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