Water movement

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Water movement

• What is a seiche? A Kelvin wave?
• What forces cause waves and currents in lakes?
• What depths in lakes are affected by different
  types of water movements?
• What factors control wave height in lakes?

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Energy sources Wind

• Wind drives waves and currents
• Energy transfer ? Wind speed, fetch, duration
• Energy of water movement ? ?

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Long Standing Waves (seiches)
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Surface Seiches
Wind Wind Stops
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Large, Destructive Seiche, Lake Michigan, 1954
Small Seiche, L. Superior
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Lake Ontario internal wave
Ontario surface seiche of 1m
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Thermocline depression can be 10-100 times
surface seiche amplitude
Wind
Water column mass ?surface H1 ?hypolim
H2
H1 H2

• Thermocline depression
• (Hsurface x ?hypolim) / (?hypolim - ?epilim)

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Effect of Water Movements on Plankton
Upwelling
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Kelvin Wave
Coriolis force balances gravity acting on surface
height of basin-scale wave
Gravity
Coriolis
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Coriolis Force
v

• Conservation of angular momentum
• m1r1v1 m2r2v2
• If r decreases (while moving toward pole), v
  increases (in direction of rotation)

m
r
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Surface kelvin waves

• Basin gt 15 km
• Coastally-trapped
• seich return
• counterclockwise
• coastal wave

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Surface waves

• Wave energy transferred to depth
• Water velocity becomes neglible at ? /2
• Water depth lt ? /2, erosion

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Shallow-water wave
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Surface waves

• Controls on h
• Fetch
• Windspeed
• Water Density
• Water Depth

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Lake Surface Currents

• Forced by wind, inflow, density gradients
• Directional flow horizontal and/or vertical
• Can be affected by Coriolis force

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Winds and Currents
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Langmuir Currents and Streaks

• Langmuir convergence/ divergence affects
  distribution of stuff
• High wind speeds disrupt streaks

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Langmuir currents
Helical currents from wind above 3 m/s Helical
cells can extend to thermocline
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Currents from inflows
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Thermal resistance to mixing and stability
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The Richardson Number

• Determines whether or not two fluids will mix
• Ratio of buoyancy (gravity-driven forces) to
  kinetic energy
• Ri(gd?/dz)/(?ave(dU/dz)2
• gacceleration of gravity ? is density U is
  horizontal water current speed

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Schmidt Stability
Work required to instantaneously completely mix a
lake
?
V1, A1, ?1
?ave
Zave, Aave, ?ave
V25, A25, ?25
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Wedderburn Number

• Integrates density/energy gradients with lake
  morphology influences
• W (H2g)/(Lµ2)
• H mixed layer depth
• g change in gravity due to density change at
  pycnocline
• g 1000g(?h - ?e)/(?ave) 9.8(?h - ?e)/(?ave
  )
• L Fetch
• µ2 (0.00123)CDUw2
• CD 0.0013 for large lakes, 0.001 for small
  lakes
• Uw wind speed in m/s

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Summary
Wind-driven turbulence Waves, currents, langmuir
Seiches and kelvin waves
Density flows

• Turbulence that mixes surface layer from
  wind-driven water movements
• Turbulent energy dissipated by friction in
  water, on bottom, on thermocline
• Internal seiches and currents can affect bottom
  of lake below surface mixed layer