Process Geomorphology
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Transcript of Process Geomorphology
Process Geomorphology
9/20/2011
Pattern to Process,Process to Pattern
0.1
1
100 1000 10000 100000 1000000
Drainage Area m2
Slo
pe
(m/m
)
A1A4
A3A2
s1
EID1
EID21a 1b 2a
2b 2c
2d
3
2c-d
Moral of the Day
• “Process” occurs when “thresholds” are exceeded in the balance of driving and resisting forces.
Geomorphologic Processes
• FAR too broad to cover in this class• Interaction of climate, geology, vegetation,
solar energy, earth internal energy…
• Tectonic uplift creates potential energy• Climate acts on uplifted surface• Driving forces try to move material
downslope• Resisting forces try to keep material in
place
Geomorphologic Processes
• Driving and resisting forces meet in battle through MANY processes– Glacial– Aeolian– Coastal– Karst– Hillslope mass movement– Fluvial sediment transport
How does material move downslope?
• Hillslope vs fluvial• Diffusive vs incisive• Chronic vs discrete
Hillslope Processes
Hillslope Processes
• Continuum from dry to wet transport mechanisms– Landslide…debris flow…hyperconcentrated flow
Hillslope Processes
• Material must be available for transport– Tectonic uplift,– Various processes (landslides, creep, sheetwash…)
rely on different degrees of processing (weathering)
• Soil formation… (no within scope)
– We will pick up where sediment availability processes stop
• For each process, we assume the material is simply there
Fluvial Processes
• Hydraulic forces entrain or scour particles from bed and bank
Hillslope and Fluvial Processes
• Watersheds are transition “landforms” balancing uplift and denudation
• Denudation of available material involves– Initiation of motion– Translation– Deposition
• Hillslope and fluvial systems work together to denude watersheds– Hillslope deliver material to streams– Streams transport and incise, allowing for further hillslope
transport
Motion Initiation
• Initiation of motion for ALL processes is a force balance problem
• See Montgomery and Dietrich 1994– “…channel initiation mechanisms can be
considered threshold phenomena.”– Computes a critical drainage area required to
initiate a channel (threshold between diffusive and incisive erosion) based on “stability analysis”
Stability Analysis
• Where does acr come from?• Consider the engineering factor of safety
approach
– FS = (resisting forces/driving forces)
Example: Landslides
• Shallow– Masses of unconsolidated material break loose and slide
over underlying surface– Move on predefined planes– Occurs in upper soil layer– Translational
• Deep-seated– Occurs at depth, usually in clayey soils where rate of
increase in shear stress with depth exceeds rate of increase in shear strength so that at a certain deph there is a critical surface where mass is unstalbe
Example: Shallow landslides
• What are the driving and resisting forces for shallow landslide initiation?– Develop on board