Process Geomorphology
9/20/2011
Pattern to Process,
Process to Pattern
1
2b
A2
2c
A3
2c-d
s1
Slope (m/m)
3
1a
2a
1b
2d
EID2
A1
A4
EID1
0.1
100
1000
10000
Drainage Area m2
100000
1000000
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