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Kazalo
1. Uvod
2. Kras
3. Kraški vodonosniki
4. Območje Visokega Krasa
5. Izvir Hublja
6. Hubeljske jame
7. Dostop do zemeljskega zaklada
8. Zemeljski zaklad
9. Viri
10. Fotografije
Uvod
Hudourniška reka Hubelj, okrog katerega je zrasla današnja Ajdovščina, že skoraj milijon let soustvarja zgodovino prebivalcev, ki so se naseljevali ob tej Mrzli reki, kot so jo imenovali Rimljani. Konec 2. stoletja so ti na tem območju zgradili utrdbo Castra, ki jo je Hubelj kot naravni obrambni jarek varoval pred napadalci. Rimljani so v Ajdovščini pustili veliko zanimivih pečatov. Do danes se je skoraj v celoti ohranilo obzidje utrdbe s štirinajstimi stolpi. Ostanki rimskih term pričajo o družabnem življenju, ki je potekalo za zidovi trdnjave Castra. Zgodovinska bitka pri Mrzli reki (Frigidu) med rimskima cesarjema Teodozijem in Evgenijem leta 394 pa bo zaradi svojega vpliva na nadaljnji razvoj Evrope in sveta za vedno ostala zapisana v svetovni zgodovini.
Hubelj je bil zibelka industrije v teh krajih, saj so se ob njem po nekaterih virih prve fužine razvile že v 4. stoletju pr.n.št. Prvi zapisi o njihovem delovanju pa segajo v leto 1560. V 17. stoletju so ob reki stale kovačija, papirnica s papirnim mlinom, suknarna in predilnica lanu. Leta 1828 je bila zgrajena bombažna predilnica, predhodnica današnje tovarne Tekstina, ki velja za prvi industrijski obrat na Slovenskem. Leta 1855 pa je baron Henrik Rieter v Palah zgradil velik valjčni mlin. Gradbena dela je vodil inženir Pally, po katerem je predel tudi dobil ime. V tem času so v Palah stali še žaga, tovarna testenin (predhodnik današnjega Mlinotesta), tovarna konzerv, tovarna električnih naprav, galvanoplastika ter celo pivovarna. Vsi ti obrati so izkoriščali čisto in hladno vodo Hublja, ki je sicer velikokrat nagajal s svojim nestanovitnim značajem in spreminjal količino vode. Kljub temu pa so vztrajali in tako postavili temelje industrije v Ajdovščini.
Reka Hubelj izvira na stiku fliša in jurskih apnencev. Vodo zbira iz kraškega podzemlja, zato je vodotok povezan z Divjim jezerom nad Idrijo (GC20KWR). Izvira iz več lukenj na različnih višinah, po 5 km pa se izliva v reko Vipavo. Gornji del Hublja ima alpski karakter s prodnatimi tlemi in strmimi spusti, kar za Vipavsko dolino ni značilno in je zato v tem okolju naravna posebnost. Senčni in vlažni gozdovi ob Hublju prehajajo v suh kraški travnik na Školu. Drevesne vrste so tu prilagojene neprijaznemu okolju: pomanjkanju vode, slabo razvitim tlom, nenehnim erozijskim procesom. Tu uspevajo črni gaber, mali jesen, puhasti kras, mokovec in črni bor. Poleg tega uspeva tudi nekaj grmovnih vrst, kot so ruj, šipek in skalna krhlika.
Hubelj je do konca 1. svetovne vojne predstavljal deželno mejo med Kranjsko in Goriško. Današnje mesto Ajdovščina je delil na dve fari: na levem bregu Hublja so bile Šturje, ki so spadale pod Kranjsko, na desnem pa Ajdovščina, ki je spadala pod Goriško. V ljudskem poimenovanju Hubelj še vedno deli mesto na Šturje in Ajdovščino.
Kras
Dinarski svet je makroregija na zahodu, jugu in jugovzhodu Slovenije. Meri 5706 km2 in zavzema 28 % ozemlja Slovenije. V grobem sicer ne preveč izrazito, v drobnem pa izjemno razgibano površje sestavlja splet bolj ali manj visokih, redko poseljenih kraških planot brez stalnih vodotokov (na primer Trnovski gozd, Snežnik, Velika gora,…), razpotegnjenih, gosteje poseljenih podolij z nizi kraških polj s ponikalnicami, uval in vmesnih pregrad (Pivško, Notranjsko, Ribniško-Kočevsko in Dolenjsko podolje ter Loški potok) ter zakraselih ravnikov (največji med njimi je Belokranjski ravnik, velik je tudi Kočevskoreški ravnik).
Kras v Sloveniji
Čeprav se zakraselo površje nadaljuje tudi v sredozemski svet s submediteranskim podnebjem, je druga značilnost Dinarskega sveta hladno in dobro namočeno podnebje, ki je najbolj izraženo prav na najvišje vzpetem zunanjem obrobju na jugozahodu makroregije, imenovanem Visoki kraški rob. Padavinski režim se v smeri proti vzhodu spreminja od sredozemskega k celinskemu. Mreža površinskih voda se zgosti le na neprepustnih kamninah, sicer je značilen podzemeljski vodni odtok s številnimi ponikalnicami, ki oblikujejo skrivnostni svet kraških jam in brezen.
Padavine na krasu pronicajo neposredno s površja v njegovo notranjost. Najprej se pretakajo skozi nezasičeno cono, nato pa se prek zasičene cone koncentrirano iztekajo skozi kraške izvire. Padavine se glede na padavinske razmere ter zapolnjenost tal in zgornjega dela vadozne cone (epikraške cone) z vodo zelo različno pojavljajo globlje v krasu, kjer jih kot kapljanja ali curke opazujemo v podzemnih jamah. Pri tem padavine spirajo razpoložljive snovi s površja, tudi onesnaženje, ki lahko tako dosežejo kraške izvire. Preučevanje pretakanja padavin in prenosa snovi s sledenjem okoljskih parametrov in s sledilnimi poskusi z umetnimi sledili so pokazali sorazmerno hitro pretakanje in hiter prenos snovi v času izdatnejših in pogostejših padavin jeseni (od 0,7 do 4,3 m/h) ter počasnejši in dolgotrajnejši prenos snovi v bolj sušnih razmerah.
Za kraški svet je značilno, da padavine neposredno prenikajo skozi tla in karbonatne kamnine ter napajajo kraške vodonosnike. Padavine po prehodu tal vstopajo v nezasičeno cono, kjer je kamnina v stiku z vodo in zrakom. Nezasičena cona lahko obsega od nekaj pa do več sto metrov debele karbonatne kamnine. Pomembno je zadrževanje padavin v zgornjem delu vadozne cone, v epikraški coni, medtem ko se po prepustnejših razpokah voda pretaka hitreje. Seveda pa so hitrosti pretakanja zelo odvisne od hidroloških razmer. Padavinska voda, ki doteka v kraški vodonosnik, pogojuje najprej iztekanje stare, kasneje pa tudi nove vode skozi kraške izvire, od katerih so številni zajeti za oskrbo prebivalstva s pitno vodo. Onesnaženje ogroža kakovost kraških izvirov, zato je zelo pomembno poznavanje načina pretakanja in prenosa snovi v krasu, kar nam omogoča ustrezno načrtovanje aktivnosti na površju, v primeru nesreč pa hitro in pravilno ukrepanje.
