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SUMMARY:Density stratification in gravity currents\; topographic interacti
 ons\, shear layer stability\, and impact on the global sediment budget - P
 rofessor Ben Kneller\, University of Aberdeen
DTSTART:20171019T103000Z
DTEND:20171019T113000Z
UID:TALK78341@talks.cam.ac.uk
CONTACT:Catherine Pearson
DESCRIPTION:Gravity currents are major agents of transport of particulate 
 materials on the Earth’s surface\, where they commonly interact with sur
 face relief. The vertical mean velocity profile of particulate gravity cur
 rents can be treated as essentially a combination of a logarithmic boundar
 y layer resulting from its no-slip lower boundary\, and an error function 
 velocity profile across the shear layer in the upper part of the current. 
 The density stratification results from a balance between particle settlin
 g and an upward flux due to turbulent diffusion.\n\nWhere the height of th
 e topography is on a smaller scale than the thickness of the boundary laye
 r\, approximate scaling laws can be derived by applying models for interac
 tions between topography and stratified flow borrowed from atmospheric sci
 ence. This is illustrated across a range of scales by examples from the 19
 80 Mt St Helens lateral blast\, and by inferred interactions between turbi
 dity currents and small-scale sand dunes on the bed. \n\nThe overwhelming 
 majority of material removed from the continents by erosion and subsequent
  transport by rivers ultimately forms large submarine fans (O 10^6 km3) in
  the deep ocean off the continental margins. The subaqueous part of the tr
 ansport is effected mainly	 by turbidity currents that flow through sinuou
 s submarine channels that may be hundreds to thousands of kilometres long.
   In many laboratory or numerical experiments on gravity currents\, shear 
 at the boundary with the overlying ambient fluid is sufficient to destabil
 ise the density stratification. The consequent Kelvin-Helmholtz instabilit
 ies result in entrainment of ambient fluid\, and in turbulence whose visco
 us dissipation constitutes an effective drag on the current. This drag and
  the vertical expansion of the flow due to entrainment would render long r
 ange sediment transport by such flows impossible on the extremely low grad
 ients of submarine fan channels. A model is presented here for currents in
  which the upper flow boundaries are stable (high gradient Richardson numb
 ers)\, a condition favoured by the low gradients of submarine fans\, and b
 y the presence of fine-grained suspended sediment in the upper part of the
  flow.\n
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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