Markarfljótsaurar og Öræfajökull
Þessa vefsíðu má einnig lesa á ensku.
Inngangur
Þetta verk er hluti af eldgosahættumati á Íslandi, en hér eru jökulhlaup samfara hugsanlegum eldgosum í Öræfajökli könnuð auk áhrifa stórhlaupa frá Kötlu á Markarfljótsaurum.
Fjallað er um tjónmætti þeirra, berstöðu (e. exposure) fólks og rýmingar. Hvað varðar Markarfljótsaura þá er mat á stærð og útbreiðslu hlaupa fengið úr skýrslu frá 2005. Verkefnið var styrkt af Ofanflóðasjóði, Landsvirkjun og Vegagerðinni.
Útgefið efni
Tenglarnir hér undir opna kynningarefni, þar sem hægt er að sækja heildartexta eða valda kafla.
- Skýrsla á ensku
- Íslensk samantekt
Bókin er á ensku og er um ítarefni að ræða en mikilvægt er að geta miðlað þessu málefni út fyrir landsteinana og jafnframt að geta nýtt erlenda þekkingu sem fyrir liggur.
Heildartexti (pdf 23 Mb)
Bókfræðilegar upplýsingar (pdf 180 Kb)
Kafli I. Volcanogenic floods in Iceland: An exploration of hazards and risks (pdf 1,06 Mb)
Bls. 7–16
Emmanuel Pagneux, Sigrún Karlsdóttir, Magnús T. Gudmundsson, Matthew J. Roberts og Víðir Reynisson
The background, goals and methodological framework of the project are introduced, in addition to highlighting the findings of subsequent chapters. The chapter concludes with a series of recommendations, based on the overall conclusions of the project. Chapter 1 is an ideal starting point for a quick overview of the project.
Kafli II. Öræfajökull Volcano: Geology and historical floods (pdf 5,43 Mb)
Bls. 17–44
Matthew J. Roberts og Magnús T. Gudmundsson
Ágrip
Despite the documented severity and lasting geomorphic imprint of the 1362 and 1727 jökulhlaups from Öræfajökull, there is scant information about the routing and extent of these floods. Using field observations, aerial photographs, and modern-day analogues, the goal of this chapter is to constrain the duration, extent, composition, and maximum discharge of the 1362 and 1727 jökulhlaups. It is proposed that floodwater draining from the caldera region would have broken through the ice surface at ~1,500 m elevation. Consequently, supraglacial outbursts of floodwater would have dominated the onset of both jökulhlaups. Geomorphological evidence suggests that jökulhlaups in 1727 reached 4x104 m3/s at Kotá and that 1362 events had substantially higher discharge. The findings of this chapter provide constraints for estimating the melting potential of Öræfajökull eruptions; they are also pertinent to the simulation of volcanogenic floods from Öræfajökull. Furthermore, insights into flood extent, floodwater composition, and the prevalence of ice blocks provides an empirical basis for the rating of flood hazards in the Öræfi region.
Kafli III. Öræfajökull Volcano: Eruption melting scenarios (pdf 3,33 Mb)
Bls. 45–72
Magnús T. Gudmundsson, Þórdís Högnadóttir og Eyjólfur Magnússon
Ágrip
The potential for ice melting during eruptions in Öræfajökull is assessed using calorimetric estimates of melting for given eruption rates constrained by empirical data from eruptions and considerations of the efficiency of the melting process. Three main types of scenarios are considered for catchments between Svínafellsjökull in the west to Kvíárjökull in the east: (1) A large caldera eruption; (2) a flank eruption on a radial fissure; and (3) the melting from pyroclastic density currents (PDCs). The onset and propagation times of floods arising from subglacial eruptions in Öræfajökull are estimated as being 15-45 minutes from the onset of eruption until a flood starts propagating down the steep flanks. The discharge of jökulhlaups caused by basaltic flank eruptions is considered to be 3,000-6,000 m3/s, a basaltic fissure eruption within the caldera as <20,000 m3/s, and a major explosive caldera eruption, such as the 1362 CE eruption of Öræfajökull, could result in flooding exceeding 200,000 m3/s. Jökulhlaups arising from PDCs may reach 4,000-17,000 m3/s.
