(SI) Loško polje je lep primer kraškega prelivnega polja, na katerem si je vredno ogledati vsaj dve zanimivosti: glavni odtok, ki ga predstavlja ponorna jama, z vhodom kar 9 m nad dnom doline ter presenetljivo majhno območje z gosto razporejenimi požiralniki pri vasi Dane. Opisujemo še zelo malo poznano, suho ali polsuho stransko dolino, z začetkom pri vasi Podlož, ki izjemoma lahko postane celo rečna dolina.
PRED ZACETKOM:
Opis tega Geološkega zaklada je dolg. Prosimo, preberite ga in si zapisite naloge za vpis se pred obiskom zaklada!
NALOGE ZA VPIS OBISKA
1. Opišite kakšne so bile vodne razmere na koordinatah Geološkega zaklada v času vašega obiska. Koliko je bilo vode in kam je odtekala? Če so požiralniki povsem suhi, na enako vpašanje odgovorite na točki CHANNL.
2. Na točki BRIDGE si oglejte umetni kanal. Kakšen je?
3. Zelo bomo veseli, ce kaksno zanimivo fotografijo z mesta zaklada prilozite svojemu vpisu (logu) obiska.
POMEMBNO!
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(ENG) Loško polje is a good example of karst overflow polje, where you are invited to visit at least two sites of interest: main drainage – sink cave, with an entrance full 9 m above the polje plane, and surprisingly small area which contains almost all swallow holes of the entire Loško polje. We are also describing practicaly unknown dry or semi-dry valley under the Podlož village, which could even become pocket valley with a surface flow in the extreme conditions.
BEFORE THE BEGINNING:
This EarthCache has a long description! Please, read it and write down logging tasks before visiting the EarthCache site!
LOGGING TASKS
1. Describe water conditions at the EarthCache coordinates at the time of your visit. How much water was there and where did it flow to? If there is no water at EarthCache coordinates, answer the same question at the waypoint CHANNL.
2. Take a look at the artificial channel from the waypoint BRIDGE. What does it look like?
3. We would be glad if you added interesting photos from the EarthCache location to your log.
IMPORTANT!
- Send answers through GC profile or directly to e-mail vane.si.geo@gmail.com
before logging. After that you can log - you don't need an additional permission to log. Please, upload photos if you have taken them.
- Logs without an accompanying e-mail containing correct answers will be encrypted and later deleted!
- Logs containing answers will be deleted!
slovensko besedilo / Slovenian text
tocke poti in dnevniki / waypoints and logs
Visit our web-site Geološki zakladi / EarthCaches
for a complete list of our EarthCaches and a lot of additional information. You can find a lot about Ljubljanica river there and about other EarthCaches connected to this famous kastic river (LJUBLJANICA-Retovje, RAK-Rakov Skocjan, UNICA-Planinsko polje).
(ENG) LOŠKO POLJE
Ljubljanica is not just a short river from Vrhnika to river Sava! In fact, it is a mysterious river with at least seven names. It flows over Loško polje under a name Obrh.
For easier undestanding, just a short explanation of two scientific terms used in text:
POLJE ... Large flat-floored closed karst depression, with sharp slope breaks between the commonly alluviated floor and the marginal limestone. Streams or springs drain into poljes and outflow is underground through ponors.
PONOR ... Hole or opening in the bottom or side of a depression where a surface stream or lake flows either partially or completely underground into the karst ground-water system.
AN OVERFLOW POLJE
Loško polje is a great example of an overflow polje. Short description of this type of poljes:
Idrija fault zone has been the main factor in the formation of a whole chain of karts poljes, where bottom has sunk as a result of tectonics. Counting them from the first springs of Ljubljanica in the southeast, they are: Babno polje, Loško polje, Cerkniško polje, and Planinsko polje.
An OWERFLOW polje, such as Loško polje, is defined as: Flat-floored closed karst depression, with sharp slope breaks between the commonly alluviated floor and the marginal limestone. Streams or springs drain into poljes and outflow is underground through ponors. Commonly the ponors cannot transmit flood flows, so many poljes turn into wet-season lakes.
