Water Movements

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Water Movements Mixing in Lakes

• 29 Jan 2007
• AWER 4500

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Announcements

• Homework due Wednesday
• Mail to wurts_at_cc.usu.edu
• Subject Line Homework_ your name
• Exam, Monday 5th
• Seminar Brian Bledsoe
• Considering vegetative influences in stream
  restoration design
• Engineering 101, 400 PM

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3 ways that water moves

• Waves
• Turbulence
• Currents

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Spatial Scales of Water movement

• Lake currents larval fish dispersal, km-scale
• Wave motion effect movements of submerged
  plants at shoreline, m-scale
• Microdiffusion algal cells, µm or mm-scale

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Largest Scale - Currents

• Caused by wind
• Large vs. Small Lakes

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Gyres

• Wind currents, modified by Coreolis effect

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Surface Waves
Wave action dissipates quickly with depth
Decreases by ½ for each depth increase of
L/9 Ex. L 18 m, H 1 m, then at 2
m 0.5 4 m 0.25 8 m 0.12 m
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Langmuir Circulation Cells

• Combination of Surface and subsurface currents

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Quake Lake, Montana
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Subsurface Currents
Return Flows
Wind
Upwelling
Downwelling
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Surface Seiche
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Surface Seiche Period
Where t time from one high point to the
next l basin length (cm) h mean depth
(cm) g acceleration due to gravity (981 cm
sec-2)
e.g. Bear Lake, 28 km, 28 m t 2 2,800,000
v981 2800 3,380 seconds 56
minutes
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Internal Seiche
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Temperature isopleths, Lake Ontario
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Importance of seiches

• Rapid horizontal currents, can rival surface
  currents
• Vertical excursions of algae and other organisms
• Primary movement in hypolimnion
• Mix sediments
• Turbulent mixing

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Eddy Diffusion
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Eddy Diffusion

• Important for mixing particles of water and
  solutes across boundaries
• Coefficient of eddy diffusion (K)

Example Suppose we have a nutrient gradient,
with one area having a concentration of 0 mg NH4
/ cm3, and a second area 1-cm away having a
concentration of 0.5 mg NH4 / cm3 (e.g.
zooplankton pee)
Approx. diffusion rate without eddy diffusion
0.001 mg / cm2 sec
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• In lakes, increasing water velocity causes
  greater turbulent flow across boundaries such as
  the thermocline
• Increasing water velocity leads to an increase in
  eddy diffusion
• Therefore, water movements such as seiches,
  internal currents, and hypolimnetic entrainment
  increase eddy diffusion rates and the movement of
  water and solutes between the different lake
  layers

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Wind
Eddy diffusion across thermocline
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Relative rates of movements
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