Controls on supraglacial outlet development during glacial outburst floods
Roberts, Matthew James
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This project arose from the need to evaluate existing knowledge of intraglacial floodwater routing and supraglacial outlet development during glacial outburst floods (jökulhlaups). The overall aim of this thesis is to produce a refined model of the controls on supraglacial outlet development during jökulhlaups. The following hitherto unknown findings constitute a significant advance in our understanding of jökulhlaups hydrodynamics. (i) Transient increases in basal water pressure, can induce the temporary, time-transgressive formation of linked cavity drainage across large zones of the glacier bed. Linked cavity drainage facilitates hydraulic jacking and associated down-glacier transfer of potential energy in the form of a basal flood wave; consequently, supraglacial jökulhlaup outbursts form in response to transient hydraulic conditions imposed at the front of a basal flood wave. (ii) The gradient of excess basal water pressure over glaciostatic pressure governs the propensity for englacial floodwater routing. Rapid, steady increases in basal hydraulic pressure enable retro-feeding of pre-flood intraglacial drainage circuits. Conversely, rapid, unsteady increase in basal hydraulic pressure, which exceed glaciostatic pressure and fracture toughness at regular intervals through the glacier profile, facilitate intrusive hydrofracturing from glacier bed to ice surface. (iii) Retro-feeding of englacial drainage produces supraglacial outlets that have a plan and profile morphology dictated mainly by the shape of near-surface drainage structures. Where hydrofractures reach the glacier surface, their strike and planimetric length is an important control on outlet morphology. (iv) The depth beneath the glacier surface at which single hydrofractures transform into complex fracture networks is fundamental in determining the plan and profile morphology of fracture outlets. (v) A qualitative inverse-relationship exists between the magnitude of near-surface floodwater pressure and the extent to which pre-existing glaciological structures control floodwater routing and outlet morphology. (vi) Once basal water pressure exceeds the weight of a water column extending vertically from glacier bed to ice surface, the prime control on outlet evolution is the rapidity of water-pressure-increase during the period of supraglacial discharge. Comparatively rapid rates of increase in supraglacial discharge are associated with sudden changes in outlet morphology, and vice versa for lower rates of discharge increase.