Mysteries of the Schenk’s siphon in the Bull Rock Cave.

Summary

A rigorous analysis of historical accounts from hydrological, geological and, above all, just logical points of view.

The Bull Rock Cave (jeskyně Býčí skála) is located in the Josefov area of the Křtiny Valley in the central part of the Moravian Karst. It represents approximately a half of the total length of the Jedovnice Creek cave system. The system is ca. 15 km long, second longest in the Czech Republic. It is located under the Rudice Plateau, 60 - 220 m beneath the surface. The 3.8 km long (straight length) Rudické propadání is the ponor cave. The Bull Rock Cave represents an underground streambed of the Jedovnice Creek and, for a part, also a paleo-resurgence cave of the underground Jedovnice Creek. It consists of several distinguished units, each of them bearing its own name. The first section of the cave, the Old Bull Rock Cave, represents the paleo-resurgence passage. However, during extraordinary floods it also serves as an active karst spring.

Fig. 1. Map of the Old and New Bull Rock Cave at a normal hydrographical situation (normal water level).

Systematic speleological exploration of the Bull Rock Cave began in 1902. It was carried out by members of the Verein der deutschen Touristen in Brünn, Gruppe für Höhlenforschung (VdT-GfH, a German caving group in Brno, under the leadership of Günther Nouackh). In 1912, the cavers focused their attention on the key problem - the terminal point of the cave at the time, the Šenkův (Schenk's, Schenk) siphon. In 1920 their effort was crowned with a success - the overcoming of the siphon and the discovery of the New Bull Rock Cave with the underground Jedovnice Creek. This included diving (G. Nouackh), water pumping and, finally, drilling and blasting of submerged parts of the siphon roof (ceiling). Since 1947, namely from 1973 to 1985, Czech cavers gradually discovered the underground stream of the Jedovnice Creek between the New Bull Rock Cave and the Rudické propadání. They also have discovered the underground course of the creek between the Bull Rock Cave, the Bar (Barová) Cave and the springs in Josefov.

Fig. 3. Front (entrance) part of the Schenk

Normally, the siphon is fed with water that infiltrates through fluvial sediments and/or fracture or tiny karst conduits in the underlying limestone rock, from the streambed of the Jedovnice Creek in the New Bull Rock Cave. Atmospheric (meteoric) water dripping from two chimneys in the proximity of the siphon is another source. The most substantial influx comes from the so-called Kaple (Chapel); this influx, in turn, is sustained by meteoric waters descending from the surface to the Augiášův dóm (Augeias Dome, Fig. 16).

Fig. 5. Blasting - ripping of the roof - of the Schenk

Fig. 6. Blasting of the roof, continued.

The normal, naturally stabilized water level in the siphon rests at 308.5 m a.s.l. - that is, at the level of the watershed (divide) in the New Bull Rock Cave. This watershed, called Rozvodí, separates waters of the underground Jedovnice Creek from the siphon (Fig. 1 and 2). During floods, however, water may overflow the Rozvodí, fill up the siphon and rise further to ca. 311 m a.s.l., to the watershed in the Old Bull Rock Cave. A further increase in the water level leads to an overflow into the main passage of the Old Bull Rock Cave and, eventually, discharges from the cave entrances.

Fig. 7. Partly emptied (pumped-out) Schenk

Fig. 8. Partly pumped-out and bridged Schenk

Since 1947, a submersible electrical pump keeps the water in the siphon at a navigable level of 305 ± 0.5 m a.s.l. The term "navigable" refers to the state after 1920, after blasting part of the roof.

Fig. 9. Map of isohyets of the anomalous precipitation during 22. - 23. 7. 1972 in the drainage area of the Jedovnice and Křtiny creeks. Precipitanion data in milimeters.

Fig. 10. The Bull Rock Cave - hydrographical situation on July 23rd, 1972. Explanatory notes: 1-dry rooms; 2-torrential flow, Q ~ 0.25 m3/s; 3-river flow, depth in meters; 4-siphons formed during the flood; 5-water reservoirs ("lakelets") existing at a normal hydrographical situation; 6-water surface under pressure due to air trapped in enclosed roof cavities, up to 1.3 atm. (1.34 kg/cm2, 132 kPa (N/m2)); 7-newly formed erosional cuts up to 2 m deep; 8-new accumulations of fluvial sediments, mostly sandy gravel; 9-water volume and water surface of Schenk

The overflow of the watershed Rozvodí and flooding (filling up) the Schenk's siphon occur more frequently (twice in 2006) than floods and flow through the main passage of the Old Bull Rock Cave. The last such flood and through-flow occurred in 1972 (Fig. 9-12). It was preceded by floods in 1947, 1883 and, possibly, in 1832.

Fig. 11. Relationship between volume of pumped-out water and decrease of water level in the Schenk

Fig. 12. Jedovnice Creek and Bull Rock Cave: relationship between flow rate (Q) and hydrological phenomena. Flow rate is related/refers to springs-outlets at the village of Josefov. Explanatory notes: from the top, line 1-logaritmic scale of flow rate Q in l/s; line 2-theoretical classification of flow rates according to Iszkowsky (in Burkhardt 1953b); line 3-actual flow rate, estimated (upper scale) and measured (lower scale); line 4-velocity of underground flow (cm/s); line 5-frequency of high flood events (yrs.); line 6-water level at the beginning of the passage Kaňony (Canyons, vide Fig. 1 and 2); graph/schema at bottom-function of the Josefov springs I and II, flow through the Mořské oko (Eye of the Sea, vide Fig. 1 and 2) and flow through the main passage of the Old Bull Rock Cave.

A number of historical accounts, beginning with that by J. F. Hertod von Todtenfeld in 1669, describe visits to the Old Bull Rock Cave and the Schenk's siphon. Some of them, e.g. André (1804), Hurban (1839), Hanuš K. (1847), Černík (1894) and others mention crossing the siphon on a watercraft - a small boat or raft. The descriptions are short and vague, yet might allow for the interpretation that, at an extremely low water stand, it was possible to reach the farther shore of the siphon and thus, land in the New Bull Rock Cave.

$Fig. 15. Detail geologic (fracture tectonic) map of the Schenk$

A word by word, sentence by sentence analysis of these old accounts along with the reconstruction of the pre-1920 morphology of the siphon as well as that of the siphon's geological setting and recent and past hydrography (including inundations and floods), leads the present author to two major conclusions:

(1) During the last 250 years or so there has not been a natural water level in the Schenk's siphon as low as to allow any watercraft to cross the entire length of the siphon and reach the New Bull Rock Cave.

(2) The credit for overcoming the siphon and the discovery of the New Bull Rock Cave undoubtely belongs to the VdT-GfH cavers in 1920.

Fig. 13. Main passage of the New Bull Rock Cave - longitudinal cross-section of the cave floor according to test pits by Martin Kříž (1892).

Fig. 14. Downstream (southern) bank/slope of Schenk