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LECTURE 1 WHAT IS A TURBIDITY CURRENT?

CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS. LECTURE 1 WHAT IS A TURBIDITY CURRENT?. Turbidity current driven by crushed coal moving down bottom of flume containing fresh water: Garcia Tank, SAFL, University of Minnesota. A TURBIDITY CURRENT IN ACTION.

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LECTURE 1 WHAT IS A TURBIDITY CURRENT?

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  1. CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS LECTURE 1 WHAT IS A TURBIDITY CURRENT? Turbidity current driven by crushed coal moving down bottom of flume containing fresh water: Garcia Tank, SAFL, University of Minnesota

  2. A TURBIDITY CURRENT IN ACTION Turbidity current driven by plastic particles: Experiment of O. Sequeiros and H. Naruse conn13.avi

  3. AN ANALOG OF A TURBIDITY CURRENT: POWDER SNOW AVALANCHE Video clip courtesy P. Gauer AvalancheFin01GauerP.avi

  4. AN STARTING POINT: THE BOSPHORUS Black Sea Bosphorus Istanbul TURKEY Sea of Marmara

  5. THE SETTING: THE MEDITERRANEAN AND BLACK SEAS Black Sea Istanbul Sea of Marmara Mediterranean Sea

  6. DIFFERENCE BETWEEN THE MEDITERRANEAN SEA AND BLACK SEA Mediterranean Sea Bosphorus Black Sea Sea of Marmara

  7. THE MEDITERRANEAN SEA AND THE BLACK SEA • The Mediterranean Sea receives little freshwater inflow, and has a high evaporation rate. • Mediterranean water is thus rather salty. • The Black Sea receives a substantial flow of fresh water. • Black Sea water is thus less salty. • The saltier the water, the higher is the density. • The net flow through the Bosphorus is from the Black Sea to the Mediterranean Sea (through the Sea of Marmara). Black Sea: less salty flow Sea of Marmara: more salty

  8. FLOW THROUGH THE BOSPHORUS The flow through the Bosphorus is so strong that in ancient times, ships could neither row nor sail into the Black Sea.

  9. SO HOW DID SHIPS GET UP THE BOSPHORUS TO THE BLACK SEA IN ANCIENT TIMES? Black Sea Byzantium/ Nea Roma/ Constantinople/ Istanbul Sea of Marmara

  10. CURRENT AND COUNTERCURRENT! The strong surface flow of less salty water from the Black Sea to the Sea of Marmara is accompanied by a less strong (but still very strong) flow of more salty water from the Sea of Marmara (ultimately Mediterranean Sea) to the Black Sea: Dense bottom flow.

  11. WATER SAIL! THE ANCIENT SOLUTION: THE WATER SAIL!

  12. TURBIDITY CURRENTS OBTAIN THEIR DRIVING FORCE FROM THE EXTRA WEIGHT OF SEDIMENT IN SUSPENSION

  13. ARCHIMEDES’ PRINCIPLE body = density of material in a “body” (control volume? sediment grain?) amb = density of the ambient fluid in which it is immersed V = volume of the “body” g = gravitational acceleration Fbuoy The weight of the body W is given as The buoyant force Fbuoy acting on the body is given as W The effective immersed weight of the body Wimm is then given as

  14. FRESH AND SEA WATER DENSITY Fresh water density depends on Temperature (C) Sea water density depends on: Salinity (mg/l of salt ~ ppm of salt) Temperature (C) “Standard” density of fresh water: 1.00 ton/m3 = 1000 kg/m3 “Typical” density of salt water: 1.027 tons/m3 = 1027 kg/m3 (but can vary considerably) Dead Sea saltwater density: ~ 1.17 tons/m3 Dead Sea http://www.reliefmart.com/dead-sea/dead_sea_sunset.jpg Water density calculator: http://www.csgnetwork.com/h2odenscalc.html

  15. WHAT CAUSES FLOW DOWN A SLOPE? CASE: WATER UNDER AIR ~ RIVER a = density of air ( ~ 1.2 kg/m3: use gas law) w = density of water ( ~ 1000 kg/m3 for fresh water)  = bed slope angle, so that slope S = tan The control volume is full of water under air. It has length L and cross-sectional area A. The immersed weight of the control volume is The downslope component of this immersed weight Fgd drives the flow downslope: Fgd Wimm

  16. BUT FOR WATER UNDER AIR, a = density of air ( ~ 1.2 kg/m3: use gas law) w = density of water ( ~ 1000 kg/m3 for fresh water) So The immersed weight of the control volume is The downslope component of this immersed weight Fgd drives the flow downslope: Fgd Wimm

  17. WHAT CAUSES FLOW DOWN A SLOPE? CASE: WATER UNDER THE SAME WATER w = density of water ( ~ 1000 kg/m3 for fresh water)  = bed slope angle, so that slope S = tan The control volume is full of water under the same water. It has length L and cross-sectional area A. The immersed weight of the control volume is The downslope component of this immersed weight Fgd drives the flow downslope: Fgd Wimm No flow!

