Flowing Water: Sediment Transport and Landforms. Medium-term Plan 10/27Lecture 13. The Sediment...
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Transcript of Flowing Water: Sediment Transport and Landforms. Medium-term Plan 10/27Lecture 13. The Sediment...
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Flowing Water: Sediment Transport and Landforms
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Medium-term Plan
10/27 Lecture 13. The Sediment Factory: Source to Sink
11/01 Lecture 14. Flowing Water: Sediment Transport and Landforms
11/03 Lecture 15. Depositional Systems(HW #4 assigned) 11/08 Lecture 16. Dating the Sedimentary Record (Thompson)
(HW #4 due) 11/10 Lecture 17. Ice Age Cycles (Thompson)
(HW #5?) 11/15 Lecture 18: Waves and Coastal Morphodynamics 11/17 Lecture 19: Shorelines (HW #6 assigned)
11/22 The Anthropocene: Humans as Geomorphic Agents
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Reynolds number (laminar vs. turbulent flow)
• u=flow velocity; l=characteristic length (flow depth); =kinematicviscosity (dynamic
viscosity/fluid density)(water ~ 1.5x10-6 m2/s)
• Turbulence is promoted by high flow velocities and flow depths, and low viscosities (Re>2000); laminar flow occurs at Re<500
• Air and water are nearly always turbulent
ul
Re
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River Transport
Transport of sediment depends upon
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Settling Velocity and Cohesion
Play bdld.mov
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Rivers: Sediment transport
• Three modes:• Dissolved load/wash load (ions in solution - pollution)
• Suspended load
– Fine particles (sand, silt & clay)
– Turbulent eddies pick up, carry upward if vel. > settling vel.
• Bedload
– On/near bed; rolling, bouncing (‘saltating’), etc.
• Suspended and bedload increase rapidly with flow strength (nonlinear relationship)
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Rivers: Two main kinds
• Alluvial rivers; bed consists of sediment (‘alluvium’ = river-associated sediment)– Downstream reaches
• Bedrock rivers; part of the bed is bare rock, where river cutting down – generally in upper reaches of rivers
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Bedrock Rivers
• Erosion rate depends on slope
• Presence of sediment (‘tools’) increases erosion
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Photo by Duncan Heron
Alluvial Rivers
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Landform: Floodplain
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Landform: Floodplain
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Braided stream
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• Braided streams are bedload dominated
• Nonlinear sediment transport laws result in dynamic feedbacks
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Meandering stream; Point Bar and Cut Bank
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Photo by Duncan Heron
Point Bars and Cutbanks along river meanders
Santee River, SC
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Neuse River, NC
Note point bars
Photo by Duncan Heron
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Oxbow lake formation
Play meander.mov, sm1.mov
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Incised Meanders
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Natural Levee formation
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Photo by Duncan Heron
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Artificial Levees
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Levee Failures
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Crevasse Splay Deposits, Mississippi River
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Natural River - 1948
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1964
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Drainage Basins
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Graded Stream Profile• Flow increases downstream (tributaries)• Velocity Increases• Equilibrium slope reduces as flow increases
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Graded Stream Profile
• Each stretch of alluvial river tends to have slope adjusted to transport sediment delivered to it
• Slope too low, sediment piles up at upstream end
-> slope increases• Slope too high, erosion (less in than out) at
upstream end
-> slope decreases
• Need steeper slope with• Less flow• Larger grains
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Base Level Changes
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Dam cuts off sediment flux
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Shelf Transport SystemShelf Transport System
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Gravity FlowsGravity Flows
• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous• Can occur on land or underwater (Pratson.mov)
• Turbidity currents have a higher proportion of water, are always subaqueous, and move due to density contrasts
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Pore PressurePore Pressure
• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous
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Pore PressurePore Pressure
• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous
• Terrestrial flows: initial sediment packing affects type of flow
Pratson.mov
Pdfmod (weak debris flow)
Pdfst6.mov (Strong debris flow)
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Gravity FlowsGravity Flows
• Debris flows have a high (>50%) proportion of sediment to water and can be both subaerial and subaqueous• Can occur on land or underwater (Pratson.mov)
• Turbidity currents have a higher proportion of water, are always subaqueous, and move due to density contrasts
• The presence of a dilute suspension of sediment in the water of a turbidity current renders it slightly heavier than the ambient water.• This results in downslope movement of both the sediment and entrained water (or vice versa).
• Sediment suspension can be from:• catastrophic event (earthquake) • flow-generated turbulence (autosuspension).• wave stirring
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Turbidity CurrentsTurbidity Currents
turbwg.mov (turbidity current)
Undf.mov (unconfined tc)
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Turbidity CurrentsTurbidity Currents
• Turbidity currents also create levees, but can overtop them frequently
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TURBIDITY CURRENTS – constructional and erosional
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Passive (NJ/NY) Shelf
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Monterey Submarine Canyon