Water movements
Announcements• Canoe trip• Tuesday's lab: pick up
syllabus
Today:Last week's Q of the dayWater movementsBrainstorming independent
project ideas
Question of the day
In the summer, in this Northfield-area lake, why does the oxygen profile indicate low oxygen below the thermocline?
0 mg/l 8 mg/l
Surface gravity waves
Maximum wave length (m) influenced by maximum fetch (km)
Max. wave height = 0.332*(Fetchmax)0.5
Whitecaps occur at wind speed >4 - 5 m/s
Wave length
Surface gravity waves
• In deep water, water 'parcels' oscillate elliptically & do not travel horizontally with wave
• In shallow water, some horizontal movement when ellipses contact sediments
• If orbital velocity is great enough, (shallow water) sediments are re-suspended
• Scan of pg 182, fig 12-3
Currents (moving water)
• Mixing requires turbulent flow, not laminar flow
Eckman spiraling currents
• Caused by the wind• Resemble a spiral
staircase of horizontal currents that ultimate move the opposite direction of the wind
• Coriolis effect - trying to go straight on a turning planet
Convection (density) currents
Result:Onshore- offshore currents that exchange water between littoral and pelagic zones
a.k.a. The Thermal Siphon
Langmuir spirals
Wind direction
Langmuir spirals
Surface seiche (standing surface wave)
• Epilimnetic water piles up on one end of the lake due to sustained strong winds
• When wind ceases, water sloshes back (think: bathtub)• Usually, but not always, visible as minor changes in water
height.• In the Great Lakes, water levels can fluctuate meters.• Short-lived.
Lake Erie storm surges (from Ohio Sea Grant's Homepage)
Internal circulations
• Things are more turbulent below the surface than we might have thought…
• Lake Kinneret in Isreal as an example(thanks to folks at the Centre for Water Research, at the University of Western Australia)
Missing: Thermal Profile of Lake Kinneret over 4 days
Thermocline seiches
• Caused by wind stress forcing epilimnetic water towards one end of the lake
• The extra water tilts the thermocline
• When the wind lessens, the thermocline tilts back and forth
• Regular displacement in Lake Kinneret due to daily sea breezes
Missing: movies of thermocline waves…
Results of thermocline tilting
• Horizontal currents• Entrainment of epilimnetic waters or hypolimnetic
waters • Transfers nutrient-rich waters, heat, etc across the
thermocline• Erodes weak thermoclines
Currents created by seiches
The Dead Zone• Exposes benthic organisms
to oxygen-depleted waters
The Dead Zonethermocline