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Frontal Dynamics of Powder Snow Avalanches

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  1. Frontal Dynamics of Powder Snow Avalanches Cian Carroll, Barbara Turnbull and Michel Louge EGU General Assembly, Vienna, April 27, 2012 Thanks to Christophe Ancey, Perry Bartelt, Othmar Buser, Jim McElwaine,Florence & Mohamed Naiim, Matthew Scase,Betty Sovilla Sovilla, et al, JGR (2010) Sponsored by ACS Petroleum Research Fund

  2. rapid eruption McElwaine & Turnbull JGR (2005) Issler (2002) Sovilla et al (2006) static pressure (Pa) height (m) depression time (s) time (s) slope Sovilla, et al JGR (2006) width distance (m) distance (m) Field data

  3. avalanche rest frame avalanche head source Issler (2002) Sovilla et al (2006) rapid eruption Consider avalanche head

  4. Principal assumptions in the cloud • Negligible basal shear stress • Negligible air entrainment • Inviscid • Uniform mixture density avalanche head source

  5. U U’ Slowing Swelling Rankine half-body potential flow Rankine, Proc. Roy. Soc. (1864)

  6. Experiments and simulations on eruption currents

  7. pressure p, air density r, cloud density r’ stagnation-source distance b’fluidized depth h’ Static pressure in the cloud prediction data: McElwaine and TurnbullJGR (2005)

  8. interface pore pressure p Pore pressure gradients defeat cohesion rapid eruption Issler (2002) height (m) time (s) Porous snow pack

  9. interface pore pressure p ts < 0 2a+2b-p t sx s1 a Pore pressure gradients defeat cohesion s s2 sy Mohr-Coulomb failure t snowpack density rc, friction me Porous snow pack

  10. avalanche head Frontal Dynamics

  11. Mass balance

  12. snowpack density rc, friction me, inclination a, entrained fraction l of fluidized depth h’ Mass balance

  13. snowpack density rc, friction me, inclination a, entrained fraction l of fluidized depth h’ Cloud pressure fluidizes snowpack: Snowpack eruption feeds the cloud: Stability

  14. cloud height entrained depth density Stability diagram Ri unstable z stable stable Ri stable z unstable unstable

  15. acceleration momentum added mass weight + buoyancy Frontal Dynamics

  16. Acceleration gravity channel width W distance (m) distance (m)

  17. Other predictions

  18. Height vs distance cloud height Vallet, et al, CRST (2004)

  19. Froude number vs distance cloud Froude number Vallet, et al, CRST (2004) Sovilla, Burlando & Bartelt JGR (2006)

  20. Volume growth total volume air entrainment in the tail volume growth Measurements: Vallet, et al, CRST (2004)

  21. increasing height Impact pressure ≠ static pressure Cloud arrest Impact An impact pressuredecreasing with heightdoes not necessarily imply densitystratification.

  22. Air entrainment

  23. source radius rc Ancey, JGR (2004) Air entrainment into the head

  24. Conclusions • Our model of eruption currents is closed without material input from surface erosion or interface air entrainment. • Porous snowpacks synergistically eject massive amounts of snow into the head of powder clouds. • Suspension density swells the cloud and weakens its internal velocity field. • Mass balance stability sets cloud growth. • Changes in channel width affect acceleration. • Experiments should record cloud density and pore pressure.

  25. Thank you Barbara Turnbull Betty Sovilla Cian Carroll