The Devon Karst Research Society.
Part 5.3.  Eastern Herzegovina.
Cernica Polje, BiH-RS Trebinje Region - Gacko Municipality.

Updated 13 November 2006.

Cerničko Polje  -  Церничко Поље





2.1...SUMMARY :
Other than water derived directly from atmospheric precipitation, all incoming water originates either in Gatačko (Malo) Polje to the north and north-west, appearing in Cerničko Polje via the giant Vrelo "Vilina Pećina" karst resurgence spring, or as karst groundwater within the authigenic zone of the Jurassic Limestone Massif of Mount Baba along the Northern perimeter of the karst polje, appearing as permanent karst exsurgence springs along the base of the whole length of the northern perimeter.
Downstream, all water leaving Cerničko Polje eventually reaches the River Trebišnjica either directly or via other karst polja at lower elevations.

1...The Austro-Hungarian Monarchy karst engineer Philipp Ballif, (1896)*, gives us an appraisal of the karst hydrology of Cerničko Polje at the time of his extensive karst melioration works in the nearby area of Gatačko Polje :-
....."Water Supply :..Through springs, the Stepen Brook and moreover, through the Vilić Pećina in the northwest edge of the basin, at the base of a precipitous cliff, and which probably connects with the ponors draining the Mušnica River.
......Drainage :..Through Ponors, in which both the Stepen and Jasovica Brooks are also swallowed.
......Degree and Duration of Inundation :  In the east part of the polje, by the Stepen Brook, though most part inundated annually, this is only for a shorter duration than the one in the west part by Zagradci; there, the impact of the Vilić Pećina from the second half of October through to the middle of April causes the same situation, when the great amount of water coming out of it flows into the great ponor by Zagradci until it can no longer absorb the water, at which time the whole polje then becomes flooded."

2...An Hungarian intellectual Asbóth, János, wrote the following about Cerničko Polje in his travels through "Bosznia és a Hercegovina" during 1884* :-
"…..  To the west from the castle [=Grad Kljucs] two 3-m high and 3-m thick walls emerge forming a natural defence for the formal stable and gardens. Under these cliffs a bigger stream flows. This stream comes out at once from the vertical wall of Baba and after six hundred footsteps disappears in a sink hole.....
…..  Before coming out from Baba, a huge cave collects the water from the depth of the mountain. The entrance of the cave appears ten metres above the surface of the valley. At low water level the stream only comes out from little holes and crevices, but after heavy rains and snow melt, the whole depth of the cave become full and across it’s enormous entrance the water starts coming out as a strong streamflow, forming a waterfall, which is unique and such things can only happen in the Karst …..
…..  At low water level when the water is only leaking from the crevices, an expert climber can reach the entrance but nobody has ever gone further. Deep under the entrance inside the cave the water surface of the lake can be seen among rocks …..
…..  It is a fact that on one side of the cave wall there is a big hole. It is known by the local folk that behind that hole a huge lake exists in the massif of Baba. If you throw a rock strongly into the hole the echo sounds like the rock has hit water surface. At high water level its possible that the water is coming from this second hole filling up the big hall of the cave…."
.....[English Language Translation by Gergely Balázs, Budapest.]

2.1...DETAILS :
[Other webpages in this Series deal with the detailed description of individual karst features mentioned in the text below.]
From our current perspective, we can state that the two historical notes above indicate great accuracy in reporting some of the karst hydrodynamics of this strange karst polje. In the dry summer months, the casual visitor would have no indication of the situation that occurs in the very wet winter months. All would be calm, peaceful and for the most part, dry! There would be no indication of the immense karst hydrological chaos that exists here in the height of the wet winter period.
Let us examine the situation step by step and thus, understand the mechanisms controlling where all the water comes from, how, where and why it appears in such a dramatic way and then, where it goes after leaving Cerničko Polje :-

a)  At an elevation of between 810 m - 850 m asl., Cerničko Polje is located at an intermediate level between Gatačko Polje (882 - 950 m. asl.) and Fatničko Polje (460 - 480 m. asl.).
b)  The Polje is formed and aligned along a major regional reverse fault, separating different lithologies. On the north side of the fault is karstified Jurassic Limestone, which outcrops and forms the high escarpment of the Baba Mountain Massif. This forms almost the entire northern perimeter wall of the Polje. On the other side of the fault-interface is non-karstic, impermeable Eocene Flysch, which extends southwards towards the southern edge of the Polje and in so doing, forms most of the polje-floor areas. The Eocene Flysch extends in a NW-SE alignment for about 5 km. and has been proven by geophysical investigations to have a vertical depth range of about 200m.
c)  The entire southern perimeter wall is formed by karstified Cretaceous Limestone, which abuts the lower-lying impermeable Eocene Flysch.

