Failure of slopes depends on direction of groundwater flow but in many cases surface parallel flow is an adequate assumption Failure also depends on a balance between the intensity and duration of rainfall ID: 276229
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Slide1
Key concepts
Failure of slopes depends on direction of groundwater flow, but in many cases surface parallel flow is an adequate assumption
Failure also depends on a balance between the intensity and duration of rainfall
Larger landslides have relatively longer
runout
distances, and therefore lower ‘effective’ friction.
Wet material can have extremely long
runout
distancesSlide2
Key concepts
Debris flows vs. landslides: It is all about the water!
Critical state porosity in soils
Characteristics of debris flows
Long
runout
Kinematic sorting
Erosion of landscapes
Supply and storage of sediment
Colluvial
hollow cycling and triggeringSlide3
Eroding Landscapes
Hillslope
sediment transportSlide4
What follows is a long derivation of the mass balance of a
hillslope
soil
Q: Why go through this?
A1: It serves as the basis for all quantitative
hillslope
studies
A2: The thought process is the same for channels, glaciers, etc.
Alan HowardSlide5
Meaningful predictions require quantitative analyses!
Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes?
Alan HowardSlide6
Meaningful predictions require quantitative analyses!
Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes?
Why would you want to know?
Landslide risk
Soil loss
Changing soil properties changes hydrology
Downstream effects
Alan HowardSlide7
Meaningful predictions require quantitative analyses!
Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes?
A1: Qualitative: More sediment will come out.
A2: Quantitative: There is a 50% chance that erosion rates will increase from 0.1mm/
yr
to 0.17mm/
yr
, leading to 30% greater risk of flooding in your town.
Alan HowardSlide8
What is this course about? Many kinds of eroding landscapes
Upper Mekong River Basin
Chinese karst terrain
Chinese Loess PlateauSlide9
Basic observations about
hillslopes and rivers
Hillslopes tend to be round.
Valleys frequently have regular spacing.
Roering
et al. (2007), EPSLSlide10
Basic observations about
hillslopes and rivers
Slope area data (sometimes) shows a ‘boomerang’ pattern.
Grieve, PhD (former GPG student!)Slide11
Now for the mass
balance (this will be done on the board but these notes accompany the lecture)
Alan HowardSlide12
Here is a landscape with some rivers and
hillslopesSlide13
Zoom inSlide14Slide15
For convenience, pick a planar hillslope
(i.e., not convergent, not divergent)Slide16
A generic
hillslope
stripSlide17Slide18Slide19Slide20
Now look from the sideSlide21
Side viewSlide22
Look at mass in and outSlide23Slide24
Ignore mass exchange at the surfaceSlide25Slide26Slide27
Aside: dimensions and units
Express dimensions in
M for mass
L for length
T for time
Units can be
Mass: kilograms, grams, etc.
Length: metres, cm
Time: seconds, hours, days, etcSlide28
Mass in box: Check dimensions
Mass in box is:
ρ
s
*h*
dA
Dimensions of
ρ
s: M/L
3Dimensions of h: LDimension of dA: L2
So dimensions
ρ
s
*h*
dA
: M*L*L
2
/L
3
= M
In my personal experience, this is the easiest way to check if you are doing things correctly. Slide29Slide30
r
is the density of the parent materialSlide31Slide32Slide33Slide34Slide35Slide36Slide37Slide38Slide39Slide40Slide41Slide42Slide43
What could determine
q
s
?
What we need is a ‘sediment flux law’Slide44
Sediment flux laws
A sediment flux law tells us how
q
s
varies as a function of the properties of the landscapeSlide45
Sediment flux laws
So, you tell me the slope angle, or the amount of overland flow, or the number of gophers, and I’ll tell you how much dirt is moving on your
hillslopeSlide46
A very simple case
The steeper the slope, the more sediment fluxSlide47
A very simple case
The steeper the slope, the more sediment flux
This is called a ‘linear’ sediment flux law
Proposed by Culling, W.E.H., 1960. Analytical Theory of Erosion. Journal of Geology, 68(3): 336-344.Slide48
A very simple case
The steeper the slope, the more sediment flux
This is called a ‘linear’ sediment flux law
Proposed by Culling, W.E.H., 1960. Analytical Theory of Erosion. Journal of Geology, 68(3): 336-344.Slide49
Slightly more complex
When the slope gets very steep (near the angle of repose), particles start to slide downslope. This sediment transport gets very fast as the slope approaches some critical value
Proposed by
Andrews, D.J. and Bucknam, R.C., 1987. Journal of Geophysical Research-Solid Earth and Planets, 92(B12): 12857-12867.