  18. WHAT CAUSES FLOW DOWN A SLOPE? CASE: SALINE WATER UNDER FRESH WATER f = density of fresh water ( ~ 1000 kg/m3) sal = density of saline water ( ~ 1027 kg/m3 for sea water)  = bed slope angle, so that slope S = tan The control volume is full of saline water immersed in fresh water. It has length L and cross-sectional area A. The immersed weight of the control volume is The downslope component of this immersed weight Fgd drives the flow downslope: Fgd Wimm

  19. LET’S COMPARE Fresh water flowing under air = open channel flow = RIVER Saline water flowing under less saline (e.g. fresh) ambient water = SALINE BOTTOM UNDERFLOW Let’s compare the downslope driving force Fgd,saline for standard seawater under fresh water versus fresh water under air Fgd,river Using f = 1000 kg/m3, sal = 1027 kg/m3 and a = 1.22 kg/m3, The saline underflow has only 2.7% of the driving force of a corresponding river!

  20. WHAT CAUSES FLOW DOWN A SLOPE? CASE: SEDIMENT-LADEN WATER UNDER SEDIMENT-FREE WATER: TURBIDITY CURRENT w = density of water ( ~ 1000 kg/m3) t = density of underflowing water + sediment = density of turbidity current > w How do we compute t? sed = density of sediment (quartz ~ 2650 kg/m3) c = volume concentration of sediment in suspension c = (volume sediment)/[volume sediment + volume water] Density f of sediment-water mixture is given as

  21. WHAT CAUSES FLOW DOWN A SLOPE? CASE: TURBIDITY CURRENT w = density of ambient water ( could be fresh or saline: ~ 1000 kg/m3) t = density of turbid water in flow  = bed slope angle, so that slope S = tan The control volume is full of turbid water. It has length L and cross-sectional area A. The immersed weight of the control volume is The downslope component of this immersed weight Fgd drives the flow downslope:

  22. CASE: TURBIDITY CURRENT Driving force of a turbidity current Thus Compare the driving forces of a river and a turbidity current: So how large can c be?

  23. HOW LARGE CAN THE SEDIMENT CONCENTRATION IN A TURBIDITY CURRENT BE? The sediment in a turbidity current that derives it must be in suspension, i.e. dispersed in the water column away from the bed. Both rivers and turbidity currents carry suspended sediment To qualify as a river suspension or turbidity current, the concentration of suspended sediment must be dilute, so that Thus since R ~ 1.65, A turbidity current thus has much less driving force than a river carrying the same concentration of suspended sediment!

  24. WHAT HAPPENS IF THE CONCENTRATION IS NOT DILUTE? • Turbidity currents and submarine (subaqueous) debris flowsare members of the class of dense bottom flows that includes thermohaline bottom flows (e.g. Straits of Gibraltar or Bosporus). • The presence of a dilute suspension of • sediment in the water of a turbidity current • renders it slightlyheavier than the ambient water. • Asubmarine (subaqueous) debris flows consists of a dense sediment-water slurry that is much heavier than the ambient water, so creating its own sediment-water rheology. • In both cases gravity pulls the sediment downslope, and sediment pulls the water downslope. • Turbidity currents and submarine (subaqueous) debris flows differ from a thermohaline underflows in that it is free to exchange sediment with the bed. • A turbidity current is the subaqueous analog of a river. A submarine (subaqueous) debris flow is the subaqueous analog of subaerial debris flow.

  25. AN EXAMPLE OF A SUBAERIAL DEBRIS FLOW rte-bookjapandebflow.mpg

  26. AN EXAMPLE OF A SUBAQUEOUS DEBRIS FLOW IlstMain_cam3.avi

  27. THE FUNDAMENTAL DIFFERENCE BETWEEN A RIVER AND A TURBIDITY CURRENT A river flows downslope under the influence of gravity acting on the water. The water then drags the sediment with it. The suspended sediment it carries adds only slightly to the driving force as long as c << 1). A turbidity current flows downslope under the influence of gravity acting on the sediment. The sediment then drags the water with it. Note that turbidity currents must die, but river flows do not die, as c  0. Turbidity currents must find a way to keep their sediment in suspension if they are to sustain themselves!

  28. VELOCITY AND SUSPENDED SEDIMENT CONCENTRATION PROFILES IN RIVERS AND TURBIDITY CURRENTS A river is sediment-laden water flowing under air. A turbidity current is sediment-laden water flowing under sediment-free water. In the image below u = streamwise flow velocity, c = volume suspended sediment concentration Rivers (usually) form sharp interfaces with the air above, and turbidity currents (usually) form more diffuse interfaces with the water above.

  29. Source Material (for Parker only) TurbCurrAAPGApril06.ppt ExxonMobilShortCourse06.ppt TurbidityCurrentMinutes.ppt

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