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Water Movements14 September 2004

pp. 50-61 in Dodson

1. What is the difference between a wave and a current?

3. Which water movements are most important in the mixing

of heat, dissolved substances, etc.?

2. Which water movements are waves and which are currents?

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Wind blows across the surface of the lake

At the air-water boundary wind velocity decreases due to

friction

This produces shear forces at lake surface, which produce:

1. Waves

2. Currents

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Waves and currents usually occur together, but there

are differences

Water can also be set in motion by:

Changing atmospheric pressure

Influx of water to a lake (i.e. river)

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Waves: Periodic motion of the water

Rise and fall of water particles with little net flow

Unless it is a breaking wave, there little disturbance of

the deeper water

Direction of the wind

= wavelength

h = wave height

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The diameter of the circle is halved for each increment of/9Example: If = 18m and h = 1m

Top circle has a vertical oscillation of 1m

Bottom circle (4m) has a vertical oscillation is 25 cm

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White-caps are the result

of extreme turbulence at

the air-water interface,not wave size

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2. h is proportional to the square root of the fetch

h = 0.105* (fetch)

What factors influence wave height (h)?

1. h is proportional to lake depth

This is most true in deep lakes. In shallow lakes, thebottom also influences wave height

In general, the bigger the lake, the bigger the waves

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In general, little energy is transferred from the wind in

surface waves, because the water molecules justmove up and down in a circle.

In a breaking wave, top water falls into the waterbelow and energy is transferred to the water as

eddies

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Eddyan area of random, chaotic tumbling of water particles

Eddies result in Turbulent Mixing

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Laminar flow is smooth and orderly,dominated by viscous forces (Re < 500)

Turbulent flow contains a swirling of water

and chaotic eddies (Re > 2000)

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Surface currents move water, which results in mixing

Currents: Non periodic net unidirectional flow of water

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In a stratified lake, the wind sets up currents in the

epilimnion and hypolimnion

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When the wind blows for a long time, water piles

up at the downwind end of the lake

Some

mixing at

boundary

May be

some heatgain to the

hypolimnion

Reaches a stable configuration in a steady wind

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What happens when the wind stops blowing?

The current has set up a long wave

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The water will flow back towards its original

position, but because of momentum it

overshoots the starting point

Internal seiche rocking of the

thermocline. An internal wave

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Can see the seiche with temperature profiles

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Lake Ontario

Kalff 2002

Hypolimnetic water is often rich in limiting nutrients

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How might these internal waves

influence the biology of

organisms?

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In large

lakes, the

seiche can

move due

to the

Coriolis

Force

Kelvin Wave

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Can have internal breaking waves.

Breaking waves result in mixing between the

epilimnion and hypolimnion

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Surface Seiche a standing surface wave

A seiche can persist for several days

In a stratified lake, a surface seiche can start an

internal seiche

A 1 cm surface seiche can produce a 6 m

internal seiche

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Where:

The period of oscillation of a surface seiche:

t = period (seconds)L = length of the basin (m)

g = acceleration due to gravity (9.81 m sec-2)

t = 2 L

z = mean depth (m)

(g z)

F i t l i h d t id d it

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For an internal seiche, need to consider density

differences between water masses

ze= thickness of epilimnion

zh = thickness of hypolimnion

De = mean density of water in epilimnion

Dh

= density of water in hypolimnion

t =

Distance traversed

Velocity of the internal wave=

2L

g (Dh De)

Dh

zh

De

ze+

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Langmuir Circulationparallel helical currentsKalff 2002

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Kalff 2002

The local areas of upwelling and downwelling leads to a sorting of particles

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Can see streaks or windrows on the

surface of the lake

http://homepages.cae.wisc.edu/~chinwu/CEE514_Coastal_Engineering/2001_Students_Web/Dave_Calkins/streaks.gif

Currents in large lakes:

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The displacement gets accentuated down the water

column

Currents in large lakes:

Ekman Spiral

The water moves 45 to the wind due to the Coriolis

Force.

Right in N. hemisphere, left in S. hemisphere

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Get turbulence and mixing due to shear

force at the boundary of water masses

In an Ekman Spiral there is return

flow at the thermocline

Think of it as different water massesmoving in slightly different directions

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Currents and breaking waves

cause mixing

Mixing moves heat, gases, nutrients

etc. throughout the water column

Mixing influences thermal stratification

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Terms to know

wave

current

fetch

eddy

laminar flowturbulent flow

Reinternal seiche

surface seicheLangmuir circulation

Ekman spiral