Estuarine Circulation and the Knudsen Relation

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Estuarine Circulation and the Knudsen Relation

• PSO 2009

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The Knudsen Relation
Volume conservation Q1 Q2 QR
Salt conservation 0 S2Q2 S1Q1
Combining these two it is easy to show that
Q2 (S1/ds)QR
Q1 (S2/ds)QR
and

• For many estuaries (like Puget Sound) dsltltS1 and
  dsltltS2 (like 1ltlt30)
• Hence Q1 and Q2 are VASTLY AMPLIFIED over QR
  !!!!!!
• These relationships are only diagnostic no
  info about physics
• If you know the average stratification then you
  can estimate the exchange flow

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A simple story for why the exchange flow exists

• The along-channel salinity (and hence density)
  gradient is a key factor

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Thinking more about the physics of the exchange
flow

• The along-channel pressure gradient forces the
  flow
• Remember, these are all for tidally-averaged
  fields

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Puget Sound

• Salinity sections
• July, August, September

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Turbulent mixing makes smooth curves out of the
2-layer exchange flow

• The turbulence is driven by the tidal currents,
  especially as they rub on the bottom boundary

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Admiralty Inlet

• Salinity sections along the Inlet, at different
  stages of the tide

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Admiralty Inlet

• Tidally-averaged velocities
• Inflow happens during Neap tides because there is
  less turbulent mixing (which slows down the flow)