Kafli IV. Öræfajökull Volcano: Numerical simulations of eruption-induced jökulhlaups using the SAMOS flow model (pdf 5,89 Mb)
Bls. 73–100
Ásdís Helgadóttir, Emmanuel Pagneux, Matthew J. Roberts, Esther H. Jensen og Eiríkur Gíslason
Ágrip
Regions around Öræfajökull Volcano that would be liable to flooding during a subglacial eruption are identified. Jökulhlaups are simulated as instant release waves over the ice surface using the SAMOS 2D avalanche model, using estimates of the magnitude of historical jökulhlaups in the region. Simulations are made for jökulhlaups caused by a caldera eruption, flank eruptions, and pyroclastic density currents. Because of the wide range of likely flow rheologies, three Manning's n values are assessed: 0.05, 0.1, and 0.15 s/m1/3. Results indicate that a large part of the lowland between the rivers Skaftafellsá and Breiðá (350 km2) is susceptible to flooding because of jökulhlaups descending the western and southern slopes of Öræfajökull. Characterised by average flow velocities in excess of 20 m/s on the slopes the volcano, the jökulhlaups are quick to hit the lowlands once floodwater has reached the glacier's surface — in as little as ~5 minutes at peak discharge.
Kafli V. Öræfi district and Markarfljót outwash plain: Rating of flood hazards (pdf 2,35 Mb)
Bls. 101–122
Emmanuel Pagneux og Matthew J. Roberts
Ágrip
A semi-quantitative, damage-oriented method for the rating of flood hazards in the Markarfljót outwash plain and the Öræfi district is presented. The presence of life-threatening debris and the temperature of floodwater are considered, along with depth of flooding and flow velocities; these factors take into account the unique nature of volcanogenic floods. Flood-hazard zones are designated accordingly. An application of the method to the two study areas indicates a potential for significant direct economic damage and fatalities: due, in particular, to excessive flood depths and flow velocities, jökulhlaups can be lethal to anybody and cause complete destruction or unrepairable damage to dwellings and outbuildings almost anywhere in sectors at risk of flooding. As a first approximation of damage potential of volcanogenic floods in the two areas, the results should be considered carefully by the local and national authorities for planning and emergency evacuation purposes.
Kafli VI. Öræfi district and Markarfljót outwash plain: Spatio-temporal patterns in population exposure to volcanogenic floods (pdf 2,75 Mb)
Bls. 123–140
Emmanuel Pagneux
Ágrip
A spatio-temporal exploration of population exposure is performed in the Markarfljót outwash plain and in the Öræfi district. An inventory of the populations exposed to floods is made for overnight conditions using daily overnight estimates weighted with road traffic data as an indicator. The main objective of the study is to provide the authorities with a realistic estimate, at different periods of the year and at particular locations within the two studied areas, of the likely number of residents and guests potentially in the path of a flood or those that would be stranded due to flooding. In the Markarfljót outwash plain, the results indicate that more than one thousand lives would be in jeopardy if the largest of the potential floods assessed were to happen without warning and evacuation, with a further 600 people being isolated due to partial destruction of the road network; In Öræfi, up to 130 lives could be lost and 240–250 people isolated.
Kafli VII. Öræfajökull: Evacuation time modelling of areas prone to volcanogenic floods (pdf 2,96 Mb)
Bls. 141–164
Emmanuel Pagneux
Ágrip
A model of evacuation routes and timing for areas exposed to floods due to eruptive activity of Öræfajökull Volcano is presented. Estimates of evacuation times are made at the onset of a volcanic eruption originating in the caldera or on the flanks of the volcano, and at the onset of pyroclastic density currents. The aim of the study is to provide the authorities with critical baseline estimates for the development of an effective flood evacuation plan. Results of the modelling suggest that areas at risk of flooding are unlikely to be successfully evacuated once an eruption has started: the only safe route for evacuation can be flooded at multiple locations within 20–30 minutes of the onset of a volcanic eruption in the caldera or on the flanks, and within 15–25 minutes at onset of a pyroclastic density current. In optimal weather conditions, a full evacuation cannot be completed in less than 30–35 minutes.
Tilmæli um að vitna til kafla bókarinnar á eftirfarandi hátt (dæmi):
Roberts, M. J., & Gudmundsson, M. T. (2015). Öræfajökull Volcano: Geology and historical floods. In E. Pagneux, M. T. Gudmundsson, S. Karlsdóttir, & M. J. Roberts (Eds.), Volcanogenic floods in Iceland: An assessment of hazards and risks at Öræfajökull and on the Markarfljót outwash plain (pp. 17–44). Reykjavík: IMO, IES-UI, NCIP-DCPEM.
