2. 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
3. Reynolds number (laminar
vs. turbulent flow)
• u=flow velocity;
l=characteristic length
(flow depth);
=kinematic
viscosity (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
6. 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)
7. 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
8.
9. Bedrock Rivers
• Erosion rate depends on slope
• Presence of sediment (‘tools’) increases erosion
30. 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
34. Gravity 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
35. Pore Pressure
• Debris flows have a high (>50%) proportion of sediment to
water and can be both subaerial and subaqueous
36. Pore 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)
37. Gravity 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