Podzemna voda se nahaja v porah geoloških plasti. Čim več por ima geološka plast, večja je njena poroznost. Celotna poroznost je razmerje med volumnom praznin in celotnim volumnom kamnine. Vendar pa sama poroznost še ne zagotavlja, da bo kamnina prepustna. Pore ali razpoke morajo biti dovolj velike, da se voda lahko skoznje pretaka, razpoke in rovi pa povezani med sabo. Za tok vode skozi kamnino pod vplivom gravitacije je pomembna efektivna poroznost, ki je razmerje med volumnom med seboj povezanih por in celotnim volumnom kamnine. Poroznost je torej lastnost kamnin, da vsebujejo praznine. Glede na morfologijo praznin ločimo: pore (medzrnska poroznost), razpoke (razpoklinska poroznost) in kanale (kraška poroznost). Glede na nastanek praznin v kamnini pa ločimo primarno in sekundarno poroznost. Primarna poroznost nastane v času odlaganja sedimentov in med diagenezo. Sekundarna poroznost pa nastane kasneje zaradi geoloških in kemičnih procesov.
Kraški vodonosniki
Prostorska porazdelitev podzemne vode je pogojena z vrsto kamninske zgradbe in vrsto njene poroznosti oz. prepustnosti. Odvisna je torej predvsem od hidrogeološke zgradbe. Večino slovenskega ozemlja prekrivajo razmeroma dobro prepustne sedimentne kamnine z medzrnsko poroznostjo (19,8 % površine Slovenije), razpoklinsko (14,2 %) in kraško razpoklinsko poroznostjo (33,2 %). Preostale dele Slovenije (32,8 %) gradijo plasti z medzrnsko ali razpoklinsko poroznostjo manjše izdatnosti in kamnine s slabšo poroznostjo. Poleg prodov in peskov pa so vodonosni tudi apnenci in dolomitizirani apnenci, peščenjaki, laporji itd. Plasti apnencev in delno dolomitov, ki so zaradi tektonskih premikanj zdrobljene in kasneje zakrasele, imajo kraško razpoklinsko poroznost. Za plasti dolomitov je značilna predvsem razpoklinska poroznost. Slabše prepustne in manj izdatne so plasti peščenjakov, laporjev s pretežno razpoklinsko poroznostjo.
Hidrogeološka zgradba Slovenije
Vodonosniki s kraško razpoklinsko poroznostjo so na velikih sklenjenih območjih v zahodnem in južnem delu Slovenije, od Julijskih Alp do visokih kraških planot. Debelina vodonosnikov s kraško in razpoklinsko poroznostjo je lahko več sto ali tudi tisoč metrov. V njih se v Sloveniji zadržuje okoli 62 % dinamičnih zalog podzemne vode. Vodonosniki z razpoklinsko in predvsem kraško poroznostjo se odlikujejo z izdatnimi izviri. Točkovno iztekanje podzemne vode iz vodonosnikov, kar izviri so, je najpogosteje povezano z vrsto kamninskih plasti, njihovo nagubanostjo, erodiranostjo, razpokanostjo, izvotljenostjo. Pojavljajo se povsod tam, kjer naravne geološke značilnosti dopuščajo ali omogočajo iztok vode iz vodonosnika. Med najizdatnejše izvire v Sloveniji se uvrščajo izviri v zahodnih alpskih in kraških predelih.
Najpogostejši med izviri so plastni izviri, ki se pojavljajo na stiku neprepustnih plasti (glina, laporji, skrilavi glinavci) s poroznimi vodonosnimi kamninami. Neprepustne plasti so nagnjene proti izviru in podzemna voda zaradi težnosti odteka skozi prepustno kamnino nad neprepustno plastjo. Zaradi sorazmerno majhne prostornine kamninskih por, ki jih izpolnjuje podzemna voda, je izdatnost plastovnih izvirov odvisna od padavin, posebno v kraškem svetu. Ob daljši suši tovrstni izviri presahnejo. Na stiku neprepustnih plasti in poroznih vodonosnih kamnin so pogosto prelivni izviri. Večkrat se pojavljajo na vznožju dolomitnih in apnenčevih gorskih masivov v tektonsko razpokanih conah. Tudi tu odteka podzemna voda v izvir težnostno, vendar je njeno gibanje delno tudi sifonsko. Skupna prostornina kamninskih por, ki jih izpolnjuje podzemna voda, je pri tej geološki zgradbi zelo velika. Zato je izdatnost teh izvirov skozi vse leto izravnana, izviri pa tudi po dolgotrajni suši ne presahnejo. Posebna oblika prelivnih izvirov so arteški izviri, kjer voda priteka iz hidravlično zaprtega vodonosnika v izvir pod tlakom.
Model kraškega vodonosnika
Kraški vodonosnik lahko razdelimo na tri dele, in sicer: zgornjo nezasičeno cono, ki vključuje vodo uskladiščeno v tleh in v epikraški coni, spodnjo nezasičeno cono, ki vključuje cono uskladiščenja v kraških kanalih in večjih razpokah ter v okoliških blokih kamnin, ter zasičeno cono, ki prav tako vključuje cono uskladiščenja v kraških kanalih in večjih razpokah ter v okoliških blokih razpokanih kamnin.
Epikraško cono gradi zgornja plast izpostavljenih kamnin kraškega vodonosnika, v katerih je prepustnost zaradi razpokanosti veliko večja in bolj enotno razporejena kot v spodnji nezasičeni coni. Coni se torej strukturno razlikujeta, razlike v razpokanosti so praviloma vidne do globine 15 do 30 m. Posledica so razlike v hidravlični prevodnosti med epikraško in spodnjo nezasičeno cono, kar omogoča nastanek visečega epikraškega vodonosnika. Zaradi zmanjševanja števila in širine razpok je razpršeno vertikalno napajanje v epikraški coni z globino vse težje, zato tok znatno pridobiva lateralno komponento, ki konvergira proti najbolj prevodnim vertikalnim tektonskim razpokam. V bazi epikraške cone se voda uskladišči in skoncentrira v smeri proti glavnim oziroma najbolj prevodnim razpokam, ki so ponavadi dovolj široke, da zagotavljajo vertikalno napajanje spodnje nezasičene cone.
Shema epikraške cone
Legenda: epikraška cona (E), koncentrirano (A), razpršeno napajanje (B)
Tako kot so heterogeni kraški vodonosniki, je heterogena tudi epikraška cona. Zveznost epikraškega vodonosnika je odvisna v največji meri od porazdelitve in hidravlične kapacitete vertikalnih poti precejanja, preko katerih se drenira voda v spodnjo nezasičeno cono.
Mehanizem toka in prenosa snovi je v kraškem vodonosniku odvisen od obnašanja epikraške cone. Po padavinah se v bazi epikraške cone skoncentrirajo predhodno uskladiščene vode v zgornji nezasičeni coni in nova voda. Epikraška cona prevaja to vodo v nižja območja vodonosnika v odvisnosti od količine vode v njej:
- če je količina vode majhna, se večina vode zadrži in uskladišči v bazi epikraške cone; ta voda se počasi izceja skozi serije ozkih razpok in razpršeno napaja slabo prepustne bloke kamnin spodnje nezasičene cone, le-ti pa razpršeno napajajo zasičeno cono vodonosnika;
- če je količina vode velika, se en del vode lahko zelo hitro drenira prek povečanih in prevodnejših tektonskih razpok v omrežje kraških kanalov in vzpostavi hiter koncentriran tok- epitok, medtem ko se drug del vode uskladišči v bazi epikraške cone.
Območje Visokega Krasa
Visoki kras
Visoki kras je najbolj zahodni del Slovenije. Leži med Jadranskim morjem in vzhodnim hribovjem južnih apnenčastih Alp. Pas karbonatnih kamnin strukturno pripada Dinarskemu gorstvu. Širok je 10 do 15 km, dolg okoli 50 km ter zavzema približno 700 km2 površja. Prevladujejo globoko zakraseli kredni in jurski apnenci ter triasni dolomiti, ki proti severozahodu potonejo pod mlajšimi, pretežno eocenskimi flišnimi kamninami. Fliš obdaja zakrasele apnence Trnovskega gozda in sosednjih planot tudi na južni in vzhodni strani in deluje kot delna, viseča in popolna hidrogeološka pregrada. Na severni strani je Visoki kras obdan s pretežno neprepustnimi srednje in spodnje triasnimi, deloma tudi permskimi in krednimi/karbonskimi kamninami. Vanje so zarezane dolina Belce, Idrijce, Trebuše, Hotenke, Kanomlje in Zale. V njihovih porečjih, predvsem na planoti Vojsko, je nekaj manjših prelomnic, jam in kraških izvirov.