In the case of Loško polje, non-permeable dolomite expands to up to 1.5 km in width, and water cannot escape into the underground. Water flows to polje from several sides. The largest is inflow from Jurassic limestone of Racna hill massif, with karstic springs of VELIKI OBRH. In the Veliki Obrh hinterland, which includes 6 km distant Trbuhovica flow, limestone prevails. MALI OBRH springs near Snežnik caslte. It has a confluence with Veliki Obrh at Pudob and than flows across wide extension of Loško polje towards swallow holes and ponors at the northwestern side of polje.
Obrh waters carry a lot of dissolved limestone. It has been calculated (by Koblenz) that Obrh carried away almost 23.000 tons of dissolved carbonatic stone, or 125 cubic meter per 1 cubic km! This is what scientists call chemical erosion.
Image 1: Topographical map and relief of Loško polje. Red circle marks rather small swallow-holes-area, and red dot position of the entrance to Golobina cave (see Images 2 and 3 for details). Blue circles mark Veliki Obrh and Mali Obrh springs, yellow line encircles dry valley under Podlož village.
SWALLOW HOLES AND PONOR CAVE – EARTHCACHE SITE
The reief and hidrology of Loško polje differs from those of Cerkniško or Planinsko polje. SWALLOW HOLES are situated at the surprisingly small area of the polje, in the narrow angle under Dane village. Just about 500m before reaching higher rocky border of polje the water sinks in the Obrh water channel and in short side “pocket valleys” on both sides of the channel. EarthCache coordinates are set at this site.
THE PONOR CAVE GOLOBINA is the largest outflow from Loško polje. The cave has a natural entrance at the height of 9 m above the level of polje, and has therefore, before man-made regulation been able to drain only high flood waters. After the Villiem Putick plans several regulations have been made in the years from 1901 to 1906. A new entrance-channel has been dug out into the cave 8 meters lower, and 500 m of an artificial channel from the swallow holes to the cave as well. Villiem Putick has previously already displayed his technical and hydrological knowledge, during water regulation works at Planinsko polje. While diverse regulation of karst water courses usually gave small or no results at all, regulations at Loško polje resulted in visible effect. Floods are not as high as before, time of flooding is shortened too. But halting of the waters could not be eliminated: whenever inflow to polje exceeds the outflow capability of swallow holes and ponors, water is halted and starts to flood the polje. When lower lying Cerkniško polje is flooded too, the outflow from Loško polje is further reduced.
The height of the natural entrance to Golobina cave could represent the fact, that the base of the Loško polje has still been sinking after the Golobina cave has been created by water.
Image 2: Swallow holes and Golobina cave presented by sketch and aerial photo:
a – cave, b – artificial cahnnel, c – swallow holes with periodical water, d – permanent swallow holes, e – rocky border of polje;
Swallow holes drain all the water at the time of low waters. Water is not directed from swallow holes to neighboring Cerkniško polje, but rather directly towards Ljubljanica springs at Bistra – at the edge of Ljubljansko barje. High waters follow the artificial channel to ponor cave and spring as karst springs at Cerkniško polje. At the time of extreme flooding, water spills across the plain of Loško polje at both sides of Veliki and Mali Obrh. Floods don’t endanger settlements, since they are built at the terrace, about 10 m above the lower part of polje.
Image 3: Detail of swallow holes area - an EarthCache site.
DRY VALLEY?
Under the Križna gora, near village Podlož, where a small brook springs, 1 km long dry valley begins. While the brook usually only flows for a few hundred meters before sinking underground again, high waters flood the first part of the valley. The upper part of the valley floor is just about half a meter lower the floor of the valley, at the part leading towards Loško polje. Local inhabitants recount times when extremely high waters filled the valley high enough for surface outflow to the Loško polje – the valley thus becoming river valley.
The valley is therefore interesting example of transition from the river valley to semi-dry or dry valley.
