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