Pokrajinske enote Visokega krasa
V pokrajinskem in hidrogeološkem pogledu je Trnovski gozd sorazmerno zaokroženo gorsko kraško območje, ki je skoraj z vseh strani obdano z nižjim nekraškim obrobjem. Vse padavine s tega območja poniknejo v globok kraški vodonosnik, ki napaja izdatne kraške izvire na obrobju (pritoki Idrijce, Vipave, Soče in Ljubljanice). Manjše ponikalnice so le na zahodni in vzhodni strani Trnovskega Gozda. Glavni kraški izviri se nahajajo v dolinah na njegovem obrobju (Mrzlek, Avšček, Kajža in Vogršček vzdolž Soče, Lijak, Hubelj in Vipava vzdolž Vipave, Divje Jezero, Podroteja in Hotešk vzdolž Idrijce). Reke Idrijca, Soča in Vipava pripadajo jadranskemu porečju, Ljubljanica pa porečju Črnega morja. Znotraj Visokogorskega krasa poteka torej podzemno razvodje med Jadranskim in Črnim morjem. Kraški izviri na obrobju Visokega krasa so zajeti za vodno oskrbo naselij v Vipavski dolini, na Goriškem ter vzdolž Soče in Idrijce. Ker so to edini izdatni viri pitne vode v zahodni Sloveniji, morajo biti pred onesnaženjem varovana celotna kraška zaledja.
Meteorološke razmere
Trnovski gozd, Banjšice in Nanos so prva gorska ovira (1000 do 1500 m nadmorske višine) na poti od Sredozemlja (severnega Jadranskega morja) proti severu in severovzhodu. Pred njo je še kraški plato, vendar ta večinoma ne presega višine 600 m. Padavine so zato na Krasu skromne, postanejo pa precej obilne ob gorski pregradi, ki poteka od Banjšic do Nanosa. Ta pregrada predstavlja mejo med mediteranskim in alpskim podnebjem. Tako Vipavska dolina kot tudi goriška regija ležita na južnem robu Trnovskega gozda in sta zato pod močnim vplivom mediteranskega podnebja. Kljub temu pa imajo že Trnovski gozd, Banjšice in Nanos pravo alpsko podnebje z obilnimi snežnimi padavinami med precej mrzlimi zimami. Padavine so obilne skozi vse leto, z očitnim maksimumom v oktobru in novembru. V osrčju Trnovskega Gozda, območju Golakov, so povprečne letne padavine 3000 mm.
Tudi celotno področje Banjšic, Trnovskega gozda in Nanosa je bogato s padavinami. V povprečju so letne količine padavin na tem območju več kot 2000 mm. Za jesenske padavine je značilno, da so zelo močne in kratkotrajne ter so pogosto neenakomerno razporejene skozi opisano obdobje. Intenzivnost padavin posameznih območij se lahko močno razlikuje, kar ni značilno za konveksne padavine, ampak za orografske padavine, ki nastanejo zaradi frontalnih sistemov.
Trnovski gozd dobi večino padavin v jesenskem obdobju, ko je morje še vedno precej toplo in priteka z jugozahodnimi vetrovi iznad Sredozemlja zelo topel in vlažen zrak. Ko na svoji poti zrak doseže prvo višjo gorsko pregrado, se mora vzdigniti, da jo preide, kar se odraža v orografskih padavinah. Take situacije so pogoste med nastajanjem sekundarnih ciklonov v genovskem zalivu ali nad severnim Jadranom. To je jeseni in spomladi, ko so sekundarni cikloni najbolj pogosti, z razliko, da topel zrak jeseni vsebuje precej vlage. Vlažnost zraka je spomladi zaradi hladnejšega severnega Sredozemlja znatno nižja in zato orografske padavine niso tako zelo obilne. Na obravnavanem območju mesečna količina padavin znatno presega izhlapevanje. Kljub temu da je julij najmanj moker mesec, tudi takrat pade več kot 600 mm padavin. Hkrati je to tudi mesec z najbolj intenzivnim izhlapevanjem.
Kot je že bilo omenjeno, predstavljata Trnovski gozd in Nanos mejo med mediteranskim in alpskim podnebjem. Ko je notranjost Slovenije napolnjena z mrzlim zrakom s severa ali severovzhoda, nastajajo na prej omenjeni gorski pregradi velike temperaturne razlike kot tudi veliki gradienti tlaka. Ko se zrak spušča iznad Trnovskega gozda in Nanosa proti Vipavski dolini in Krasu, se adiabatsko ogreva, čeprav je še vedno bolj hladen kot zrak nad severnim Jadranom. Rezultat te temperaturne razlike je, da mrzel zrak pada navzdol po pobočjih in doseže velike hitrosti, medtem ko se zaradi razgibanih zemeljskih oblik razvijejo močne turbulence. Ta močan veter poznamo pod imenom burja in s posameznimi sunki vetra dosega hitrosti do 200 km/h. Relativna pogostost burje, s seveda manjšimi hitrostmi, je tudi vzrok, da je zrak nad Krasom in Vipavsko dolino bolj suh kot pa zrak nad ostalimi slovenskimi regijami.
Geološke razmere
Geološka karta in prerezi Visokega krasa
Legenda:
1- kvartar (holocen in pleistocen): obledeniška breča in pobočni grušč
2- eocen, paleocen in zgornja kreda; fliš
3- paleocen; numulitni, alveolinski, miliolidni in kozinski apnenci
4- zgornja kreda; pretežno organogeni apnenci
5- spodnja kreda; pretežno bituminozni apnenec z vložki dolomita
6- jura; apnenec in dolomit
7- zgornji trias (norij-retij); dachsteinski apnenec
8- zgornji trias (norij-retij); dolomit
9- zgornji trias (karnij); zrnati dolomit, menjavanje meljevca in peščenjaka
Obravnavano območje Visokega krasa je sestavljeno iz krednih in jurskih apnencev in zgornjih triasnih dolomitov, ki pripadajo Trnovskemu in Hrušiškemu pokrovu v narivni zgradbi zahodne Slovenije. Karbonatne kamnine so narinjene na plasti eocenskega fliša, narinjeni bloki so zlomljeni in tektonsko premaknjeni po vzdolžnih dinarskih in prečnih prelomih. Zahodni del Visokega krasa je, kot sklenjen blok zakraselih apnencev in dolomitov, v predelu med Idrijco in Vipavo širok 10 do 15 km, med Sočo in Pivko je dolg okoli 50 km in pokriva približno 700 km2 kraškega površja, ki je praktično iz vseh strani obdano z nižjim rečnim obrobjem. Flišne kamnine obdajajo zakrasele apnence kot delna ali popolna hidrogeološka zapora na zahodni, južni in vzhodni strani. Na Banjšicah, v zahodnem delu Visokega krasa, je na zakraselih apnencih ohranjen fliš kot tanki pokrov. Na nekaterih predelih se skozenj prebija zakrasela podlaga, večinoma pa deluje kot viseča hidrogeološka pregrada, pod katero prevladuje značilno kraško pretakanje vode. S severne strani obdajajo Visoki kras neprepustne srednje in spodnje triasne pa tudi permske in karbonske kamnine. Vanje so poglobile svoje površinske struge reka Idrijca in njeni pritoki Belca, Zala, Kanomljica, Hotenja in Trebušnica.
