1. Water Movements

2.  Transfer of wind energy to water  Modified by gravity, basin morphometry and differential water densities to produce characteristic water movements

3. Water Movements  Movements govern distribution of physical/chemical parameters within lake  In turn affect distribution of living organisms

4. Turbulent Movements  Nearly all water movements are turbulent (non- laminar)  Result in some degree of mixing where density gradient exists  Air-water, epilimnion- hypolimnion

5. Water Currents  Frictional force between wind and water  Wind drift  Displacement of water downwind  Speed of 2-3% of speed of wind generating it - decreases exponentially with depth  Relationship breaks down at critical wind speed - e.g., 14 mph for Lake Mendota

6. Wind Drift  What happens when the water gets to the end of the lake? It turns and flows back along the side It turns and flows back along the side Some plunges down, flows back at the bottom of the lake or epilimnion Some plunges down, flows back at the bottom of the lake or epilimnion wind In small lakes, these patterns predominate

7. Large lakes form gyres  Deflection by Coriolis force - downwind and to the right in N hemisphere

8. How much deflection is there?  In immense water bodies: 45°  Angle decreases with decreasing lake area and depth (insig. In lakes <20 m deep) e.g., in Lake Mendota (39 km 2, 26 m deep): 21° e.g., in Lake Mendota (39 km 2, 26 m deep): 21° Due to side and bottom friction Due to side and bottom friction Enough for some loops and gyres

9. Ekman Spiral  The surface current moves 45° relative to the wind due to the Coriolis force  The layer below is set in motion by the overhead drag  It takes off in the direction of the surface, but also is deflected by the Coriolis force  This process is repeated with each layer down yielding a spiral  Speed decreases with depth due to friction

10. Traveling Surface Waves  Surface motion without physically moving water downwind  Water surface set into oscillations

11. Traveling Surface Waves  Move down the lake  Have a wavelength (crest to crest)  Height (crest to trough)  Amplitude (deviation from wave axis; 1/2h)  Wavelength ~ 20X wave height (varies 10-100 X)  At <10X wave collapses

12. Why do they occur?  Wind has a vertical component  Wind tends to gust  The alternating pushing and release of the wind causes oscillations, the momentum of which passes through water as waves  Wind must be > 1 m/sec for waves to form

13. As the surface forms a wave pattern, what happens in the water below?  Water molecules move in circles Wave lifts cork in arch as it passes under it Wave lifts cork in arch as it passes under it Reaching maximum height with the wave crest Reaching maximum height with the wave crest Then cork is moved down in an arch Then cork is moved down in an arch It ends up in its starting position It ends up in its starting position

14. How deep does the wave motion go?  The direct impact extends over the height of the wave  Water travels in circles with d = wave height  Circles set in motion below the surface, moved by the circle above. Due to friction they get smaller with depth. Due to friction they get smaller with depth.

15. Traveling Surface Wave

17. Example of the depth of impact  Rule: circle diameter (wave height at surface) decreases by 1/2 for every depth interval = 1/9th of the wave’s length  Suppose a wave is 9 m long Its height (and circle d) will be 1/20th of this; ~50 cm Its height (and circle d) will be 1/20th of this; ~50 cm Depth (m) Circle d (cm) 050 125 212.5 36 43 51.5

18. How much water is carried down the lake through waves?  None  All water travels in circles, returning to where it started after the wave passes  The motion moves down the lake, but not the water  Also no mixing

19. The exception to the rule: breaking waves  When waves become so steep that L/h <10, they break  Turbulence (chaotic motion) results Stirs the water Stirs the water  When produced in open water these waves are called: White caps White caps

20. Waves also break as they approach shore  What are these called? Breakers Breakers  The circles of water below the wave begin to hit the bottom  The water is piled up, causing the wave to achieve a L/h ratio <10

21. Shoreline erosion  The circles change into elipses in shallow water, so that a back and forth motion results  It erodes fine sediment Sediment deposition occurs only below the zone of wave action

22. Breakers  As wave enters shallow water, velocity decreases, wavelength reduced  Wave height increases greatly  Wave becomes asymmetric, unstable

