Contrasting glacier behavior over deformable and non-deformable beds

1. Contrasting glacier behavior over deformable and non-deformable beds Gaute Lappegard gaute.lappegard@statkraft.com

2. Photo: Jürg Alean

3. Photo: National Snow and Ice Data Center

4. Movie courtesy: UNIS

5. Photo: Jürg Alean

6. Glaciers on deformable and non-deformable beds • Non-deformable bed • Deformable bed • Valley glaciers • Ice sheets • Valley glaciers • Ice sheets • Ice streams • Surging glaciers

7. Photo: Michael Hambrey Temperature control on basal processes z T TM (z) = - 0.00064 z TM (1000) = - 0.64 ºC Pressure melting point, TM If TBed < TM: no/few active basal processes

8. α Deformation of multilayered structures • Driving stress: τd=ρghsinα • A) Glacier/bedrock • B) Glacier/sediments • C) Glacier/water/bed

9. Glacial beds have different capabilities of handling water Photo: Frank Wilschut • Photo: Roger J. Braithwaite • No outlet streams • Porous media saturated aquifer • Surface water tunneled into a few outlet stream For both beds: The diurnal variability of melt water input can force diurnal velocity changes

10. courtesy: U.H. Fischer Non-deformable bed: High flux hydraulics R-channels: Melt enlargement and creep closure in competition Flowing water generates heat Channel enlargement into the ice Creep closure due to deformable ice Seasonal and diurnal geometry evolution Photo: Michael Hambrey Steady-state: inverse pressure-discharge relation arborescent structure low surface-to-volume ratio

11. Non-deformable bed: Low flux hydraulics Kamb, 1987

12. pw Non-deformable bed: Low flux hydraulics pi

13. pw Non-deformable bed: Low flux hydraulics pi

14. Kamb, 1987 Non-deformable bed: Low flux hydraulics Distributed system: High water pressure Low flux Proportional discharge-pressure relation Non-arborescent structure Large surface-to-volume ratio courtesy: U.H. Fischer

15. Lappegard et.al., 2005 Hubbard et.al., 1995 A non-deformable bed is kept clean by the hydraulic systems A pw is low B pw is high C pw is low

16. R-channel canal 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 • 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

17. Water pressure influence on sliding and bed deformation • 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 Blake et.al, 1994

18. Ice flow Sliding on non-deformable bed: The controlling obstacles • Water at the ice-bedrock interface smoothens the bed From fig.: pe (a) > pe (b) > pe (c) • The drag on the ice is generated by obstacles not drowned • Sliding inversely related to the effective pressure: • ub ~ τbp pe-q • For a given basal shear stress • sliding, ub, increases when the • effective pressure, pe, decreases Fowler, 1987

19. Dilatancy 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 • shear thickening • i) No free water available • porewater pressure decreases • shear strength increases • ii) Free water available • water volume increases • shear strength decreases • Sliding depends on till properties as • porosity: n=n(pe) • shear strength: τf = τf(pe) • both functionally dependent on pe

20. Deformable bed: Porewater pressure experiment Iverson et.al., 2003

21. Iverson et.al., 2003 Iverson et.al., 2003

22. 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 Sliding on deformable bed: Controlled by porewater pressure porewater pressure • Low ice flow due to: • High sediment strength discourage sediment deformation • Sliding and ploughing Ice flow

23. Erosion on non-deformable bed Photo: Jürg Alean Photo: Michael Hambrey Photo: Tom Lowell

24. 5 km

25. Courtesy: D. Robinson Landforms on deformable bed 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))

26. Glaciers on deformable and non-deformable beds • Non-deformable bed • Deformable bed • Hydraulics • Darcian flow, canals and R-channels • Hydraulics • Linked cavities and R-channels • Bed displacement • Sliding, deformation, free-slip • Bed displacement • Sliding • Landforms • streamlined forms (drumlins) • Landforms • Roches moutonnées, U-valleys