Vipavska dolina je nizko ležeče površje eocenskega fliša med Nizkim krasom na jugu in Visokim krasom na severu. Dno doline je razmeroma ozko, razen med Vipavo in Ajdovščino. Pomembno podobo ji poleg nizkega flišnega podgorja daje na severni strani strm, v pretežnem delu celo prepaden rob Visokega krasa. Na fliš narinjeni apnenci so izpostavljeni silovitemu mehanskemu razpadanju in podiranju, zato so ob njegovem vznožju nakopičene pobočne breče, melišča in podorno skalovje.
Hidrogeološke razmere
Hidrogeološka skica Visokega krasa
Legenda:
1- pobočni grušči, dobro prepustne plasti z medzrnsko poroznostjo
2- apnenci, dobro prepustne plasti s kraško poroznostjo
3- dolomiti, srednje prepustne plasti z razpoklinsko poroznostjo
4- fliš, delna "viseča" hidrogeološka pregraja
5- fliš, neprepustne plasti, bočna in talna podlaga kraškega vodonosnika
Površina Trnovskega gozda, Banjšic, Nanosa in Hrušice pokriva približno 700 km2, kar je 3,4 % slovenskega ozemlja oz. 7,8 % slovenskega kraškega površja. Celotno območje lahko razdelimo glede na prepustnost njihovih litoloških značilnosti v več hidrogeoloških enot:
- neprepustna območja z omejenimi prepustnimi vložki
- srednje prepustna območja - vodonosniki z razpoklinsko poroznostjo
- dobro prepustna območja - vodonosniki s kraško in razpoklinsko poroznostjo
- dobro prepustna območja - vodonosniki z medzrnsko poroznostjo
- slabo prepustna območja - vodonosniki z medzrnsko poroznostjo
Vodonosniki s kraško poroznostjo so iz zgornje triasnega dachsteinskega apnenca, liasnega, doggerskega in malmskega apnenca, spodnjega in srednjega krednega apnenca in zgornje krednega apnenčastega grušča. Glavne hidrogeološke enote so: kraški vodonosnik Hrušice, kraški vodonosnik Nanosa, kraški vodonosnik platoja Črni Vrh, kraški vodonosnik v razvodju Hublja in kraški vodonosnik zahodnega dela Trnovskega gozda in Banjšic. V vseh teh primerih je obstoj globokih kraških sistemov vprašljiv. Dobro prepustne kamnine z medzrnsko poroznostjo so zgrajene iz grobo zrnatih do kockasto oblikovanih kvartarnih breč in nekompaktnega holocenskega pobočnega drobirja. Pokrivajo velik sklenjen pas na južnem obrobju Trnovskega gozda in Nanosa.
Razpoklinski vodonosniki so grajeni iz dolomitov različnih starosti. Najbolj razširjen je zgornje triasni dolomit, ki omejuje kraški vodonosnik na severni strani in delno usmerja odtok podzemne vode. Neprepustne litostratigrafske horizonte oblikujejo nezdrobljene premogovne klastične kamnine, srednje permski peščenjaki, čiste tufne plasti ladinianske dobe, karnianskeklastične kamnine ter paleocenski in eocenski flišni laporji. V neprepustnih plasteh se nahajajo bolj prepustni sedimenti, ki so pomembni razpoklinski ali kraški vodonosniki med neprepustnimi kamninami.
Hidrogeološke razmere Trnovskega Gozda so odvisne od geološke strukture in litologije. Fliš Vipavske doline pripada različnim tektonskim enotam. Južna stran je del Komenske narivne plošče. Fliš na severni strani pripada po primerjavah Nanosu in Hrušici. Fliš Hrušičnega pokrova predstavlja nepropustno podlago za kraški vodonosnik Trnovskega gozda in Banjšic kot tudi nepropustno južno in jugozahodno pregrado v Vipavski dolini. Na vzhodu je pregrada stanjšana in kaže se razvoj karbonatov. Tako predstavlja le delno (lokalno) hidrogeološko pregrado.
Zgornji triasni, jurski in kredni apnenci Trnovskega gozda pripadajo tektonski enoti Trnovski pokrov in gradijo osrednji del kraškega vodonosnika. Podzemna voda leži izjemno globoko. Nivo podzemne vode je višji v zaledju Hublja. Podzemne vode kraških vodonosnikov Nanosa in Trnovskega gozda (izvir Vipave, Hublja in Lijaka) se pojavijo na površju na najnižjih točkah nepropustne flišne pregrade.
Izvir Hublja
Geološki prerez Trnovske planote čez izvir Hublja
Kraški izvir Hublja se nahaja visoko nad dnom doline. Hubelj leži v dnu izrazite, ozke in globoke strukturne depresije v narivni ploskvi Trnovskega pokrova. V sušnem obdobju se v jamah za izvirom ohrani več sifonskih jezer. Vode večinoma pritekajo po korozijsko razširjenih tankih plasteh - lezikah med slabo prepustnimi apnenci in flišno podlago. Izvir Hubelj se izmed vseh izvirov na tem območju pojavi na najvišji nadmorski višini. Leži na južni meji Trnovskega gozda. Ob visokih vodah se nivo vode dvigne za 40 m. V jugozahodni Sloveniji predstavlja kraški izvir Hubelj glavni iztok iz enega močno zakraselega kraškega vodonosnika. Pretok Hublja niha od 0,2 do 59 m3/s, medtem ko je njegov srednji pretok 3 m3/s. V zaledju izvira je visoka kraška planota Trnovski gozd, katere povprečna nadmorska višina je 900 m. Na tem območju pade povprečno 2450 mm padavin na leto. Napajalno območje izvira, ki ga gradi v glavnem jurski apnenec, obsega 50 do 80 km2.
Zaledje izvira Hubelj
Ob hudourniku je lepo urejena naravoslovna učna pot Ob Hublju. Sprehod po tej poti omogoča ogled zanimive zgradbe tal, rastlinskih združb, posameznih drevesnih vrst ter živalskega sveta na območju okrog Hublja. Biotska raznolikost je prva učna snov, ki se jo človek nauči na tej poti. Na tako majhnem prostoru se dva povsem različna svetova mešata med seboj in ustvarjata svojevrstne kombinacije. Dve različni mikroklimi, dve povsem različni geološki podlagi. Tla nižinskega gozda ob Hublju sestavlja fliš, ki ga tvorijo različne sedimentne kamnine. Na Školu pa najdemo za vodo propusten apnenec, kamnino, ki je nastala iz morskih usedlin. Oboje priča o tem, da je tu bilo nekoč morje. Pravo posebnost predstavljajo pobočne breče, posebna oblika apnenca, ki je nastal s ponovnim strjevanjem že razpadlega apnenca v manj kompaktno obliko.
