :  B. Lewarne, 17 July, 2004., Digital Sony FD75.)
A view of the damage to the Estavelle Dam and the hole, as seen from the inside.
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The effects of this significant change in the functionality of Estavela "Obod" can be seen at the base of the entrance shaft, where a large quantity of limestone pebble debris has accumulated, having been washed down from the higher levels of the entrance shaft by the increased erosive power of the incoming water during the short period when the Estavelle was in its "ponor mode". We expect this infilling process to continue and accelerate in future years until the original stable, natural condition is assumed, bearing in mind that the energy during the outflow functionality of the Estavelle remains greatly reduced by the continuing presence of the bulk of the HET Co. concrete plug !!  Photos FP8.24., FP8.25. and FP8.26.  below indicate the recent observed changes to the base of the Estavelle's entrance shaft.
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.....Photo FP8.24. (right).  (Photo :  B. Lewarne, 13 July, 2004., Digital Sony FD92.)
.....A view of the change to the base of the Estavelle Entrance Shaft from the inside.
.....There is a new accumulation of debris washed down from higher up the outside slope of the entrance shaft.
.....Photo FP8.25. (left).  (Photo :  B. Lewarne, 13 July, 2004., Digital Sony FD92.)
.....A closer view of the change to the base of the Estavelle Entrance Shaft from the inside.
.....There is a new accumulation of debris washed down from higher up the outside slope of the entrance shaft.
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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ .............8.5.6.   PHYSICO-CHEMICAL CHARACTERISTICS OF THE WATER IN (OR ISSUING FROM) ESTAVELA "OBOD"  :
Constructing a detailed quality profile of the water within or issuing from the Estavela "Obod" System, has to be accomplished over a minimum period of 10 "hydrological years". The factors that we are interested in monitoring are related to two particular aspects :-
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..........a)  the contribution of the Estavela "Obod" water to rates of karst corrosion, and
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..........b)  the suitability of the Estavela "Obod" water as a habitat.
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In March 2002, we had access to the HET Company Water Analysis Laboratory, situated near the Gorica Dam. We were shown their technology and discussed their sampling methodology. Although we now have access to their data, we do not agree with their water sampling methodology and we are not convinced of the accuracy or reliability of their results, having observed at first hand the condition of some of the analytical equipment. It is therefore pointless to compare their results with our own.
The Link below will take authorized Project workers into the Fatničko Polje Karst Water Quality and Species Sightings Records Database.
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  *..Fatničko Polje Karst Water Quality Database and Species Sightings Records.
(Status :  Active - Authorized users only)
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Additionally, we are analysing ash samples from the waste dumps arising from the TE Gacko I thermo-electric generating plant in Gatačko Polje. The ash is stored in the catchment area for Estavela "Obod". This ash is derived from the low quality lignite fuel used in the power plant furnaces and the ash dumps have polluted the Estavelle about 3 years ago (1999) and caused the Gaovica Fish to become temporarily extinct at this location. We have learned from a local farmer, Mr Stanko Dučić in Fatnica, that at the very beginning of March 2002, he caught 66 Gaovica Fish in the Estavela "Obod" Entrance Shaft Lake. He is of the opinion that, although the fish have now returned to the site, they are poisoned because when left lying around on the ground, they are no longer taken by wild animals. If this pollution-event has indeed affected the water quality of the site to such an extent, we thought it unlikely that we would be able to confirm that Niphargus and Proteus are in residence here, at least in the medium-term future. However, the results of two seasons of cave diving in the permanently phreatic passages of the estavelle have revealed the abundant presence of crustacea. The results of the chemical analysis of the power station ash will be published after the chemical analysis has been completed. See also Fatničko Polje Webpages, Section 13., for further details about this situation in Gatačko Polje. (A separate Main Section of the Website is currently under development and will be dedicated to the karstography of Gatačko Polje).
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8.5.7.   BACTERIOLOGICAL CHARACTERISTICS OF THE WATER IN (OR ISSUING FROM) ESTAVELA "OBOD"  :
As of 2004, we shall be including this site in the range of locations subjected to Baceriological Examinations by an accredited external laboratory. The results are available to Project Workers via a Link in Section 21. of the Fatničko Polje Webpages but will not yet be made available for Public Access.
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FATNIČKO POLJE - ФАТНИЧКО ПОЉЕ 8.6.   ESTAVELA "OBOD" SUB-PROJECT REPORTS, DOCUMENTATION, FILES, STILL IMAGES AND MOVING IMAGES
...........for Access by Project Workers and Authorized Users :
These are all located via Links in Section 21., where all digitized Sub-Project Reports and Documentation dealing with the investigation of Fatničko Polje are located. Please remember that individual Reports may not be available to unauthorized users.
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