HELGI BJÖRNSSONABSTRACT. Jökulhlaups drain regularly from six subglacial geothermal areas in Iceland. From Grímsvötn in Vatnajökull, jökulhlaups have occurred at a 4-6 year interval since the 1940s with a peak discharge of 1,000 - 10,000 m3/s, 2 -3 weeks duration and total volumes of 0.5-3 km3. Prior to that, about one jökulhlaup occurred per decade with an estimated discharge of 5 km3 of water and a peak discharge of approximately 30,000 m3/s. Clarke½s (1982) modification of Nye½s (1976) general model of discharge of jökulhlaups gives in many respects satisfactory simulations for jökulhlaups from Grímsvötn. The best fit is obtained for the Manning roughness coefficients n = 0.08-0.09 m-1/3 s and a constant lake temperature of 0.2 °C (which is the present lake temperature). The rapid ascent of the exceptional jökulhlaup in 1938, which accompanied a volcanic eruption, can only be simulated for a lake temperature of the order of 4 °C.
University of Iceland
Dunhaga 5, 107 Reykjavík, Iceland.
Jökulhlaups originating at geothermal areas beneath ice cauldrons located 10-15 km northwest of Grímsvötn have a peak discharge of 200 - 1,500 m3/s in 1-3 days, the total volume is 50-350x106m3, and they recede slowly in 1-2 weeks. The form is in that respect a mirror image of the typical Grímsvötn hydrograph. The reservoir water temperature must be well above the melting point (10-20 °C) and the flowing water seems not to be confined to a tunnel but spread out beneath the glacier and later gradually collected back to conduits.
Since the time of the settlement of Iceland (870 A.D.), at least 80 subglacial volcanic eruptions have been reported, many of them causing tremendous jökulhlaups with dramatic impact on inhabitated areas and landforms. The peak discharge of the largest floods (from Katla) has been estimated at the order of 100-300,000 m3/s, duration was 3 -5 days and the total volume of the order of 1 km3. It is now apparent that the potentially largest and most catastrophic jökulhlaups may be caused by eruptions in the voluminous ice-filled calderas in northern Vatnajökull (of Bárdarbunga and Kverkfjöll). They may be the source of prehistoric jökulhlaups, with estimated peak discharge of 400,000 m3/s.
At present, jökulhlaups originate from some fifteen marginal ice-dammed lakes in Iceland. Typical values for peak discharges are 1,000 - 3,000 m3/s, duration 2-5 days and total volumes of 2,000 x 106 m3. Hydrographs for jökulhlaups from marginal lakes have a shape similar to those of the typical Grímsvötn jökulhlaup. Simulations give reasonable ascent of the hydrographs for constant lake temperature of about 1 °C but fail to show the recession. Some floods from marginal lakes, however, have reached their peaks exceptionally rapidly, in one day. That ascent could be simulated by drainage of lake water of 4-8 °C.
An empirical power law relationship is obtained between peak discharge and total volume of the jökulhlaups from Grímsvötn; Qmax = K Vtb, where Qmax is measured in m3/s, Vt in 106 m3, K = 4.15 10-3 s-1 m-2.52 and b = 1.84.
In general, the jökulhlaups (excepting those caused by eruptions) occur when the lake has risen to a critical level, but before a lake level required for simple flotation of the ice dam is reached. The difference between the hydrostatic water pressure maintained by the lake and the ice overburden pressure of the ice dam is of the order 2-6 bar.