Hubeljske jame
Hubeljske jame je izdolbla reka Hubelj, ki izvira visoko nad dnom doline med 220 in 235 m nad morjem, na pobočju Gore pod Otlico in nad Ajdovščino. To je skupno ime za tri jame, in sicer Veliki Hubelj, Hubljeva kuhna in Pajkova reža. Vse tri jame so nastale v zatrepu izvira Hublja. Vzhodno steno zatrepa predstavljajo stene iz pobočne breče, severne stene pa so zgrajene iz apnenca. Izviri so nastali, ker je fliš, na katerega je narinjen apneniški pokrov, v tem delu najnižji in si je zato voda tu izdolbla pot na prosto. Največja jama je Veliki Hubelj na nadmorski višini 260 m. Najnižje leži jama Pajkova reža na nadmorski višini 250 m, najvišje pa je Hubljeva kuhna 280 m nad morjem. Vse jame so vodne in občasni izviri, tudi ob suši pa se v vseh jamah nahaja voda. V Pajkovi reži je ob normalnem vodostaju to tekoča voda 243 m nad morjem, ki odteka proti bližnjim izvirom. V ostalih dveh pa dosežemo nivo kraške vode. Meritve so pokazale, da se voda v najnižjem delu Hubljeve kuhne nahaja na nadmorski višini 263 m, v Velikem Hublju pa se v ozki dvorani nahaja na nadmorski višini 264 m. Razlika 1 m je verjetno posledica napake v meritvah. Pajkova reža je očitno spodnja aktivna etaža. Tik pod to jamo se nahajajo tudi največji izviri. Zanimivo je, da voda nikoli ne zalije Dvorane prepirov v Pajkovi reži. Ravno tako nikoli ne teče skozi prekopan vhod v jamo, čeprav je ta nižje od vhoda v Veliki Hubelj. Iz vhoda v Veliki Hubelj voda ob povodnjih bruha v velikem slapu, kar kaže na to, da so povezave med Pajkovo režo in Velikim Hubljem zelo ozke. Ožje, kot so rovi med Pajkovo režo in izviri, tako da voda iz Pajkove reže brez težav odteka. Zanimiv pojav je bil opažen tudi v Hubljevi kuhni, kjer ob začetku poplav začne voda pritekati iz grušča v vhodnem rovu. Od tam teče nazaj v jamo, šele potem, ko voda dovolj naraste, pa začne teči iz jame.
Morfologija vseh treh jam je zelo podobna. Vhodni rovi ležijo v smeri vzhod-zahod in so malce večjih dimenzij. Notranji deli jam so večinoma vertikalne razpoke približno v smeri sever-jug. Vse tri jame so nastale ob sekundarnih prelomih. Rovi večjih dimenzij so nastali s podori. Višje ležeči rovi, ki jih voda redko ali nikoli ne doseže, so ponekod zasigani. Dvorani v Velikem Hublju in v Hubljevi kuhni, ki ležita najgloblje v notranjosti masiva, sta močno blatni. Videti je, da se tam voda dvigne skozi rove v dnu, ob tem pa odloži blato. Voda tu naraste zaradi ovire, ki preprečuje odtok večjim količinam vode. Verjetno je ta ovira porušena cona ob prelomu.
Poligon Hubeljskih jam
Dostop
Do tega zemeljskega zaklada pridete iz središča Ajdovščine, kjer je dobro označen odcep do izvira Hublja. Odcep je pred mostom, če prihajate iz smeri Vipave, oziroma takoj za mostom, če prihajate iz smeri Nove Gorice ali avtoceste. Po nekaj kilometrih cesta pripelje do območja, imenovanega Pale, kjer sta mladinski hotel in turistično-informacijski center.
Za ogled tega zaklada priporočamo ogled lepo označene naravoslovne učne poti Ob Hublju, ki se začne v neposredni bližini mladinskega hotela (točka HUBELJ PARK1). Naravoslovna učna pot Ob Hublju je 3,1 km dolga krožna pot, ki vodi od Pal po desnem bregu Hublja do njegovega izvira, ter se od tam po suhem kraškem travniku na Školu vrne na izhodišče. Na poti je 24 dobro označenih postaj, kjer si lahko ogledate različne naravne posebnosti, tako geološke kot rastlinske.
Naravoslovna učna pot Ob Hublju
Alternativni dostop je od izvira Hublja (točka HUBELJ PARK2), do katerega se pripeljete po cesti mimo Pal in se potem naravoslovni učni poti pridružite v točki, na kateri se ta obrne nazaj proti izhodišču. Izvir Hublja je ponavadi precej obljuden, saj je tam priljubljena izletniška točka in izhodišče planinskih poti na Goro, kot se imenuje planota nad Ajdovščino med Colom in Predmejo.
V neposredni bližini je nekaj tradicionalnih zakladov, ki jih morda velja kombinirati s tem.
Zemeljski zaklad
Za registracijo tega zemeljskega zaklada morate odgovoriti na naslednja vprašanja:
1. Pojdite do točke HUBELJ1, kjer boste v bližini videli informacijsko tablo, ki označuje neko geološko posebnost. Katero?
2. Pojdite do točke HUBELJ2, kjer boste v bližini videli informacijsko tablo, ki označuje neko geološko posebnost. Katero?
3. Pojdite do točke HUBELJ3. Ocenite višino slapu, ki pada iz osrednje jame izvira (tiste z rešetko).
4. Kot neobvezno nalogo pa naredite fotografijo vas ali vašega GPSja pri izviru in jo pripnite vašemu vpisu.
Odgovore pošljite skozi GC profil. Potem lahko zabeležite vaš obisk, ne da bi čakali na posebno dovoljenje. V kolikor bo z vašimi odgovori kaj narobe, vas bomo kontaktirali.
Če ste fotografirali, priložite zanimive fotografije.
Vpisi brez poslanih spremljajočih odgovorov, ali vpisi, ki vsebujejo odgovore na vprašanja, bodo izbrisani!
(Skok na začetek teksta) (Skok na fotografije / Go To Photos) (Skok na konec teksta)
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Table of Contents
1. Introduction
2. Karst
3. Karst Aquifer
4. The High Karst area
5. Hubelj Spring
6. Hubelj Caves
7. Access
8. EarthCache
9. Sources
10. Photos
Introduction
It's been almost million years since Hubelj has been co-creating the history of the inhabitants, settling along the Cold River, as once called by the Romans. And it was them who actually built the fortified settlement of Castra in this region by the end of the 2nd century. Thus, Hubelj served as a natural defense moat, guarding the fort from its numerous invaders. The Romans left indeed quite a number of interesting stamps of the period in the town of Ajdovščina. The walls of the fort with its fourteen towers have been preserved almost entirely till nowadays. The remains of the Roman baths bear witness to a lively social life, once unwinding behind the strong walls of Castra. And the historical battle on the banks of the Cold River (i.e. Fluxus Frigidus), fought between the Roman Emperors of Theodosius and Eugenius in A.D. 394 shall remain noted down in the world’s history forever because of the influence it had on the further development of Europe and indeed the world.
According to some sources, the hammer-mills developed along the River of Hubelj in the 4th century B.C. The first notes of their activity go back to 1560. In the 17th century there were a blacksmith’s workshop, a paper factory with a paper mill, a cloth factory and a flax mill along the river. In 1828, a cotton mill was built that later became the textile factory Tekstina, which is now considered to be the first industrial plant ever built in Slovenia. In 1855, Baron Henrik Rieter built a larger rolling mill in Pale. The region of Pale was named after Pally, the engineer conducting the building works in the area. At that time Pale also had its own sawmill, noodle factory, cannery, electrical appliances factory, galvanoplasty and even a brewery, thanks to Hubelj’s clear and cool waters. All those plants made a good use of Hubelj’s force and impetuosity. Although the torrent’s unsteady nature often caused lots of troubles, as it kept changing its amount of water, they managed to hold out and thus making Pale a genuine industrial centre.
The Hubelj River takes its source on the juncture of flysch and Jurassic limestone. It gathers its waters from the karst subsoil; therefore its watercourse is connected with the Divje jezero (the Wild Lake) above the town of Idrija (GC20KWR). It rises from a number of holes scattered at different altitudes and outfalls into the Vipava River. The upper part of the Hubelj River bears the alpine character with its gravelly soil and steep descents, quite untypical for the Vipava Valley and thus makes a natural curiosity in this particular environment. Shady and humid forests near the Hubelj River pass on the dry karst meadow at Škol. Here the tree species have adapted themselves to the unpleasant living conditions: lack of water, poorly developed soil and constant erosive processes. The species flourishing at Škol are black beech, small ash, downy oak, white beam and black pine. Quite special for this particular region are the following shrub species: sumach, alder buckthorn and dog rose.
