contrasting glacier behavior over deformable and non-deformable beds gaute lappegard...
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Contrasting glacier behavior over deformable and non-deformable beds
Gaute Lappegard
gaute.lappegard@statkraft.com
Photo: Jürg Alean
Photo: National Snow and Ice Data Center
Movie courtesy: UNIS
Photo: Jürg Alean
Glaciers on deformable and non-deformable beds
Deformable bed Non-deformable bed
Ice streams
Surging glaciers
Valley glaciers
Ice sheets
Valley glaciers
Ice sheets
Temperature control on basal processes
T
z
Pressure melting point, TM
If TBed < TM:
no/few active basal processes
Photo: Michael Hambrey
TM (z) = - 0.00064 z
TM (1000) = - 0.64 ºC
Deformation of multilayered structures
A) Glacier/bedrock C) Glacier/water/bedB) Glacier/sediments
Driving stress: τd = ρ g h sin α α
Glacial beds have different capabilities of handling water
Photo: Frank Wilschut
No outlet streams
Porous media saturated aquiferSurface water tunneled into a few
outlet stream
For both beds: The diurnal variability of melt water input can force diurnal velocity changes
Photo: Roger J. Braithwaite
Non-deformable bed: High flux hydraulics
Photo: Michael Hambrey
R-channels: Melt enlargement and creep closure in competition
Flowing water generates heatChannel enlargement into the ice
Creep closure due to deformable ice
Seasonal and diurnal geometry evolution
Steady-state:
inverse pressure-discharge relation
arborescent structure
low surface-to-volume ratio
courtesy: U.H. Fischer
Kamb, 1987
Non-deformable bed: Low flux hydraulics
pi
pw
Non-deformable bed: Low flux hydraulics
pi
pw
Non-deformable bed: Low flux hydraulics
Kamb, 1987
Non-deformable bed: Low flux hydraulics
Distributed system:
High water pressureLow flux
Proportional discharge-pressure relation
Non-arborescent structureLarge surface-to-volume ratio
courtesy: U.H. Fischer
Lappegard et.al., 2005
A non-deformable bed is kept clean by the hydraulic systems
A pw is low
B pw is high
C pw is low
Hubbard et.al., 1995
Deformable bed: Darcian flow, canals and R-channels
Thin sediment layers can not transport large fluxes of water
the drainage capacity will be exceeded by the water supply
water will start flowing along the ice-till interface
R-channelcanal
For small surface slopes (<0.1)
water will drain in canals of high water pressure eroded into the sediments
For large surface slopes (>0.1)
water will drain in R-channels eroded into the ice
Water pressure influence on sliding and bed deformation
Blake et.al, 1994
Glaciers on both deformable and non-deformable beds can respond temporally with increased velocity to a rapid increase in water pressure
Effective pressure is defined as:
pe = pi – pw
pe - indicates level of buoyancy
(if pe = 0, the glacier floats!)
pi - applied load (ice overburden)
pw – either water pressure in the drainage system or porewater pressure of the till
Ice flow
Sliding on non-deformable bed: The controlling obstacles
Water at the ice-bedrock interface smoothens the bed
Fowler, 1987
From fig.: pe (a) > pe (b) > pe (c)
For a given basal shear stress
sliding, ub, increases when the
effective pressure, pe, decreases
Sliding inversely related to the effective pressure:
ub ~ τbp pe
-q
The drag on the ice is generated by obstacles not drowned
Sliding on deformable bed: Controlled by porewater pressure
Small scale roughness absent
Drag by particles/rocks reduced significantly due to deforming till
Shear stress from the ice transmitted to the till
Sliding depends on till properties as
porosity: n = n ( pe)
shear strength: τf = τf ( pe)
both functionally dependent on pe
Dilatancy
shear thickening
i) No free water available
porewater pressure decreases
shear strength increases
ii) Free water available
water volume increases
shear strength decreases
Deformable bed: Porewater pressure experiment
Iverson et.al., 2003
Iverson et.al., 2003
Iverson et.al., 2003
Sliding on deformable bed: Controlled by porewater pressure
Low ice flow due to:
High sediment strength discourage sediment deformation
Sliding and ploughing
porewater pressure
High ice flow due to:
Low sediment strength encourage sediment deformation
Dilatation and transition to pervasive ductile flow
High ice flow due to:
Decoupling and reduction of basal deformation rates
Ice
flow
Erosion on non-deformable bed
Photo: Michael Hambrey
Photo: Jürg Alean
Photo: Tom Lowell
5 km
Landforms on deformable bed
Courtesy: D. Robinson
Streamlined subglacial bed forms (drumlins, flutes and Rogen moraines) explained by an instability in the laminar flow of ice over a deformable substrate (Hindmarsh (1998), Fowler (2000))
Glaciers on deformable and non-deformable beds
Deformable bed Non-deformable bed
Bed displacementSliding, deformation, free-slip
HydraulicsDarcian flow, canals and R-channels
HydraulicsLinked cavities and R-channels
Bed displacementSliding
Landformsstreamlined forms (drumlins)
LandformsRoches moutonnées, U-valleys
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