Popularized by
Roering, J.J., Kirchner, J.W. and Dietrich, W.E., 1999. Water Resources Research, 35(3): 853-870.Slide50
Slightly more complex
When the slope gets very steep (near the angle of repose), particles start to slide downslope. This sediment transport gets very fast as the slope approaches some critical value
Proposed by
Andrews, D.J. and Bucknam, R.C., 1987. Journal of Geophysical Research-Solid Earth and Planets, 92(B12): 12857-12867.
Popularized by
Roering, J.J., Kirchner, J.W. and Dietrich, W.E., 1999. Water Resources Research, 35(3): 853-870.Slide51
Sediment flux laws: we’ll come back to this later
First lets go back to the hillslope mass balanceSlide52
Introduce a concept: Steady state:
Steady state refers to a condition in which something doesn't change in time.
What is ‘something’?
Topography?
Topography relative to some moving datum?
Soil thickness?
Soil production?Slide53
Soil production
Soil coming in minus soil going out
Change in the amount of soil in the boxSlide54Slide55Slide56Slide57Slide58Slide59Slide60
but
h
new
=h
oldSlide61Slide62Slide63Slide64Slide65
Okay, lets go back to looking at the
hillslope
stripSlide66Slide67
Because it is at the divideSlide68Slide69Slide70Slide71Slide72
What came out of box 1 goes into box 2.Slide73Slide74Slide75Slide76Slide77Slide78
Sediment flux laws: reminderSlide79Slide80
Check dimensionsSlide81Slide82
Implication?
Slope increases as you move away from the divide!!!Slide83Slide84Slide85Slide86
A steadily eroding hillslope with linear creep is convex up!Slide87
So that explains the top part of this profile, but not the bottomSlide88Slide89
Simplest creep flux law:
What about water?
Probably also depends on slope.
But also depends on how much water is there Slide90
What about water?
Simplest sediment flux law that accounts for water:
Greater slope, more transport.
More drainage area, more water
More water, faster erosionSlide91Slide92Slide93Slide94Slide95Slide96Slide97Slide98
This is a fundamental result in geomorphology!!Slide99
This is a fundamental result in geomorphology!!
Convex: creep like processes dominate
Concave: fluvial and wash processes dominateSlide100Slide101Slide102Slide103Slide104
Roughening and smoothing
Creep like processes smooth the landscape
Wash and fluvial processes roughen the landscapeSlide105
Roughening and smoothing are in competition!!!
In balance at the channel head!Slide106
Two end members
In creep-like domain:
Greater area means you need greater slope to erode at the same rate, since you have more sediment to transport and sediment transport depends only on slope
In fluvial/wash domain:
Greater area means you need a gentler slope to erode at the same rate because the water you gain is very efficient at eroding and transporting sedimentSlide107
So, looking at slope area plots:Slide108
Slope-area plots can be used to compare landscapes, and the competition between creep and fluvial processesSlide109
Relationship between valley spacing and uplift rates unclear (hard to distinguish from climate, parent material,
etc
)
But transition between channel and
hillslope
can now be determined for large areas using digital elevation models
Roering et al. (2007), EPSLSlide110
Process and form
Creep only
Threshold landslidingSlide111
Process and form
On a theoretical basis, one could pick out process transitions from slope – contributing area relationshipsSlide112
Key points
Creep leads to fundamentally different topography than water transport
Creep
smoothes
the landscape, water roughens it
One can search for different process regimes by looking at S-A plots
Reading: Anderson and Anderson Chapter 10 and section on page 592 about derivativesSlide113
Calculus note