Jökulhlaups drain regularly from six subglacial geothermal areas in Iceland. From Grímsvötn in Vatnajökull, jökulhlaups have occurred at 4 to 6 yearly-intervals since the 1940s with peak discharges of 600 to 10000 m3s−1, durations of 2 to 3 weeks and total volumes of 0.5 to 3.0 km3. Prior to that, about one jökulhlaup occurred per decade, with an estimated discharge of 5 km of water and a peak discharge of approximately 30000 m3s−1. Clarke’s (1982) modification of Nye’s (1976) general model of discharge of jökulhlaups gives, in many respects, satisfactory simulations for jökulhlaups from Grímsvötn the best fit being obtained for Manning roughness coefficients n = 0.08 to 0.09 m−1/3s and a constant lake temperature of 0.2°C (which is the present lake temperature). The rapid ascent of the exceptional jökulhlaup of 1938, which accompanied a volcanic eruption, can only be simulated by a lake temperature of the order of 4°C.
Jökulhlaups originating at geothermal areas beneath ice cauldrons located 10 to 15 km northwest of Grímsvötn have a peak discharge of 200 to 1500 m3s−1 in 1 to 3 days, with total volume of 50 to 350 × 106m3, and they recede slowly in 1 to 2 weeks. The form of the hydrograph has reversed asymmetry to that of a typical Grímsvötn hydrograph. The reservoir water temperature must be well above the melting point (10 to 20°C) and the flowing water seems not to be confined to a tunnel but to spread out beneath the glacier and later gradually to collect back to conduits.
Since the time of the settlement of Iceland (870 AD), at least 80 subglacial volcanic eruptions have been reported, many of them causing tremendous jökulhlaups with dramatic impact on inhabited areas and landforms. The peak discharges of the largest floods (from Katla) have been estimated at the order of 100 000 to 300 000 m3 s−1, with durations of 3 to 5 days and 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 Bárdharbunga and Kverkfjöll). They may be the source of prehistoric jökulhlaups, with estimated peak discharge of 400 000 m3 s−1.
At present, jökulhlaups originate from some 15 marginal ice-dammed lakes in Iceland. Typical values for peak discharges are 1000 to 3000 m3s−1, with durations of 2 to 5 days and total volumes of 2000 × 106 m3. Hydrographs for jökulhlaups from marginal lakes have a shape similar to those of the typical Grímsvötn jökulhlaup. Simulations describe reasonably well the ascending phase of the hydrographs assuming a constant lake temperature of about 1°C; but they fail to describe the recession. Some floods from marginal lakes, however, have reached their peaks exceptionally rapidly, in a single day. Such rapid ascent can be simulated by assuming drainage of lake water at 4 to 8°C.
An empirical power-law relationship is obtained between peak discharge, Q
max, and total volume V
t of the jökulhlaups from Grímsvötn: Q
max = KV
b, where Q
max is measured in m3s−1, V
t in 106m3, Κ = 4.15 × 10−3s−1 m2 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 to 6 bar.