IV. ÖRÆFAJÖKULL VOLCANO: NUMERICAL SIMULATIONS OF ERUPTION-INDUCED JÖKULHLAUPS USING THE SAMOS FLOW MODEL Ásdís Helgadóttir *, Emmanuel Pagneux *, Matthew J. Roberts *, Esther H. Jensen *, Eiríkur Gíslason * * Icelandic Meteorological Office Introduction Results on minimum surface transport time found in this study are used, along with This study identifies regions around estimates of eruption onset time, subglacial Öræfajökull Volcano that would be liable to retention time and subglacial transport time flooding in the event of a subglacial eruption. (Gudmundsson et al., 2015), in an assessment Melting scenarios (Gudmundsson et al., of the time available for evacuating the areas 2015) are used to simulate the routing of at risk of flooding (Pagneux, 2015b). glacial outburst floods (jökulhlaup) over the ice surface and the propagation of floodwater Past volcanogenic floods from the base of the glacier on the western and southern flanks of the volcano. Since Iceland was first populated in 874 CE, Jökulhlaups are simulated as fluids using the two eruptions have occurred beneath the SAMOS numerical model, developed for Öræfajökull ice-capped stratovolcano (Figure shallow and fast moving granular gravity IV-1). The first observed historical eruption currents (Zwinger et al., 2003). The occurred in mid-June 1362, and the second uncertainty in rheology of the floods is dealt eruption began on 7 August 1727 with by using predefined Manning’s n (Thorarinsson, 1958). Both eruptions were coefficients ranging 0.05–0.15. Simulations accompanied by a massive, short-lived are made for outburst floods caused by: (i) a jökulhlaup that inundated several areas caldera eruption, (ii) flank eruptions, and (iii) simultaneously. Accounts of the 1727 pyroclastic density currents. eruption reveal that it rose rapidly, within hours, to a maximum discharge that was The main objective of the study is to provide exceptionally large compared to the volume information on inundation extent, maximum of floodwater drained. For the 1362 depths of flooding, maximum flow speeds, jökulhlaup, Thorarinsson (1958) estimated a and minimum surface transport times, maximum discharge of ~100,000 m3/s, computed for several scenarios and aggre- attained within a matter of hours. Debris gated into thematic datasets. Aggregated transport was also a significant factor during results on inundation extent are used in an both jökulhlaups. Debris-laden flows would assessment of the populations exposed to have comprised juvenile eruptive material, floods (Pagneux, 2015a) while information glacial ice, and glaciofluvial sediments, as on maximum flood depths and maximum described in chapter III (Roberts and flow speeds serve as input for rating flood Gudmundsson, 2015). hazards (Pagneux and Roberts, 2015). Öræfajökull Volcano: Numerical simulations of eruption-induced jökulhlaups using the SAMOS flow model 73 The 1362 jökulhlaup is thought to have burst of the ice cap at high elevation. Flood primarily from the glaciers Virkisjökull, sediments from the 1362 jökulhlaup extend Falljökull, and Kotárjökull (Thorarinsson, over a much greater area than those from the 1958). Apparently, the 1727 jökulhlaup from 1727 jökulhlaups, especially towards the Kotárjökull was comparable in size to the northwest and west of Falljökull (Thora- 1362 jökulhlaup from the same glacier rinsson, 1958). Pyroclastic flows would have (Thorarinsson, 1958). However, the 1362 been prevalent during eruptions of Öræ- jökulhlaup from Falljökull was much larger fajökull. These flows would have scoured than the 1727 jökulhlaup there. Similar to large zones of the ice cap, causing significant modern-day volcanogenic floods from steep, and pervasive ice-melt. ice-capped volcanoes (Tómasson, 1996; For a full description of the 1362 and 1727 Magnússon et al., 2012a; Waythomas et al., jökulhlaups, see chapter II (Roberts and 2013), it is probable that the 1362 and 1727 Gudmundsson, 2015). jökulhlaups burst initially through the surface Figure IV-1: Öræfajökull ice-capped stratovolcano, shown by a black triangle, is a separate accumulation area of the Vatnajökull ice cap in south-east Iceland. 