Violent glacial bursts (jokulhlaup) are huge floods produced by subglacial volcanic eruptions. They are well known in Iceland. The biggest jokulhlaups come from Katla and can discharge water masses on a scale of several cubic kilometers into the ocean in just a few hours. Subglacial volcanoes can melt the glacier fast enough to produce 100,000–300,000 m³/s discharges for a long enough time to produce such floods. Geological evidence of such large floods exists, and it is concluded that historical floods like the one produced by the Katla eruption of 1918 have most likely been of this order of magnitude. Results from a computational fluid dynamics (CFD) simulation of a jokulhlaup down the Markarfljot valley in Iceland are shown. The flood is a translatory wave, and the existence of the translatory wave is established for an inclined plane in a prismatic channel and in a funneling channel as an initial value problem. Inherent instabilities of the wave tail are discussed. The energy and power emitted by the flood wave into the ocean is estimated, and transformation into a wave group suggested as a method to find the height of a tsunami-like wave hitting the Vestmannaeyjar Islands. Edge waves along the coast are discussed. Probabilities of jokulhlaups that might cause tsunami-like waves are reported.