23. There are two types of breakers Plunging (front curls over) Spilling

24. Plunging and spilling

25. Short, deepwater surface waves  Wavelength < water depth  Ripples or capillary waves Wavelength < 6.28 cm (2  ) Wavelength < 6.28 cm (2  ) Water returned from crest by surface tension Water returned from crest by surface tension  Gravity waves > 6.28 cm in length > 6.28 cm in length Pulled down by gravity Pulled down by gravity

26. Long, shallow water surface waves  Wavelength > 20X water depth  Velocity proportional to square root of depth

27. What determines how high waves can get?  Fetch (uninterrupted distance over which wind can blow) H (cm) = 0.105 square root of fetch (cm)  Temperature Warmer means higher Warmer means higher  Depth (if the lake is large) Deeper lakes mean higher waves Deeper lakes mean higher waves

28. What determines how high waves can get?  Lake Superior fetch = 482 km  Max. predicted wave height = 7.3 m  Max. observed wave height = 6.9 m  East Lake Winona (2253 m, 49.8 cm)  West Lake Winona (965 m, 32.6 cm)

29. Langmuir Spirals  First described by Irving Langmuir early in 20th century Observed Langmuir streaks while on a cross-Atlantic cruise Observed Langmuir streaks while on a cross-Atlantic cruise Formed a theory and tested it on Lake George Formed a theory and tested it on Lake George

30. What are they?  Wind drift and waves interact at wind speeds >2- 3 m/s (4.5-6.5 mph) to produce a spiral motion along a horizontal plane Vertical helical currents Vertical helical currents Many spirals span lake, alternating in spin direction Many spirals span lake, alternating in spin direction Diameter the depth of the epilimnion Diameter the depth of the epilimnion Speed in cm/s Speed in cm/s Unlike waves, LS carry water down the lake, as well as mix it downward

31. Langmuir streaks (wind rows) occur where two spirals converge at the surface  Foam and debris are swept here and remain buoyant and trapped  At wind speed > 7 m/s (15.5 mph), debris is forced down and no streaks are seen But the spirals are still there But the spirals are still there Pratt Lake, MI - July 2005

32. Whole Lake Water Movements  Entire lake basin commonly set into motion by wind, change in pressure  Movement detected by changes in surface level or level of thermocline  Long standing waves with wavelengths in range of entire basin length  See-saw-like movement about a line of no vertical movement (node)

33. Unimodal Seiche

34. Bimodal Seiche

35. Seiches  Up to 17 nodes have been detected in some basins  Basin oscillates until damped out by friction, gravity (may take weeks)

36. Period of Unimodal Seiches  Lake Erie - 400 km long, 21 m deep, period = 786 min or 14 hours  Lake Michigan (EW) - period = 132 minutes  Shorter period with each oscillation Period = 2 X basin length (cm) square root of g X mean depth (cm)

37. Amplitude of Unimodal Seiches  Smaller lakes like Lake Mendota - only 1-2 mm  Larger lakes like Lake Erie - possibly >2 m  Alternating flooding and dewatering - flushes sediment from river deltas, but havoc for marinas

38. Internal Seiches  Internal seiches can be produced during stratification  Oscillation along thermocline  Period and amplitude much greater than on surface  Small (<2 km) lake can have surface seiche <1 mm with 5-min period, and internal seiche of 1 m and 4-h period

39. Internal Seiches  Amplitudes in larger lakes (Lake MI) may be in excess of 10 m, with currents >10 cm/sec near nodes  The major deepwater movements in lakes  Important in distribution of heat, dissolved substances

40. Rotating Seiches  Surface and internal seiches are affected by Coriolis force  Back and forth rocking rotates to the right (clockwise) in northern hemisphere  Turbulent mixing at epilimnion/metalimnion interface

41. Other Movements  Currents generated by inflowing rivers  Waters flow into those of similar density (temperature, dissolved substances, suspended sediments)  Overflow, interflow, underflow

42. Other Movements  Currents under ice  Horizontal and vertical currents (horizontal greater)  Thermally induced by convection from accumulated heat flowing from sediments