Hillslopes - moving mass: creep,slides and flows

1. Force balance - driving vs resisting forces

2. Soil creep - slope dependent, diffusive processes

3. Mass movements - landslides and rock falls

4. Flows - of debris and mud

FORCE BALANCE

GRAVITY

SHEAR STRENGTH (FRICTION

AND COHESION or VISCOSITY)

DRIVING VERSUS RESISTING

http://ees.as.uky.edu/sites/default/files/elearning/module11swf.swf

Processes that move mass on slopes

first consider infiltration rate

Rainsplash (bare soil only)Rainsplash (bare soil only) Channels (ephemeral, permanent) Channels (ephemeral, permanent) Creep (grab-bag, many processes)Creep (grab-bag, many processes) Mass movement (landsliding = translational, Mass movement (landsliding = translational,

rotational and rockfall)rotational and rockfall) Flowage (debris and mud)Flowage (debris and mud)

Slope types – connect to process

Rain splashRainsplash - splatering, downslope component

2

Where flow is thin, rainsplash disturbance prevents channelization

Incision begins when shear stress exceeds

strength

2

Critical Distance from drainage divide = channelization2

Soil Creep - evidence

Marathon Basin, west Texas. J.S. Aber.Marathon Basin, west Texas. J.S. Aber.

3

Which way is soil moving?

Rate is a function of slope Q= ks

Other biotic effects - vary by climate and location

3

Mammals (gophers here)3

NET DOWNSLOPE TRANSPORT

Rotational failure - clay, silt, homogeneous material

Rotational Failures

Bishop method of slices

Importance of unique events/thresholds

Discrete vs. continuous processesStorms

DisturbanceLandslides

Debris Flows

2000

2600

5900680082009400

The Ritterbush Record 50 cm

Cal

end

ar Y

ears

BP

AMS 14C Analysis

Storm Frequency

• 4 maxima; currently increasing—observed?

0 2 4 6 8 10 12

Calendar kyr BP

NorthAtlanticClimate

GISP2 nssK

GISP2 ssNa

Coolings

Storm

Freq.

StormsFloods

0 2 4 6 8 10 12Calendar kyr BP

Hillslope- channel transition, colluvial hollows

Bank erosion?