[Summarizing so far, we have a long but narrow low-lying basin of non-karstic, impermeable rock (Eocene Flysch), forming the polje-floor of Cerničko Polje, which is bounded all around by mountainous karstified limestone facies. Please refer to the Hydrogeological schema below, which illustrates a Plan View and Cross Section of this scenario.]

d)  Sandwiched between karstified mountain massifs, the non-karstic, impermeable Eocene Flysch, therefore, forms a locally important hydrogeological barrier (aquiclude).
e)  The Baba Mountain Massif along the northern perimeter of the Polje separates Cerničko Polje from the adjacent higher elevation of Gatačko (Malo) Polje. 
f) In the adjacent Gatačko (Malo) Polje during the dry summer months, only a few of the ponors are active when the River Mušnica is at its minimum flow.
One of these is the terminal Šabanov Ponor, which is permanently active and feeds directly and only to the Trebišnjica River Springs at Bileća.
There are also permanently-active low-absorption capacity ponors in the area of Bašići and even the Srđevići (Jama) Ponor can become active at just above minimum water flow of the Mušnica River, but none of these ponors would appear to contribute to the hydrology of Cerničko Polje in the dry summer months, although we are investigating the ponors around Bašići to confirm this. So the water from Gatačko (Malo) Polje all travels around the Eocene Flysch barrier of Cerničko Polje to appear at the Trebinjica River Springs at Bileća.
g)  However, when the flow of the River Mušnica in Gatačko (Malo) Polje increases above its minimum, other ponors start to become active.
The total quantity of water within the aquifer rises to a level whereby it activates a route in the direction of Cerničko Polje. The water cannot pass beneath Cerničko Polje because of the depth of the impermeable Eocene Flysch hydro-geological barrier, so it rises up within the karstified Jurassic Limestone near its northern boundary of the impermeable barrier to appear directly in Cerničko Polje via the main karst resurgence spring, Vrelo "Vilina Pećina".
The water then flows the short distance across the top of this Eocene Flysch hydro-geological barrier in Cerničko Polje via the Ključke Rijeka, only to sink abruptly again into the karst underground of the Cretaceous Limestone via the Ponor "Ključke Rijeka" on the far southern side of the Eocene Flysch barrier. By this route, some of the Gatačko Polje ponor water feeds the giant estavelles of Fatničko Polje about 15 km. distant.

The output of the combined flow of Vrelo and Izvor "Vilina Pećina" into the Central depression of Cerničko Polje is about 50 m³ sec­¹. The absorption capacity of the Ponor "Ključka Rijeka" is only about 20 m³ sec­¹. For this reason, an intermittent lake appears in the Central depression which back-flows in an easterly direction into the Eastern Section of Cerničko Polje, where another intermittent lake is then created. The connection point between the Central and Eastern Sections of the Polje is illustrated in Section 1. of this series of webpages. It takes the form of a small karst canyon, the origin and functionality of which would be completely obscure, unless seen in the very wet winter months!

Karst hydrological activity in the Eastern Section of Cerničko Polje is less dramatic but, nonetheless, very interesting. Here are groups of ponors centred around the two principle ones of Jasovica Ponor and Šukovići Ponor. Both these ponor groups have a limited absorption capacity. Currently, we have been unable to visit these ponor areas due to the adverse environmental conditions prevailing during our reconnaisance of the polje.


.....Date of Test  =  29 November 1961.
.....Method  =  Na-fluorescein.
.....Water level  =  high.
.....i)...Connection with  =   Estavela "Obod" and Estavela "Baba Jama" in Fatničko Polje;
..........Straight-line distance, sink to rising  =..14.0 km.
..........Vertical range, sink to rising  =..342 metres (1122 feet).
..........Water Velocity (sink to rising)  =  11.27 cm sec -¹.
.....ii)..Connection with  =...Izvorište Trebišnjice.
..........Straight-line distance, sink to rising  =..25.60 km.
..........Vertical range, sink to rising  =..493 metres (1617 feet).
..........Water Velocity (sink to rising)  =  12.59 cm sec -¹.

In a Paper given by Borislav Petrović during the Treći Jugoslavenski Speleološki Kongres (1962), this dye test was cited as having fast underground connections with "Izvorište Trebiišnjice" (Trebišnjica Springs) and to a marginally lesser degree with Estavela "Obod" in Fatničko Polje. This latter point, if correct, would indicate that Estavela "Obod" may not be a principal outlet but probably an overflow off one side of the master karst conduit aquifer between Gatačko / Cerničko Polja and the main outlet at the Trebišnjica Springs. This insight into the karst hydrology of the area may prove to be valuable to us in our investigations.
The dye-tracing tests below also indicate faster flow-times from the ponors in the Eastern Section of the polje to Izvorište Trebišnjice, than to Estavela "Obod".

