Transcript 6-Hillslopes.ppt

Hillslope
Processes
Hillslope: the
elevated land between
valley bottoms.
Hillslope Processes:
movement of rock and
soil by mass wasting,
rain splash, and
overland flow
2 fundamental kinds of hillslope:
Bedrock
soil production < erosion
little to no soil cover with bedrock at surface
sediment transport is weathering limited
slope form controlled by rock properties
Soil-mantled
soil production ≥ erosion
surfical layer of soil covers bedrock
sediment transport is transport limited
slope form controlled by soil properties,
production, and transport rates
Hillslope Evolution
Weathering-Limited (bedrock)
steep slopes
thin to absent soil mantle
common in arid climates
also occur in humid areas
with rapid uplift and
downcutting low rate of
weathering (i.e., strong rocks)
rockfall and slides parallel
retreat at threshold angles
Bedrock Hillslopes
Bedrock slopes typical of arid and semi-arid
areas due to slow soil production
Totem Pole formation, Monument Valley, Navajo Tribal Park
Bedrock hillslopes, Brice Canyo, Utah
Hillslope Evolution
Transport-Limited (soil mantled)
gentle slopes
thick soil mantle
common in warm,
humid climates
soil creep and slides
convex profiles
Soil-Mantled Hillslopes
Soil-mantled slopes typical of humid and tropical
regions due to high pace of soil production
Soil-mantled hillslopes, Bolivia
Strength of Materials
Some basic concepts:
Stress - force per unit area
recall that force = mass * acceleration
(F = ma)
Normal stresses - forces acting perpendicular to area
Shear stresses - forces acting parallel to area
Strength of Materials
() angle of internal friction (a.k.a. friction angle)
characterizes the frictional resistance among particles
in a material
(C) cohesion
the inherent strength of a material in the absence of
any load
shear strength
the ability to resist deformation and fracture without
significant failure is a function of both  and C.
Strength of Materials
Mohr-Coulomb shear strength equation
The total shear strength (s) of a slope material is given by
s
=
C + ' tan
where ' is effective normal stress
Soil Strength Properites
Soil Strength:
typical values for sand and clay
Material
Friction Angle (°) Cohesion (kPa)
Dense uniform sand
Sandy glacial outwash
Soft organic clay
Stiff glacial clay
32 - 40
36 ± 11
22 -27
30 - 32
0
12 ± 9
20 - 50
70 - 150
Rock Strength Properties
Rock Strength:
intact versus field strength
Material
Intact sandstone (lab)
Intact shale (lab)
sandstone (field)
shale (field)
Friction Angle (°) Cohesion (kPa)
35 - 45
10,000 - 30,000
25 - 35
1,000 - 20,000
17 - 21
120 - 150
20 ± 6
69 ± 3
Rock mass strength = f (scale) due to discontinuities
Slope Stability
Slopes fail when shear stress > shear strength
Ratio of driving to resisting forces = factor of safety (FS)
FS = shear strength
shear stress
at failure FS = 1.0
slope stable if FS ≥ 1
slope unstable if FS ≤ 1
Shallow (soil) Landslides
Shallow Landsliding
mobilization of shallow material - primarily regolith
major geomorphological agent in mountain
landcapes
Occur during rainfall due to reduced shear
strength
Deep-Seated (bedrock) Landslides
Deep-Seated Landsliding
mobilization of deeper material - including bedrock
major geomorphological agent in mountain
landcapes
Occur during rainfall and earthquakes due to
reduced shear strength
Infinite-Slope Model
Driving force (shear stress):
t =
rsgd sinq
Resisting force (shear strength):
SS = C + ( – m) tan
where
 = rsgd cosq
and
m = rwgh cosq
And thus,
SS = C + (rsgd cosq – rwgh cosq) tan
Infinite-Slope Model
Hence the factor of safety or the ratio of resisting to driving
stresses is given by:
C + (rsgd cosq – rwgh cosq) tan
FS = SS / t =
rsgd sinq
which may be simplified to:
FS = SS / t =
where m = h/d
At failure FS = 1.0
C + gd cosq (rs – mrw) tan
rsgd sinq
C + gd cosq (rs – mrw) tan
FS =
rsgd sinq
Maximum stable angle for dry cohesionless material
(C = 0; m=0; FS = 1)
1=
rs g d cosq tan
rs g d sinq
=
cosq tan
sinq
=
tan
tanq
Hence, tanq = tan
The maximum stable angle for dry cohesionless material is
equal to the friction angle
C + gd cosq (rs – mrw) tan
FS =
rsgd sinq
Maximum stable angle for saturated cohesionless material
(C = 0; m = 1; FS = 1)
1=
g d cosq (rs - rw) tan
rs g d sinq
= (rs-rw) tan
rs tanq
Hence, tanq = [(rs-rw)/ rs] tan
Since rs ≈ 2 rw ,
tanq = tan
2
The maximum stable angle for saturated cohesionless
material is equal to half the friction angle.
Infinite-Slope Model
Debris flows typically occur on slopes of 26° to 45°.