Í samantektini er gert grein fyrir öllum niðurstöðum verksins á Íslensku. Markmiðið er að þar séu aðgengilegar fyrir allan almenning á Íslandi.
Heildartexti (pdf 138 Mb)
Jökulhlaup vegna eldgosa og jarðhita eru algengasta gerð náttúruvár vegna eldvirkni hér á landi. Hér er fjallað um hlaup sem orðið geta vegna eldgosa í Öræfajökli og hvaða áhrif geta orðið af slíkum hlaupum. Jafnframt er tjónmætti vegna stórhlaupa niður Markarfljót metið.
Öræfajökull er eitt stærsta eldfjall landsins og jafnframt hæsta fjall Íslands. Þar gýs að meðaltali á 500–1000 ára fresti og gosefnaframleiðslan er lítil í samanburði við mikilvirkustu eldstöðvakerfi landsins. Þó hafa tvö gos orðið á sögulegum tíma, 1362 og 1727. Gosið 1362 er talið öflugasta sprengigos sem orðið hefur hér á landi á sögulegum tíma. Mat hefur verið lagt á stærðir og gerðir eldgosa sem orðið geta í Öræfajökli, stærðir jökulhlaupa vegna eldgosa auk þess sem útbreiðsla, framrásartími og vatnsdýpi á áhrifasvæði hlaupa hefur verið metið með hermunarlíkaninu SAMOS. Þá hefur verið lagt mat á líkleg áhrif á byggðir og mannvirki nærri fjallinu auk þess tíma sem það tæki að rýma byggðina á áhrifasvæðum jökulhlaupa.
Skipta má gerðum hlaupa vegna eldgosa í Öræfajökli í þrennt. Í fyrsta lagi eru hlaup vegna sprungugosa undir tiltölulega þunnum jökli í hlíðum fjallsins, í öðru lagi hlaup vegna gosa undir þykkum jökli í öskjunni og í þriðja lagi hlaup vegna bráðnunar á yfirborði jökulsins í heitum gjóskuflóðum. Metið er versta tilfelli sem einhverjar líkur eru á að geti átt sér stað. Niðurstöðurnar benda til þess að sprungugos í hlíðum geti valdið hlaupum af stærðargráðunni 3.000–6.000 m3/s. Stórgos í öskjunni geta orsakað hamfarahlaup, stærri en 100.000 m3/s. Stærðargráða hlaupa vegna gjóskuflóða í stórgosum er metin 10.000–20.000 m3/s. Fram¬rásar¬tími hlaupa yrði í öllum tilvikum stuttur, eða að lágmarki 20–30 mínútur frá upphafi goss þar til hlaup næði að þjóðvegi 1.
Hætta vegna jökulhlaupa við eldgos í Öræfajökli er metin mikil eða geysimikil á um 340 km2 svæði sem nær yfir svo til allt láglendi austan Skaftafells austur fyrir Kvíárjökul. Bæirnir í Svínafelli, Hofi og Hnappavöllum liggja þó rétt utan þess. Fjöldi þeirra sem gista yfir nótt á svæðinu austan Skaftafells að Kvískerjum fer vaxandi með auknum ferðamannastraumi og um há-ferðamannatímann er hann langt umfram fjölda þeirra sem búa á svæðinu að staðaldri. Full rýming Öræfasveitar milli Skaftafellsár og Fjallsár við bestu aðstæður tekur að lágmarki 35–40 mínútur. Þessi tími er lengri en stysti mögulegi framrásartími hlaupa sem undirstrikar nauðsyn þess að forboðar séu greindir rétt. Ef forða á heimafólki og ferðamönnum áður en hlaup næði niður á láglendi þarf að hefja rýmingu áður en gos hefst.
Áhrif vegna hugsanlegra hamfarahlaupa niður Markarfljót vegna Kötlugosa í vestanverðri Kötluöskjunni eru metin, en þar er byggt á eldri niðurstöðum hermunar á hlaupum á því svæði. Greiningin sýnir að ef stórhlaup kæmi niður Markarfljót vegna goss í Kötlu væri hætta mikil eða geysimikil á um 800 km2 lands. Á þessu svæði býr mun stærri hópur fólks en í Öræfasveit og fjölsóttir ferðamannastaðir myndu einangrast í slíku hlaupi.