Reference:
- The ponor cave Golobina at Loško polje, Peter Habič, Acta Carsologica XVI, The Scientific research centre of the Slovenian Academy of Sciences and Arts, Ljubljana, 1987.
- Ivan Gams, Kras v Sloveniji (Karst in Slovenia), ZRC Publishing, The Scientific research centre of the Slovenian Academy of Sciences and Arts, Ljubljana, 2004.
- Photos: all aerial photos, reilef and topografical map are clippings from web atlas Geopedia, www.geopedia.si.
- Sketch at Image 2 is taken from article in reference No. 1.
Slovensko besedilo
Obiscite naso spletno stran Geološki zakladi / EarthCaches
za seznam vseh nasih Geoloških zakladov in mnogo dodatnih podatkov. Tam izveste tudi vec o reki Ljubljanici in ostalih Zemeljskih zakladih iz nase serije o Ljubljanici (LJUBLJANICA-Retovje, RAK-Rakov Skocjan, UNICA-Planinsko polje).
(SI) LOŠKO POLJE
Ljubljanica ne tece samo od Vrhnike naprej. V resnici je to skrivnostna reka z najmanj sedmimi imeni. Pod imenom Unica se pretaka cez Planinsko polje.
PRELIVNO KRAŠKO POLJE
Idrijski prelom je botroval nastanku cele verige kraških polj, katerih dno se je pogreznilo kot posledica tektonskih premikov. Če naštejemo polja povezana s tokom Ljubljanice od njenega prvega izvira na jugovzhodu proti severozahodu, so to: Babno, Loško, Cerkniško in Planinsko polje. PRELIVNO POLJE prepoznamo po naslednjih dejstvih: Vode se do polja pretakajo pod povrsino, skozi rove ustvarjene v vodoprepustnih kamninah. Na stiku z vodoneprepustnimi kamninami ali naplavinami na dnu kraskega polja, se del voda prelije na povrsje. V odvisnosti od kolicine vode, te bolj ali manj stalno vijugajo po povrsini polja, ob vecjih dezevjih pa ga tudi preplavljajo. Ko na obrobju dosezejo vodoprepustne kamnine, znova poniknejo
V Loškem polju se pas triasnega, slabo prepustnega dolomita razširi na 1,5 km, in preprečuje odtekanje vode v podzemlje. Na polje pritekajo vode z več strani. Največji dotok je iz jurskih apnencev masiva Racne gore (1.140 m) v kraškem izviru VELIKEGA OBRHA, s povprečnim pretokom 3,5 m3/sek. V njegovem zaledju, ki zajema tudi 6 km oddaljeno Trbuhovico, prevladujejo apnenci. Pri gradu Snežnik izvira MALI OBRH. Po sotočju z Velikim Obrhom pri Pudobu, teče po širokem podaljšku polja, proti požiralnikom in ponorom na severozahodni strani, pri vasi Dane.
Vode Obrha prenašajo veliko raztopljenih karbonatnih kamnin. Izračuni za leto 1976 (Koblenz) so pokazali, da je Obrh odnesel skoraj 23.000 ton raztopljenih karbonatnih kamnin, oziroma 125 m3/km3! Temu strokovnjaki pravijo kemična erozija vode.
Slika 1: Loško polje na topografski karti in s prikazom reliefa. Z rdečim krogom je označeno razmeroma zelo majhno območje požiralnikov in z rdečo piko vhod v jamo Golobina (podrobneje prikazano na slikah 2 in 3). Modra kroga označujeta izvire Velikega in Malega Obrha, rumeno je očrtana suha dolina pod vasjo Podlož.
POŽIRALNIKI IN PONORNA JAMA - MESTO GEOLOŠKEGA ZAKLADA
Loško polje se po reliefu in vodnih razmerah precej razlikuje od Cerkniškega in Planinskega polja. POŽIRALNIKI so razporejeni v razmeroma ozkem kotu pod vasjo Dane. Le dobrih 500 m pred višjim skalnim obodom na koncu polje voda ponika v sami strugi Obrha in v kratkih stranskih zatrepih ob njej. Sem so postavljene tudi koordinate Zemeljskega zaklada.
