Process Geomorphology 9/20/2011

Pattern to Process, Process to Pattern

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

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 a cr 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