Fine-Sediment Transport in the Coastal Ocean: the Amazon and the Atchafalaya Systems

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Fine-Sediment Transport in the Coastal Ocean the
Amazon and the Atchafalaya Systems

• Gail C. Kineke
• Dept of Geology Geophysics
• Boston College
• Acknowledgements
• National Science Foundation
• Dick Sternberg
• Office of Naval Research
• Boston College Coastal Processes Lab

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Rouse Parameter, P Suspended load
We have equations tocalculate velocity and
concentration profiles throughout the water
column with some direct measurements
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Formation?
Resuspension by currents
OR
Resuspension by waves
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Thanks Geyer and WHOI Graphics
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Processes and Products

• Fluid muds form at the bottom salinity front on
  the inner shelf, largely due to estuarine
  trapping
• Flow downslope when thick and dense enough to
  overcome baroclinic forcing, and/or when onshore
  forcing relaxes (weakening of alongshelf wind)
• Supply submarine delta with sediments
• Affect tidal wave, limit growth of the boundary
  layer, buffer between seabed and water column,
  cross-shelf integrator and remineralizer of
  organic carbon

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Study site Atchafalaya River and shallow LA shelf

• Fine sediments discharged directly onto shallow
  shelf
• Previous observations of fluid muds (Wells and
  Kemp, 1986)
• Accreting coastline (Roberts et al. 1989)

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Most of Louisiana shoreline is eroding, section
west of Marsh Island is accreting
Westphal et al. 1991
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Erosion rates of m/yr
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Localized areas of accretion
young marsh
mud on sandy beach
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Prior observations of muddy surface plume and
emplacement of muds on beach point to importance
of cold front passages
Satellite Images
Walker et al.
Roberts, Huh, et al.
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Kemp and Wells
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Cold Front Passages
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east
west
onshore
offshore
Velocity, 5 hr averages
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Discussion

• Waves combined with currents cause greatest
  resuspension and mixing with onshore winds.
• Once the winds change to northerly direction,
  waves decrease and stratification becomes
  re-established and sediments settle rapidly.
• Net transport for these events will typically be
  onshore and to the west.
• Waves enhancing resuspension has long been
  recognized, but there is an additional influence.

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• Waves combined with currents cause greatest
  resuspension and mixing with onshore winds.
• Net transport typically onshore and to the west.
• Once the winds change to northerly direction,
  waves decrease and stratification becomes
  re-established and sediments settle rapidly,
  forming fluid mud (thin layer, nearbed).
• Northerly winds drive upwelling with onshore
  currents nearbed, transporting high
  concentrations of sediment

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• Waves enhancing resuspension has long been
  recognized, but there is an additional influence.

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Discussion

• Observations of cross-shelf wave characteristics
  from a prior experiment indicate significant
  onshore attenuation during these events, likely
  due to interaction with unconsolidated seabed or
  fluid muds.
• This creates a positive feedback, preventing wave
  energy from reaching the coast, increasing the
  likelihood of coastal accretion.
• Results from Sheremet compare wave conditions
  over sandy vs muddy seabed.

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Dean and Dalrymple
LFDT, Elgar and Guza
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Implications

• The cycle of cold front passages combined with
  wave attenuation enhances transport to the coast.
• Perhaps unusually energetic winters would lead to
  greatest accretion, in stark contrast to sandy
  coasts where successive winter storms can cause
  greatest erosion.

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Work in progress

• Cross-shelf wave attenuation (Eron Higgins,
  Masters 2003)
• Nearshore clinoform, consolidation (Dave Velasco,
  Masters 2003)
• Coastal change (Amy Draut, Ph.D., WHOI 2003)
• Dispersal via surface plume and coastal mudstream
  (Katie Hart, Masters 200?)

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Summary

• Amazon
• Fluid muds found on shelf, importance of
  convergence, salinity stratification and fronts,
  thickness O(tidal boundary layer), persistent
• Atchafalaya
• Fluid muds very close to shore or onshore
  (mudflats), associated with rapid settling
  following resuspension events, thin O(wave
  boundary layer)?, ephemeral

Concentration of source waters not as important
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Some Thoughts

• Coastal evolution depends on whats happening
  offshore
• Interdiscplinary issues

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Eastern chenier plain inner shelf
V.E. 1300x
Sigmoidal clinoforms Convex cross-shore profile
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Central chenier plain inner shelf
V.E. 540x
Reflectors truncated by sea floor eroding
clinoforms? Concave cross-shore profile
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