Water MovementsWater Movements

Water MovementsWater Movements

Transfer of wind Transfer of wind energy to waterenergy to water

Modified by gravity, Modified by gravity, basin morphometry basin morphometry and differential water and differential water densities to produce densities to produce characteristic water characteristic water movementsmovements

Water MovementsWater Movements

Movements govern Movements govern distribution of distribution of physical/chemical physical/chemical parameters within parameters within lakelake

In turn affect In turn affect distribution of living distribution of living organismsorganisms

Turbulent MovementsTurbulent Movements

Nearly all water Nearly all water movements are movements are turbulent (non-turbulent (non-laminar)laminar)

Result in some Result in some degree of mixing degree of mixing where density where density gradient existsgradient exists

Air-water, epilimnion-Air-water, epilimnion-hypolimnionhypolimnion

Water CurrentsWater Currents

Frictional force between Frictional force between wind and waterwind and water

Wind driftWind drift Displacement of water Displacement of water

downwinddownwind Speed of 2-3% of speed Speed of 2-3% of speed

of wind generating it - of wind generating it - decreases exponentially decreases exponentially with depthwith depth

Relationship breaks down Relationship breaks down at critical wind speed - at critical wind speed - e.g., 14 mph for Lake e.g., 14 mph for Lake MendotaMendota

Wind DriftWind Drift What happens when the water gets to the What happens when the water gets to the

end of the lake?end of the lake? It turns and flows back along the sideIt turns and flows back along the side Some plunges down, flows back at the bottom Some plunges down, flows back at the bottom

of the lake or epilimnionof the lake or epilimnion

wind

In small lakes, these patterns predominate

Large lakes form gyresLarge lakes form gyres

Deflection by Coriolis force - downwind and Deflection by Coriolis force - downwind and to the right in N hemisphereto the right in N hemisphere

How much deflection is there?How much deflection is there?

In immense water bodies: 45°In immense water bodies: 45° Angle decreases with decreasing lake area Angle decreases with decreasing lake area

and depth (insig. In lakes <20 m deep)and depth (insig. In lakes <20 m deep) e.g., in Lake Mendota (39 kme.g., in Lake Mendota (39 km22, 26 m deep): 21°, 26 m deep): 21° Due to side and bottom frictionDue to side and bottom friction

Enough for some loops and gyres

Ekman SpiralEkman Spiral The surface current moves The surface current moves

45° relative to the wind 45° relative to the wind due to the Coriolis forcedue to the Coriolis force

The layer below is set in The layer below is set in motion by the overhead motion by the overhead dragdrag

It takes off in the direction It takes off in the direction of the surface, but also is of the surface, but also is deflected by the Coriolis deflected by the Coriolis forceforce

This process is repeated This process is repeated with each layer down with each layer down yielding a spiralyielding a spiral

Speed decreases with Speed decreases with depth due to frictiondepth due to friction

Traveling Surface WavesTraveling Surface Waves

Surface motion Surface motion without physically without physically moving water moving water downwinddownwind

Water surface set into Water surface set into oscillationsoscillations

Traveling Surface WavesTraveling Surface Waves Move down the lakeMove down the lake Have a Have a wavelengthwavelength (crest to crest) (crest to crest) HeightHeight (crest to trough) (crest to trough) AmplitudeAmplitude (deviation from wave axis; 1/2h) (deviation from wave axis; 1/2h) Wavelength ~ 20X wave height (varies 10-100 X)Wavelength ~ 20X wave height (varies 10-100 X) At <10X wave collapsesAt <10X wave collapses

Why do they occur?Why do they occur? Wind has a vertical componentWind has a vertical component

Wind tends to gustWind tends to gust

The alternating pushing and The alternating pushing and release of the wind causes release of the wind causes oscillations, the momentum of oscillations, the momentum of which passes through water as which passes through water as waveswaves

Wind must be > 1 m/sec for Wind must be > 1 m/sec for waves to formwaves to form

As the surface forms a wave pattern, As the surface forms a wave pattern, what happens in the water below?what happens in the water below?

Water molecules move inWater molecules move in circles circles Wave lifts cork in arch as it passes under Wave lifts cork in arch as it passes under

itit Reaching maximum height with the wave Reaching maximum height with the wave

crestcrest Then cork is moved down in an archThen cork is moved down in an arch It ends up in its starting positionIt ends up in its starting position

How deep does the wave motion go?How deep does the wave motion go?

The direct impact extends over the height of the waveThe direct impact extends over the height of the wave Water travels in circles with d = wave heightWater travels in circles with d = wave height

Circles set in motion below the surface, moved by the Circles set in motion below the surface, moved by the circle above. circle above. Due to friction they get smaller with depth.Due to friction they get smaller with depth.