74 Öræfajökull Volcano: Numerical simulations of eruption-induced jökulhlaups using the SAMOS flow model Melting scenarios floods due to a flank eruption, and floods due to pyroclastic density currents. Post-eruptive Ten melting scenarios relating to volcanic floods, as well as syn-eruptive floods due to eruptions of various sizes, types, and precipitation, were not considered in the locations were considered in the modelling of modelling. Meltwater volume and maximum floods due to eruptions of Öræfajökull peak discharge were determined for each volcano. Full description of the scenarios is scenario using an order-of-magnitude given in chapter III (Gudmundsson et al., approach (Table IV-1, Figure IV-2). A 2015). comparison can be made with the explosive Flow simulations were restricted to primary eruptions of Mount Redoubt in 2009, which jökulhlaups, i.e. floods induced by the produced lahars having volumes of 107–108 eruption itself. A distinction was made m3 and peak discharges of 104–105 m3/s between floods due to a caldera eruption, (Waythomas et al., 2013). Table IV-1: Melting scenarios, with special reference to risk source, meltwater origin and peak discharge (Gudmundsson et al., 2015). Scenario Glacier Peak discharge Risk source Meltwater origin ID catchment (m3/s) S01c Virkisjökull – Caldera eruption Falljökull – Virkisjökull 105 Falljökull (VIR) S01f Flank eruption Falljökull – Virkisjökull 104 S02c Caldera eruption Kotárjökull 105 S02f Suðurhlíðar Flank eruption Kotárjökull 104 (SUD) 4 S03f Flank eruption Stigárjökull 10 S03p Pyroclastic flow East from Rótarfjallshnúkur* 3·104 S04c Kvíarjökull Caldera eruption Kvíarjökull 105 (KVI) S04f Flank eruption Kvíarjökull 104 Svínafellsjökull, south from 4 S05p Svínafellsjökull Pyroclastic flow Svínafellshryggur Ridge 10 (SVI) Svínafellsjökull, north from S06p Pyroclastic flow Svínafellshryggur Ridge 104 *Kotárjökull excluded Öræfajökull Volcano: Numerical simulations of eruption-induced jökulhlaups using the SAMOS flow model 75 Figure IV-2: Hypothetical eruptive fissures proposed by Gudmundsson et al. (2015). Delineation of the caldera rim is based on ice thickness estimations by Magnússon et al. (2012b). Ice divide is based on airborne LiDAR survey performed in 2011 (see section 3.2.2). Modelling assumptions hlaups in Iceland and Alaska (Magnússon et al., 2012a; Waythomas et al., 2013). Recent observations of high-magnitude Significant volumes of snow and ice would jökulhlaups due to volcanism have shown be incorporated into a surface-based (supra- that floodwater often bursts through the glacial) flow. We make no attempt to surface of steeply sloping glaciers (Roberts, incorporate the dynamic effects of ice-block 2005; Magnússon et al., 2012a). This was transport and floodwater bulking. However, also the case for large jökulhlaups from the the increased friction resulting from this is outlet glaciers Kötlujökull (Mýrdalsjökull) taken into account indirectly by using a and Skeiðarárjökull (Vatnajökull) in 1918 higher Manning's roughness coefficient (n). and 1996, respectively (Roberts, 2002). The geomorphic consequences of ice-block Likewise, anecdotal accounts of the 1727 deposition are addressed by Roberts and jökulhlaup from Öræfajökull describe water Gudmundsson (2015). draining from the glacier. Given that ice thicknesses on the upper slopes of Öræfajökull, outside the volcano’s caldera, SAMOS modelling are widely less than 100 m (Magnússon et al., Several numerical flow models have been 2012b), it is likely that floodwater would used in recent years in the modelling of emerge from crevasses at elevations volcanogenic floods, including LaharZ (e.g. exceeding 1,000 m AMSL. Hence, for the Hubbard et al., 2007; Capra et al., 2008; simulations in this study, floodwater Muños-Salinas et al., 2009; Magirl et al., descends initially from the surface of the ice 2010; Muños-Salinas et al., 2010), Titan2D cap at predetermined elevations. In reality a (e.g. Charbonnier and Gertisser, 2009; fraction of the flood would also propagate Charbonnier et al., 2013), and VolcFlow (e.g. across the glacier bed, but such routing is not Charbonnier et al., 2013). considered here. This is in agreement with recent observations of volcanogenic jökul- 76 Öræfajökull Volcano: Numerical simulations of eruption-induced jökulhlaups using the SAMOS flow model In this study, the SAMOS numerical model is Constraints and limitations used for the simulation of jökulhlaups. Several constraints or limitations inherent in SAMOS is a two dimensional depth-averaged using SAMOS should be named. First, numerical avalanche model initially supraglacial floods are simulated as instant developed for the Austrian Avalanche and release waves and the effects of sediment Torrent Research Institute in Innsbruck to bulking and de-bulking (erosion and model dry-snow avalanches (Sampl et al., entrainment) are not taken into account. 2004; Sampl and Granig, 2009; Zwinger et Hydraulic equations at each location are al., 2003). The model has been used solved using a digital surface model that intensively in Iceland in the assessment of remains unchanged during simulations. run-out zones of snow avalanche (Gíslason During such sediment-loaded floods, pro- and Jóhannesson, 2007), and occasionally in nounced landscape change is likely to occur the assessment of floods caused by volcanic (Roberts and Gudmundsson, 2015), thus eruptions