Analysis of the first jo ¨ kulhlaup at Bla ˚mannsisen, northern Norway, and implications for future events Rune Verpe ENGESET, 1 Thomas Vikhamar SCHULER, 2 Miriam JACKSON 1 1 Hydrology Department, Norwegian Water Resources and Energy Directorate (NVE), PO Box 5091 Majorstua, NO-0301 Oslo, Norway E-mail: rue@nve.no 2 Department of Geosciences, University of Oslo, PO Box 1047 Blindern, NO-0316 Oslo, Norway ABSTRACT. The first known jo ¨ kulhlaup from the Bla ˚mannsisen ice cap in northern Norway began on 6 September 2001. It lasted 35 hours and emptied the lake Øvre Messingmalmvatn (4.0  10 7 m 3 ). Before the event, the lake drained steadily via a rock spillway into Sweden. The water from the jo ¨kulhlaup drained into the hydropower reservoir Sisovatn, and so was financially beneficial to Norway. Glaciological data show evidence of glacier retreat and thinning during the last four decades. Glacier thickness decreased in the ablation zone, reducing ice-barrier stability. The lake drained at a water level 40 m below that required to equalize the ice overburden pressure. Measurements show an ice-barrier thinning of 3.5 m since the jo ¨ kulhlaup occurred. Climate scenarios indicate future negative mass balance and further thinning. The lake volume was 82% full 2.5 years after the event, suggesting a probable repeat interval of 3 years. Future jo ¨kulhlaups may be triggered at lower water levels and produce lower discharges. INTRODUCTION Jo ¨ kulhlaup mechanisms A jo ¨ kulhlaup is an outburst flood caused by the sudden drainage of a glacier-dammed lake. During such an event, the discharge from a glacier can increase by more than one order of magnitude within a short time (e.g. Bjo ¨ rnsson, 2002). Because of their far-reaching impact and their erratic nature, jo ¨ kulhlaups can pose a significant hazard and cause substantial loss of human life and damage to agricultural land and infrastructure. Reviews of the geographic distri- bution and drainage characteristics of ice-dammed lakes are provided, for example, by Liestøl (1955), Ro ¨ thlisberger (1981), Walder and Costa (1996), Tweed and Russell (1999) and Bjo ¨ rnsson (2002). Drainage from ice-dammed lakes occurs when the lake level is high enough to overcome a potential barrier at the glacier bed, thus enabling discharge underneath the glacier (e.g. Bjo ¨rnsson, 1974; Nye, 1976; Fowler, 1999). There are different mechanisms of flood initiation: some lakes start to drain when the water level reaches the flotation pressure of the ice dam, whereas others release their water at a lower level (e.g. Bjo ¨ rnsson, 1992; Walder and Costa, 1996; Jo ´hannesson, 2002). A characteristic of the latter is the progressive melt enlargement of the ice-walled drainage channel due to dissipation of energy from turbulent flood- water (e.g. Ro ¨thlisberger, 1972; Nye, 1976). Usually, the flood terminates abruptly as the lake is emptied or the drainage channel collapses. Overtopping of the ice dam (Walder and Costa, 1996; Raymond and Nolan, 2000; Mayer and Schuler, 2005) or excavation of a breach between the ice and the adjacent rock wall (Walder and Costa, 1996) has also been reported. Former jo ¨kulhlaups in Norway There are at least 19 glacier-dammed lakes in Norway. The most dangerous jo ¨kulhlaups in Norway were from lake Demmevatn at Hardangerjøkulen in southern Norway. Written sources describe late-summer jo ¨ kulhlaups from this lake since the 18th century. After a catastrophic jo ¨ kulhlaup in 1893, a rock tunnel was constructed to drain the lake artificially. However, a further decrease in glacier volume inflicted another two serious floods in 1937 and 1938 (Elvehøy and others, 2002). Studies of jo ¨ kulhlaups in relation to hydropower plants have been undertaken (e.g. Huggel and others, 2003), but tend to be focused on the hazards, rather than benefits. However, some knowledge of the possibility of reoccurrence in this case is useful not only in planning how to make the most efficient use of this additional water for hydropower, but also for risk mitigation. The jo ¨ kulhlaup at Bla ˚mannsisen 2001 The first known jo ¨ kulhlaup from the ice cap Bla ˚ mannsisen in northern Norway occurred from 6 to 7 September 2001 (Fig. 1). Bla ˚mannsisen is the fifth largest glacier in Norway, covering 87 km 2 . The water drained from the lake Øvre Messingmalmvatn (67820 0 N, 16805 0 E), which is located adjacent to and dammed by the 12 km long outlet glacier Rundvassbreen (Fig. 1). The water drained under Rundvass- breen and then to the Sisovatn reservoir, where the water level rose 2.4m. Previously, the lake had drained steadily in the opposite direction, over the border into Sweden. Fortunately, there were no casualties or material damage from the jo ¨kulhlaup; on the contrary, it increased the volume of water in the reservoir that is used to supply a hydropower plant operated by Elkem Energi Siso AS, so the flood was financially beneficial. A preliminary description of the jo ¨kulhlaup is given by Engeset (2002, 2003). Study outline The unprecedented drainage of Øvre Messingmalmvatn poses a number of questions. Why did the ice dam fail although it had been stable before? What changes of the glacier or the lake might explain the sudden instability? Which drainage mechanism governed the flood? What is the likelihood of future jo ¨ kulhlaups, how often might they be Annals of Glaciology 42 2005 35