The role of sedirn.ent transport in the rn.echanics of jokulhlaups

The classical theory of jokulhlaups used Rothlisberger's earlier theory of ice-channel drainage to describe the development of the Oood hydrograph. This theory has some drawbacks: the mechanism o[ initiation (breaking the seal) is opaque, the l\i[anning roughness coefficient is too large and the hydrographs can reveal a sudden switching from channel opening to channel closure which is not simulated by the model. In this paper, we examine these features by exploring a more detailed model, which takes into account the physics of sediment erosion and its effect on channel morphology. \Ve propose a theory in which channels need not be semicircular, but have shapes determined by a local balance between closure and melting, and in which erosion of the tunnel margins is taken into account; in particular, we deri\'e theoretical predictions for sediment discharge, and we also propose a mechanism whereby the pressure seal over the caldera rim at Grimsvotn in Vatnajokull, Iceland, can be broken when the lake-level water pressure is still some 6 bar below the maximum overburden ice pressure.