A view of the confluence where water from the Stržen (left) and the Cerkniščica (right) flows into Velika Karlovica, enclosed by wooden “rakes.” (Photo: M. Blatnik)
Within the LIFE Tršca project, part of the research is dedicated to the flood dynamics of the Cerknica plain, which are closely connected to wetland habitats. The flood dynamics themselves are the result of weather conditions (especially the amount and distribution of precipitation) and the consequent inflow of water into the plain, as well as the outflow capacity – where, in the case of the Cerknica plain, the Karlovice play the most important role.
The Karlovice are a fairly complex network of cave passages with two larger cave entrances, called Mala Karlovica and Velika Karlovica, and an additional artificial entrance, Nova Karlovica. The total length of the known cave passages is 8.1 km with an elevation difference of 20 m, with the most distant passages from the sinkhole area almost connecting to the passages of the 7.3 km long Zelške caves. Between these two cave systems lies the collapse doline Šojica, where the fallen rock blocks prevent cave divers from passing through.
Most of the Karlovice passages (and also those of the Zelške caves) are spacious and horizontal, yet not easy to explore. A large part of the passages is covered with mud and collapsed blocks, while even more passages are partially or completely flooded, requiring diving equipment for exploration. Most of the cave passages accessible by boat were surveyed in the 1960s, while diving explorations took place later and at a somewhat slower pace.
Penetrating through water-filled passages of Velike Karlovice.
Karlovice represent the most important ponor area for the outflow of water from the flooded Cerknica Field. Because the flood dynamics of the Cerknica Field often did not align with the wishes of the local population, Karlovice have over time been subjected to various human interventions.
Especially in the 19th century and also in the early 20th century, interventions prevailed in the desire for faster drainage of water from the field and the associated more intensive drying of agricultural land. The cave entrances were expanded and deepened, and even several hundred meters into the interior of the caves, stone blocks were removed from the floor to make drainage more efficient. Numerous interventions were also carried out on the surface, primarily in the form of regulating riverbeds.
A view of the natural entrance to Velika Karlovica around 1930 (unknown photographer) and the entrance, which was initially widened and later partially dammed.
In the 1960s, there were ambitions to retain water on the plain for longer periods, with the idea of further using it for electricity production. In 1969, the entrances to Velika and Mala Karlovica were therefore partially dammed. At the site where the cave passages of Velika Karlovica come closest to the edge of the Cerknica plain (300 m north of the entrance to Velika Karlovica, once known as Rakovski mostek), a tunnel approximately 26 m long and 3 m wide was built. In front of the tunnel, a sluice gate was installed, which could be fully opened (for faster water outflow) or closed (to retain water), depending on the needs. The nearby Narti sinkhole area was also dammed. Before and after damming the sinkhole areas, researchers carried out several flow measurements in the downstream Rakov Škocjan and found that the maximum sinking capacity had decreased by about 30 m³/s. After several years of experimentation, it turned out that the interventions worked only partially – the floods were prolonged, but during peak drought the lake still dried up, making it unsuitable for sufficient electricity production.
These results, together with strong opposition to such interventions from various interest groups, led to the abandonment of the experiment and the partial removal of the blockages in the sinkhole areas. The somewhat deteriorated sluice gate is still in use today, although it leaks and lets through a considerable amount of water even when closed.
The interior of the artificially constructed tunnel that connects the passages of Velika Karlovica with the edge of the Cerknica plain.
The Karlovice area is still the subject of various studies. In terms of speleological activities, the Rakek Caving Club is especially active, having discovered several fairly long and deep caves that come very close to the groundwater level and the existing passages of the Karlovice. The Karst Research Institute ZRC SAZU has also carried out several series of continuous water-level measurements in the passages of Velika and Mala Karlovica. These show that the area is characterized by quite intense water mixing, partly coming through the Cerkniščica from the eastern edge of the Cerknica plain catchment, and partly from the Stržen, which supplies water from the southern part of the catchment.
Depending on where the larger amount of water flows in from, the proportion of water in the underground changes, and periodically the gradient or flow direction of the water shifts as well. Further measurements will make it possible to develop a hydraulic model for a better understanding of flood dynamics and underground water flow.
Fluctuations in water level on the surface of the Cerknica plain (gauging station Dolenje Jezero) and in the water-filled passages of Mala and Velika Karlovica.
References:
The article was prepared by ZRC SAZU.
The implementation of extensive nature conservation projects exceeds the financial capacity of the local community, therefore Notranjska Park regularly applies for projects with the possibility of co-financing by the European Union.
The implementation of large-scale conservation projects exceeds the financial capabilities of the local community, therefore Notranjska Park regularly applies for projects with the possibility of co-financing from the European Union.