For coarse sandy soils  ≤ 45°and so tan ≤ 1.0. The
maximum steepness of most soil-mantled slopes is about
45°.
Half the friction angle (tan/2 = 0.5) equates to a slope of
26°. Debris flows rarely initiate on slopes less than 26°.
Threshold Slopes
Once slopes reach their upper limiting stable
angle erosion rate becomes dominated by
landslide frequency rather than slope steepness.
Types of Mass Movement
There are 4 major mechanistic styles of mass movement:
Flow
Slide
Heave
Fall
Rates of mass movement vary substantially among these
types.
Mass Movement
Flow
Shear occurs throughout the
moving mass and there is
no well-defined shear plane
Shear is maximum at base
of flow
Mass Movement
Slide
Failure occurs along a welldefined shear plane.
Shear uniform through flow.
Resistance to movement
falls after failure; movement
continues until encountering
sufficient resistance, often
due to decreased slope.
Mass Movement
Heave
Downslope movement
arises from cyclic expansion
and contration biased by
gravity.
Typically results in slow
creep.
Mass Movement
Fall
Downward motion of rock or
soil through the air.
Only occurs from very steep
faces or cliffs.
Erosional Processes
•
•
•
•
Rainsplash
Soil “Creep”
Overland Flow
Mass Wasting
Rainsplash The transportation of sediment through the
sudden impact of a water droplets on unconsolidated
sediment.
Overland Flow
Erosion by overland flow is rare
in forested mountain
landscapes because:
Rainfall tends to infiltrate into
the ground
The ground has substantial
erosion resistance due to
vegetation
Erosion by overland flow is
most common in disturbed or
semi-arid landscapes
200 Meters
from Divide
~400 Meters
from Divide
~800 Meters
from Divide
1200 Meters
from Divide
Soil Creep
Slow -- too slow to perceive without measurement or
other indicator
Soil creep
i)
typically associated with tilted fence posts,
concave tree trunks, etc...
typically occurs at rates of mm's /yr or less
ii) Solifluction: slow flow of water-saturated
cohesive material, often aided by frost
(gelifluction)
Soil Creep
Figure 13.27
Pistol Butt trees, Nevada (soil creep)
“Terracettes”
terracettes
Rock Creep
Rock Creep
Slow gravity-driven
deformation of
mechanically
oversteepened
slopes can lead to
rock creep
Rock creep, Marathon, Texas
Rock Creep
Debris Flows
Debris flows
Slow to very rapid flow of
water-saturated soil and
rock debris down distinct
channels
Typically occur in coarsegrained soils
Tolt River, Washington
Debris Flows
Lots of internal deformation
Relatively high water
content / fluid-like flow
Failure typically occurs
along well-defined shear
plane at soil-bedrock
interface.
Tolt River, Washington
Debris Flows
Debris flows: rapidly moving slurries of soil, rock, and water
than can travel far.
Source area: controlled by soil moisture and slope
steepness; typically originate on slopes of 26° to 45°.
Scour and Transport zones: some shallow landslides travel
far and scour material from steep valley bottoms - they can
grow to 100 to 1000 times their initial volume.
Deposition zone: debris flows deposit once they reach
slopes low enough so that their shear strength can resist
further motion - typically 3° to 6°.
Debris Flows
Debris Flow Runout
Bouldery debris
flow runout,
eastern Sierra
Nevada south of
Reno, Nevada
Debris flow runout
Venezuela, 1999 after 8
inches of rain followed in two
Venezuela
weeks by 36 inches of rain.
Slides - slumps
Slumps slipping of
coherent rock or
soil units often
with a backward
rotation
Toe
Scarp
Slumps
Slumps exhibit:
Headscarp
Toe
Internal rotation of
blocks
Highly variable internal
deformation
Earth Flows
Earth flows - water-logged material moving slowly
typically occur in fine grained soil or rock
Slump / earthflow, central California
Translational Landslides
Translational, like a board sliding past another board
Landslide Damage
El Salvador
following a
magnitude 7.7
earthquake, 2001
Landslide Damage
La Conchita Landslide,2005
(10 fatalities, 13 homes destroyed)
La Conchita Landslide,1995
(destroyed 9 homes)
Rotational slump
Rotational slump in
Alaska
Note how slide has
pushed stream to far
side of valley bottom
Complex slump, Central California
Shallow, planar slide in remolded “soil” on glacial till, at a road cut in Vermont
Rotational
landslide,
Anchorage,
Alaska, 1964
Note older
landslide scarp
along
escarpment
The biggest landslide events are
termed “Rock Avalanches”
Volumes larger than 5 million cubic meters
Scarp
Rock Avalanche
Bedrock landslides can
limit the relief of
mountain ranges
Runout
zone
Mt. Cook, New Zealand:
– top 10 meters of
summit fell away in a
massive debris
avalanche on
December 14, 1991.