Traveling Surface WaveTraveling Surface Wave

Traveling Surface WaveTraveling Surface Wave

Example of the depth of impactExample of the depth of impact Rule: circle diameter (wave height at surface) Rule: circle diameter (wave height at surface)

decreases by 1/2 for every depth interval = 1/9th decreases by 1/2 for every depth interval = 1/9th of the wave’s lengthof the wave’s length

Suppose a wave is 9 m longSuppose a wave is 9 m long Its height (and circle d) will be 1/20th of this; ~50 cmIts height (and circle d) will be 1/20th of this; ~50 cm

Depth (m)Depth (m) Circle d Circle d (cm)(cm)

00 5050

11 2525

22 12.512.5

33 66

44 33

55 1.51.5

How much water is carried down the How much water is carried down the lake through waves?lake through waves?

NoneNone

All water travels in All water travels in circles, returning to circles, returning to where it started after where it started after the wave passesthe wave passes

The motion moves The motion moves down the lake, but down the lake, but not the waternot the water

Also no mixingAlso no mixing

The exception to the rule: The exception to the rule: breaking wavesbreaking waves

When waves become so steep When waves become so steep that L/h <10, they breakthat L/h <10, they break

Turbulence (chaotic motion) Turbulence (chaotic motion) resultsresults Stirs the water Stirs the water

When produced in open water When produced in open water these waves are called:these waves are called: White capsWhite caps

Waves also break as they Waves also break as they approach shoreapproach shore

What are these called?What are these called? BreakersBreakers

The circles of water The circles of water below the wave begin to below the wave begin to hit the bottomhit the bottom

The water is piled up, The water is piled up, causing the wave to causing the wave to achieve a L/h ratio <10achieve a L/h ratio <10

Shoreline erosionShoreline erosion The circles change The circles change

into elipses in shallow into elipses in shallow water, so that a back water, so that a back and forth motion and forth motion resultsresults

It erodes fine It erodes fine sedimentsediment

Sediment deposition occurs only below the zone of wave action

BreakersBreakers

As wave enters As wave enters shallow water, shallow water, velocity decreases, velocity decreases, wavelength reducedwavelength reduced

Wave height Wave height increases greatlyincreases greatly

Wave becomes Wave becomes asymmetric, unstableasymmetric, unstable

There are two types of There are two types of breakersbreakers

Plunging(front curls over)

Spilling

Plunging and spilling Plunging and spilling

Short, deepwater surface wavesShort, deepwater surface waves

Wavelength < water depthWavelength < water depth RipplesRipples or or capillary wavescapillary waves

Wavelength < 6.28 cm (2Wavelength < 6.28 cm (2)) Water returned from crest by Water returned from crest by

surface tensionsurface tension

Gravity wavesGravity waves > 6.28 cm in length> 6.28 cm in length Pulled down by gravityPulled down by gravity

Long, shallow water surface wavesLong, shallow water surface waves

Wavelength > 20X water Wavelength > 20X water depthdepth

Velocity proportional to Velocity proportional to square root of depthsquare root of depth

What determines how high waves What determines how high waves can get?can get?

Fetch (uninterrupted distance over Fetch (uninterrupted distance over which wind can blow)which wind can blow)

H (cm) = 0.105 square root of fetch (cm)H (cm) = 0.105 square root of fetch (cm)

TemperatureTemperature Warmer means higherWarmer means higher

Depth Depth (if the lake is large)(if the lake is large) Deeper lakes mean higher wavesDeeper lakes mean higher waves

What determines how high waves What determines how high waves can get?can get?

Lake Superior fetch = 482 kmLake Superior fetch = 482 km Max. predicted wave height = 7.3 mMax. predicted wave height = 7.3 m Max. observed wave height = 6.9 mMax. observed wave height = 6.9 m East Lake Winona (2253 m, 49.8 cm)East Lake Winona (2253 m, 49.8 cm) West Lake Winona (965 m, 32.6 cm)West Lake Winona (965 m, 32.6 cm)

Langmuir SpiralsLangmuir Spirals

First described by Irving Langmuir early First described by Irving Langmuir early in 20th centuryin 20th century Observed Langmuir streaks while on a Observed Langmuir streaks while on a

cross-Atlantic cruisecross-Atlantic cruise Formed a theory and tested it on Lake Formed a theory and tested it on Lake

George George

What are they?What are they?