Till the end of WWI, Hubelj was actually drawing a provincial border between the Kranjska and Goriška regions. Thus, it was dividing the today’s town of Ajdovščina into two parishes: on the left river bank was Šturje, which once belonged to the Kranjska region and on its right bank Ajdovščina, which was part of the Goriška region. According to the folk denomination the River of Hubelj is still dividing the town into Šturje and Ajdovščina.
Karst
Dinaric Alps are a macro-region in the west, south and southeast Slovenia. It measures 5706 km2 and occupies 28% of the territory of Slovenia. This extremely rugged surface brings together several sparsely-populated karst plateaus without permanent watercourses (e.g. Trnovski gozd, Snežnik, Velika gora, ...), elongated, densely populated valleys with a series of karst fields with underground streams (Pivka, Notranjska, Ribnica-Kočevje and Dolenjska valley and Loški potok) and karst plains (the largest of which is Bela Krajina).
Karst in Slovenia
Although the karstified surface continues in the Mediterranean world with sub-Mediterranean climate, another feature of the Dinaric is wet and cold climate, which is most pronounced at the highest ascending fringe on the southwest of the macro-region, called the High Karst edge. Precipitation regime in the east direction varies from Mediterranean to continental. A network of surface water is concentrated only on impermeable rock; otherwise underground waters with numerous streams are characteristic for the region, forming a secret world of karst caves and potholes.
Rainfall in the Karst permeates directly from the surface into the underground. First the water flows through the unsaturated zone, then through the saturated zone and finally it flows out through karst springs. Depending on precipitation conditions and on how much the soil and upper vadose zone (epikarst zone) is filled with water, precipitation penetrates to various depths in karst areas. Precipitation can be observed in the underground caverns as dripping or flushes of water. Rain washes out the material from the surface, together with pollutants, which can eventually reach karst springs. The study of water streaming and mass transfer by tracking the environmental parameters and experiments with artificial tracers showed relatively fast streaming and fast mass transfer at the time of larger and more frequent rainfall in the autumn (from 0.7 to 4.3 m / h) and a slower and longer-term mass transfer in the more arid conditions.
It is characteristic for Karst that rainfalls permeate directly through soil and carbonate rocks and feeds the karst aquifers. After passing through soil, precipitation enters the unsaturated zone, where the rock is in contact with water and air. The unsaturated zone can range from a few to several hundred meters of carbonate rocks. Very important is retention of precipitation in the upper vadose zone, the epikarst zone, while through cracks water flows faster. However, the flow velocity is dependent on hydrologic conditions. Rain water flows into the karst aquifers and first forces the old, and later the new water to flow out through karst springs. Many of those are used as a supply of drinking water. Pollution threatens the quality of karst springs, so it is very important to know how water flows and how material is transported in karst. This allows appropriate planning of activities on the surface and, in case of accidents, assures that speedy and proper actions are taken.
Groundwater is located within the pores of geological layers. The more pores the geological strata has, the greater the porosity. Total porosity is the ratio of the volume of voids and total volume of rock. However, porosity itself does not guarantee that the rock is permeable. Pores or cracks should be large enough that water can flow through them and cracks and tunnels should be connected to each other. For the flow of water through the rock under the influence of gravity the effective porosity is important, which is the ratio of the volume of interconnected pores and the total volume of rock. Porosity is thus characteristic of rocks that contain gaps. Given the morphology of the voids we can distinguish: pores (intergranular porosity), cracks (fissure porosity) and channels (karst porosity). Depending on the formation of voids in the rock we distinguish primary and secondary porosity. Primary porosity occurs during the deposition of sediments and during the diagenesis. Secondary porosity arises later due to geological and chemical processes.
Karst Aquifer
Spatial distribution of groundwater is subject to the type of lithology and type of porosity or permeability. Therefore it depends largely on the hydrogeological structure. The majority of Slovenian territory is covered with relatively highly porous sedimentary rocks with intergranular porosity (19.8% of Slovenia), fissure (14.2%) and karstic fissure porosity (33.2%). The remaining parts of Slovenia (32.8%) are covered with layers with intergranular or fissure porosity of lower intensity and rocks with low porosity. In addition to gravel and sand aquifers can also form in dolomitized limestones and limestones, sandstones, marls, etc...
Hydrogeological structure of Slovenia
Karst aquifers with fissure porosity are in large contiguous areas in western and southern parts of Slovenia, from the Julian Alps to the high karst plateaus. The thickness of aquifers with karst and fissure porosity can be hundreds or even thousands of meters. They hold about 62% of dynamic groundwater reserves in Slovenia. Aquifers with fissure and even more those with karst porosity are characterized by abundant springs. Springs are just spot leakages of groundwater from aquifers and they are most often dependant on the type of rock layers and on their level of erosion and hollowness. They occur wherever natural geological features permit or allow effluent water from the aquifer. Among the most abundant springs in Slovenia are those in the western Alps and karst areas.
The most common are layered springs that occur at the junction of impermeable layer (clay, marl, slate) and porous aquifer. Impermeable layers are inclined towards the source and ground water flows by gravity through permeable rock above the impermeable layer. Because of the relatively small volume of the rock pores, which are filled with the groundwater, the flow of the layered springs depends on rainfall, particularly in Karst. After longer dry periods such springs dry up. At the junction of impermeable layers and porous aquifers are often overflowing springs. They often occur at the base of dolomite and limestone mountain ranges in tectonically fractured zones. Here, too, groundwater flows into the source because of gravity, but its movement is partly also siphonic. The total volume of pores in the rock, which are filled with the groundwater, is very high at this structure. Therefore, the yield of these springs is in balance throughout the year and also after prolonged drought they don’t dry up. A special form of overflow springs are artesian wells, where water flows to the spring under pressure from the hydraulically closed aquifer.
Model of the karst aquifer
Karst aquifer can be divided into three parts, namely: the upper unsaturated zone, which includes water storage in the soil and epikarst zone, the lower unsaturated zone, which includes a storage area in karst channels and major fissures in the surrounding rocks, and the saturated zone, which also includes a storage area in karst channels and major fissures and cracks in the surrounding blocks of rock.
Epikarst zone consists of the upper layer of the exposed rocks of the karst aquifer, in which due to cracking the permeability is much larger and more uniformly distributed than in the lower unsaturated zone. The two zones are therefore structurally different, the difference in cracking is normally visible to a depth of 15 to 30 m. Due to the differences in hydraulic conductivity between the epikarst zone and the unsaturated zone below it, the formation of a hanging epikarst aquifer is possible. Because of reduction in the number and width of cracks, scattered vertical flow is getting harder with depth; therefore a significant lateral flow component is produced, which converges to the most conductive vertical tectonic cracks. In the base of the epikarstic zone water is stored and concentrated in the direction of the principal or most conductive cracks, which are usually wide enough to provide vertical flow to the lower unsaturated zone.
Model of the epikarst zone
Legend: epikarst zone (E), concentrated (A), scattered flow (B)
Just as karst aquifers are heterogeneous, also the epikarst zone is heterogeneous. Continuity of the epikarst aquifer depends largely on the distribution and the hydraulic capacity of vertical percolation paths through which water drains into the lower unsaturated zone.
The mechanism of flow and solute transport in the karst aquifer depends on the behavior of the epikarst zone. After precipitation, previously stored water in the upper unsaturated zone and the new water concentrate in the base of the epikarst zone. It then conducts the water to the lower area of the aquifer depending on the amount of water the epikarst zone holds:
- In case of a small amount of water, most water is retained and stored in a base of the epikarst zone; this water then slowly percolates through a series of narrow cracks into the low permeable rock of the lower unsaturated zone and after that into the saturated zone of the aquifer;
- In case of a large amount of water, one part quickly drains through enlarged and conductive tectonic faults into the network of karst channels and creates a concentrated, fast stream of water –epikarstic flow, while another part of the water gets stored in the base of the epikarst zone.
