Effects of bed depressions upon floods from subglacial lakes

At the beginning of a subglacial flood, storage of floodwater in bed depressions is very important. Storage delays the extension of a channel and forces the flood into a water sheet mode everywhere except upstream from the first depression encountered by the flood. In order for a grounded R-channel to evolve, it must extend its length continuously downstream by incorporating many subglacial ponds in its path. Simultaneous with R-channel extension, water sheet elimination works its way downstream. It is likely that limitations on the length of R-channels leading from the flood lake exist under ice sheets. In addition, volume discharge from a lake might be severely limited if a second lake exists downstream and close by. Bed depressions and humps are shown to delay the evacuation of water from the bed after a water sheet flood. Both channel and water-sheet flows are can exist beneath ice sheets. Ice displacement by uplift is important in creating cavities for water storage. Subglacial floods under ice sheets are likely to have a duration of years as compared to the 1972 Grimsvötn flood of 10 days. Any flood to the bed from a supraglacial lake has a duration of less than 3 days. Flood volumes are reduced for lakes located in the interior of ice sheets, particularly near ice divides.