Paleopoplave v porečju kraške Ljubljanice

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Založba ZRC, 1. sep. 2016 - 187 strani

 Paleopoplave v porečju kraške Ljubljanice so bile preučevane na območju najpomembnejšega sotočja vod, med Pivško kotlino ter Cerkniškim in Planinskim kraškim poljem. Poplavne oblike se na tem območju pojavljajo tudi na bistveno višjih nadmorskih višinah kot jih dosežejo najvišje poplave v današnjih hidroloških razmerah. Te oblike, ki so danes nad hidrološko aktivnim nivojem, so obravnavane kot sledi paleopoplav. Z več metodami je bil ugotovljen prostorski obseg ter dinamika sedimentacije gradiva ob paleopoplavah. Časovna umestitev paleopoplav je temeljila na datacijah sige z radiometričnima metodama 14C in U-Th. Ugotovljeno je bilo, da je bila jakost poplav v preteklosti bistveno večja, a je bila dinamika pojavljanja paleopoplav podobna današnjim. Tako kot ob današnjih poplavah v porečju Ljubljanice se je tudi ob paleopoplavah iz suspenzije odlagalo drobnozrnato gradivo. Hidrološki razvoj porečja se je skladno z dinamiko geomorfnih procesov spreminjal skozi več faz paleookoljskih sprememb v kvartarju.

V znanstveni monografiji je predstavljena raziskava paleopoplav v Pivški kotlini ter na Cerkniškem in Planinskem polju, kjer je najpomembnejše sotočje vod v porečju kraške Ljubljanice. Na tem območju se značilne poplavne oblike, kot so poplavne ravnice, rečne terase, korozijske skalne zajede na jamskih stenah ter drobnozrnati sedimenti z vodoravno stratigrafijo, pojavljajo tudi na nadmorskih višinah, ki so bistveno nad najvišjimi recentnimi znanimi poplavami. Njihov prostorski obseg je bil ugotovljen z morfometričnimi analizami oblik na površju in v jamah, z modeliranjem paleopoplav pa izračunana njihova prostornina in površina. S petrološkimi analizami je bil ugotovljen izvor poplavnega gradiva in dinamika sedimentacije, s čimer je bil dokazan obstoj paleopoplav. Morfokronološke analize so temeljile na datacijah sige med poplavnimi sedimenti z radiometričnima metodama 14C in U-Th. Ugotovljeno je bilo, da je bila jakost poplav v preteklosti bistveno večja od jakosti recentnih poplav. Za Pivško kotlino so bila ugotovljena tri obdobja s paleopoplavami, v katerih se je višina poplav sčasoma zniževala. Dinamika pojavljanja paleopoplav je bila kljub višjim poplavam v preteklosti podobna današnjim. Plastovitost gradiva dokazuje ponovljivost paleopoplav, njegova debelina pa dolgotrajnost teh procesov. Te ugotovitve so v nasprotju s starejšimi razlagami paleohidroloških razmer v porečju Ljubljanice, ki so govorile o trajni ojezeritvi kraških polj v preteklosti. Sklepamo, da se je skladno z dinamiko geomorfnih procesov porečje Ljubljanice hidrološko razvijalo skozi več faz paleookoljskih sprememb v kvartarju.

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The scientific monograph ('Palaeofloods in karstic Ljubljanica River Catchment') presents a research on palaeofloods in the main water confluence in the Ljubljanica River Catchment, namely the area between the Pivka Basin, Cerknica Karst Polje, and Planina Karst Polje. Several geomorphological forms genetically related to floods have been identified, e.g. floodplains, river terraces, corrosion notches on cave walls, as well as laminated fine-grained sediments. Since they were found at higher altitudes than the present floods reach, they were considered as palaeoflood features. Morphometric analysis of the studied geomorphological forms was used to determine the volume of palaeofloods and their altitudinal range. The hydrometric characteristics of the palaeofloods where calculated by computer modelling. Petrological analyses of fine-grained sediments from the surface and caves were used to determine the origin of sediments and their settling velocities, hence to prove their flood origin. Furthermore, morpho-chronological analyses with radiometric methods 14C and U-Th on flowstones interlaying with flood sediments where carried out. The results show that the volume and altitude of palaeofloods throughout the study area fairly exceeded the highest known recent floods. In the Pivka Basin three palaeoflood periods were determined showing a decline of palaeofloods' altitude through time. Although palaeofloods reached higher altitudes, the dynamics of palaeofloods was similar to the dynamics of recent floods: laminated fine-grained sediments were deposited from suspension during palaeofloods, which is also characteristic of recent floods in the Pivka Basin and of other karst poljes of the Notranjska Region. Laminated material confirms periodical flooding, while its depth and thickness prove the durability of the processes, contradicting older theories which assumed the karst poljes where permanently flooded in the past geomorphological periods. In conclusion it was established the Ljubljanica River Catchment developed in several stages, with changing dynamics of the geomorphic processes reflecting palaeoenvironmental change in the Quaternary.

 

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