Wind drift and waves Wind drift and waves interact at wind speeds >2-interact at wind speeds >2-3 m/s (4.5-6.5 mph) to 3 m/s (4.5-6.5 mph) to produce a spiral motion produce a spiral motion along a horizontal planealong a horizontal plane

Vertical helical currentsVertical helical currents

Many spirals span lake, Many spirals span lake, alternating in spin directionalternating in spin direction

Diameter the depth of the Diameter the depth of the epilimnionepilimnion

Speed in cm/sSpeed in cm/s

Unlike waves, LS carry Unlike waves, LS carry water down the lake, as water down the lake, as well as mix it downwardwell as mix it downward

Langmuir streaks (wind rows) Langmuir streaks (wind rows) occur where two spirals converge occur where two spirals converge

at the surfaceat the surface Foam and debris are Foam and debris are

swept here and remain swept here and remain buoyant and trappedbuoyant and trapped

At wind speed > 7 m/s At wind speed > 7 m/s (15.5 mph), debris is (15.5 mph), debris is forced down and no forced down and no streaks are seenstreaks are seen

But the spirals are still But the spirals are still therethere

Pratt Lake, MI - July 2005

Whole Lake Water MovementsWhole Lake Water Movements

Entire lake basin Entire lake basin commonly set into motion commonly set into motion by wind, change in by wind, change in pressurepressure

Movement detected by Movement detected by changes in surface level changes in surface level or level of thermoclineor level of thermocline

Long standing waves with Long standing waves with wavelengths in range of wavelengths in range of entire basin lengthentire basin length

See-saw-like movement See-saw-like movement about a line of no vertical about a line of no vertical movement (movement (nodenode))

Unimodal SeicheUnimodal Seiche

Bimodal SeicheBimodal Seiche

SeichesSeiches

Up to 17 nodes have Up to 17 nodes have been detected in been detected in some basinssome basins

Basin oscillates until Basin oscillates until damped out by damped out by friction, gravity (may friction, gravity (may take weeks)take weeks)

Period of Unimodal SeichesPeriod of Unimodal Seiches

Lake Erie - 400 km Lake Erie - 400 km long, 21 m deep, long, 21 m deep, period = 786 min or period = 786 min or 14 hours14 hours

Lake Michigan (EW) - Lake Michigan (EW) - period = 132 minutesperiod = 132 minutes

Shorter period with Shorter period with each oscillationeach oscillation

Period = 2 X basin length (cm) square root of g X mean depth (cm)

Amplitude ofAmplitude ofUnimodal SeichesUnimodal Seiches

Smaller lakes like Lake Smaller lakes like Lake Mendota - only 1-2 mmMendota - only 1-2 mm

Larger lakes like Lake Larger lakes like Lake Erie - possibly >2 mErie - possibly >2 m

Alternating flooding and Alternating flooding and dewatering - flushes dewatering - flushes sediment from river sediment from river deltas, but havoc for deltas, but havoc for marinasmarinas

Internal SeichesInternal Seiches

Internal seiches can be Internal seiches can be produced during stratificationproduced during stratification

Oscillation along thermoclineOscillation along thermocline Period and amplitude much Period and amplitude much

greater than on surfacegreater than on surface Small (<2 km) lake can have Small (<2 km) lake can have

surface seiche <1 mm with 5-surface seiche <1 mm with 5-min period, and internal min period, and internal seiche of 1 m and 4-h periodseiche of 1 m and 4-h period

Internal SeichesInternal Seiches

Amplitudes in larger lakes Amplitudes in larger lakes (Lake MI) may be in (Lake MI) may be in excess of 10 m, with excess of 10 m, with currents >10 cm/sec near currents >10 cm/sec near nodesnodes

The major deepwater The major deepwater movements in lakesmovements in lakes

Important in distribution of Important in distribution of heat, dissolved heat, dissolved substancessubstances

Rotating SeichesRotating Seiches

Surface and internal Surface and internal seiches are affected by seiches are affected by Coriolis forceCoriolis force

Back and forth rocking Back and forth rocking rotates to the right rotates to the right (clockwise) in northern (clockwise) in northern hemispherehemisphere

Turbulent mixing at Turbulent mixing at epilimnion/metalimnion epilimnion/metalimnion interfaceinterface

Other MovementsOther Movements

Currents generated Currents generated by inflowing riversby inflowing rivers

Waters flow into those Waters flow into those of similar density of similar density (temperature, (temperature, dissolved substances, dissolved substances, suspended suspended sediments)sediments)

Overflow, interflow, Overflow, interflow, underflowunderflow

Other MovementsOther Movements

Currents under iceCurrents under ice Horizontal and Horizontal and

vertical currents vertical currents (horizontal greater)(horizontal greater)

Thermally induced by Thermally induced by convection from convection from accumulated heat accumulated heat flowing from flowing from sedimentssediments