Transcript Debris flows

Flowing Water: Sediment
Transport and Landforms
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
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)
Re 
ul

• 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
River Transport
Transport of sediment depends upon
Settling Velocity and Cohesion
Play bdld.mov
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)
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
Bedrock Rivers
• Erosion rate depends on slope
• Presence of sediment (‘tools’) increases erosion
Alluvial Rivers
Photo by Duncan Heron
Landform: Floodplain
Landform: Floodplain
Braided stream
• Braided streams are
bedload dominated
• Nonlinear sediment
transport laws result in
dynamic feedbacks
Meandering stream; Point Bar and Cut Bank
Point Bars and Cutbanks along river meanders
Santee River, SC
Photo by Duncan Heron
Neuse River, NC
Note point bars
Photo by Duncan Heron
Oxbow lake formation
Play meander.mov, sm1.mov
Incised Meanders
Natural Levee formation
Photo by Duncan Heron
Artificial Levees
Levee Failures
Crevasse Splay Deposits, Mississippi River
Natural River - 1948
1964
Drainage Basins
Graded Stream Profile
• Flow increases downstream (tributaries)
• Velocity Increases
• Equilibrium slope reduces as flow increases
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
Base Level Changes
Dam cuts off sediment flux
Shelf Transport System
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
Pore Pressure
• Debris flows have a high (>50%) proportion of sediment to
water and can be both subaerial and subaqueous
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)
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
Turbidity Currents
turbwg.mov (turbidity current)
Undf.mov (unconfined tc)
Turbidity Currents
• Turbidity currents also create levees, but can overtop them
frequently
TURBIDITY CURRENTS – constructional and erosional
Passive (NJ/NY) Shelf
Monterey Submarine Canyon