PONORNA JAMA GOLOBINA predstavlja največji odtok z Loškega polja. Svoj naravni vhod ima 9 m nad dnom polja, zato je pred človeškimi posegi lahko odvajala le precej visoke poplavne vode. Osem metrov pod vhodom v jamo so po načrtih Viljema Puticka, med letoma 1901 in 1906, izkopali nov vhodni rov in do njega kanal vse od zadnjih stalnih požiralnikov na polju. Viljem Putick je svoje tehnično in hidrološko znanje pred tem že pokazal pri urejevanju odtokov na Planinskem polju. Raznovrstni posegi na nekaterih kraških poljih praktično niso spremenili stanja, na Loškem polju pa so z izvedeni ukrepi želene rezultate vsaj delno dosegli. Poplave ne segajo več tako visoko kot prej, skrajšali so tudi čas poplavitve polja. Vendar zastajanja vode ni bilo mogoče odpraviti: kadar dotok na polje preseže sposobnosti odtoka v požiralnikih in ponorih, voda zastane in poplavlja polje. Ko se napolni dolvodno ležeče Cerniško polje, pa je odtok še dodatno otežen.
Po višini naravnega vhoda v Golobino lahko sklepamo tudi, da se je dno polja po nastanku jame še zniževalo.
Slika 2: Požiralniki in jama Golobina predstavljeni na skici in zračnem posnetku:
a – jama, b – umetni kanal, c – občasni požiralniki, d – stalni požiralniki, e – skalni obod polja;
Požiralniki s polja v celoti odvedejo nizke vode, ki potem večinoma ne odtekajo proti sosednjemu Cerkniškemu polju, temveč neposredno proti izvirom Ljubljanice pri Bistri. Visoke vode odtekajo po izkopanem kanalu v ponorno jamo in privrejo kot kraški izviri na Cerkniškem polju. Ob izjemnih poplavah se voda razlije čez ravnico Loškega polja ob obeh Obrhih. To ne ogroža naselij, saj ležijo 10 metrov nad nižjim delom polja, na vrtačasti terasi.
Slika 3: Detajl območja s požiralniki - točka Zemeljskega zaklada.
SUHA DOLINA?
Pod južnim pobočjem Križne gore, kjer na koncu pasu jurskega zrnatega dolomita pri vasi Podlož izvira potoček, se začenja 1 kilometer dolga suha dolina. Potoček običajno ponika kmalu pod vasjo, ob visoki vodi pa zalije začetni del sicer suhe doline. Dolinsko dno je na zgornji tretjini za pol metra nižje kot v nadaljevanju do Loškega polja. Izredno visoke, stoletne vode, dolino zapolnijo do zadostne višine, da ta površinsko odteče na Loško polje – takšna so pričevanja domačinov, ki jih je v svojo knjigo vključil krasoslovec Ivan Gams. Dolina je torej zanimiv primer prehajanja rečne doline v polsuho, in ob suši, ko izvir povsem presahne, v suho dolino. V povprečju je to najdlje suha in polsuha dolina, le enkrat na mnogo let pa postane rečna, zatrepna dolina.
Viri
- Ponorna jama Golobina na Loškem polju, Peter Habič, Acta Carsologica (krasoslovni zbornik) XVI, Inštitut za raziskovanje krasa ZRC SAZU, 1987.
- Ivan Gams, Kras v Sloveniji v prostoru in casu, Zalozba ZRC, Znanstvenoraziskovalni center Slovenske akademije znanosti in umetnosti, Ljubljana, 2004.
- Slike: vsi zračni posnetki, relief in topografska karta so izseki iz spletnega atlasa Geopedia, www.geopedia.si.
- Skica na sliki 2 je povzeta iz članka navedenega v viru 1.
tocke poti in dnevniki / waypoints and logs