Idealized numerical modeling of river plumes

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Idealized numerical modeling of river plumes

• Rob Hetland
• and
• Rocky Geyer

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Outline

• Estuary mouth conditions and estuarine scales
• Tides in the estuary
• Low resolution vs. High resolution
• Effect of winds.

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(No Transcript)
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Connection between estuaries and river plumes

• Estuarine circulation sets the strength and
  density anomaly of the flow at the estuary mouth.
• Yankovsky and Chapman (1997)
• Burger and Rossby (or Froude) numbers dependence
  at estuary mouth sets type of plume (i.e.,
  surface or bottom trapped)
• Yankovsky (2000)
• Baroclinic exchange at estuary mouth is important
  in upstream plume propegation.
• Garvine (2001)
• Outflow structure and coastal wall depth affect
  plume evolution.
• Nof and Pichevin (in press, JPO)
• Vorticity structure of outflow affects bulge
  evolution.

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Idealized domain
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Motivation

• Explicitly modeling the estuary would place
  severe constraints on model resolution.
• Question Is it possible to finesse the
  estuary such that the river plume will be
  accurately simulated, while ignoring the details
  of the estuarine circulation?
• YES

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Estuarine scales

• Classic analytical models of estuaries i.e.,
  Hansen and Rattray (1965) and Chatwin (1976)
  give scales for estuarine length scale,
  stratification, and estuarine circulation.
• Numerical experiments corroborate these scalings
  for simple domains with constant vertical mixing.

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Analytic scaling

• L Kv-1 QR-1/3
• ?S QR2/3
• Ue QR1/3

Not dependant on Kv!
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Numerical results vs. scaling
Estuarine circulation
Length
Stratification
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Estuarine scales - summary

• Stratification and estuarine circulation are not
  dependant on mixing -- only the length scale
  varies.
• Thus, for steady fresh water forcing, high mixing
  may be set in the estuary, so that the estuarine
  length is reduced, while the outflow properties
  remain unchanged.

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Tides in the estuary

• If tides are weak on the shelf, tides may be
  finessed by varying the river discharge at a
  tidal frequency.
• Tides increase shear, which increases mixing,
  reducing the density anomaly of the plume.

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Tidal estuary plume
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Vertical salt flux
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Froude number
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Wind effects
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Conclusions

• Estuaries may be shortened by increasing mixing
  in the estuary.
• Tides may be finessed by modulating the riverine
  discharge.
• Wind effects cannot be ignored in realistic
  simulations.