Deposit
Debris Avalanche
St. Elias Range, Alaska
Nevado Huascaran
Cordillera Blanca, Peru
1970 Earthquake
Failed near summit
Vertical drop of 4,100 m
Horizontal distance of
16 km
Velocities > 186 mph
Killed > 20,000 people
Rock Avalanches
Avalanches
Mass wasting of snow enhanced
by density differences between
layers from different snowfall
events…
Rock Falls
Falls
common in arid and
semi-arid lands, but
can be triggered by
earthquakes from
steep faces in any
environment
rapid and observable
Pasig-Potrero River, Philippines
Rock Fall
Rock fall produced by frost
heave, Yosemite NP(1996).
Glacier Point climbing area.
162,000-ton granite slab.
160 mph speed.
Killed several people.
Rockfall, Pasig-Potrero River, Philippines
Rockfall in action
• Tend to be minor events, but collectively can
generate substantial landscape change in some
locations.
Talus Slopes
Talus accumulates where basal removal < supply rate from face
Judean desert, Israel
Root Reinforcement
Vegetation Type
C' (kPa)
Coast Douglas fir (stumps)
Rocky Mt. Douglas fir (stumps)
SE Alaskan forest
16.7 ; 2.3
8.3 ; 2.9
Reference
Burroughs & Thomas (1977)
Burroughs & Thomas (1977)
3.4 - 5.9
Wu et al. (1979); Swanston (1970)
mixed hardwoods
alder
birch
hardwoods
Dunford(1983)
13.1
2 - 12
1.5 - 9.0
4.3
Reneau & Dietrich (1987)
Endo & Tsuruta (1969)
Takahasi (1968)
Riestenberg & Sovonick-
chaparral
burned chaparral
grass
swordfern
0 - 3.0
0.4 - 0.8
0.5 - 0.7
1.7
Terwilliger & Waldron (1991)
Terwilliger & Waldron (1991)
Terwilliger & Waldron (1991)
Burroughs (1984)
Debris flow scars at CB1 and CB2, Coos Bay, Oregon
CB2 debris flow, Coos Bay, Oregon
Headscarp of CB2 debris flow, Coos Bay, Oregon
CB2 debris flow, Coos Bay, Oregon
Debris flow occurred at base of hillslope hollow at transition
from hillslope to channel network
Hillslope Hollows
Problem: How do you get valleys to persist if they
naturally fill up with stuff?
Solution: you need another process - big rivers
clean out valleys, but what about small headwater
valleys that lack a channel?
Topographic convergence also generates wet
areas and the position of the water table
influences slope stability -- steep, convergent
areas tend to fail.
Hillslope Hollows
Cross-section of a colluvium-filled hollow, Washington
Hillslope Hollows
Gullies incised into hollow fills, Pacifica, California
Development downslope of hollows, Pacifica, California
1982 debris flow, Pacifica, California
Hollow drained by
constructed channel,
Northern California
Channel Initiation
Channel head: the upstream
limit of concentrated water
flow between banks
– transition between hillslopes
and channels
– “pivot point” in sediment
transport between diffusive
process and incisive process
Channel initiation requires
runoff
Channel initiation occurs by:
– saturated overland flow
– seepage erosion
– shallow landsliding
Xc is the critical distance from a drainage divide that it takes
to carve a channel.
Xc is the critical distance from a drainage divide that it takes
to carve a channel.
Cinder cone with rills, Kohala, Hawaii
Unchanneled valleys occur where the erosion resistance of
the ground surface is high relative to the amount of
overland flow  Xc is very large.
Unchanneled valleys
upslope of channel heads
are often sites of landslide
initiation due to the
convergence of both
colluvium (soil) due to soil
creep and to flow
convergence that
saturates accumuatled
soil (colluvium).
Convex hillslopes, Marin
County, California
Rills occur on slopes outside of valleys where the erosion
resistance of the ground surface is low relative to the
amount of overland flow  Xc is very small.
Rills, New Mexico
Rills on a roadcut, Brazil
Channels often begin at the base of hollows.
14 m across
3200 m across
Landscapes consist
of ridge and valley
topography at all
scales, but only finest
scale reveals the
actual valley network
and defines the
transition between
hillslopes and valley.
Montgomery and Dietrich, 1992, Science
c
a
b
d
Channel networks are
of finite extent.
The spacing of the
finest-scale valleys
depends on the
competition of valley
cutting and hillslope
eroding processes.
Fractal analysis breaks
down at the channelhillslope transition.
Channel Head Location
and Topography
Montgomery and Dietrich, 1989
Channel Initiation
– Channels initiate where sufficient
runoff accumulates to incise the
ground surface… by landsliding or
overland flow
– It takes less drainage area to
incise a channel on steep slopes
than on gentle ones.
Low-gradient, seepage controlled channel heads
Gully headcut, Tennessee Valley, California
Low-gradient, overland flow
controlled channel heads
United Kingdom
Tennessee Valley, California
Steep, landslide controlled
channel heads
Channels begin when enough runoff accumulated to overcome the
erosion resistance of the ground surface
Oregon Coast Range
N. California
channeled
channel head
unchanneled
S. California
Summary
Montgomery and Dietrich, 1992, Science
channeled
Application of
slope- area
channel
threshold to a
digital terrain
model
unchanneled
= hillslope
transition
Montgomery and Dietrich, 1992, Science
Observed
channel in the
field
On to rivers…