Buoyant Plumes

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Buoyant Plumes

• Positively and negatively buoyant plumes
  contribute to particle transport across and along
  shelves and to density stratification of coastal
  waters.
• Plumes are normally sharply bounded offshore by
  fronts, which can serve as permeable barriers to
  across-shelf transport.

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• Positively buoyant plumes are the most common
  and are attributable to low salinity river mouth
  or estuarine effluents.
• Negatively buoyant plumes can also occur and are
  caused by high suspended sediment concentration,
  brine extrusion from freezing sea ice, and
  intense cooling of coastal waters by cold air
  outbreaks.
• On leaving the confines of a river, a positively
  buoyant plume will initially expand horizontally,
  and thin vertically, and eventually detach from
  the seabed.
• Further, the plume spreads more gradually and
  undergoes anticyclonic Coriolis turning tending
  toward alongshore coastal currents.

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Plume generally constrained close to the coast -
for many shelves this is on the inner shelf.
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• Sediment settling from the plume tends to do so
  in shallow water, on the inner shelf. For
  energetic coastal environments, this sediment may
  then undergo secondary processes, e.g.,
• Cross-shelf diffusion
• Advection in bottom boundary layer
• Advection in density underflows (still within
  bottom boundary layer, but different mechanisms
  at work).

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• Negative buoyancy (less prevalent than positive
  buoyancy)
• responsible for the downslope transport of
  dense plumes, including - in the extreme case,
  turbidity currents.
• although previously thought to only occur in
  submarine canyons and on the continental slope,
  they are presently being investigated in many
  inner shelf environments.
• can be formed in situ (e.g., wave-supported
  fluid mud) or by direct discharge of high
  concentrations into the marine environment
  (hyperpycnal plume)

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Negatively Buoyant Plumes
(Wright, 2000)
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Plume velocity can be evaluated using Chezy
Equation Fg Fd Where Cd bottom drag
coefficient (0.0025 0.0050) Et
interfacial drag coefficient (0.0004 0.0015)
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• Simple Chezy model ignores
• Coriolis turning
• Plume interactions with tidal or currents
  (including waves)
• More complete (yet still relatively simplistic)
  analytical modeling is being undertaken by
  Friedrichs Wright. They imply that
  density-driven processes may control
• the across-shelf transport (these processes move
  a large amount of sediment in directions
  perpendicular to prevailing currents) and,
• the location of many flood deposits around the
  world.

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Forms of negatively buoyant sediment flows Fluid
Mud - Conc 10-300 g/l of sediment, downslope
flow is non-erosive. Convergent processes
Wave-supported - Direct Hyperpycnal Plume
Conc gt 40 g/l of sediment in fresh water plume to
overcome fluid density difference. Turbidity
Currents after initial trigger, downslope flow
sustained by erosion of the seabed (generally
coarse-grained)