.....Date of Test  =  21 February 1964.
.....Method  =  Na-fluorescein.
.....Water level  =  0.50 m³ sec -¹.
.....i)...Connection with  =...Izvorište Trebišnjice.
..........Straight-line distance, sink to rising  =..25.60 km.
..........Vertical range, sink to rising  =..485 metres (1591 feet).
..........Water Velocity (sink to rising)  =  8.15 cm sec -¹.
.....ii)..Connection with  =...Estavela "Obod", Fatničko Polje.
..........Straight-line distance, sink to rising  =..19.80 km.
..........Vertical range, sink to rising  =..334 metres (1096 feet).
..........Water Velocity (sink to rising)  =  6.11 cm sec -¹.

.....Date of Test  =  26 March 1965.
.....Method  = Na-fluorescein.
.....Water level  =..0.05 m³ sec -¹.
.....i)...Connection with  =...Estavela "Obod", Fatničko Polje.
..........Straight-line distance, sink to rising  =..15.40 km.
..........Vertical range, sink to rising  =..334 metres (1096 feet).
..........Water Velocity (sink to rising)  =..0.88cm sec -¹.
.....ii)..Connection with  =...Izvorište Trebišnjice.
..........Straight-line distance, sink to rising  =..24.60 km.
..........Vertical range, sink to rising  =..485 metres (1591 feet).
..........Water Velocity (sink to rising)  =..6.7cm sec -¹.

after P.T. Milanović.

...............PLAN VIEW.....................................................................................................................CROSS-SECTION (North-South)

.....KEY / LEGEND :
.....Cross Section :-
.....1...Jurassic and Cretaceous Karstified Limestone  /  Jura i kredni okršeni krečnjak  /  јура i кредни окршени кречњак.
.....2...Eocene Flysch sediments (hydrogeological barrier)  /  Eocenski fliš (hidrogeološke barijere)  /  Еоценски флиш (хидрогеолошке баријере).
.....3..."Ključka River" Ponor  /  Ponor "Ključka rijeka"  /  Понор "Кључка ријека".
.....4..."Vilina Cave" karst spring  /  Vrelo "Vilina Pećina"  /  Врело "Вилина Пећина".
.....5...Continuous surface flow  /  Stalni površinski tok  /  Стални површински ток.
.....6...Reverse (Overthrust) Fault  /  Reversni rasjed  /  Реверсни расјед.
.....Plan View :-
.....E...Eocene Flysch sediments (hydrogeological barrier)  /  Eocenski fliš (hidrogeološke barijere)  /  Еоценски флиш (хидрогеолошке баријере).
.....J....Karstified Jurassic Limestone  /  Jura okršeni krečnjak  /  јура окршени кречњак.
.....K...Karstified Cretaceus Limestone  /  Kredni okršeni krečnjak  /  кредни окршени кречњак.

Much of the geological information about what lies below the polje-floor was recovered from borehole data and geophysical surveying.

Cerničko Polje  -  Церничко Поље


There is much Hydrometric Infrastructure around the Ponor complex in the Central Vrtača. For practical reasons, it was found to be easier to measure the ponor-input rate rather the individual yield of Vrelo and Izvor Vilina Pećina. The Hydrometric Infrastructure consists, firstly, of a gigantic concrete weir across the ponor-approach canyon, through and over which the entire flow of the Ključka Rijeka has to pass until the ponor becomes inundated. Secondly, there is a whole series of water-level guages rising up the southern perimeter wall of the polje from near the position of the weir in the ponor-approach canyon. Of course, the water-level indicator guages can tell nothing about flow-rates but can indicate when the level is reached whereby the backflow or overflow commences from the Central Doline or depression into the Eastern Section of the Polje.

Photo CP2.1. (above-left) - A view of the concrete hydrometric weir at the Ponor "Ključka Rijeka", looking down from above. ..(Photo :  Igor Trklja, 02 September 2006.)
Photo CP2.2. (above-right) - A long view of the upstream side of the concrete weir, with the water-level guage staffs rising up the side of the ponor-approach canyon on the left side...(Photo :  B. Lewarne, 27 August 2005.)

Photo CP2.3. (above-left) - A view of the position of the lowest water-level guage staff upstream of the hydrometric weir at the Ponor "Ključka Rijeka", looking down from above...(Photo :  B. Lewarne, 02 September 2006.)
Photo CP2.4. (above-right) - A long view looking down at the water-level guage staffs rising up the side of the south wall of the polje above the ponor-approach canyon. .(Photo :  B. Lewarne, 27 August 2005.)

We currently have no information about the possible presence of hydrometric infrastructure at the principal ponors in the Eastern Section of the polje.

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