The High Karst Area
The High Karst
The High Karst is the most western part of Slovenia. It lies between the Adriatic Sea and the eastern hills of southern limestone Alps. Belt of carbonate rocks structurally belongs to the Dinaric Alps. It is 10 to 15 km wide, about 50 km long and covers approximately 700 km2. High Karst is dominated by deep karstified Cretaceous and Jurassic limestone and Triassic dolomites, which on the northwest sink beneath the younger, predominantly Eocene flysch rocks. Flysch is surrounding karst limestone of Trnovski gozd and adjacent plains on the south and east sides and acts as a hydrogeological barrier. On the north side the High Karst is mostly surrounded by an impermeable middle and lower Triassic, Permian and partly Cretaceous / carbon rocks in which river valleys of Belca, Idrijca, Trebuša, Hotenka, Kanomlja and Zala are incised. In their river basins, especially on the Vojsko plateau, there are a few minor faults, caves and karst springs.
Units of the High Karst
From the geographic and hydrogeological point of view Trnovski gozd is a relatively rounded mountain karst area, which is almost completely surrounded on all sides by lower non-Karst fringe. All precipitation in this area ends up in a deep karst aquifer that supplies water to several strong karst springs (tributaries of Idrijca, Vipava, Soča and Ljubljanica). Smaller streams can only be found on the west and east side of Trnovski gozd. The main karst springs are located in the valleys on its outskirts (Mrzlek, Avšček, Kajža and Vogršček along the Soča River, Lijak, Hubelj and Vipava along Vipava River, Divje jezero, Podroteja and Hotešk along Idrijca River). Idrijca, Soča and Vipava belong to the Adriatic basin, whereas Ljubljanica belongs to the Black Sea basin. Within the High Karst there is therefore an underground watershed between the Adriatic and the Black Sea. Karst springs on the outskirts of the High Karst are used for the water supply of settlements in the Vipava Valley, Goriška region and along the Soča and Idrijca rivers. Since these are the only abundant sources of drinking water in western Slovenia, entire karst hinterland must be protected from contamination.
Meteorological Conditions
Trnovski gozd, Banjšice plateau and Nanos form the first mountain barrier (1000 to 1500 m above sea level) on the route from the Mediterranean (northern Adriatic Sea) to the north and northeast. Before it there is karst plateau, but it does not exceed 600 meters in height. Because of this, rainfall is modest in the Karst region, but becomes quite abundant on the mountain barrier, which runs from Banjšice to Nanos. This division is the boundary between the Mediterranean and Alpine climates. Both Vipava Valley and Goriška region lay on the southern edge of the Trnovski gozd and are therefore strongly influenced by the Mediterranean climate. Despite this, already Trnovski gozd, Banjšice plateau and Nanos have a real alpine climate, with plentiful snow during very cold winters. Rainfall is abundant throughout the year, with a clear maximum in October and November. In the heart of Trnovski gozd, in the Golak area, the average annual rainfall is 3000 mm.
Also, the entire area of Banjšice plateau, Nanos and Trnovski gozd is rich in precipitation. The average annual rainfall in this area is more than 2000 mm. For autumn rainfall it is characteristic to be very strong and short, and often unevenly distributed throughout the period. The intensity of individual rainfall areas can vary significantly, which is not characteristic for convex rainfall, but rather for the orographic precipitation, caused by frontal systems.
Trnovski gozd gets the most rainfall in the autumn period, when the sea is still quite warm and thus very warm and humid air flows from the Mediterranean with Southwest winds. Once this air reaches the first mountain barrier it must lift to get over it, which is resulting in orographic precipitation. Such situations are common during the formation of secondary cyclones in the Gulf of Genoa or over the northern Adriatic. Secondary cyclones are most frequent in autumn and spring; however, the warm autumn air contains much more moisture. Humidity in the spring is significantly lower due to cooler northern Mediterranean and therefore orographic precipitation is not so plentiful. In this area monthly precipitation significantly exceeds evaporation. Despite the month of July is the least wet, the area still gets more than 600 mm of rainfall during the month. July is also the month with the most intense evaporation.
As already mentioned, Trnovski gozd and Nanos form a boundary between the Mediterranean and Alpine climates. When the interior of Slovenia is filled with cold air from the north or northeast, this results in large temperature differences, as well as large pressure gradients at the aforementioned mountain barrier. As the air descends from above Trnovski gozd and Nanos to Vipava Valley and Kras region, it heats adiabatically but it is still colder than the air over the northern Adriatic. The result of this temperature difference is that cold air falls down the slope and reaches high speeds, while the undulating land below causes strong turbulence. This strong wind is known as burja and its gusts reach up to 200 km / h. Relative frequency of burja, although mostly with lower speeds contributes to the air above the Karst and Vipava valley being drier than the air above the other Slovenian regions.
Geological conditions
Geologic map and sections of the High Karst
Legend:
1 - Quaternary (Holocene and Pleistocene: glacier breccias and slope rubble
2 - Eocene, Paleocene and Upper Cretaceous; flysch
3 - Paleocene; various limestones
4 - Upper Cretaceous; mostly organogenic limestones
5 - Lower Cretaceous; mainly bituminous limestone with dolomite spots
6 – Jurassic; limestone and dolomite
7 - Upper Triassic; Dachstein limestone
8 - Upper Triassic; dolomite
9 - Upper Triassic (Carnian); fine-grained dolomite, alternation of sandstone and siltstones
The High Karst area consists of Cretaceous and Jurassic limestones and upper Triassic dolomites belonging to Trnovo and Hrušica cover the thrust structure of western Slovenia. Carbonate rocks are thrust into layers of Eocene flysch; thrust blocks are fractured and tectonically moved along the longitudinal and transverse faults. The western part of the High Karst is between Idrijca and Vipava 10 to 15 km wide, between Soča and Pivka it is about 50 km long and covers approximately 700 km2 of karst terrain, which is practically surrounded on all sides with the lower river fringes. Flysch rocks surround the karst limestone as a partial or complete hydrogeological barrier on the west, south and east sides. On Banjšice plateau, in the western part of the High Karst, flysch is preserved as a thin cover. In some parts the karst base is breaking through it, but mostly it acts as a hanging hydrogeological barrier, below which the characteristic karst streams of water can be found. On the north side the High Karst is surrounded by impermeable middle and lower Triassic, Permian and Carboniferous rocks. Idrijca and its tributaries Belca, Zala, Kanomljica, Hotenja and Trebušnica have formed their surface channels there.
Vipava Valley is a low-lying surface of Eocene flysch between the Low Karst on the south and the High Karst on the north. Bottom of the valley is relatively narrow, except between Vipava and Ajdovščina. An important addition to the image of the valley is a steep, for the most part even precipitous edge of the High Karst on its north side. Limestones, thrust on the flysch, are exposed to an intense mechanical disintegration and breaking, so at its foot slope breccias, scree and rockfalls are accumulated.
Hydrogeological Conditions
Hydrogeological sketch of the High Karst
Legend:
1 - slope rubble, high porosity layers with intergranular porosity
2 - limestones, well-permeable layers with karst porosity
3 - dolomites, medium-permeable layer with fissure porosity
4 - flysch, partly "hanging" hydrogeological barrier
5 - flysch, impermeable layers, side and floor base of the karst aquifer
Area of Trnovski gozd, Banjšice plateau, Nanos and Hrušica covers approximately 700 km2, which represents 3.4% of Slovenian territory or 7.8% of Slovenian karst surface. The whole area can be divided according to their lithological permeability into several hydrogeological units:
- Impermeable areas with permeable inserts
- Medium-permeable zone - aquifers with fissure porosity
- Permeable areas - aquifers with karst and fissure porosity
- Permeable areas - aquifers with intergranular porosity
- Poorly-permeable areas - aquifers with intergranular porosity
Aquifers with karst porosity are made of the upper Triassic Dachstein limestone, the lower and middle Cretaceous limestone and upper Cretaceous limestone rubble. The main hydrogeological units are: karst aquifer Hrušica, karst aquifer Nanos, karst aquifer Črni vrh, karst aquifer in the Hubelj watershed and karst aquifer of western Trnovski gozd and Banjšice. In all these cases the existence of deep karst systems is questionable. Well permeable rocks with intergranular porosity are constructed of coarse-grained to the checkered shaped quaternary breccias and loose Holocene slope debris. They cover a large zone on the southern outskirts of Trnovski gozd and Nanos.
