A6 Jökulhlaups and Glacial Floods

Topic

A *jökulhlaup*—an Icelandic term for a sudden glacial flood—is the rapid discharge of water previously stored within or atop a glacier; such a sudden release can generate flow rates of tens of thousands of cubic meters per second in just a few hours. A *jökulhlaup* can be triggered in three distinct ways: by the failure of an ice dam that had been retaining the water; by the drainage of a lake that had accumulated beneath the glacier and was being heated by geothermal or volcanic activity; or by the breach of a dam formed by moraine sediments deposited by the glacier itself.

The mechanism driving a subglacial *jökulhlaup* involves a pressurized water channel carving its way beneath the glacier: the pressurized water flowing through the channel melts the surrounding ice walls via heat generated by the friction of the moving water, and this melting widens the channel. A wider channel reduces the hydraulic pressure of the water flowing within it; this lower pressure draws even more water from the subglacial storage system into the channel, and the increased volume of water further accelerates the *jökulhlaup* discharge, completing a self-reinforcing cycle of thermal and hydraulic feedback. This same thermal-hydraulic feedback cycle produces the characteristic shape of a *jökulhlaup* hydrograph: a slow, gradual initial rise followed by a catastrophic peak. In contrast, when a *jokulhlaup* originates from the draining of a glacial lake dammed at the glacier's terminus, the failure of the barrier retaining that lake follows the same physical principles governing water flow in an open channel, combined with upstream-migrating erosion that deepens the breach in the barrier as more lake water escapes through it. A standard model describes the evolution of the subglacial channel as the *jokulhlaup* progresses, while another set of standard equations describes the propagation of the flood wave once the *jokulhlaup* has exited the glacier and is moving down the downstream channel.

The expected behavior of a *jokulhlaup*—whether originating beneath the ice or from the draining of a dammed glacial lake—follows a well-established empirical correlation: the greater the volume of water accumulated prior to drainage, the higher the peak discharge the *jokulhlaup* reaches during its brief release—the very same sudden discharge of tens of thousands of cubic meters per second mentioned at the beginning of this description.

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