Fissured aquifers are constructed from the dolomites of different ages. The most common is the upper Triassic dolomite, which limits the karst aquifer on the north side and partially directs the outflow of groundwater. Impervious lithostratigraphic horizons consist of uncrushed coal clastic rocks, central Permian sandstones, and Paleocene and Eocene flysch marls. In the impermeable layers there are more permeable sediments, which form important fissured or karstic aquifers between impermeable rocks.
Hydrogeological conditions of Trnovski gozd depend largely on the geological structure and lithology. Flysch in the Vipava valley belongs to different tectonic units. South side is part of the Komen thrust plate. Flysch on the north side belongs to Nanos and Hrušica. Flysch of Hrušica cover forms impermeable base for the karst aquifer of Trnovski gozd and Banjšice plateau as well as south and southwest barrier in the Vipava Valley. To the east barrier is thinned and shows the development of carbonates. Thus, it represents only a partial (local) hydrogeologic barrier.
Upper Triassic, Jurassic and Cretaceous limestones in Trnovski gozd belong to the tectonic unit of Trnovo cover and build a central part of the karst aquifer. Groundwater lies very deep there. Its level is higher in the Hubelj hinterland. Groundwaters from karst aquifers Nanos and Trnovski gozd (sources of Vipava, Hubelj and Lijak) appear on the surface at the lowest points of impermeable flysch barrier.
Hubelj Spring
Geological cross-section of Trnovo plateau over the Hubelj spring
Karst spring of Hubelj River is located high above the valley. Hubelj lies on the bottom of a distinctive, narrow and deep structural depression in the thrust plane of Trnovo cover. In the dry season several siphon lakes are retained in the caves behind the spring. Water flows in between poorly permeable limestones and flysch base. The Hubelj source occurs at the highest altitude of all the springs in the area. It lies on the southern border of Trnovski gozd. At high water the water level rises for 40 m. Flow of Hubelj varies from 0.2 to 59 m3 / s, while its mean flow is 3 m3 / s. Behind the source is the high Karst plateau of Trnovski gozd with the average height of 900 m. This area gets an average of 2450 mm of rainfall per year. The catchment area of the Hubelj spring consists mainly of the Jurassic limestone and covers an area of 50 to 80 km2.
Catchment of the Hubelj spring
A walk through the naturalistic educational trail along the torrent may offer a close examination of an interesting geological ground structure, flora communities, individual tree and fauna species in the land around the Hubelj River. The biological diversity is the first subject of instruction a stroller can get on this trail. The two entirely different worlds are blending into each other on this small piece of earth, creating some unique combinations. Two different microclimates and two entirely different geological substrata. The ground in the lowland forest near Hubelj is composed of flysch, created by various sedimentary rocks. At Škol one can run across permeable limestone, a rock, created by marine sediments. Both these facts actually prove that these lands were once covered by the sea. Truly unique are the slope breccias – a special form of limestone, arising from the resumed coagulation of limestone, which already dilapidated into a less solid form.
Hubelj Caves
Hubelj caves were created by the Hubelj River, which springs high above the valley floor between 220 and 235 m above sea level, on a mountainside of Gora pod Otlico and above the town of Ajdovščina. Hubelj caves are the common name for three caves, namely Veliki Hubelj, Hubljeva kuhna and Pajkova reža. All three caves were created behind the Hubelj source. East wall of the gable walls represent the slope breccias, northern walls are built of limestone. Springs are generated because the flysch, in which the limestone cover is thrust, is the lowest here and thus water has carved a path to the outside. The largest cave is Veliki Hubelj at an altitude of 260 m. The lowest lying is Pajkova reža at an altitude of 250 m, and the highest Hubljeva kuhna - 280 m above sea level. All three are water caves and are occasionally sources of water. Even during the dry periods there is water in all three of the caves. Just below the Pajkova reža cave are the largest sources of Hubelj. Interestingly, the water never fills Hall of quarrels in that cave. It also never passes through the entrance that was artificially dug into the cave, although it is lower than the entrance to the Veliki Hubelj. From the entrance to the Veliki Hubelj cave water flows in a large waterfall during wet periods, which indicates that the connection between Pajkova reža and Veliki Hubelj is very narrow.
Morphology of the three caves is very similar. The input shafts are located in the east-west direction and are of slightly larger dimensions. Internal parts of the caves are mostly vertical cracks around the north-south direction. All three caves were created along secondary faults. Passages of large dimensions have been generated by rockfalls. Elevated passages that water seldom or never reaches are in some places full of calcareous sinter. Halls in Veliki Hubelj and in Hubljeva kuhna, which lie the deepest in the massif, are very muddy. It seems that there the water rises through the holes in the bottom and deposits mud in the process. The water there rises due to the barriers that prevent the outflow of large amounts of water. It is likely that barrier is a demolished zone at the fault.
Polygon of the Hubelj caves
Access
One can reach this EarthCache from the center of Ajdovščina, where there is a well-marked turnoff to the Hubelj spring. If you are coming from Vipava you should turn before the bridge, or immediately after the bridge, when you are coming from Nova Gorica or the highway. After a few kilometers the road brings you to an area called Pale, where the youth hostel and tourist information center are located.
To log this EarthCache we recommend taking the well-marked naturalistic educational trail Ob Hublju, which begins near the youth hostel (waypoint HUBELJ PARK1). Naturalistic educational trail along the Hubelj River is 3.1 km long circular route that leads from Pale on the right bank of Hubelj River to its spring. From there it returns to the starting point through the dry karst meadow of Škol. There are 24 well-marked stations on the trail, where you can see a variety of natural features, both geological and vegetal.
Naturalistic educational trail Ob Hublju
Alternative access is from the Hubelj spring (waypointHUBELJ PARK2), which you can reach by driving along the road past Pale and then join the naturalistic educational trail at the point where it turns back toward the starting point. The Hubelj spring is usually quite busy, because it is a popular destination for locals and starting point for hiking to Gora, as the plateau above Ajdovščina between Col and Predmeja is called.
There are several traditional caches nearby that you can combine with this EarthCache.
EarthCache
To register this EarthCache you must answer the following questions:
1. Go to waypoint HUBELJ1. There you will see an info table, marking a geological peculiarity. What is it? (If you need help translating, Google Translate will do the job.)
2. Go to waypoint HUBELJ2. There you will see an info table, marking a geological peculiarity. What is it? (If you need help translating, Google Translate will do the job.)
3. Go to waypoint HUBELJ3. Estimate the height of the waterfall falling from the central cave of the spring (the one with iron bars).
4. Optionally, take a photo of yourself or your GPS at Hubelj spring and attach it to your log.
Send your answers in Slovene of English through my GC profile. After that you can log your visit, don't wait for permission from me. If there’s something wrong with your answers I will contact you.
If you took photos, attach them to your log.
Logs without the answers sent to me or logs containing the answers will be deleted!
(Go To English Text)
Viri / Sources:
- Wikipedija
- Jamarsko društvo Danilo Remškar, Ajdovščina
- Nika Tomažič, 2007. Ugotavljanje vpliva gnojenja z mineralnim gnojilom na kakovost vode v nezasičeni coni kraškega vodonosnika trnovske planote, Diplomsko delo
- Občina Ajdovščina
- Turistično informacijski center Ajdovščina (english)
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